Topband: Shared Apex Loop Array

[bruce whitney]

Tom, 
Good to hear you chime in on this topic - I listen very intently when the oracle of  Barnesville speaks...
 
I have been trying to learn more about the SAL array from the 4NEC2 models - as it seems to outperform anything else I have to listen with at present. The models show narrower forward directivity from the SAL than a 4-square - which seems to be confirmed by my subjective observations. Can this be explained by the very close spacing of the 'virtual' verticals as well? 
 
"Transmission line" mode (in the closely spaced vertical wires of the loops) to me implies equal and opposite currents and yet the antenna performance would seem to be based on currents out of phase - hence the delay line to combine/cancel them for directivity?
 
Regardless, I am not sure if any sort of "descriptive logical explanation" will ever allow me to understand what is going on with this antenna but I am beginning to get comfortable with the modeling. The bigger question to me is; do you think the 4NEC2 model results faithfully represent what is going on with this antenna?
 
I am wondering how an 8 circle array of these SAL antennas - fed 4 at a time in the classical broadside/endfire way, like individual verticals - would perform? (Maybe not use the amplifiers or check that they have identical phase delay?) I think the single support makes something like that mechanically feasible - not much different than top loaded verticals with 4 support wires/guy ropes.
 
Bruce W8RA
 


________________________________
From: Tom W8JI <w8ji at w8ji.com>
To: bruce whitney <zuceman at yahoo.com>; Carl Luetzelschwab <carlluetzelschwab at gmail.com>; topband at contesting.com 
Sent: Monday, February 10, 2014 1:36 PM
Subject: Re: Topband: Shared Apex Loop Array


> Is is a two element crossfire vertically polarized array made from loops.
> 
> Personally, if I were going to build one, I would spread the loops apart. I would make the loop centers as far apart as possible, not to exceed 1/4 wavelength on the highest band.
> 

Let me expand a bit how it works. Because the SA array has such close spacing, the center vertical element wires cancel. They just vanish from any significant pattern effect. The result of the shared apex is the loops function as two short ground independent verticals that are phased.

The axis null in a loop forms because all opposing sides carry 180 degree out-of-phase currents.  If we push two small  loops together, the vertical center wires carry mostly opposing currents, and go into transmission line mode. They are too close for any useful pattern control, so the loop changes mode.

The reason I suggest wider spacing, if I was doing all that electrical hardware, is pushing the loops apart keeps the loop elements in loop mode, so far as pattern is concerned. This will add two nulls through the loop axis. Although the nulls are rather small area, any new null or additional null not in signal direction is a good null.

The no free lunch rule is, as usual, in full force. We simplify by using one support and decrease length, and we lose other nice things.

We have two short verticals phased, instead of what would be three or four with wider spacing.

73 Tom