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[TowerTalk] Tennadyne LPDA

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Subject: [TowerTalk] Tennadyne LPDA
From: (Robert Hummel)
Date: Fri, 10 Apr 1998 10:10:18 -0400
At 08:01 AM 4/10/1998 -0400, you wrote:
>If I am not mistaken, it is never mentioned that the impedance is 50
>ohms. In fact, he specifically states that it is the correct value of
>377 ohms in the product brochure. A balun is required to bring it to the
>proper feedline impedance.

I think you are mistaken, Anthony. Tennadyne sells their T-10 HF log as
having a 50-ohm nominal impendance suitable for direct feed with 50-ohm
coax. They suggest the usual 10-turn coax balun as an RF-choke.

BTW, there is no "correct" impedance for a log. Since the feedpoint
impedance of a log is simply that of its transmission line (boom), you can
design the line to be any impedance you want. I designed a real TV log (ch2
- 13) a few years ago for a guy in a fringe TV area. First I designed it
for a 300-ohm feed. Then he wanted coax, so I redesigned it for 75-ohm.

Designing a log is a cookbook design. I wrote a basic program decades ago
(man, I've been doing this a long time) that will tell you the boom
spacing, size, element positions and lengths for any frequency range and
gain. If there's interest, I could dig it out.

I reverse-engineered the T-10 and found it to be a good design except for
the feed impedance. When I spoke to the designer, he claimed that you could
indeed have a 50-ohm feed with a circular-cross-section 2-line boom feed.
That was my first clue that he really didn't understand the design and
couldn't help me with the poor SWR.

Finally, remember that anything within a boom's length of a log will skew
everything. I put a 40-2CD above it and varied the separation distance
until the log's SWR was okay.

Robert L. Hummel (WS1A)

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