CB Radio

[BigMike]

well i got a CB Radio for graduation. but i know nothin about them. it came with a magnetic antenna base and a 2'6" antenna. now how do i go about calibrating it? what kinda antenna can i get for better reception? anything else i should know? it came with no instructions, slightly used but other wise in good condition. oh where have you guys been mounting the radio it self? there seems to be a lack of space for it in the 1st generation Pathfinder.

[88pathoffroad]

Mine's mounted right below the stock stereo, but in the 90-up Pathys, there's another DIN-sized changeholder/storage deal under there so there's no room. I dunno. You'll have to hold the CB in several different places to see where it'll fit.

 

Here's some very useful CB radio information.

 

No-Ground-Plane antenna systems have a very specific purpose. They should be the system of choice when you have no other choice. When your vehicle has little or no metallic surface area for the antenna to use as its NEEDED counterpoise your decision process should be;

 

1. I just won't have a CB in my vehicle.

2. I will use a ground plane (GP) system and suffer the possible consequences.

3. I will use a no-ground-plane (NGP) system and be able to use my CB.

 

In short, the NGP system (we call it a system because the antenna and coaxial cable are a matched set that may not be interchanged with other NGP antennas and coax assemblies) is a problem solver. If your vehicle does not provide sufficient ground plane for a regular GP antenna, the NGP system will solve the problem. Who should use an NGP system? Here are some probabilities.

 

* Fiberglass or plastic vehicles

* Hot air balloons

* Wilderness back pack frames

* Fiberglass pick-up bed caps

* Aluminum and/or fiberglass boats and canoes

* Aluminum and/or fiberglass cab-over campers (antenna on camper)

* Aluminum and/or fiberglass travel trailers

 

The NGP systems are not "required" on metal vehicles. They will work but you are almost better off using a GP set-up if you have the reflective surface available. The ground wave field strength of a GP antenna on a metallic surface is about 15% stronger than a NGP system on the same vehicle in the same location. This is directly due to the way that the radio's power is delivered to the antenna via the special cable. There is some energy absorption within the cable assembly. However, and again we stress the specific purpose of the NGP system, it is better to have some energy absorbed in the cable assembly than it is to have no communications, or very poor performance with a GP set-up.

 

If your vehicle fits one of the above mentioned profiles and you are doing the first install, you should think of using an NGP system. Furthermore, if your vehicle fits the profile and you already have a disappointing performing antenna, you are a prime candidate for a change. Do keep in mind that the NGP antenna system will not fix the problems that are due to poor installation locations. That is, if you've mounted a GP antenna in a manner that prevents it from radiating energy into free space (usually shows up as an unmanageable SWR problem), the NGP antenna will fail as well.

 

As with most matters involving communications, we try to lay down some basic rules to help you before you get into too much trouble. While theory is okay for grasping a basic concept, if you let it give you tunnel vision you will probably run into a problem from time to time. We like a foundation of theory but cannot ignore over 30 years of actual experience in matters involving CB communications. Accordingly, we know there are exceptions to everything we write and say. But, when we write we need to aim the content towards the majority. It is beyond our capacity to write installation guidelines for every vehicle on the road and every possible location that an antenna could be mounted on the vehicle. For instance, on some fiberglass motorhomes with a steel substructure it is possible to tap into the underlying structure and get a GP antenna to work just fine. Likewise, many Corvette owners have attached a long mounting bracket to the right-rear frame and the good chassis ground allows the use of a regular GP antenna set-up. But, how do you explain to someone who just spent half a day routing cables and installing mounts that their problem is due to insufficient ground plane and they need to tear the whole thing apart and start all over.

 

There are some things you can do before you get too involved in the installation. First of all, find the metal. If you can't … don't even fool with a GP set-up. If you do, make ABSOLUTELY sure that your antenna mount is grounded to the vehicle in some manner, if not directly then with a short grounding braid or wire (minimum of 12 gauge). The fact that the coax cable may be grounded at the radio connection is NOT sufficient and does not exempt you from having a good chassis ground on the mount of a GP set-up. If you mount a GP set-up on an insulated roof rack, ladder, or spare tire rack (most of which have no or intermittent grounding), metal vehicle or not, you must run a ground from the vehicle to the mount. If you aren't sure what to do, you should find someone who can give you some help. Worse case, if you have a GP set-up and are wondering if it will work, than do a temporary installation. That is, put the mount in the area that you plan to make a permanent installation, ground the mount, route the coax from the mount to the radio through a window or door and do a SWR check. This could save you a lot of trouble and keep you from yanking us from your Christmas card list.

 

And last but not least, regardless of the antenna system selected, ALL transmitting antennas MUST be TUNED in their final mounting location. We thought that we would be able to stop mentioning this fact around 1978, but there isn't a week that goes by that someone doesn't says "Huh!" when we ask them if the antenna was tuned. Not tuning your antenna is the same as not putting air in your tires after they are installed. A tire without air is a flat … an antenna without tuning is a stick.

 

Q The antenna I bought claims to be pre-tuned. Do I need to do anything?

A Yes. You must tune your antenna to your vehicle. The antenna is pre-tuned on a test bench to make certain it is within the general frequency specifications. It will be somewhat different on your vehicle because of the difference in the ground plane and surroundings. Always check your antenna ... even if you move it from one location on your vehicle to another.

