INTRODUCTION TO SWR

General Information

As in most matters involving the technical side of antennas, "SWR" can be complex beyond the understanding and concern of many radio communicators.In this discussion we will attempt to simplify the SWR subject. If you need or want more detailed information, there are plenty of good books that are loaded with antenna testing information. In the meantime, this information will help the newcomer understand the basic concept.

"SWR" is the common abbreviation for "Standing Wave Ratio". It may, in some circles, be referred to as "VSWR (voltage standing wave ratio). Never the less … it is the same. For the most part the term has been reduced to SWR since the calculation can involve either voltage or current. The voltage/current is measured in a transmission line scenario that involves a source (in this case, a radio capable of transmiting radio signals) and a load (everything from the coaxial connector at the radios output to the end of the antenna).

In simple terms … SWR compares the maximum amount of voltage or current that can be delivered by the radio to the minimum voltage or current that actually leaves via the transmission line and antenna. If everything, from the radio end connector to the base of the antenna is perfect, and if the antenna is perfectly tuned to the testing frequency, SWR will be 1:1. That is, every bit of power that the radio has to deliver is leaving the radio. For frequencies below 200 MHz (CB, VHF, etc), SWR below 2.0:1 is acceptable. Granted, lower SWR means more power leaving the radio but, at 2.0:1 the loss does not amount to more than 1/2 dB, even on very long transmission lines. A loss of 1/2 dB is an undetectable change in actual signal strength. Short of having an easy to tune antenna, fighting to achieve SWR in the 1.3:1 to 1.6:1 range when it is already at 2.0:1 probably isn't worth the time and effort.

Many people believe that SWR only involves the antenna but that is an inaccurate assumption. When you place an SWR meter between the radio and the antenna it cannot decipher one component from another. A defective connector, bad coaxial cable, faulty or improperly installed stud mount or a defective and perhaps untuned antenna may all return the same results on the meter. Not only that, but how the antenna is mounted will also play a big part in the resulting SWR readings. For instance, on a ground-plane dependent mobile antenna, if the mount doesn't have a chassis ground the results could be higher than expected SWR. Just as well, if the antenna is mounted on the vehicle in a way that hampers its ability to radiate the applied energy into free space, you will experience less than optimum SWR.

Antennas, for the purpose of this discussion, will only be resonant at one primary frequency. For example, on a 40 channel CB, channel 19 is the center frequency (27.185 MHz). If the antenna is tuned to be resonant on channel 19, the further away from that channel you are, the higher the SWR will be. On occasion you may hear someone say the "SWR should dip at the center frequency". With the antenna tuned at channel 19, if you were to plot the SWR from channel 1 to channel 40, the results would be curved line with both channel 1 and channel 40 being equal but higher than channel 19. The steepness of the line as it leaves the center frequency indicates the bandwidth of the antenna. The flatter the line, the broader the bandwidth. See the following chart for visual assistance.

 

CB Specific Information

Note: Even though this section focuses on CB frequencies, do note that the general concept applies to all transmitters and their associated antennas. What applies here also applies to other frequenciews as well.

To accomplish the lowest SWR on channel 19 (center frequency dip) you need to check SWR on channels 1 and 40. The SWR difference between these two channels will tell you if the antenna is electrically short (channel 19 resonant frequency too high) or electrically long (channel 19 resonant frequency too low). With that in mind, the SWR meter is placed in-line between the radio and the coaxial cable that feeds the antenna. The next step involves meter calibration. In this case, calibration involves measuring the radios maximum output potential. Depending upon the meter design, the switch on the meter will be in the Reference (REF) or Forward (FWD) position (in this position the radios maximum power is established). With the radio set to channel 1, the microphone button is depressed and the calibration knob on the meter is turned until the meter needle aligns itself with the calibration line on the meter face. This is establishing a reference point for the radios maximum output at channel 1, whatever it may be. Once calibrated, the meter switch is changed to the Reverse (REV) or SWR position and the SWR reading is noted and recorded. Now change the radio to channel 40, place the meter into the FWD or REF position and recalibrate for channel 40 by depressing the microphone button and turning the meter adjustment knob. Once calibrated, switch meter to the REV or SWR position and record the SWR reading.

NOTE: If at anytime during the SWR measurement process the meter needle jumps to the extreme right edge of the scale (pegging the needle), do not operate the radio. You most likely have a short in the coaxial cable, connections or the antenna stud mount that must be repaired prior to any further action.

