On 3/6/17 9:16 AM, Jim Thomson wrote:
Date: Mon, 6 Mar 2017 07:43:32 -0800
From: jimlux <jimlux@earthlink.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 4-square questions
To my mind, the big advantage of directional antennas on HF is not so
much forward gain, but the fact that they have nulls that can be steered
to block undesired signals.
You can have fairly big phasing errors and the forward gain doesn't
change much (tenths of a dB), but a phasing error can kill the null
depth. I suspect that this is why some people swear by 4-squares and
others swear at them.
## If the 4 sq is used on TX only, any nulls is sorta a moot point.
On a yagi, even with lousy FB or FS, at least they will have a good null
off each side. Usually you dont hear of folks with 80m or 40m yagis,
who also use a dedicated RX ant.
Yes, they'll have a null "somewhere" on each side. And given that a Yagi
is a bunch of dipoles, off the end of the dipole is a likely candidate.
The same is theoretically true of a 4 square.. you can get N-1 nulls
with N elements.
In the long run, I think the strategy will evolve to "get some forward
gain on Tx, and use nulls on Rx to reject interferers" which is sort of:
A) do approximate phasing on Tx, e.g. a 90 degree hybrid, which gets you
the best forward gain without superdirectivity
B) use a software defined radio approach to generate nulls on Rx
Most amateur phased arrays have the elements far enough apart that
getting a "superdirective array" (which is what a high performance Yagi,
particularly with 4-5 elements) is hard. Superdirective arrays have
coupling among the elements and stored energy in the array, and most
simple phasing schemes don't do that well. You're not going to be
getting 8-9 dBi from a conventional 4-square. But then, a 9dBi 80meter
Yagi is a behemoth.
The background noise is high enough on the low bands that you don't need
a really hot ADC for the SDR approach, so a low cost 4-8 channel system
isn't hard to put together. I don't know if you could do it with
RTL-SDR USB or pods at $20 each, but it's in that ballpark, say
$50/channel. (the SDRPlay is $150) After that it's software, which, once
someone develops it, is potentially free.
A nice approach would be a lumped LC band select filter in front of the
cheap ADC (so you don't have to worry about BC and SW interference),
maybe with a low noise preamp. And in fact, why even use the same
antennas.. on receive, a short whip and a preamp would work.
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