 

Q Can I use any kind of wire to hook my antenna to my radio?

A No! For single antenna installations we recommend RG-58 A/U type coaxial cable. If you are running dual antennas (co-phased) you must use RG-59 A/U type coax. Make sure you buy the best cable too. We see numerous problems caused by low grade coax. Don't cut corners when it comes to coax.

 

Q Is the length of the coax cable important?

A We find that it is very important ... especially with high performance top-loaded antennas. Your safest bet is to use 18 feet (5.5 meters) coaxial leads on all of your CB installations.

 

Q I only needed 9 feet of coax to go from my radio to my antenna. How should I handle the excess?

A What ever you do, do not roll it into a small convenient coil. It will become an RF choke. If you cannot let it lie loose under a seat or in a headliner, wrap it into a yarn-like skein of about 12 to 16 inches, put a wire tie in the center and tuck it under your dash, seat, etc.

 

Q My system has very high SWR, but I only talk very short distances. Since distance isn't important, should I be concerned about high SWR?

A Absolutely! High SWR will limits distance and may cause serious damage to your transmitter. The time spent tuning your antenna is time well spent. Don't take the chance.

 

Q What is are the most common errors you find on CB installations?

A In order of most to least common, 1) antenna not tuned to vehicle, 2) mounting locations chosen for convenience or appearance versus effectiveness, 3) coax cable ... low quality, worn out, wrong length, or severely pinched, 4) standard antennas used on vehicle with no ground plane instead of special no-ground-plane system.

 

Q There are a lot of different antennas available for CB. Are some better than others?

A Without a doubt, top loaded antennas are better than center loaded antennas, and center loaded antenna are better than base loaded antennas. Also, within each style, the taller the antenna the better it will generally perform.

 

Q How important is it to have the antenna mount grounded to the vehicle?

A Unless you are using a no-ground plane system, it is extremely important. Ungrounded mounts will usually cause SWR to be high across all channels.

 

Every industry has its bottom dwellers. We cannot protect you from them. Consumers who make decisions based strictly on price, or on what someone says instead of what they can do, will often fall prey to the bottom dwellers.

 

Beware of information from "experts" (real or self-proclaimed). There is antenna theory and there is antenna reality. We have yet to see a vehicle that simulates a lab. While theory is a good starting place...experience is invaluable when it comes to real problems. The knowledge gained from the best book on theory will not necessarily produce the best antenna design.

 

Some "experts" may "claim" 5/8 wave mobile antennas are not possible because they would need to be 23 feet high. They are wrong! Physical length and ground wave performance are not the same. If you ever hear someone make that claim, ask them how a handheld CB can have a 1/4 wave antenna 8 inches long and mobile 1/4 wave antennas can be anywhere from 12-60 inches long in spite of the fact that a physical 1/4 wave is 108 inches.

 

Never key up or attempt to operate your CB without a working antenna or "silly load" (non-radiating antenna simulating device) connected to the radios antenna jack, unless you have extra money to buy another radio, or know a good repairman.

 

All mobile and base transmitting antennas need counter-poise, more commonly called ground plane. The antenna is the reactive unit, the ground plane is the reflective unit. Neither is more important than the other. In mobile installations with standard antenna systems, the vehicle metal (body, frame, etc.) acts as the ground plane. In "no-ground-plane" systems, the coax shield is used for counterpoise.

 

Most, but not all, manufacturers pre-tune their mobile antennas on a test bench. To protect your radio's circuitry and achieve optimum performance, mobile transmitting antennas (CB, cell phone, amateur, etc.) need to be tuned on the vehicle.

 

Before transmitting, you should check your antenna system for shorts or opens. If you have continuity between the center pin of the connector and the outer threaded housing, you may have a short. Don't transmit! If you do not find continuity between the center pin of the coax and the antenna base, you have an open. Fix it. (See "Testing Continuity") Exceptions: Some base loaded antennas use a center tap design and there will be continuity from ground to center conductor.

 

SWR that pegs the needle on all channels almost always indicates a short in your antenna system. Do not attempt to tune the antenna until the short is fixed. Operating with high SWR will probably damage your CB's internal circuits.

 

Make sure that the antenna you are using is the right antenna for your application. Don't use a TV antenna or an AM/FM antenna for your CB. Do not operate your CB without an antenna or silly load.

 

Transmitting antennas are sensitive to objects in their "near field of radiation." Tune your antennas in an open area. Never tune inside or next to a building, near or under trees, near or under power lines, and never with a person holding or standing next to the antenna. Try to simulate normal operating conditions.

 

If you mount two or more antennas close to each other, you will alter the transmission patterns of each one. The affect may be either positive or negative. A recommend minimum of 12" should exist between your CB antenna and other types of antennas.

 

Your radio cannot tell one component from another. As far as the radio is concerned, the coax, stud mount, mounting bracket, antenna and vehicle is ONE unit. Don't be too quick to fault your antenna until you are sure that all of the other components have been given equal consideration.

 

In almost every instance, once you get the same SWR reading on channels 1 and 40, further antenna tuning will not improve the readings. If the SWR is still over 2:1, you have other problems to conquer. Exception: There are rare occasions when the ground plane is so small or large that the system is way out of phase (especially with high-performance antennas). If you have high SWR on all channels and have confirmed that you have no opens or shorts in the feedline, try making a small tuning adjustment in the antenna. There are times when the SWR will drop equally across all channels under unusual ground plane conditions. If you find this to be the case, carefully adjust the antenna.

