Topband: 4SQ around tower

[Tom W8JI]

> Thanks for the great reminder! The chapter does in fact describe exactly 
> what I have been planning. It also gave me the most important piece of 
> information- as long as the tower is grounded, which is the case, it has 
> no impact on the 80m 4SQ.
>


If that is the actual statement, it is not a correct statement except in a 
very specific case.  If the tower is empty of any other antennas or guy 
lines and the tower is around 100-150 feet tall and grounded at the base, it 
would have the minimum effect on an 80M 4 square.

Every area of the tower and the guy lines and everything else around the 4 
square must not be resonant on the band the 4 square is on.

>>A couple of points:

>1) The 4SQ is for 80, not 160. I am trying to decide if I should clear some 
>forest to make room for it or just place around the tower. If it goes in 
>the forest, radials become a nightmare

>2) Currently the tower is just a tower, not a 160m TX antenna. If the 80m 
>4SQ would suffer from gamma matching the tower than I have the option of 
>building a dedicated 160m vertical in a new location. A lot of work, but 
>less work than trying to lay radials in the forest.
>

Let me explain the issue a little better.

It doesn't matter one bit what anyone else does with a **different tower** 
on a different band or even on the same band. The center area of the 4square 
has very high field levels when the 4sq is active. The 4sq will couple into 
whatever is inside the 4 square, or around the 4sq on the outside.

If you just simply had a tower with insulated non-resonant guy wires, and if 
that tower was 120-140 feet tall and reasonably well grounded at the base, 
it would be almost perfectly non-resonant on 80M. It would have only a 
minimal effect on the 4 square.

The moment you add anything  to the tower that is metallic, the 
anti-resonant frequency will shift. How much anti-resonance shifts naturally 
depends on what is placed on the tower and where it is placed on the tower. 
This would even include feedlines that are not bonded to the tower, because 
they can act like parallel stubs. If they happen to shift the anti-resonance 
caused by the tower being about 1/2 wave long and grounded at the base, then 
you are in the soup. The first change will be a modification of F/B ratio, 
because the nulls are most sensitive to current ratios that will be upset by 
the presence of the tower. It does not take much re-radiation to hurt the 4 
square nulls. It takes more re-radiation to actually damage the gain.

A worse case example of this would be a tower 130 feet long grounded at the 
base. If I put a large 20M Yagi with grounded elements on the tower anywhere 
except down near ground level, the tower will no longer be anti-resonant on 
80. Moving the Yagi up and down the tower will change how the tower 
interacts, and there can be some very sour locations for the Yagi that would 
grossly affect the 4 square.

The thicker the tower is, the more difficult it is to make invisible. The 
more feedlines and antennas, the worse it is.

I have a 160 4 square surrounding a bare ~200 ft tall tower.  If I 
completely float that tower from ground, it just kills the performance of my 
4 square.  If I ground the tower, it knocks about 10-15 dB of F/B out of the 
4 square. I have to find a "sweet" value of reactance to put between the 
tower base and ground to make the 4 square behave.

If I put one Yagi on it, the Yagi mounting height and size would affect the 
impedance needed at the base to detune the tower. If I loaded that tower 
with Yagi's, I might never get it detuned. If I ran a feedline down the 
tower on the outside that was not bonded to the tower every 1/4 wave or 
closer on 160 (in my case where it is on 160), that might also cause an 
issue.

The case where we can carte blanche say there is minimal interaction would 
be when the tower is not too large a cross section, there are no metal guy 
lines attached, there is nothing else anywhere near resonance around, and 
the tower is about 1/2 wave tall and grounded at the base.

The mechanism that decouples the tower is the ground at the base plus the 
transmission line effect of the tower tries to create a very high impedance 
1/4 wave up, but that high impedance is "shorted" by the 1/4 wave section 
going up from the center to the open end. The top half tries to create a low 
impedance right where the bottom half tries to create a high impedance. 
This "detunes" the tower.

The thinner the tower, the better this works. Stick something else on the 
tower, and it all can change.

73 Tom