Cable Length
From my experience, it is best to use the coax in multiples of ¼, ½ or 1 wave length of the center frequency. For VHF 2M, the center frequency is 146MHz. This is derived for the 2M VHF of amateur allocation, 144 MHz – 148 MHz. 144, 145, 146, 147, 148. Now you can clearly see that 146 MHz is the center frequency.
So, coax of 1 wavelength = 300/146 which equals about 2.05 Meters. To get the optimum performance this value has to be multiplied with the coax velocity factor. If your coax has a velocity factor of 0.92, then coax of 1 wavelength = 2.05 * 0.92, which equals to 1.886 Meters. Now you know how long one wavelength of your coax is, you should cut it in multiples of 1.88 Meters or ¼ or ½ of that value.
Example, you need to use approximately 26 Meters of cable.
First, find out how many wavelengths that is.
26 Meters/1.88 = 13.82 wavelengths
13.82 wavelengths is not multiples of 1.88.
So you could use 14 wavelengths. This works out to be 26.23 Meters.
NOTE: The velocity factor used as example one. Below is a table of commonly used coax and velocity factor. For detail info please download the data sheet from you coax manufacture.
| COAX | VF | VF x CF |
| RG-8 | 0.66 | 1.35 |
| LMR-400 | 0.85 | 1.74 |
| RG-11 | 0.75 | 1.54 |
| RG-58 | 0.66 | 1.35 |
| RG-213 | 0.66 | 1.35 |
| RG-400 | 0.695 | 1.42 |
Testing Coax
Now that you have your coax ready with connectors soldered on both ends, it is time to test if there is an electrical short or open circuit. For this task you will need a multi-tester.
The outer braiding in the coax is called the ground (GND) and the inner core is the positive (+). The same is true for the connector, the outer part of the connector is the GND and the inner part is the positive (+).
Set the tester to Ωx1 or to the buzzer position.
Touch each end of the coax GND to the testers’ tip. Either the buzzer will sound or the indicator on the tester will move. This indicates that the coax GND is has connectivity.
Do the same for the positive part of the connector, the result should be same.
Finally test the GND and (+) for electrical short. This time, there should be no sound or activity from the tester. If there is, you need to rectify the problem before testing SWR. Most likely the soldering is bad.
Tuning SWR
There is a misconception that tuning SWR is tuning your antenna. The cable, mounting and other factors also play a role in tuning the SWR. SWR is affected by many factors and not just the antenna.
Mount the antenna at the place you intend to use it.
Connect the coax from the antenna port marked ANT on the SWR meter.
Connect the rig/radio to XMIT/Transmit port of the SWR meter. I suggest that this coax be at least ½ wavelength. Most people use ¼ wavelength and it tends to be too short, especially when testing in cars.
Turn on rig and in VFO mode set to 144MHz.
Set the meter to
Press PTT and calibrate the meter.
Release the PTT and set the mode to REF (Reference/Reflected).
Press the PTT and make note of the reading.
Repeat this again wit 146 MHz and 148 MHz.
144 MHz is the lowest end if 2M band.
146 MHz is the center of 2M band.
148 MHz is the highest of 2M band.
If all goes well, there should be little or no movement on 146 MHz. SWR on 144 MHz and 148 MHz should be 2.0 or less. If above 2.0 then you need to check the antenna mounting, check the grounding of the mounting or maybe place the antenna in different location etc.
Some SWR meters have no switches and buttons, for more information on how to use and read the meters, please refer to your SWR meter manual or ask a fellow HAM for help. I use the Diamond SX-400 in my example.
Tuning Antenna
If SWR on 144 MHz is more than 148 MHz, you will need to lengthen your antenna. Especially for jalopies, this can be done by adding additional spring mounting. For base antennas refer to the length chart that comes with the antenna.
If SWR on 148 MHz is more than 144 MHz, your will need to trim your antenna. Refer to the cutting chart/length adjustment chart that comes with the antenna. Some antennas have a small Allen screw to adjust the length.
If SWR is above 3.0, check for electrical shorts, improper grounding, etc.
Factor that contribute to high SWR
Low quality coax has high loss.
Coax is too long.
Bad soldering.
Excess coax is coiled.
Antenna length not tuned according to length chart..
Little or no ground plane.
Location of antenna is not the best.
When testing SWR on jalopies, do in open area with doors shut.
Antenna mounting not grounded or shorting.
There are other factors too, but these are the most common ones.
I hope I have not made things too complicated for the newcomer. I could get more technical but I don’t want to. Best of luck tuning your SWR. If you need help, you could leave me a comment.
73s to you and all monitoring stations.
9W2GU.
If you have an questions, please leave your comments.
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