 

Setting the SWR of Your Antenna

from Firestik®; Antenna Co. Technical Support
Copyright © 1996 Firestik® Antenna Company

SWR (standing wave ratio), is a measurement of how efficiently your antenna system will radiate the power available from your radio. In simple terms, your radio would like to radiate all of its power, but can only do so if the other components cooperate. Bad coax and mounts, or inefficient antennas and ground plane can cause system bottlenecks. The easiest way to understand the concept is to think of it in terms of water flow. That is, if you put a one inch faucet on a two inch pipe, your potential output will be restricted by the one inch outlet. So goes antenna systems. Setting your antennas SWR will reduce the restriction of radiated power.

If all radios only transmitted on one channel, it would be a much easier task to design antennas. As it is, on CB alone, there are 40 channels to contend with. Mobile antennas can only be made to resonate at one specific frequency (channel). The goal of the antenna manufacturers is to build the antenna to resonate at a frequency in the middle of the available band (channel 19 on CB) and make it broad- banded enough to keep the off-frequency related SWR at the two extreme ends of the band below 2.0:1. It should be noted that if you communicate on one or two adjacent channels anywhere within the band, you can tune your antenna to achieve optimum performance on those channels. Most people, however, prefer to use the entire bandwidth when tuning.

THINGS YOU WILL NEED

  1. Knowledge of what not to do .... read previous sections.
  2. Properly installed antenna system (mount, coax and antenna) that was made for the type of radio you will be using and has been tested for shorts and opens in continuity. (See "Testing Continuity")
  3. Functional radio.
  4. SWR meter. (See "SWR Meter Hook-Up")
  5. Short piece of coaxial cable (jumper) with PL-259 connectors on both end.

SWR METER HOOK-UP

The SWR of the antenna, without feedline, can be measured by placing the SWR meter in-line at the antenna instead of at the radio. However, the coax can help or hinder performance. In the end, your SWR should be checked at the radio end because all components will be a part of the final operational system being used.

SWR TESTING REMINDERS:

  1. Remember to check for continuity, shorts and opens in your coax and mount installation first.
  2. Take measurements in an open area with the vehicle's doors and hatches closed.
  3. All measurements should be taken with antenna tip on, unless you do not plan to use the tip in normal use.

THE SET UP

 

If already connected, disconnect the coaxial cable from the radio. Connect the coax cable that normally connects to the back of the radio to the SWR meter connector marked "Antenna" or "Ant". Now, connect one end of the jumper cable to the back of the radio and the other end to the SWR meter connection marked "Transmitter" or "Xmit". Your SWR meter is now in series (in-line) with your radio and antenna.

Since you've already read the previous parts of this pamphlet, you should now have your vehicle in an open area, with all doors closed. Turn your radio on and tune to channel one or the lowest channel on your radio. If your radio has side band operation, make sure you are in AM mode before doing SWR tests.

The following assumes that your SWR meter has a standard set of switches, knobs and meters. That is, there will be at least one switch with the marking Forward (FWD) in one position and Reference (REF or SWR) in the other. There will also be a knob or sliding controller marked "Set" or "Adjust". Most meters come with full instructions. If the common configuration does not match your meter you will need to rely on the meters manual for assistance.

With the radio on the lowest channel (1 on CB) and the SWR meters switch in the Forward (FWD) position, depress the transmit switch (key up) located on the microphone. While holding the unit in this transmit mode, adjust the meter needle to the set position using the Set or Adjust knob on the meter. As soon as the needle is in alignment with the corresponding mark on the meter face, flip the switch to the Reference (REF) position. The meter is now showing your SWR on channel one. Note the value and quickly release the microphone switch. Record this reading on your paper to the nearest 1/10th. i.e. 1.8, 2.3, 2.7, 1.4, etc.

Now, switch your radio to the middle channel (19 on CB). Place the meter switch in the Forward (FWD) position, depress the microphone switch and adjust the meter to place the needle on the Set position of the meter face. Once in the set position, place the meter switch in the Reference (REF) position and note the reading. Release the microphone switch and write this value down to the nearest tenth of a point. Note: If your antenna system is closely matched to the radio you may get little or no movement from the meter needle on this channel. This is normal.

Finally, place your radio on the highest number channel (40 on CB). Place the meter switch in the Forward (FWD) position, depress the microphone switch and adjust the meter to place the needle on the Set position of the meter face. Once in the set position, place the meter switch in the Reference (REF) position and note the reading. Release the microphone switch and write this value down to the nearest tenth of a point.

With these three readings, you can determine many things about your system. For instance ...