 

SWR that is high on all channels (over 2:1 but not pegging the needle) after the antenna has been tuned normally indicates a ground plane or coax cable problem.

 

The doors, mirrors, spare tire racks, luggage racks, etc. on many vehicles are insulated from a good ground with nylon or rubber bushings. This also stands true for fiberglass vehicles. Make sure that your antenna mount is grounded, even if it entails running a ground wire to the vehicle chassis. Bad hard ground at the mount generally equates to less than optimum performance. Exception: No ground plane antenna kits do not require a grounded mount.

 

If you are hearing whining noises from your radio while your vehicle is running, it is probably due to "dirty power" being supplied to the radio. Under dash power may be more convenient, but the "cleanest" power will be found by running the radio's power leads straight to the battery.

 

You can never buy coax cable that is too good for your system. Never compromise quality for cost when purchasing coax. Your best bet is to stick with coax that has a stranded center conductor and 90% or higher shielding.

 

Most manufacturers of high performance antennas recommend a specific length of coax cable. If your antenna manufacturer suggests a specific length, give priority to that recommendation.

 

If your ground plane is good, your mount grounded and, your antenna favorably located, coax length rarely becomes an issue. But, if one or more mismatches occur, you may find high SWR. This can often be corrected by using 18 feet lengths of high quality coax.

 

Excess coax between your radio and antenna mount should never be wound into a circular coil of less than 12" in diameter. Doing so can cause system problems. Your best option for handling excess coax is to serpentine the cable into a 12 to 18 inch yarn-like skein. Secure the skein in the center with a wire tie and tuck it away.

 

Single antenna installations require coax with approximately 50 ohm's of resistance (RG-58/U, RG-58 A/U or RG-8X). Dual antenna installations require the use of 72 ohm cable (RG-59/U or RG-59 A/U).

 

Coaxial cables with foam (polyfoam) center conductor insulation should be your last choice for use on mobile (vehicle) installations. Even though it will work initially, it has limited life and does not stand up to the conditions encountered in the mobile environment. Choose coax with polyvinyl insulation when doing mobile installs.

 

Coax cables should never be cut and spliced together like common electrical wire. Line losses will occur.

 

Coaxial cable with holes in the outer insulation, severe bends, or door, trunk or hood caused pinches will cause performance problems. Treat your coax with care.

 

If you live in an area where rain and/or sleet is common, wipe your antenna down with a rag that has been coated with WD-40, Armor-All, Pledge, light oil, etc. This trick prevents ice build up that can overload and cause your antenna to break. In an emergency use butter, cooking oil or anything else that will repel water.

 

When tuning your antenna(s), make sure that you do so with the vehicle doors, hood and trunk closed. If left open, they can cause inaccurate SWR readings. Try to simulate actual operating conditions.

 

Mobile antennas, for best performance, should have no less than 60% of their overall length above the vehicles roof line. For co-phased antennas to perform optimally, the space between the top 60% of the two antennas needs to be unobstructed.

 

Remember, all transmitting antennas need ground plane (counterpoise). Base antennas, much like "no ground plane" antennas, build it in. Do not use mobile antennas for base station applications unless you know how to build your own ground plane.

 

If you are installing a single antenna on one side or the other of your vehicle, best on-the-road performance will be realized if the antenna is on the passenger side of the vehicle (Passenger cars and light trucks) Large trucks or vehicles pulling large trailers should put the antenna on the drivers side to avoid the signal from being blocked by the trailer and to keep from hitting road side trees.

 

Co-phased (dual) antenna installations create a radiation pattern that favors communication directly in front and back of the vehicle. This is why co-phase systems are popular with people who do a lot of highway driving. Co-phase antennas must be center or top loaded. Top loaded antennas are the best.

 

Co-phase antennas can improve performance on vehicles that lack good ground plane characteristics (fiberglass motorhomes, trucks, etc.). Instead of using available metal to reflect the radiated energy, the antennas use each others field.

 

When tuning co-phased antennas (dual), it is best to adjust both antennas an equal amount to maintain equality in their individual resonant frequency.

 

On a co-phase system, if you try to tune each antenna independently using RG-58 type coax and then connect them to the co-phasing harness, you will almost always find that they will appear electrically short as a set. We recommend that you first assemble the entire system. Take all measurements and make all adjustments with both antennas in place.

 

If you are experiencing SWR that is high across the entire band and have eliminated shorts, opens, groundless mounts and coax as potential problems, suspect lack of ground plane. Try adding a spring or quick disconnect to the antenna base. In some cases, the repositioning of the antenna relevant to available ground plane will solve the problem.

 

Magnetic mounts should be used in temporary situations only. If you leave them in the same spot for a long period, the paint will not age like that of the uncovered areas and/or moisture will be trapped between the mount and vehicle causing rust or discoloration. Periodically lift the magnet and gently clean off the underside of the magnet and the vehicle surface.

 

Generally speaking, center loaded antennas perform better than base loaded antennas, and top loaded antennas perform better than all. For any given antenna design (base, center or top loaded), the taller the antenna the better. With length comes a wider bandwidth (lower SWR over more channels), more power handling capability and overall performance increases.

