Chapter M-2's Technical Corner

Howard Halasz's July 2008 Technical Article


"MINIMIZING THE CB RADIO'S SWR, USING AN SWR METER"

Note: I've edited this numerous time, hoping to avoid any omissions. I can't very well go through the actual procedure while typing, so I'm doing the best I can from memory.

In trying to be as detailed and specific as possible, I have included numerous steps that take less time to perform than they do to describe.

Please do not assume from the length of this description that this is a difficult or painstaking procedure. Try writing detailed instructions on how to put on a pair of lace-up shoes, and you'll see what I mean. There are MANY steps to describe, despite the fact that it takes less than a minute to do. First of all, SWR stands for standing wave ratio, a ratio of how much signal stands at the antenna to how much signal gets broadcast away from the antenna. Our aim is to shoot for a ratio as close to 1:1 as possible.

Although this procedure can be done anywhere, to see the lowest possible readings, I suggest gathering up and taking with you the tools and equipment needed, and moving the bike to an open area at least 25' away from buildings, vehicles, trees, fences, giraffes, elephants, and anything over

4' tall.

1. Remove the left fairing pocket and locate the CB antenna line connection. This is a simple, round, push-pull plug-in connection. It's a round cable about 3/16"

to 1/4" in diameter.

2. Disconnect the CB radio antenna line connection. Note which end goes to the antenna (the male connector), and which end goes to the CB radio (the female connector).

3. Using the appropriate connectors, adapters, and cable(s): connect the antenna line (the male connector) to the ANTENNA connection on the SWR meter, and the radio line (the female connector) to the TRANSMITTER connection on the SWR meter. Ensure that all connections are tight and secure.

4. Set the meter's SWR switch to the CAL position. If it's not clear which is the SWR switch, check the instructions that came with the meter. (If these switches are present, set the RANGE switch to 20W, the FUNCTION switch to CAL, and the MODE switch to AVG. With this particular meter, the FUNCTION switch is both the POWER (up) and SWR (down) switch.)

5. Turn your ignition key to the Accessory position. Turn on the CB radio, and select the desired CB channel. I use channel 19, as it is halfway between channels 1 & 40. I also suggest turning the volume down to its lowest setting to silence the incoming noise. You don't need the distraction! Now key the radio (cause the CB to transmit) and check to see that the meter moves. If not, check all of your connections again.

(Note: To minimize battery drain, you may want to cut the ignition key off when not actually transmitting.)

6. From this point onward, do not allow anyone but yourself to move around near the CB antenna. Bodies absorb and reflect some of the RF radiation, causing the readings to vary. It's okay to move around when you are not transmitting, but try to position yourself in the exact same place every time you transmit. YOUR proximity to the antenna will affect the readings, too, so be consistent in where you position yourself every time you take a reading (every time you transmit). To see this for yourself, while pressing the CB radio's transmit button, watch the meter as you move your hand toward and away from the CB antenna. If everything is working correctly (connections, power, etc.), you should see the meter move as you move your hand to and from the antenna.

7. Using the appropriate Allen wrench, loosen both of the setscrews that lock the UPPER antenna shaft in the antenna coil (the black object about halfway up the antenna). Remove the upper antenna shaft, and inspect its bottom end to make sure the bottom 2" are clean and shiny. If not, use very fine sandpaper or emery cloth to clean it. Then replace the upper antenna shaft, and (VERY IMPORTANT HERE!) make sure it goes all the way to the bottom of the hole! Now tighten the lower setscrew only to the point of being snug. It is not necessary to tighten it all the way at this point, and doing so every time just increases the risk of rounding off the wrench or the set-screw. Leave the upper setscrew loose for now.

NOTE: If you have a good memory, fine. If not, use pen and paper to note the meter readings that result from each test.

8. With the meter's SWR switch in the CAL position, press and hold the CB radio's transmit button (do not release the transmit button until told to do so), while turning the SWR meter's calibration knob to position the meter's needle to the CAL indicator (far right side).

9. Now move the SWR (or FUNCTION) switch to the SWR position (usually downward). Once again, try to have your body in the same position every time you do this. Now NOTE AND RECORD the meter reading.

10. Release the transmit button, and move the SWR switch back to the CAL position.

11. Loosen the upper antenna shaft's lower setscrew, remove the upper antenna shaft, and cut 1/8" from the bottom end. I use a die grinder, but you can use whatever means you have available. If your cutting method leaves any burrs on the shaft, remove them before replacing the antenna shaft. It is very important that the antenna goes all the way to the bottom of the hole.

12. Replace the upper antenna shaft, making sure that it goes all the way to the bottom of the hole (important!), and then tighten the lower set screw only until snug.

13. With the meter's SWR switch in the CAL position, press and hold the CB radio's transmit button (do not release the transmit button until told to do so), while turning the SWR meter's calibration knob to position the meter's needle to the CAL indicator (far right side).

14. Now move the SWR (or FUNCTION) switch to the SWR position (usually downward). Once again, try to have your body in the same position every time you do this. NOTE AND RECORD the reading on the meter, release the transmit button, and move the SWR switch back to the CAL position.

15. If this latest reading is LOWER than the previous reading, repeat steps

11 through 14.

If this latest reading is HIGHER than the previous reading, then continue to step 16.

If the latest reading is IDENTICAL to the previous reading, then skip ahead to step 17.

16. If your latest reading was HIGHER than the previous reading, your upper antenna shaft is now too short, but only up to 1/8", which is less than the antenna holder's range of adjustment. The ideal position for the upper antenna shaft will now be somewhere between the very bottom of the hole, and the slightly higher point where the lower set screw will still tighten onto the antenna shaft. Spend as much time as you like experimenting within this narrow range of adjustment to get the lowest possible reading on the SWR meter. Then proceed to step 17.

17. Now that you have minimized the SWR...

a. Turn off the ignition key b. Tighten both of the upper antenna set screws c. Disconnect the SWR meter from the antenna line d. Re-connect the antenna line connectors e. Test the CB radio to ensure that it works f. Replace the fairing pocket g. Put away your tools and equipment h. Go riding NOTE: If you hold the glass of a 48 inch fluorescent tube against the upper portion of the antenna, key the radio, and see that the fluorescent tube is glowing, you can rest assured that your radio is transmitting properly.

RADIO SHACK PART NUMBERS

2780121 BNC to PL259 adaptor $3.49

2780208 PL259 to Motorola adapter $1.99

2780117 BNC PLG-MOTO Socket $3.59

2780968 RG58/U Cable Assembly $4.99

2100534 SWR/PWR Meter $29.95 (there's also a $19.95 SWR meter, but I don't have its part number)

This took a long time!! I hope it helps someone!

And if anyone notes something I've overlooked, please post a correction as soon as you can.

Thanks.

Gordon Banks, "Crimson Ride"

Edited by Howard Halasz, Chapter M2 Technical Coordinator