Tuning a CB Antenna

Revision: 06/17/2003

Introduction:

I want to point out that I have a none-Honda CB on my GL1800. I created these notes from various sources, on the Internet, to help me understand SWR (standing wave ratio). SWR (measured with an SWR meter) indicates how well the coax, antenna mount, ground plane and antenna matches the output capability of the CB radio.  Meaning, how much of the radio's transmit power is reflected from the antenna back into the antenna cable, generating heat and wasting power. The purpose of tuning the antenna is to get the antenna and radio combination to the maximum power that it capable of producing. My goal was to achieve a ratio below 2:1 on channels (1) and (19), because I use channel (1) 99% of the time.

Before You Start:

Before measuring and setting SWR, I reviewed these tips to get a better idea of the CB radio’s relationship with the antenna.

  1. Never operate your CB without a working antenna.
  2. Manufacturers pre-tune their mobile antennas on a test bench, but for the best performance the antenna needs to be tuned on the vehicle.
  3. SWR that pegs the needle on all channels almost always indicates a short in your antenna system. Find the short first before tuning the antenna.
  4. Transmitting antennas are sensitive to objects in their "near field of radiation." Tune your antennas in an open area.
  5. Make sure that your antenna mount is grounded, even if it entails running a ground wire to the chassis.
  6. You can never buy coax cable that is too good for your system. Your best bet is to stick with coax that has a stranded center conductor and 90% or higher shielding.
  7. 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.
  8. 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).
  9. On wire-wound antennas that require wire removal for tuning purposes, best overall performance will be achieved by keeping the loose end of the wire pressed down tightly against the wire coil.
  10. 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.
  11. Wire wound antennas with a plastic outer coating will greatly reduce audible RF static when compared to metal whip antennas.
  12. On antennas that are topped off with a vinyl tip, make sure that you take your SWR measurements with the tip in place.
  13. On a budget? Buy a cheap radio and a good antenna. Aside from added bells and whistles, all CB's are FCC regulated to transmit no more than 4 watts of power. A good antenna on an inexpensive radio will almost always outperform a bad antenna on an expensive radio.
  14. Clean and check all grounds. To ensure a good ground it may be necessary to run a wire from an antenna mounting-bolt to the M/C frame under the seat.
  15. Check antenna for continuity. Two checks, from the tip of the antenna to the end of the cable center plug, and from the base of the antenna mount to the outside portion of the cable end.
  16. Check for infinite resistance between the antenna element and the base. If there is continuity, don't try to transmit. Possible cause may be a crushed antenna cable causing an electrical short.
  17. Check cable (coax connections) clean, tight, with no moisture.
  18. Do not touch the antenna while transmitting.
  19. Objective is to obtain an SWR of less than 2 to 1 (2.0:1) on all channels from 01 to 40. If you experience a high SWR on all channels, you are probably experiencing a ground plane deficiency. RE-CHECK your GROUNDS.
  20. If the SWR on channel 40 is greater than that on channel 01, the antenna is considered to be 'long' and reduction of physical length is necessary. 
  21. If the SWR on channel 01 is greater than that on channel 40, your antenna is considered to be 'short'.
  22. Follow the instructions for your SWR. You probably will have to 're calibrate' the meter each time you make a major change in frequency (channel change) or adjustment in antenna length.
  23. Keep the connecting cable between the CB and the SWR meter short.
  24. Keep the coax cable length in 3ft increments, 3, 6, 9, 12, etc. to maintain the transmission wave pattern unaltered.

 

i

Connecting the Meter

I had to make a set of connecting coax cables, for the SWR meter.

  1. Make sure the CB radio is powered off.
  2. Connect the SWR meter in-line with the CB and the antenna using the coax connector cables.

 

If you have the GL1800 Honda CB installed on your bike you will need the following, to make the SWR meter connecting cables.

