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Re: [TowerTalk] log periodic beam performance

Subject: Re: [TowerTalk] log periodic beam performance
From: jim Jarvis <>
Date: Sun, 13 Dec 2009 22:17:33 -0500
List-post: <">>
w4boh wrote:

I'd appreciate hearing of experience with LP beams, especially performance 
vs number of elements.  The little T6 spec's are remarkably similar to the 
T10, but some experienced people are pretty adamant that six elements don't 
do a good job.
Anything will be a lot better than the little low endfed I use now, but I'd 
like to have some real idea of what to expect.

The LP is great for getting me WARC coverage and avoiding traps!

Thanks,   Wilson   W4BOH 



I've had a cushcraft lpda, as well as  a Tennadyne T8, along with  a host of 
and a SteppIR 3 element yagi.    Hopefully, my empirical comments may have 

1)  The LPDA was as good as any of the kt34a's I had up, and better than any 
tribander.   With the log at 70', I was highly competitive, and highly agile.   
In switching
from a kt34a @ 60' to a T8 at the same height, I didn't feel I gave up any 
But I couldn't do an A/B comparison.    The antenna changeover took about 3 
and propagation had shifted in that time.     

2)  The Tennadyne boom-feed system is superior to the Cushcraft system with 
connections.   less lossy, and easier to assemble, too.   

3)  I can't speak to the M2 logs, although Mike has a good reputation, and his 
work bears
consideration.   Having owned and refurbed 4 kt34xa's, and a couple of kt34a's, 
I have
an appreciation for his design work.    If you compare Tennadyne to M2,  look 
at the boom and feed systems.   

4)  In the discussion about out of band interference, it IS true that the log 
does not serve
as an RF filter.   This was never a major problem, although I DID see baseline 
on my 756pro band scope, when shortwave broadcast was 60 over 9 on 21450, and I 
working 15m cw.   Never HEARD any interference... just saw elevated baselines 
in the
broadband scope receiver.   

I perceive this as a non-problem, with contemporary receivers and roofing 

5)  Apart from lower losses than trap bearing antennas, and broad frequency 
the principal virtue of the LPDA is its electrical simplicity.   In the 
Tennadyne design, this
also translates to lower losses.

6)  I also felt the T6 was a bit light on elements, and went with a T8, at the 
time.   Now, I 
think I'd go for a T10.   One of the benefits of more elements is a smoother 
vswr curve, within
the one octave coverage of the log.   I have a friend who has had a t6 for 
years, and been
quite pleased with it.   

7)  As a practical matter, you're going to see 20dB f/b ratio  and reasonable 
forward gain.  
One nice thing about the log is, you're likely to see that over most of the 
band.  A trapped
tribander, or even a monoband antenna is likely to have f/b,  vswr, and gain 
at different frequencies.   So you never quite know what you're going to get 
for performance.

8)  My 3 element steppIR replaced the LPDA,  and performance went from a 2 
element beam
equivalent to a 3 element beam.   It was a noticeable improvement, at 
considerable expense,
and with the potential for mechanical failures.   As it turned out, I had no 
problems with the SteppIR, 
in the time I owned it, and in 2 VT winters.

That said, my next antenna will be an LPDA.  Probably a T10.   Principal 
consideration is
robustness of design and efficiency.




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