[TowerTalk] 160 M OCF antenna

W4ZW w4zw at comcast.net
Sat Feb 25 16:20:07 EST 2006

Jerry,  It was originally 271' and had a SWR match of 1:1 at 2.1 MHz.  I
added 25' and that brought the 1:1 match down to 1.950 MHz.   Note, I am
saying SWR match and not resonance which I used before and, of course, could
be misleading.  I have misplaced my MFJ 259 meter so I'm relying on a Daiwa
SWR bridge to measure this guy.  The resonance at 296'  should be much lower
than 2 MHz but I don't know the velocity factor of this insulated wire which
should push the frequency even lower, not higher.  My ground here is nearly
perfect since I'm Oceanside about 13' ASL and surrounded by ocean on all
sides since I'm on the southern tip of the island,  except to the NNW as
this key runs.

I just moved the long end to about 20' elevation and removed the 25' adder
section.  With this configuration, the SWR is now centered at 1.3:1 at 2.04
MHz.  Still too high for a 271' long OCF dipole. I think the idea that the
proximity to ground is the culprit here is probably correct.

Jon Hamlet, W4ZW

Casey Key Island, Florida
"A little piece of paradise in the Gulf of Mexico" 

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of K4SAV
Sent: Saturday, February 25, 2006 5:02 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] 160 M OCF antenna

Wait a minute!

You originally had a 281 ft OCF dipole that resonated at 2.1 MHz.
You then shortened it by 10 ft and moved the resonance to 1.95 MHz?

Kinda blew my mind there for a second.

Assuming you are giving us good data, then there are two possibilities.

1. What you are getting is not the resonant frequency of the antenna, What
you are getting is the impedance of the antenna, transformed by a highly
mismatched transmission line. This impedance can be almost anything
depending on the length of the feedline.

2. The balun is not working as a current balun, and the feedline is acting
as part of the antenna. These effects will be difficult to predict.

Jerry, K4SAV

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