 

When ultimate mobile performance is desired, function should be given precedence over mounting location convenience and appearance. Don't confuse SWR with overall performance. You should seek SWR of 2:1 or lower on channel 1 and 40, but keep in mind that best performance may not be found at the lowest SWR readings. For the most part, if you get your SWR below 2:1, on both ends of the band, don't be overly concerned about using meter tricking procedures that bleed off energy.

 

The SWR meters built into CB radios are okay for general readings, but are rarely sensitive and/or accurate enough for fine tuning of antennas. Use them mostly to indicate serious high SWR problems only.

 

Aside from cost, the type of wire used in or on antennas (copper, silver, aluminum, gold, tinned, etc.) has negligible effect on antenna performance. The antenna must be designed to resonate with the wire type and gauge chosen by the designer. However, larger wire gauges will normally increase the bandwidth and heat dissipation abilities of the antenna.

[mikesmaximase]

A Firestick antenna, I have a 3 foot one mounted to my spare tire carrier, so far its not attatched to anything. But looks great, hahaha. I got it from my buddy who drive triaxles and semis for a living and he said firestick is a great brand and thats what he uses on his trucks.

[k9sar]

Dang Aaron... all that in one post. I'd have thought a "post whore" (not my words) like you would have used at least a dozen posts for all that information

[deej]

Cut and paste are his friend Now fess up..what site is that from?

[88pathoffroad]

Not telling!

 

k9sar: Shush!

[statikuz]

It's mostly from the Firestik site. =P

[gacruiser]

Depending on the radio's size, my '91 has plenty of room to mount a small to medium sized radio below the brake antilock module (just ahead of the auto tranny lever). Another potential mounting point is on the back of the center console, mounted vertically. If the radio's really small, you could mount it in the E-brake cubbyhole below and to the right of the steering column.

 

As for antennas, I recommend you ditch the magmount. For decent range, you'll need a grounded antenna...the longer the better. You have a couple of options for mounting. Mount a firestick or whip antenna to either your spare tire carrier's cast mounting bracket, or get a mirror mount and mount it to the tubular carrier itself. Coax can be run into the body through the taillight housing. Another option is to get a trunk-lid mount and mount the antenna to the upper edge of the rear hatch at the roofline (I have enough room for this to still clear the air deflector). Or you could mount a trunk-lid mount under the edge of the hood.

[BigMike]

for now i have it slid under the driver seat. the radio itself is a Radio Shack TRC-446. i tried to put it in the dash but its a little too big. i mma try to look for any info of the radio online.

[JeepRescueService14]

i just installed a cb in my beast and i mounted it where...?? behind the console.. and for the antenna.. i mounted it on my roof rack.. looks like a giant R/C car

[BigMike]

lol, wheres the remote control ?

[Guest 95PathDN]

I was going to put mine in the ashtray but it was just a tiny bit too big. So my radio is just loosely sitting between drivers seat and the center console.

 

I got a mount that attaches to the back hatch area.

 

http://www.firestik.com/CatalogFrame.htm

 

Its the door jamb mount under mounts and accessories.

 

My antennea is a Wilson. I wanted a Firestik, but the company I purchased from was out of them, so they sent me a 4 ft. Wilson and said it would be better anyways. Its been great and I think better than my friend's Firestik. I'm just running a cheap $25 radio, so a good antennea will take you pretty far.

 

Also, get a spring for the mount, because eventually you WILL break the antennea if you don't have the play of a spring. Whether its a tree off road or the low clearance parking garage at the mall (don't ask ) you will want a spring!

[Mr. Pickles]

Hey, on this train of thought, I've been wanting to finally get a cb, but I'm not sure what to get. Never really been around them or know what to look for. Anyone have any recommendations of brand and features to look for and stay away from? I think I'm good on antenna and such, just mainly the unit itself. Also, how does one "tune" the cb?

[94extreme]

or the antenna?

[Guest 95PathDN]

I'm sure the more you spend, the better radio you will get and the more features you can enjoy. I just got a $25 Maxon (cheapest and smallest I could find) and its worked great. I only use it offroad when driving down the freeway in a caravan to the trail or on the trails themselves, therefore I don't need anything great because I'm usually right next to the people. But I have gotten 3-4 miles consistently with the CB and Wilson 4ft. antennea.

 

You tune the antennea with (I think) an allen wrench. We had a friend with a decent CB unit and tuned the tip of the antennea until we got the highest number of bars (reception) on his CB. So when his CB said he was receiving our signal the strongest it was tuned. Or you could just find someone with a tuner (name?)

[88pathoffroad]

The information I posted above tells IN GREAT DETAIL how to use and tune your CB antenna.

[94extreme]

The information I posted above tells IN GREAT DETAIL how to use and tune your CB antenna.

In too much great detail. :o I just cound't read all of that at once. Quick scan didn't reveal anything about it. Anyway thanks 88. good info as always.

[Mr. Pickles]

The information I posted above tells IN GREAT DETAIL how to use and tune your CB antenna.

Yeah, but where's the "beginner's" version? I guess I'll just go for the size/price factor and see where that gets me.

[Nismojunky]

whats swr?

[fleurys]

whats swr?

here's a good explanation if you want to know the whole story,,,,

 

SWR, Return Loss, and Reflection Coefficient

 

This paper is intended to give the newcomer to RF terminology a brief overview of SWR, return loss and reflection coefficient. Instead of concentrating on mathematical derivations or formulas which can make simple ideas seem complicated, this paper will endeavor to explain the fundamental principals and physical relevance of the terms.