 

Radio Shack RG-58U coax cable Two 1ft cables
Radio Shack PL-259 connector
2
278-191
Radio Shack Female Motorola type connector
1
274-710
Radio Shack Male Motorola type connector
1
274-709

 

i

 

I have a handheld CB installed on my GL1800. I used the following, to make the SWR meter connecting cables:

 

Radio Shack RG-58 coax cable 1ft cables
Radio Shack PL-259 connector
1
278-191
Radio Shack Twist-On Male BNC connector
1
278-103
Radio Shack BNC Adapter
1
278-121
Radio Shack Female Motorola type connector
1
274-710
Radio Shack RG-58 Coax Cable Assembly 12Ft
1
278-965

 

i

Getting the Initial SWR Readings

  1. Set the CB radio to channel 1.
  2. Switch the SWR meter to FWD (FS) position.
  3. Depress the PTT and while holding the unit in transmit mode, adjust the meter needle to the set position (or CAL position).
  4. When the needle is in alignment with the corresponding mark on the meter face, flip the switch to the Reference (REF or SWR) position. Note the SWR value and release PTT switch. Record the reading to the nearest 1/10th. i.e. 1.8, 2.3, 2.7, 1.4, etc.
  5. Now, switch your radio to channel (19), and repeat steps 2 thru 4. 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.
  6. Finally, place your radio to channel (40), and repeat steps 2 thru 4.

Interpreting SWR Readings

An ideal SWR reading is 1.0, but this reading is usually possible only under laboratory conditions or with a dummy load. Actual antenna installations have higher readings. The information below will help you interpret the readings you get.

 

SWR Efficiency Interpretation
1.0 to 1.5 Excellent The antenna cable and the antenna length match the transmitter's output requirements almost perfectly.
1.5 to 2.0 Very good The antenna, the cable, and the transmitter operate very efficiently.
2.0 to 3.0 Acceptable The antenna, the cable, and the transmitter operate with some loss. If possible, adjust your antenna or antenna mounting system to improve.
Above 3.0 Inefficient Adjust your antenna or antenna mounting system to improve efficiency.

 

* ERP = Percentage of Effective Radiated Power

SWR READING
% OF LOSS
ERP*
WATTS AVAILABLE
1.0:1
0.0%
100.0%
4.00
1.1:1
0.3%
99.7%
3.99
1.2:1
0.8%
99.2%
3.97
1.3:1
1.7%
98.3%
3.93
1.4:1
2.7%
97.3%
3.89
1.5:1
3.0%
97.0%
3.88
1.6:1
5.0%
95.0%
3.80
1.7:1
6.0%
94.0%
3.76
1.8:1
8.0%
92.0%
3.68
2.0:1
11.0%
89.0%
3.56
2.2:1
14.0%
86.0%
3.44
2.4:1
17.0%
83.0%
3.32
2.6:1
20.0%
80.0%
3.20
3.0:1
25.0%
75.0%
3.00

Tuning the Antenna

To tune the antenna to the CB, the goal is to get the SWR on channel (1) and channel (40) the same.

If you transmit on one channel more often than any other, select that channel. If you transmit on several channels, choose a frequency in the middle of the range of channels you use. (For example, if you transmit on all 40 CB channels, choose Channel 20, because it is midway between Channel (1) and Channel 40.) Antenna manufactures pre-tuned their antennas to channel (19) that is one reason people experience better transmission and reception on channel (19).

NOTE: The use channel (1) 99% of the time, and channel (19) the rest of the time. I tuned my antenna for channel (9), which is the mid-point between channel 1 and 19.

If SWR on channel (1) is higher than channel (40), your antenna system appears to be electrically short. Increasing the physical and/or electrical length of the antenna is required to correct this situation.

If SWR on channel (40) is higher than channel (1), your antenna system appears to be electrically long. Decreasing the physical height and/or conductor length will correct this situation.

If you have to cut the antenna make sure to cut in increments of no more than a 1/8 of an inch.  A 1/8 of an inch can make a big difference in the SWR.