[TowerTalk] Isolating adjacent 4-Squares

[Lux, Jim]

On 12/22/20 7:09 AM, Ignacy Misztal wrote:
> Actually, Charlie's problem is an exciting one to look at and experiment.
> Not sure what he means that the array is not working. I suspect poor F/B or
> high SWR. Many ways to troubleshoot when the weather is nice. Necessary
> items are antenna analyzer and large snap-on toroids.
>
> 1. Is the impedance at resonance at each feed point same for all the
> verticals and close to 40 Ohms?

Most 4-squares have significant interaction among the elements, so you 
need some way to take the other elements out of the system while 
measuring the Z.

Or you need to measure the full 4x4 mutual impedance/admittance matrix.  
Which means measuring element 1, while terminating elements 2,3,4 in all 
8 combinations of short and open (or load).

Realistically, the easiest way to do this is to cal your analyzer to 
include the feedlines, and then do all the measurements at the central 
point.



> a) if some impedances are widely different,  check the hardware.
> a) If the same but the impedance is not 35-45 Ohms, the coupling structure
> is symmetrical to all the verticals (most likely the tower).
> b) if impedances are not the same, the coupling is likely unsymmetrical.
> Try to disconnect other antennas, put snap on toroids on various cables and
> wires, and see whether it makes a large difference in impedances or SWR.
> If it something does, you have a culprit.
>
> 2. Are impedances similar at the feedpoint?
> If not, check the phasing lines.
>
> 3. Have a switch to listen to each vertical separately. If there any
> directivity?
> a) If not,  the tower is likely not in resonance.
> b) If yes, the tower is likely in resonance, and the regular 4sq box would
> not work well.
> However, a custom box or just custom phasing lines could bring performance
> above the regular 4 sq since the tower will be used as part of the antenna.
> One way to check for it would be to use only two inline verticals and apply
> a custom shift box a la W9AD unit:
> https://www.arraysolutions.com/antennas-phasing/w9ad-sys
>
> I had trouble with my 80 4 sq. The problem was a feeder to an 80m dipole
> off the top of the tower. I also tried a 2 el 160m array to top of 100 ft
> trees with 4 elevated radials. But one such vertical was 8 db below the
> 100' shunt fed tower so the effort would not pay off. Combined effects of
> tree proximity and elevated radials.  In both cases the simulation would be
> fruitless.



Note that one of the nice things about the 1/4 wavelength feedline  (aka 
current forcing) approach is that the current in each element is the 
right value, regardless of the element's feedpoint impedance, and since 
current is what radiates, variations in Z don't make a huge difference.

Granted, large differences in measured Z might indicate some bigger 
problem (i.e. a broken element)