----- Original Message -----
From: Tom Osborne <w7why@frontier.com>
To: Towertalk <towertalk@contesting.com>
Sent: Thu, 22 Dec 2011 22:23:56 -0000 (UTC)
Subject: [TowerTalk] Elevated vertical
Hi All
I just put up an elevated 40 meter vertical. The feedpoint is around 15
feet or so up and I have 8 radials on it right now. I cut the radials the
same length of the vertical section. I have a coil of around 8 turns of
coax for a choke.
It works good, as the first day I put it up, I worked some Europeans from
here in Oregon.
The thing that seems strange is that at the low end of 40, the SWR is around
1.3:1. As I go up the band, it goes up to about 1.5:1, then when I get to
the top of the band, it goes back down to 1.2:1.
Is that because some of the radials are resonating at different parts of the
band?? 73
Tom W7WHY
>>>> Tom, I can think of a couple of possible explanations for this to occur.
1. Do you have a 75-80 meter dipole in the vicinity of the vertical? If so,
perhaps one side of the dipole is resonate at one of the low SWR frequencies of
your vertical and causing RF energy to be reflected back into the vertical.
2. Perhaps the total length of each pair of your radials is resonate at one of
the low SWR frequencies while the vertical element combined with the radials is
resonant at the other low SWR frequency. Perhaps that is the reason why some
references suggest making the radials a little longer than the vertical element.
When I put up an elevated vertical, I first put up a pair of radials and tuned
them to resonate at the desired frequency with an antenna analyzer. I then cut
the other radials to the same length.
Incidentally, I found that the elevated vertical with 16 radials rarely
performed any better than my 40 meter inverted V dipole with the apex at about
60 feet so I took the vertical down.
Don K9MUF
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