[TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please

[Jim Lux]

----- Original Message -----
From: "Rick Bullon" <kc5ajx at hotmail.com>
To: <Towertalk at contesting.com>
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
difference
> in the strength of unwanted signals reaching the receiver front-end.
>
> --------------------------------------------------------------------------
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> 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
Q
> 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.