What I do is use an antenna analyzer (like an MFJ 259B or RigExpert AH-54)
to find the antenna's current resonant frequency (where the SWR dips), then
calculate the length of wire I need to add or subtract.
If you don't have such an instrument, measure the SWR using the transmitter
(at low power) at the low, middle, and high frequencies of 40 meters, and
add or subtract a few feet at a time depending on those results.
For example, you might observe SWR 3.5 at the low end, 4.2 in the middle,
and 5.8 at the high end of the band. That shows you that the antenna is
closer to being "right" at the low end than the high end of the band, but
its resonant frequency is lower than the low end of the 40 meter band.
Since the antenna's resonant frequency is lower than you want, the antenna
needs to be shorter. Remove a few feet and repeat the test. Keep notes so
that you know what the measurements were before, the change you made, and
the measurement afterward. This helps you understand cause and effect.
When I "remove" antenna length I usually bend the end back on itself and
wrap the end a bit so that I can easily extend the length later. I start
out intentionally long and then finally cut it when I have the length right.
If the antenna's bandwidth is likely to be quite narrow (which it won't be
with your full-sized dipole), then you'd need to make measurements at more
places in the band to make sure that you don't miss the resonant dip.
73 de Dick, K6KR
>>I'm helping a friend set up a 40 m dipole on his tower. It hangs at
approximately 30ft. The dipole is in a flat top configuration. There is some
droop on the ends, but not too much. We have tried it at the 33' per side
length as well as 30' per side. Neither gives reasonable swr. Both show 3:1
Any ideas to try?
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