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Re: [TowerTalk] dipole z vs height

To: towertalk@contesting.com
Subject: Re: [TowerTalk] dipole z vs height
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Wed, 20 Feb 2013 10:19:38 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 2/20/2013 7:18 AM, Jim Lux wrote:

It will have an effect, but I don't know about a huge one.

I've done a lot of modeling, both with NEC and with SimSmith to study the effects of voth antenna height and transmission lines of different Zo and lengths. One thing that jumps out at you is that the (mis)match between the antenna and the line, combined with the impedance transformation of the line, will usually cause an APPARENT shift in the resonant frequency as observed on an SWR indicator in the shack. The shift can easily be 100 kHz on 80M.

To get a true picture of what the antenna REALLY looks like, I measure complex Z in the shack using a Vector Network Analyzer (a Vector Antenna Analyzer also works), find the electrical length of the line making a TDR measurement, export that data to SimSmith, and use SimSmith to "subtract out" the line by entering a line with negative length equal to what I've just measured. I can than add that line back in to find the loss in the line, and I can try various matching schemes.

If I see that the Z of the antenna is closer to 50 ohms, I may choose to feed it with 50 ohm coax.

There's a VERY slick broadband matching scheme for antennas close to 75 ohms that involves using a section of 50 ohm line that is some multiple of half-wavelengths followed by a quarter wave of 75 ohm line. This works VERY well on 80M for 50 ohm sections of a half wave, full wave, and 3/2 wavelength, such that an antenna cut for about 3675 kHz is better than a 2:1 match to a 50 ohm transmitter from 3500 kHz to 3875 kHz. It does NOT reduce the LOSS in the line due to mismatch between the line and the antenna, but it can eliminate the need for an antenna tuner. Around Northern California, Dave Leeson, W6NL, is often given credit for it but Dave says the technique goes back to at least the 1940s. I've included it in my tutorial on using SimSmith to design matching networks.

http://audiosystemsgroup.com/PacificonSmithChart.pdf

73, Jim K9YC


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