[TowerTalk] Liberal Arts Major - feed point impedance

[Robert Chudek]

Lee,

You are getting some good advice here. I went through a similar "learning curve" with a 160m dipole last fall.

Here's a couple of things to keep in mind.

1) As already stated, the feedpoint values (all of them) will vary depending upon height, nearby resonant antennas, and to a minor extent the wire and insulation on the dipole itself.

2) The most accurate location for your Antenna Analyzer is at the feed point of the antenna. Any other location and the measurements will vary, depending upon feedline impedance, velocity factor, and line length.

3) If you can't take measurements at the feedpoint, the second best choice is to use a feedline that is 1/2 wavelength on your target resonant frequency. You can use your Antenna Analyzer to trim a feedline to 1/2 wavelength by shorting the opposite end and finding the lowest "resonant" frequency of that coax.

4) It was stated to cut a feedline to 3.525 MHz and it will perform on the higher bands. (This is true in a practical sense.) But using test equipment, I did not find this to be the case.

I tried various feedlines (rg-58, rg-59, rg-8x, rg-8, and 9913) and they all "skewed" on my analyzer. That is to say, if I cut the feedline for half wavelength at 3.500 MHz, you would expect the resonances to appear at precisely 7, 14, 21, and 28 MHz. They did not, to my surprise! The resonances tended to shift up by several hundred KHz each time the frequency doubled. So in practical terms using coax of 1/2 wavelength on the lowest band WILL work fine, just don't bet a lot of money on the concept if you are in a highly technical discussion!

Bob, W9GE is right on target... throw it up in the air, if your transmitter and/or antenna tuner is okay with the match... start making some QSO's... Forty meters is a great band... You should be able to work EU with that dipole, even if it is a "sky warmer"!

73 de Bob - K0RC


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Message: 9
Date: Thu, 01 Jun 2006 17:09:37 -0400
From: bob finger <finger at goeaston.net>
Subject: Re: [TowerTalk] Liberal Arts Major - feed point impedance
To: "'TowerTalk Reflector'" <towertalk at contesting.com>
Message-ID: <447F5791.4050103 at goeaston.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Is this a test?  If you use a line flattner so you can cover the entire 
band with that dipole it ain't gonna care.  Field strength measurements 
would be insignificantly different with either equal quality 50 or 75 
ohm coax and line flattner or not if PO remains constant.  We are 
talking mice nuts here.  Load er up and make some noise.  73 bob de w9ge