[TowerTalk] 4-square questions

[jimlux]

On 3/6/17 9:16 AM, Jim Thomson wrote:
> Date: Mon, 6 Mar 2017 07:43:32 -0800
> From: jimlux <jimlux at earthlink.net>
> To: towertalk at 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.