A while ago, I bought an MFJ antenna analzyer, but I hadn’t really done much with it. I wanted a short, simple project over the weekend, so I decided to check out my 40m dipole. A quick sweep revealed that it was resonant off the top end of 40m, around 7.350 or so, and that down at the bottom end of the CW portion of the band, it was about a 3:1 SWR. I’ll take better notes later tonight when I get home, and maybe even produce a small graph.
This dipole is actually a premanufactured one from radiowavz.com. It’s just a basic 40m dipole with balun, fed by 50 ohm coax. In my case, its mounted quite low: Its about 20 feet off the ground at one end, but it is tied to a short tree up my hill. so the middle is maybe 10 feet above ground, and the far end maybe only 6 feet.
So, the open question is: will raising the far end of the antenna lower its resonant frequency? Or is the antenna just cut wrong for the CW portion of the band?
Bets anyone?
Addendum: This website does show that as the antenna gets lower, the feedpoint impedance drops significantly, and also shows that dependent on height, the resonant frequency of the antenna can vary by more than 100khz. This suggests (unsurprisingly) that I should try to mount this antenna higher to provide an easier match in the 40m band. It also suggests that measuring an antenna close to the ground isn’t a good idea.
As that site shows, impedance and therefore resonance has sort of a cyclical relationship with height. Your point of resonance could go up or down, depending on how much higher you go up.
My advice would be to get it as high as possible, measure it with your analyzer, and then trim based on that. You might want to iterate a couple of times if you need to trim a lot.
You can actually get a dipole too high, but it’s really hard on 40m.
This mirrors some of my experiences with tuning my 40m dipole. When I had it arranged in a crazy shape only a few feet above my flat apartment building roof, it measured horribly on an antenna analyzer, and performed just as bad. Now that I’ve managed to get it up about 10′ off the roof, it performs much better. I bet it will still look pretty bad on an analyzer, but I’m not hopeful that I can improve it much more.
>>It also suggests that measuring an antenna close to the ground isn’t a good idea
I have the same issue with an attic antenna, and I’m trimming it according to the antenna analyzer.
If close to the gound is what you’re stuck with then that’s the place to measure it (and trim it).
>> If close to the gound is what you’re stuck with then that’s the place to measure it (and trim it).
Really?? I’ve never heard this (I’ve only been licensed for 9 months though). It doesn’t seem like adjusting it when the ground is affecting the pattern would reduce the degree to which that was happening.
>>It doesn’t seem like adjusting it when the ground is affecting the pattern would reduce the degree to which that was happening.
Despite being licensed for 25 years (but inactive for the middle 22), I am no expert on antenna theory (not even close). So take anything I say with a grain of salt.
I wasn’t thinking of pattern. And I’d certainly never want to measure and trim in a poor location and then publish the results as typical. I just meant that anything that affects your measurements of a particular installation of a particular antenna will also affect your operation.
What I meant was that to minimize loss in your (presumably coax) feedline you will want as good of an impedance match at the antenna feedpoint as possible. If you know that you are stuck operating the antenna in a certain location (as poor as that location may be), then you’d want to adjust the antenna for best possible match at that location. Assuming a simple dipole with either direct coax feed, or a simple balun, then that adjustment takes the form of element trimming. If you were to use balanced feedline (with low loss even at high SWR) or a remote antenna tuner at the feedpoint then your priorities may be different.