40m dipole not really tuned for 40m… or is it?
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.