City Park Antenna Tests

After Field Day I went to Whitney Park here in town. At one time it was the city airport so although there are not a lot of tall trees there are some around the north edge of the park. That day I put up my fifteen and twenty meter folded dipoles around twenty feet. The MFJ-259b did not give me good news. The actual antennas are 450-ohm ladder line shorted at the ends and fed with 300-ohm ladder line. I use the Ten-Tec center to give the center connections support. At that time I did not have a balun so I just fed the 300-ohm line to ground and the center pin of the SO-239 on the top of the unit. The analyzer said that the fifteen meter antenna was resonant around forty megahertz and the twenty meter antenna did not seem to have a null at all.

Yesterday, in between some rain, I had an opportunity to retry my previous measurements. This time I also took a ten meter dipole with a 1:1 balun at the center. I also had a 4:1 balun to test the ladder line fed antennas. This time I was able to get one end of the antennas up about forty feet. It was forty feet at the tree with the antenna sloping gently down toward another tree.

The first test was the ten meter dipole. I made two measurements. The first was with twenty five feet of some very old Tandy RG-8 mini from the analyzer to the balun. The antenna had been cut for around 28.3 mHz if I remember correctly. When I cut an antenna I use 468/f mHz. For HF I also tend to be a bit sloppy on the long side, maybe as much as three to six inches, because you can always cut some off easier than it is to put it back on. The antenna showed resonance at 28.65 mHz at fifty ohms. Then I replaced the fifty ohm coax with thirty one feet of RG-6. The resonant frequency showed  28.15 at fifty ohms. As I moved the frequency to 28.6 the standing wave only went as high as 1.7:1 at about seventy five ohms.

For those who don’t memorize such things RG-8 is fifty ohm coax and RG-6 is seventy five ohm coax. RG-6 is commonly used for cable TV and is very well shielded and cheap to buy. I have been using it for years for my antennas.¬†Most radios, as I understand, are supposed to be able to handle fifty to seventy five ohms at the antenna terminal. Dipoles are naturally resonant with a seventy two ohm center resistance. A tuner can handle the difference if the radio can not.

A few hours later after a brief rain I went back to the park. I left some fishing line in the tree so putting the antennas up with the same nice height was just a matter of pulling string back through with the fishing line. It’s still there by the way. So I put up the fifteen meter dipole mentioned above, but this time using a 4:1 balun and a short coaxial jumper to the MJF-259. The antenna showed resonance at the bottom end of the fifteen meter band which would be consistent with my antenna cutting method. I did hook up the Icom 703 for a few minutes. The antenna was oriented for north-south reception. There was very little signal, but I did hear a CT1 chatting with stations on the east coast. He was 100% copy and if my antenna had been situated properly I might have chatted with him. That was all I heard on fifteen meters.

I then put up the twenty meter antenna and found it resonant at around seventy megahertz. So there is something wrong with that antenna. I took it apart this afternoon and could find nothing obvious. I will remeasure and solder all of the connections to check it tomorrow.

There are people living across the street from where I was working on the antennas. It was at about this time that a young woman came over letting me know that they had been curious as to what I was doing. After explaining, I packed up as it began looking like there might be more rain coming.

So I learned that I need to work on the twenty meter folded dipole and that, as experience has taught me, using RG-6 for feed line is just fine.

Tim

About Tim

Amateur Radio operator, television broadcast engineer living in Minnesota.
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