[TowerTalk] terrain for a 4SQ system

[jimlux]

On 5/16/19 1:00 PM, Jorge Diez - CX6VM wrote:
> hello
> 
> I want to build a 4SQ system, with four aluminium vertical antennas
> 
> My terrain is not flat, so I will do some work to level the ground
> 
> My question: is just enough to level the terrain where the 4 antennas will
> be placed or is also necessary to level all the field included the field
> occupied by the radials?


A 4 square is 4 vertical antennas with the element currents set by a 
phasing network - typically, the phasing network uses 1/4 wavelength 
lines to "force" the currents to be the right ones, using some form of 
90 degree hybrid at the common feedpoint.

The answer to your question has two parts:
1) Assuming the element currents are phased as expected, how much will 
the pattern be degraded by non-ideal elements?
2) Is the element current phasing perturbed by non-ideal elements?

For #1 - 1/4 wave monopoles are pretty tolerant of their radial field 
variations - the gain and phase don't change very much - it's all about 
the current distribution in the vertical element and the "sort of 
complex" reflections off the soil surface in the near field.  You can 
look at N6LF's page for a discussion of the (small) changes in pattern 
of a 1/4 wave vertical.

My opinion - not a big deal as long as you're not talking about putting 
it on the side of a cliff, or the "lumps and bumps" in the soil are a 
"significant fraction of a wavelength"

For #2 - non-uniform ground systems and installation in general will 
change the mutual impedances of the antennas, which will change the feed 
point impedance.  If you were using a simple feed system that assumed 
everything had 50 ohm feeds, it would definitely be degraded. Note well 
that a typical 4SQ in ideal situation has feedpoint impedances that are 
nowhere near 50 ohms.


  However, most 4SQ setups use the 1/4 wave (or 3/4 wave) "current 
forcing" scheme.  So now, you're really talking about how well does your 
90 degree hybrid work when working into an unexpected load impedance. 
And that's something that's hard to know - I suspect a lot of folks have 
this, and aren't aware of the problem.

And here we come to the actual question - what performance number do you 
care about?
If it's forward gain - 4SQ type phased arrays can tolerate huge changes 
in phase angle/amplitude without greatly affecting the gain. We're 
talking 0.3 dB kinds of changes with a 30 degree phase error 
[(3+cos(theta))/4 = 3.87/4]

If it's sharp nulls - yeah, small changes of a few degrees turn a 20dB 
null into a 10dB or a 6dB null.










> 
> Would be very good to level ALL the terrain that will be occupied by the
> radials and antennas, but if it's enough to level just the place of the
> antennas, will save a lot of money
> 
> thanks!
> Jorge
> CX6VM/CW6W
> 
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