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Re: [TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please

To: <>
Subject: Re: [TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please
From: "Larry Phipps" <>
Reply-to: Larry Phipps <>
Date: Wed, 23 Jun 2004 03:28:40 -0400
List-post: <>
I just ran a quick sweep of a 3el SteppIR model and the loss in the feedline 
alone due to mismatch is about 4dB at 1 mHz out and 10dB at 5 mHz out. Add to 
that the pattern loss and you're well over 10dB total out-of-band loss compared 
to a LPDA. That might not seem like much, but 10dB lower interfering signal is 
roughly 20dB lower distortion products. It all adds up.

I have a friend with a good sized LPDA at 100' who watches the bands on his 
spectrum analyzer from time to time to check band activity and he reports sw 
bdcst stations that hit -35 dBm. With a typical receiver with 3rd order IP of 
+10 dBm and 10dB NF, that puts distortion products above the noise floor. With 
local contest activity, it could be worse. 

I'm not saying it should be a major design consideration, but taking into 
account all the care that goes into some contest station designs, it's worth a 
little consideration. 

Here's a little trick for you SteppIR owners to try. Point your beam at WWV at 
15 mHz with the antenna tuned for 15 mHz. Note the signal strength.  Now tune 
it to 15m or 10m and see what the signal strength is. I'll bet it drops a 
couple S-units.

Larry N8LP

----- Original Message -----
From: "Rick Bullon" <>
To: <>
Sent: Tuesday, June 22, 2004 8:07 PM
Subject: Re: [TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please

> One point needs to be made, especially for contesters. The LPDA is a very
> low Q antenna with very wide bandwidth... the SteppIR is a high Q antenna
> with narrow bandwidth that is frequency agile.
> When the bands are very crowded with big signals, there is a huge
> in the strength of unwanted signals reaching the receiver front-end.
> --------------------------------------------------------------------------
> Both are good for the causal operator depending on your interests on the
> bands. but  not one of the serious contest station use either antenna.
>   The SteppIR tunes too slow for contest work when changing bands and the
> on the LPDA is too low for contesting

Seems to me that depending on the antenna to perform the function of a
preselector is sort of silly, in a no-holds-barred system design. If you're
worried about overloading the front end of the receiver, either you need a
better front end and a receiver that has better instantaneous dynamic range,
or it shouldn't be too hard to design a fairly high Q, low loss, preselector
that would tune automatically given frequency presents.  This is a pretty
standard thing in wideband microwave receivers like spectrum analyzers,
where they use a YIG filter for preselection. Granted, spectrum analyzers
aren't particularly hot stuff noise-figure wise, but then, most HF receivers
aren't either.  Whatever the Q of a Yagi is, it's nothing as narrow as even
a crummy LC filter.  Don't forget that SWR bandwidth is not the gain
bandwidth, either.

Clearly, what's needed is a fast tuning, fairly narrow band, directional
antenna.  Hmmm.. that would be an electronically scanned active phased
array.  The SteppIR is an intermediate step to this, with mechanical tuning.


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