[TowerTalk] 4-square questions

[Bob Shohet, KQ2M]

The blue disappeared on the posting so you might have trouble figuring out where I responded.  I will try BOLD and see if that goes through....

Bob Shohet, KQ2M

From: Bob Shohet, KQ2M 
Sent: Thursday, March 09, 2017 12:43 PM
To: towertalk at contesting.com ; jimlux 
Subject: Re: [TowerTalk] 4-square questions

Hi Jim,

Thanks for your response – I have been thinking about what you wrote. I have interspersed some comments in Blue...


> 1)  Are the minimum wave angles of the transmitted and received
> signals primarily determined by the height above ground at the feed
> point?  At the top of each element?  Or some complex calculation of
> the average height along the entire length of the vertical legs?  I
> am sure that having part of two elements horizontal also affects the
> wave angles.

The latter, some complex calculation, the vertical pattern is affected 
by the "ground plane" many wavelengths out.  You'd have to model it to 
see what's really going on. And unfortunately, for a vertically 
polarized antenna, you can't use HFTA (because HFTA makes use of the 
fact that for H-pol, you can consider the earth as a perfect reflector 
for the most part)

You might be able to get a qualitative answer (see how much it changes) 
by using the NEC "ground cliff" mechanism
http://www.nec2.org/part_3/cards/gn.html

Set up just one vertical element over flat ground, then change it to a 
cliff/ledge and see what happens.

I looked at the link that you provided but I did not see anything that looked like “Ground cliff” mechanism.  Can you please be more specific?
I have an old DOS based NEC Wires program that I used to model many beverages and wire antennas but I do not recall anything that gave an option like “Ground Cliff”.

> 2) If the feed point is at the same vertical height as the top of the
> ledge when the array is pointed towards the ledge, is the minimum
> wave angle affected?

Did you answer this question as part of number 3 below?  It isn’t clear to me.

> 3) Is the minimum wave angle increased if the feed point of the
> vertical legs is below the top of the ledge?  Is it a straight
> geometry calculation – that is if the feed point were as 0 feet and
> the direction of the array was aimed pointed straight at the 12’ high
> ledge from a distance of 20’, would the effective wave angle be ~ 35
> degrees?  Or, in fact, is the minimum wave angle considerably lower
> since the broadside of the NW – SE legs are effectively radiating to
> the NE and therefore is about an additional 37’ away from the ledge
> than the NE element?

Not really. Consider an antenna as being broken up into a bunch of 
smaller antennas, each with different current and phase.  The far field 
is the sum of the radiation pattern from each of the little sub 
antennas.  This is what NEC does, it calculates the current (mag and 
phase) in each segment, then calculates the far field for each segment, 
then sums all the segments together.

With vertical antennas and uneven ground, it is almost never a simple 
geometry thing.   with horizontal antennas, all the little 
"sub-antennas" are the same height above the ground, so the calculation 
is the same for all of them. Furthermore, for Hpol, the reflection from 
the ground for almost any angle is like a perfect mirror, so geometry works.

I understand what you are saying  - but it isn’t clear to me that you answered my first question: “Is the minimum wave angle increased if the feed point of the vertical legs is below the top of the ledge?”

> 4) I would rather have a ground mounted 4-square with a ground screen
> of about 120 radials and with all four elements being fully vertical,
> but I am concerned about effects of the ledge on the wave angles when
> the array is “pointed” at the the ledge.

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.

That is one major advantage.  But the additional ~ 2 db gain over a dipole/inverted v or vertical matters too – and so does the lower angle of radiation of the 4-square compared to many other types of antennas. (depending on what your options are).  And lets not forget that the 4-square is usually a quieter antenna than an inverted V, dipole or single vertical.   And they are rather forgiving if the dimensions are not exactly right – the 4-square is antenna that WANTS to work.   :-)

So I guess the answer is that no one really knows what the effects of ledge are on the ground-mounted 4-square.  I have to believe that as the feed-point is lowered below the top of the ledge towards the ground, the minimum wave angles will be increase although by what amount and how significantly the performance may be impacted, is unknown.

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.

Unfortunately for you, your questions are one that can really be 
answered two ways: experiment (build it and see how it works) or some 
sort of finite element modeling.  And the FEM is going to be tricky if 
you want to stick with NEC, because it really doesn't deal with the 
uneven ground surface well.

As mentioned above, you can fool with NEC and use the "second ground 
surface-cliff" and model it to see.   One thing to bear in mind is that 
you can run two models and kind of combine the outputs.

Say you have your antenna system at the north end of a N-S ridge, so you 
have a cliff on 3 sides (W,N,E) and flat ground to the south. You run 
the NEC model with flat ground, and you can get the pattern to the 
south.  Then run it with the cliff (on all sides) and you get the 
pattern to the West, North, and East.

Thank you – that is a very helpful conceptualization of how to model the array.  I just don’t know where to find the “Ground Surface Cliff” that you mention.

73
Bob KQ2M

It's not perfect, but you'll at least know if it's terrible or not.

Bear in mind that the cliff ground in NEC does NOT factor into the 
element interaction or current calculations, it's far field only.  So 
the input impedances, etc. are all calculated as if you're on flat ground.

>
> I would appreciate any thoughts/comments that you might have.  Thank
> you in advance for your interest and cooperation!
>
> 73
>
>
> Bob KQ2M http://lists.contesting.com/mailman/listinfo/towertalk