I concur with Jim. I put up my full size sloper on 80 several years ago. I
started by making an inverted vee about 30 feet high. I tuned it for the
correct frequency. Then I hoisted up one end to 120 feet and pulled the
other end out to about 40 degrees. It required very little trimming to get
it to resonate at the right freq. I suspect that moving an inverted vee
from 30 ft to 60 feet will not change the freq much. An inverted vee on 80
will be easy to prune once you get it up at 60 feet anyway.
If you put a 1/4 wave of 75 ohm coax (I use RG6 because it is lighter)
between the antenna and the 50 ohm coax it will further broaden the response
of the antenna.
Bill W5VX
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Sunday, March 29, 2009 12:55 PM
To: Tower Talk List
Subject: Re: [TowerTalk] Tuning Antennas with the MFJ-259
On Sat, 28 Mar 2009 23:59:49 -0500, AD5VJ Bob wrote:
>Well one of the antennas I will be working on in the morning is
>going to be an 80 Meter dipole, but I can only get it up here at
>abt 30' then I will be taking it and putting it up at around 60
feet, so it sounds like I want to tune it high
Bob,
You will see SOME difference on 80M between 30 ft and 60 ft height,
but NOT a lot. If you tune the antenna at 30 ft, the resonance will
move up a bit when you raise it to 60 ft. I'd guess 1% or so. In
other words, if you want the antenna to resonate on 3600 kHz, tune
it to 3560 kHz at 30 ft. Lowering the ends (inverted V style) also
lowers the resonant frequency a bit. I would expect the SWR curve of
an inverted V with the ends at 30 ft and the center at 60 ft to be
not very different from a flat dipole at 30 ft.
The other thing that happens is that the IMPEDANCE at resonance
rises as the antenna is higher. The impedance of a dipole at
resonance is close to 72 ohms in free space, but gets lower when it
gets close to the earth. 75 ohm coax is the best match to a HIGH
dipole, while 50 ohm coax is the best match to a LOW dipole.
73,
Jim Brown K9YC
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