Brian, VK8BL offers some observations on recent experiments with VSWR and impedance. Thanks Brian.

Gary Gibson VK2DBN always argued that there was more to “matching” an antenna to a radio than having a low VSWR reading. Sure, a low VSWR can help when you’re trying to get your signal out, but there are at least two components that have to be taken into account, VSWR and the impedance of the antenna relative to the 50 ohm transceiver impedance. Don’t get “hung up” on the “50” ohm impedance, as I understand that there is no simple way of measuring it, so we don’t know the specific impedance of our radios. My philosophy is, that a figure between 45 and 55 is acceptable and a bit further either side is not a crime. Your radio will tell you when it is too much by reducing power. Sure, when you build an antenna try to adjust it to 50 Ohms if you have the instrument to measure it.

My recent experience with my antenna that I used for the Sunday morning callbacks may be of interest and help to some of our members.

I’m driving a five-element trapped vertical which has a very narrow band width on 80 metres with an ICOM 7300 Transceiver, using a Kenwood 230 “antenna tuner”. Be aware that while this device is called an “antenna tuner” it is in fact a device to “match” the impedance of the antenna to that of the radio. Unfortunately, it does not read/display the impedance, but only the VSWR, without respect to the impedance. It does not “tune the antenna”, you have to do that by adjusting the lengths or spacing of the elements. If you have a very well-tuned antenna, then you should get a low VSWR and an impedance in the order of 50 Ohms without adjusting the capacitance or induction of the feed line. If the match is not so good, then you can have both VSWR and the impedance out of kilter to such an extent that the radio will reduce power to protect its output stage. I have a Dick Smith ”Relative Field Strength” Meter set up beside my radio which gives me the relative strength of any transmitted signal direct from the antenna. I also have an “MFJ 269C” Antenna Analyzer which allows me to monitor both the VSWR, impedance and reactance while adjusting the Kenwood 230 “Tuner”.

I only use the 80-metre band for Sunday morning call backs, hence I have been a little slack paying attention to the manner in which it has been performing.

My routine was to adjust the VSWR of the antenna to a minimum (about 1:1.3) on 3.597 MHz (so as not to interfere with the 80-metre broadcast) and then when it came time to use it, adjusted the radio to 3.607 MHz for the call backs. The radio’s “auto tuner” was never happy with 30 watts of power and always displayed a VSWR well above probably 10, with the field strength meter indicating about 20 on the scale of 0 to 100. I often had doubts as to how well my signal was getting out, but reports were encouraging, so I continued this practice.

A couple of Thursdays ago whilst discussing my concerns with Ray, VK8RW he reminded me that the impedance was probably more important than the VSWR to get full power to the antenna. Memories of discussions with Gary some years ago came to mind and I decided that it was necessary to check the impedance as well as the VSWR on the operating frequency.

I connected the antenna analyzer in place of the radio whereby it was on the output side of the Kenwood 230 “tuner” and proceeded to adjust the tuner to give me the minimal VSWR (1:1.3) on 3.597 Mhz. The impedance was OK, I don’t recall the exact figure, but when I re-tuned to 3.607MHz I was surprised to see the impedance had skyrocketed towards the 100 Ohm mark but the VSWR was still reasonable.

Last Sunday I turned on the radio prior to the WIA news so I could use the call back frequency and had a look at the 80 metre antenna at 3.607MHz with my MFJ69C Antenna Analyser, and adjusted the “tuner” for an impedance of 50 ohms without taking any notice of the VSWR, and was surprised to see that the VSWR was about 1: 1.7. With the radio output set at about 30 watts, I was most surprised to see the field strength meter reading better than 80 on the scale rather than under 20 as previously observed, when the antenna was matched off frequency and then moved just a few kHz.

My MFJ69C was a fairly expensive instrument in the order of $600 at the time, ( before the Sark 110 and the less than $99 Nano VNA meters were available), but I was impressed by a similar meter owned by VK8TA, borrowed it a couple of times and decided It would be a good investment. It does a lot more than read VSWR and Impedance, but so do the Sark and Nano VNA meters, which will display a Smith chart, which the 69C doesn’t. I have used my 69C a lot and have no regrets at the expenditure.

I guess the moral of this story is if you have any doubts, stop, think about what you are doing, and then look at the meter readings, (impedance between 45 & 55 Ohms and VSWR below 2) rather than just going on the way you have in the past and wondering.