 

SWR, which stands for standing wave ratio, may be illustrated by considering the voltage at various points along a cable driving a poorly matched antenna. A mismatched antenna reflects some of the incident power back toward the transmitter and since this reflected wave is traveling in the opposite direction as the incident wave, there will be some points along the cable where the two waves are in phase and other points where the waves are out of phase (assuming a sufficiently long cable). If one could attach an RF voltmeter at these two points, the two voltages could be measured and their ratio would be the SWR. Identical results would be obtained by measuring currents. By convention, this ratio is calculated with the higher voltage or current in the numerator so that the SWR is one or greater.

 

Here are two examples to illustrate how the numbers work. Consider a 1 volt source driving a cable with either a short or open on the end such that all of the power is reflected. Since the reflected wave is as big as the incident wave there will be points where the two voltages cancel completely and other places where the voltage will be 2 volts. The ratio of 2/0 is infinity which is as "bad" as the SWR can be. If, instead, the load were equal to the characteristic impedance of the feed line, say 50 ohms, no power would be reflected and only a constant incident wave would appear at all points along the cable. The ratio of any two voltages would therefore be 1 which is as "good" as the SWR can be. The SWR for terminations between these two extremes may be calculated by considering the interaction of the reflected wave with the incident wave to determine the minimum and maximum voltages. But, as it turns out, the SWR is simply the ratio of the resistance of the termination and the characteristic impedance of the line. For example, a 75 ohm load will give an SWR of 1.5 when used to terminate a 50 ohm cable since 75/50 = 1.5. A 25 ohm resistor will give an SWR of 2 since 50/25 = 2. Note that the larger resistance is always used in the numerator by convention.

 

Consider that the concept of a reflected wave also works at "DC". Suppose that a long pair of superconducting jumper cables are connected to a 12 volt car battery and the far end of the cables are touched together. The battery will be "shorted out" as long as the cables are touching: that is, the battery voltage will fall to zero and the current will be limited only by the internal resistance of the battery. Another way to describe what is happening is to say that 6 volts travels down the cables where is encounters the short and is reflected back inverted in "phase". This -6 volt reflected wave cancels the +6 volts at all points on the cables. In this example, the characteristic impedance of this system is the battery's internal resistance: if a resistor of the same value is connected to the ends of the cable then the voltage will drop to 6 volts and maximum power will be delivered to the resistor. When the short is removed the 6 volts reflects off of the open circuit without inversion and it adds constructively bringing the voltage on the cable up to 12 volts. A 12 volt battery could be said to be a 6 volt source driving a poorly matched load. The battery is a 6 volt source when it is loaded by its characteristic impedance which rarely happens - most batteries couldn't withstand a "good" match for very long! The point is that SWR, return loss, etc. are valid concepts for long cables, short cables, no cables, or even ideal non-dimensional parts. And perhaps more importantly, simple Ohm's Law computations at DC will give the same results as the more mysterious RF equations involving magically reflecting signals and characteristic impedance.

 

For example, consider a 2 volt battery with a 50 ohm internal resistance driving a 50 ohm load through 50 ohm coax cable. (Follow along on a piece of scratch paper!) The match is optimum and the maximum power of 20 mW is delivered to the load. (1 volt squared / 50 ohms.) Now consider a 100 ohm load. The current is 2/150 = 13.3ma and the resulting voltage across the 100 ohms is 1.33 volts. The power dissipated in the resistor is 1.33 x 13.3 ma = 17.8mw. Since incident and reflected power concepts are valid at DC it could be said that 20 mW arrives at the 100 ohm resistor which absorbs 17.8 mW and reflects the remaining 2.2 mW. The reflected 2.2 mW has a voltage of 0.33 volts in the 50 ohm cable. This reflected voltage adds to the 1 volt incident wave to give 1.33 volts. For a very low frequency there would also be a point along a sufficiently long cable where the voltages would subtract giving 0.67 volts. (As the frequency approaches DC, the required cable length approaches infinity!) The SWR is therefore: 1.33/.67 = 2. It is indeed easier to calculate the ratio of the resistors as mentioned earlier! Obviously, at DC the wavelength is infinite and only the voltage addition is observed. Note that the reflected wave is not inverted when the resistance is greater than the characteristic impedance of the cable! (Here is a memory aid: remember the DC example where a short reflected a canceling negative voltage. Obviously, a lower resistance reflects an inverted wave.) Also, notice that the voltage across the 100 ohm resistor (1.33 volts) is equal to the voltage that would appear across a 50 ohm resistor (1 volt) added to the reflected voltage (0.33 volts). Although this description may seem like an artificial construction, consider what happens when the battery is first connected. With a fast oscilloscope connected at a midpoint on the cable, the 1 volt could be observed as it passes as a step increase. When the 1 volt arrives at the load, 0.33 volts is reflected and is observed a short time later bumping the 1 volt up to 1.33 volts where the scope is connected. The voltage does not simply go up to 1.33 volts in one step!

