You can raise the impedance of your vertical to 50 ohms by installing a hair
pin coil at the base which may make you and your transmitter happier. It
won't effect the antenna performance. I agree with Jim that the antenna on
the metal roof probably worked fine. Too bad you didn't compare it to your
reference dipole.
John KK9A
To: "towertalk" <towertalk@contesting.com>
Subject: [TowerTalk] Fw: Fw: elevated radials vertical
From: "EC1CT Fernando" <ec1cwg@dxhunters.com>
Date: Sun, 10 Jun 2007 11:33:54 +0200
List-post: <mailto:towertalk@contesting.com>
Hello Jim,
Thank you very much for the input. Well, I made a few mistakes on my
previous message. I should have said that the SWR was 1.7 (3.790) up in the
roof and the resistance was nearly 20 ohmns (not the impedance). Down in the
gound the resistance (on the MFJ analizer) was 35 ohmns and got 1.1 (3.790).
So, I should assume that the ground plane was better on the roof?....I´m not
an expert but I guess I should have connected directly to the radio. I would
expect to have at least the same signal on both the dipole and the vertical.
73s!
BTW: I got some photos of the antenna over the roof that I´m trying to
upload to any site for your views...
EC1CT Fernando
E-mail: ec1ct@ure.es
I'll bet that the vertical on the metal roof works just fine! Have you
tried transmitting through it? Your mistake is thinking that SWR and
impedance defines how well the antenna works. WRONG! A vertical on a metal
roof SHOULD read 20 ohms or so if it working properly, because the
impedance of the antenna itself is around 20 ohms, and the roof gave you a
very low (GOOD) ground impedance -- a few ohms. When you put the same
antenna on the ground, the earth added 30 ohms of resistance, which wasted
2/3 of your power, but gave you a low SWR, making you think that was good.
But it's BAD!
See http://www.w2du.com/Chapter05.pdf
If you had connected the vertical on the metal roof (using the roof as the
ground) to your transmitter and tuned it with an antenna tuner, you would
probably found that it worked VERY well.
SWR is NOT an indicator of good antenna performance. It is only an
indicator of the MATCH between the antenna and the transmitter. That match
can very easily be fixed by an antenna tuner.
73,
Jim Brown K9YC
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