[TowerTalk] Portable Phased Arrays!

[Jim Lux]

At 09:08 AM 6/28/2006, Paul Playford wrote:
>I have experience with 4 square phased arrays.  Over a dozen so far.
>
>The advantage of parasitic feeding is simplicity.
>
>I erect the 4 elements with elevated radials in their calculated positions 
>(.22 wavelength on a side).  Then I take two of the radials that run in 
>opposite directions, feed them as if they were a half wave dipole and trim 
>them to resonance.  Then I adjust all of the other radials to that length.
>
>Then I trim one of the vertical elements for resonance against it's 
>counterpoise, and adjust the remaining vertical elements to the same length.


And this strategy works well for a single band, and for a nice symmetric 
layout.

When you start looking at multiband operation and arbitrary layouts, you 
wind up with situations where "you can't get there from here" using only 
parasitic coupling.

I went through a whole batch of Monte Carlo modeling where I would:
1) determine semirandom placements of elements
2) optimize the tuning (either active or parasitic) for several "beam 
directions" and "null directions"
3) see what the tolerances were
4) repeat...

The real hangup comes when you put the elements, say, 5-10 meters apart... 
that's 1/2-1 wave on 10, 1/4-1/2 wave on 20, and 1/8-1/4 wave on 40... Lots 
of coupling on 40, not much on 10.  And, of course, the soil properties, 
adjacent trees, cars, playground equipment, houses, etc. all have an 
effect.  And, then, there's also the RF safety aspect, which can be quite 
constraining in some environments.

yes, supporting arbitrary element positioning is a significant design 
constraint, however, after having fooled with some other field deployed 
arrays which required careful positioning or monoband operation, I decided 
that it was important.

Contemplate these scenario:
1) Family outing to a horse show.. many hours of tedious waiting and plenty 
of time to operate, but somewhat constrained in antenna placement options
2) Beach trip or camping where you have to work around other people.
3) Rover style contest operation (for this, monoband elements may be a 
better choice)

It all boils down to needing a lot of flexibility.



>The disadvantage is they are not broad banded.  One array will not cover 
>both 80m and 75m, but one array will cover all of 40m.

Precisely