 

In cases involving RF signals, some time will pass during the 'round trip of the reflected energy and the phase of the reflection will also depend upon this length of time. Imagine that a resistor in a black box is at the end of a length of cable. From the outside world this length of cable will give the reflection from the resistor a phase shift since the signal must make a round trip through the length. If a 100 ohm resistor has an SWR of 2, a cable long enough to invert the signal after the round trip will make it look like a 25 ohm resistor, also with an SWR of 2 but with inversion (a cable with a multiple of 1/4 wavelength would do the trick). Since the impedance looking into this black box is a function of the SWR and the cable length, it can be seen that intentionally mismatched lines can be used to transform one impedance into another. Notice that the 1/4 wave cable inverts the impedance and preserves the SWR. This impedance inversion may be used to match two impedances at a particular frequency by connecting them with a 1/4 wave cable with an impedance equal to the geometric mean of the two impedances. (The geometric mean is the square-root of their product.) A 50 ohm, 1/4 wave cable will match a 25 ohm source to a 100 ohm load : sqrt(25 x 100) = 50. Of course, it is not always easy to find the desired impedance cable!

 

Multiples of 1/2 wavelength will give enough delay that the reflection is not inverted and the impedance will be the same as the load. Such cables may be used to transfer the load impedance to a remote location without changing its value (at one frequency).

 

Other cable lengths will transform an impedance which differs from the cable's impedance with a reactive component. If the load is a lower impedance than the cable, a length below 1/4 wave will have an inductive component and above 1/4 (but below 1/2) wave a capacitive component. If the load is a higher impedance than the cable, the reverse is true. Above 1/2 wavelength, the reactance will alternately look capacitive and inductive in 1/4 wave multiples. This reactance will combine with the load's reactance and offers the possibility of resonating the reactive component of the load. Therefore, a cable with the "right" length and impedance can match a source and load with different resistance and reactance values. Obviously, these calculations can become quite involved and most engineers resort to a Smith chart, a computer program or perhaps the most common method, trial and error with a SWR meter or return loss bridge! In most cases, it is most desirable to match every component of a system to the chosen system impedance so that device matching is not frequency sensitive and critically dependent upon the cable lengths.

 

SWR is a useful number for evaluating the actual voltages and currents present along transmission lines and SWR can be directly measured in many cases but it is often more convenient to work with other, equivalent measures. For example, the voltage reflection coefficient is the fraction of the incident voltage that is reflected. If 0.2 volts reflects from a load with a 1 volt incident wave then the reflection coefficient is 0.2. This number conveys the same information as the SWR but is often more easily calculated and observed. And the terms 'power transmitted', 'transmission loss' and 'power reflected' need no explanation beyond explaining that they are usually percentages. The return loss is simply the amount of power that is "lost" to the load and does not return as a reflection. Clearly, high return loss is usually desired even though "loss" has negative connotations. Return loss is commonly expressed in decibels. If one-half of the power does not reflect from the load, the return loss is 3 dB.

 

Return loss is a convenient way to characterize the input and output of signal sources. For example, it is desireable to drive a power splitter with its characteristic impedance for maximum port-to-port isolation and , therefore, it may be desireable to check the output return loss of an oscillator or other source. Theoutput return loss is measured by applying a test signal to the oscillator through a directional coupler or circulator:

 

 

 

Any reflected energy appears at the test port and will be x dB below the input. This dB drop is the return loss (after correcting for the coupler's loss). The test signal frequency is swept through or adjusted to be near the oscillator's output frequency. A spectrum analyzer connected to the test port of the coupler will display the output of the oscillator and the reflected test signal. The dB drop in the reflected test signal below the applied level is the output return loss. The baseline is easily determined by disconnecting the oscillator so that nothing is connected to the coupler's test port. Since there is no load all of the energy will reflect and a 0 dB return loss reference may be established. In situations where an open is unacceptable due to high power levels an intentional mismatch will provide a known return loss. For example, a 75 ohm resistor will exhibit a 14 dB return loss in a 50 ohm system while reflecting only 4% of the test power.

 

An isolator is a seemingly magical device which allows energy to flow in only one direction so reflected energy from a load at the test port does not return to the signal generator but is passed on to the output port regardless of the impedance at any of the ports! This "magic" defies linear "common sense" for passive networks but isolators are highly non-linear devices employing special ferrite in powerful magnetic fields. Engineers who design circuits and systems operating above 500 MHz enjoy the utility of the ferrite isolator but these marvelous devices become impractical below about 200 MHz. Circuits are available in the technical library which simulates the ferrite isolator for frequencies below 300 MHz. The RF op-amps can handle signals approaching 12 dBm so this isolator is only suitable for bench testing low-power RF devices. The attenuation through a directional coupler or return loss bridge can make measurements difficult when the return loss is high and the test signal is small but the isolator has no "loss" and will work well with very small signals. It is also desirable to use small signals when testing antennas for obvious reasons. The isolator exhibits a good output return loss at its test impedance and its good input return loss provides an excellent termination for a long cable from a generator with a questionable SWR.

[SteeevO]

here's a good explanation if you want to know the whole story,,,,

 

SWR, Return Loss, and Reflection Coefficient

 

This paper is intended to give the newcomer to RF terminology a brief overview of SWR, return loss and reflection coefficient. Instead of concentrating on mathematical derivations or formulas which can make simple ideas seem complicated, this paper will endeavor to explain the fundamental principals and physical relevance of the terms.

