In this post, I will describe basic radio antenna tuning using an SWR meter. For best transmission and reception, antennas must be “tuned” to the frequencies of the radio. When you tune a radio antenna, you are adjusting the length of the antenna so the radio transmits most efficiently.
Even though this is the Ham Radio section of the watermanatwork.com blog, we are going to be working on a CB radio and and antenna. I have several ham radios but none of them have a tunable antenna. Most antennas for handheld(HT) and mobile UHF/VHF ham radios do not require tuning.
I wanted a cheap, portable CB setup for road trips. The radio is a Uniden Pro 505 XL. The antenna is the TRAM 703 Magnet Mount. The TRAM 703 is a reasonably priced and pretty good performing ground plane antenna, but for maximum efficiency, the antenna needed to be tuned to match the radio.
To tune the antenna, I needed an SWR(Standing Wave Ratio) meter. We’re using a Surecom SW-111. This SWR meter only works for CB radios. To tune ham radios, you need a different meter.
A jumper cable is needed to hook up the SWR meter between the radio and the antenna. Hook everything up before you power up the radio.
Turn on the radio. The first thing that needs to be done is to calibrate the SWR meter. On the SW-111, the slide switches are in the “FWD” and “SWR” positions. SWR adjustment is all the way to the left. Key the microphone and use the SWR adjustment to move the needle all the way to the right. Now ready to make the first SWR reading.
Move the SW-111 slide switch from “SWR” to “REF” so the slide switches are in the “FWD”-“REF” positions. Key the transmitter. Read the meter. This is the SWF.
As far as SWR goes, anything above 2:1 is a problem. That means that there is power loss of 11%, not acceptable. The measurement above is from the TRAM 703 antenna straight out of the package with the tunable section of antenna stuck all the way in. The SWR is nearly 3:1, way out of range with a 25% power loss. With a 4 watt CB radio, you could probably yell further.
The TRAM 703 antenna is adjusted with a small set screw. The small hex wrench comes with the antenna, which is cool. Sliding the tunable section up and down is how you dial in the tuning.
I found that pushing the tunable section in, making it shorter, there was better performance with the higher frequency CB channels like 30 to 40, but the lower frequency channels 1 to 15 had very high SWR. Pulling the tunable section out, making it longer, dropped the SWR in the lower channels and sent it out of the park with the higher channels, so it was pretty clear what needed to be done.
With this antenna and radio, you could have a higher SWR or the lower channels or the upper channels, or a little off each end. I went for slightly higher SWR on the lowest and highest channels with very good performance everywhere else.
After a bit of trial and error, I found the SWR “sweet spot” for this antenna. On Channel 1, the SWR is 1.3:1, not too bad, well within the acceptable range.
Channel 10 is 1.1:1, which is very good.
Channel 20 is about the same. Doesn’t go much lower.
On Channel 30, the SWR rising to 1.3:1. Still OK.
On Channel 40 the SWR is 1.7:1, which is a bit high, but still acceptable, losing about 6% power.
That’s all there is to it. The extra $30 for the SWR meter and jumper cable was money well spent. Also using the SW-111 I was able to measure the radio output power. It was reading about 3.5 – 4 watts, which is not a tremendous amount of power, so high SWR can really cut down the range.
Got Your Ears On? Put the Hammer Down!
This is the ham radio section of the watermanatwork.com blog, but I wanted a CB radio to go along with my ham radios. The watermanatwork.com radio shack is 100% mobile and powered completely by solar power. All my radio setups are portable.
This is the TRAM 703 antenna. For about $35USD, pretty impressive. Only about 2′ high, this ground plane mag mount antenna has good results across the entire CB band. I’ve read the magnet will hold up to 100mph. I don’t plan to check it out.
The antenna is covered with pipe insulation. I don’t want to have to take the antenna apart every time I use it, so the pipe insulation fits perfect and provides good protection for the antenna.
With the antenna wrapped up with insulation, it goes in a bag normally used for camera tripods and light stands.
The Uniden Pro 505 XL radio, TRAM 703 antenna and SW-111 SWR meter all fit in a couple nondescript bags. Total cost for everything, including nondescript bags, about $140USD.
As always here on watermanatwork.com, we buy all this stuff just like you would have to, so you’re getting the straight scoop.
10-7.