ADJUSTING LONG ANTENNAS

If the SWR on channel 40 is greater than that on channel 1, your antenna is considered to be "LONG" and reduction of physical height and/or conductor length will correct this situation. Depending upon antenna model, this entails screwing down the tunable tip (Illustration #1: Firestik II, Firefly), or, removing the tip, making short slits in the plastic covering and unwinding and clipping off wire (Illustration #2: Firestik, Road Pal). Firestik Designer Series antennas require loosening the allen screws and lowering the metal whip (Illustration #3).

ADJUSTING SHORT ANTENNAS

If SWR on channel 1 is greater than that on channel 40, your antenna is considered to be "SHORT" and increasing the physical and/or electrical length of the antenna is required to correct this situation. Because we make our antennas extra long, readings which indicate "Short" normally stem from ground plane deficiency (lack of vehicle metal surface for the antenna to reflect its signal rom). This condition is often corrected by adding a spring and/or quick disconnect to increase the physical height. Ground plane deficiencies can also be compensated for by using dual (co-phased) antennas or special no-ground-plane antenna kits.

Lengthening of the Firestik II and Firefly is accomplished by turning the tuning screw further out (Illustration #1). On Firestik and Road Pal models, it requires tip removal, short slits in the plastic covering and, the separation and upward repositioning of three or more wire turns (Illustration #4). Firestik Designer Series antennas require loosening the allen screws and raising the metal whip (Illustration #3).

NOTE: The shorter the antenna, the more sensitive it is to adjustments. For example, removing two wire turns on a 4 foot antenna might move the SWR by 0.3; the same amount removed from a 2 foot antenna may move the SWR by 1.0. Make smaller adjustments on shorter antennas.

DUAL ANTENNAS

Measurements and determination of short or long conditions are the same as the single antenna procedure. However, when tuning co-phased antennas, if you adjust one antenna, it is advisable to adjust the other in equal amounts to keep them in perfect balance.

 

 

Introduction to SWR Testing of CB Antennas

From Firestik® Antenna Co. Technical Support. Copyright © 1996 Firestik® Antenna Company

SWR (standing wave ratio) is a term every CB'er should know. SWR (measured with an SWR meter) shows you how well your coax, antenna mount, ground plane and antenna match the output capability of your CB radio. It is a ratio of maximum voltage or current to minimum voltage or current. Generally speaking, the lower the ratio the better. Defective or inferior components, bad installations and antennas not tuned to the specific location on the vehicle increases the SWR. You should strive to achieve a ratio below 2:1 on all channels.

Before measuring and setting SWR, it is helpful to know some of the things that can cause problems. Spend the time to read the information we have provided on this site and you will go into your project with valuable knowledge. It could save you a lot of time, effort and possibly, money.

Unlike receive only antennas (AM/FM radio, TV, scanner), antennas that transmit require tuning. Antenna manufacturers can supply you with an antenna that is pre-tuned to the general frequency of the intended equipment, but they cannot promise you perfect performance "out of the box". The six needed components for a mobile installation are the radio, power feed, coaxial cable, antenna mount, antenna, and ground plane (counterpoise). Even if the manufacturer supplies you with an entire kit (coax, mount & antenna), this, in most cases, leaves you without the counterpoise requirement. There is an exception to this general rule; antenna systems that use the coax cable as a counterpoise. These no-ground-plane kits are generally marketed to owners of fiberglass vehicles. However, for most applications, the mobile antenna will use the vehicle's body as counterpoise. Unless otherwise stated, the following information pertains to installations that DO NOT use coaxial counterpoise antennas.

The vehicle, in mobile installations, is just as important as the antenna and other components. The antenna is the radiating unit, the vehicle is the reflecting unit. All transmit antennas need a reflective unit. What effect does the vehicle have? Plenty! If you were to install and tune your antenna on a bumper mount then move it to the roof, you would see a change in the SWR. This is due to the change in the antenna's position relative to the vehicle surface. Location is important.

It isn't always feasible, or practical to mount the antenna in the optimum position on any vehicle. Nonetheless, whenever you set aside performance for convenience, you will need to settle for what you can get. Regardless of location, tune your antenna. Tuned antennas will give you the best performance you can expect from an antenna mounted in any given location. Most of all, remember that untested installations can cost you big bucks. Operating transmitting radios when the SWR is over 3:1 can damage the radios circuits. Always check your SWR and tune your antenna.

There is a lot to know about transmitting antennas. Even if you have your system installed by a professional, it is helpful for you to know what can affect its performance. The technical staff at Firestik® has compiled 20 plus years of technical support files. Some of our findings seem contrary to theory, but the purpose of this information is solve real problems, not preach theory. We have tried to concentrate on the areas where most of the performance robbing problems crop up. We highly recommend that you read all of the supplied information in the technical section of our web site. Starting with knowledge is always better than learning from disappointment.