 

SWR, which stands for standing wave ratio, may be illustrated by considering the voltage at various points along a cable driving a poorly matched antenna. A mismatched antenna reflects some of the incident power back toward the transmitter and since this reflected wave is traveling in the opposite direction as the incident wave, there will be some points along the cable where the two waves are in phase and other points where the waves are out of phase (assuming a sufficiently long cable). If one could attach an RF voltmeter at these two points, the two voltages could be measured and their ratio would be the SWR. Identical results would be obtained by measuring currents. By convention, this ratio is calculated with the higher voltage or current in the numerator so that the SWR is one or greater.

 

Here are two examples to illustrate how the numbers work. Consider a 1 volt source driving a cable with either a short or open on the end such that all of the power is reflected. Since the reflected wave is as big as the incident wave there will be points where the two voltages cancel completely and other places where the voltage will be 2 volts. The ratio of 2/0 is infinity which is as "bad" as the SWR can be. If, instead, the load were equal to the characteristic impedance of the feed line, say 50 ohms, no power would be reflected and only a constant incident wave would appear at all points along the cable. The ratio of any two voltages would therefore be 1 which is as "good" as the SWR can be. The SWR for terminations between these two extremes may be calculated by considering the interaction of the reflected wave with the incident wave to determine the minimum and maximum voltages. But, as it turns out, the SWR is simply the ratio of the resistance of the termination and the characteristic impedance of the line. For example, a 75 ohm load will give an SWR of 1.5 when used to terminate a 50 ohm cable since 75/50 = 1.5. A 25 ohm resistor will give an SWR of 2 since 50/25 = 2. Note that the larger resistance is always used in the numerator by convention.

 

Consider that the concept of a reflected wave also works at "DC". Suppose that a long pair of superconducting jumper cables are connected to a 12 volt car battery and the far end of the cables are touched together. The battery will be "shorted out" as long as the cables are touching: that is, the battery voltage will fall to zero and the current will be limited only by the internal resistance of the battery. Another way to describe what is happening is to say that 6 volts travels down the cables where is encounters the short and is reflected back inverted in "phase". This -6 volt reflected wave cancels the +6 volts at all points on the cables. In this example, the characteristic impedance of this system is the battery's internal resistance: if a resistor of the same value is connected to the ends of the cable then the voltage will drop to 6 volts and maximum power will be delivered to the resistor. When the short is removed the 6 volts reflects off of the open circuit without inversion and it adds constructively bringing the voltage on the cable up to 12 volts. A 12 volt battery could be said to be a 6 volt source driving a poorly matched load. The battery is a 6 volt source when it is loaded by its characteristic impedance which rarely happens - most batteries couldn't withstand a "good" match for very long! The point is that SWR, return loss, etc. are valid concepts for long cables, short cables, no cables, or even ideal non-dimensional parts. And perhaps more importantly, simple Ohm's Law computations at DC will give the same results as the more mysterious RF equations involving magically reflecting signals and characteristic impedance.

 

For example, consider a 2 volt battery with a 50 ohm internal resistance driving a 50 ohm load through 50 ohm coax cable. (Follow along on a piece of scratch paper!) The match is optimum and the maximum power of 20 mW is delivered to the load. (1 volt squared / 50 ohms.) Now consider a 100 ohm load. The current is 2/150 = 13.3ma and the resulting voltage across the 100 ohms is 1.33 volts. The power dissipated in the resistor is 1.33 x 13.3 ma = 17.8mw. Since incident and reflected power concepts are valid at DC it could be said that 20 mW arrives at the 100 ohm resistor which absorbs 17.8 mW and reflects the remaining 2.2 mW. The reflected 2.2 mW has a voltage of 0.33 volts in the 50 ohm cable. This reflected voltage adds to the 1 volt incident wave to give 1.33 volts. For a very low frequency there would also be a point along a sufficiently long cable where the voltages would subtract giving 0.67 volts. (As the frequency approaches DC, the required cable length approaches infinity!) The SWR is therefore: 1.33/.67 = 2. It is indeed easier to calculate the ratio of the resistors as mentioned earlier! Obviously, at DC the wavelength is infinite and only the voltage addition is observed. Note that the reflected wave is not inverted when the resistance is greater than the characteristic impedance of the cable! (Here is a memory aid: remember the DC example where a short reflected a canceling negative voltage. Obviously, a lower resistance reflects an inverted wave.) Also, notice that the voltage across the 100 ohm resistor (1.33 volts) is equal to the voltage that would appear across a 50 ohm resistor (1 volt) added to the reflected voltage (0.33 volts). Although this description may seem like an artificial construction, consider what happens when the battery is first connected. With a fast oscilloscope connected at a midpoint on the cable, the 1 volt could be observed as it passes as a step increase. When the 1 volt arrives at the load, 0.33 volts is reflected and is observed a short time later bumping the 1 volt up to 1.33 volts where the scope is connected. The voltage does not simply go up to 1.33 volts in one step!

 

In cases involving RF signals, some time will pass during the 'round trip of the reflected energy and the phase of the reflection will also depend upon this length of time. Imagine that a resistor in a black box is at the end of a length of cable. From the outside world this length of cable will give the reflection from the resistor a phase shift since the signal must make a round trip through the length. If a 100 ohm resistor has an SWR of 2, a cable long enough to invert the signal after the round trip will make it look like a 25 ohm resistor, also with an SWR of 2 but with inversion (a cable with a multiple of 1/4 wavelength would do the trick). Since the impedance looking into this black box is a function of the SWR and the cable length, it can be seen that intentionally mismatched lines can be used to transform one impedance into another. Notice that the 1/4 wave cable inverts the impedance and preserves the SWR. This impedance inversion may be used to match two impedances at a particular frequency by connecting them with a 1/4 wave cable with an impedance equal to the geometric mean of the two impedances. (The geometric mean is the square-root of their product.) A 50 ohm, 1/4 wave cable will match a 25 ohm source to a 100 ohm load : sqrt(25 x 100) = 50. Of course, it is not always easy to find the desired impedance cable!

 

Multiples of 1/2 wavelength will give enough delay that the reflection is not inverted and the impedance will be the same as the load. Such cables may be used to transfer the load impedance to a remote location without changing its value (at one frequency).

 

Other cable lengths will transform an impedance which differs from the cable's impedance with a reactive component. If the load is a lower impedance than the cable, a length below 1/4 wave will have an inductive component and above 1/4 (but below 1/2) wave a capacitive component. If the load is a higher impedance than the cable, the reverse is true. Above 1/2 wavelength, the reactance will alternately look capacitive and inductive in 1/4 wave multiples. This reactance will combine with the load's reactance and offers the possibility of resonating the reactive component of the load. Therefore, a cable with the "right" length and impedance can match a source and load with different resistance and reactance values. Obviously, these calculations can become quite involved and most engineers resort to a Smith chart, a computer program or perhaps the most common method, trial and error with a SWR meter or return loss bridge! In most cases, it is most desirable to match every component of a system to the chosen system impedance so that device matching is not frequency sensitive and critically dependent upon the cable lengths.

 

SWR is a useful number for evaluating the actual voltages and currents present along transmission lines and SWR can be directly measured in many cases but it is often more convenient to work with other, equivalent measures. For example, the voltage reflection coefficient is the fraction of the incident voltage that is reflected. If 0.2 volts reflects from a load with a 1 volt incident wave then the reflection coefficient is 0.2. This number conveys the same information as the SWR but is often more easily calculated and observed. And the terms 'power transmitted', 'transmission loss' and 'power reflected' need no explanation beyond explaining that they are usually percentages. The return loss is simply the amount of power that is "lost" to the load and does not return as a reflection. Clearly, high return loss is usually desired even though "loss" has negative connotations. Return loss is commonly expressed in decibels. If one-half of the power does not reflect from the load, the return loss is 3 dB.

 

Return loss is a convenient way to characterize the input and output of signal sources. For example, it is desireable to drive a power splitter with its characteristic impedance for maximum port-to-port isolation and , therefore, it may be desireable to check the output return loss of an oscillator or other source. Theoutput return loss is measured by applying a test signal to the oscillator through a directional coupler or circulator:

Any reflected energy appears at the test port and will be x dB below the input. This dB drop is the return loss (after correcting for the coupler's loss). The test signal frequency is swept through or adjusted to be near the oscillator's output frequency. A spectrum analyzer connected to the test port of the coupler will display the output of the oscillator and the reflected test signal. The dB drop in the reflected test signal below the applied level is the output return loss. The baseline is easily determined by disconnecting the oscillator so that nothing is connected to the coupler's test port. Since there is no load all of the energy will reflect and a 0 dB return loss reference may be established. In situations where an open is unacceptable due to high power levels an intentional mismatch will provide a known return loss. For example, a 75 ohm resistor will exhibit a 14 dB return loss in a 50 ohm system while reflecting only 4% of the test power.

 

An isolator is a seemingly magical device which allows energy to flow in only one direction so reflected energy from a load at the test port does not return to the signal generator but is passed on to the output port regardless of the impedance at any of the ports! This "magic" defies linear "common sense" for passive networks but isolators are highly non-linear devices employing special ferrite in powerful magnetic fields. Engineers who design circuits and systems operating above 500 MHz enjoy the utility of the ferrite isolator but these marvelous devices become impractical below about 200 MHz. Circuits are available in the technical library which simulates the ferrite isolator for frequencies below 300 MHz. The RF op-amps can handle signals approaching 12 dBm so this isolator is only suitable for bench testing low-power RF devices. The attenuation through a directional coupler or return loss bridge can make measurements difficult when the return loss is high and the test signal is small but the isolator has no "loss" and will work well with very small signals. It is also desirable to use small signals when testing antennas for obvious reasons. The isolator exhibits a good output return loss at its test impedance and its good input return loss provides an excellent termination for a long cable from a generator with a questionable SWR.

 

Dude,

You're either a ham radio operator, or planning on working for NASA. Great write-up

 

for those that felt the gust of wind as this flew over their head...

 

SWR is basically a way to measure how much power is going to the antenna and how much is being reflected back.

ideally you want all the power going out the antenna and None coming back being a 1:1 SWR. the bigger the ratio, the more power is coming back to the radio, not contributing to the transmitted signal, and potentially harming your radio equipment.

[beastpath]

looks like 88 and fleurys are competing for longest post!!! lol

[colbywan]

Here's where I've got mine mounted. It wasn't exactly easy to fit it in there but with a little trimming and patience you'll have no problem. I mounted my scanner below the CB so I've got all radios in the same area.

 

 

 

 

-Colby