Topband: Dual band array

[Tom Rauch]

Hi Roger,

> I've been contemplating an 80m 4 square for a couple of years. Been
> playing with all the modelling programs. It appears that the high angle
> lobe off the back can be reduced by either changing the driving phase to
> -110, -220 or thereabouts or by increasing the element spacing. Rather

I have no idea why everyone ever settled on 1/4 wl on a side and 
90/180 shift. The phase is clearly wrong for the spacing since the 
mean element spacing is closer than 1/4 wl when fired across the 
corners.

Stretching the array out, assuming you don't run out of feedline, or 
more phase delay is better.

I use 120/240 delay or so on mine. It has more gain and a tighter 
pattern.

> dual band element. Base loading would be the easiest but least efficient.
> Top loading is better but requires some sort of switch. A  trap reduces
> bandwidth too much. I was wondering about a vacuum relay but not sure of
> what voltage to expect. Assuming a 65 ft 3 in dia element with 1kw input
> any educated guesses? Any alternative ideas?

You'd have about 2800 volts RMS at the end with 1500 watts input 
with a three inch diameter element in a single vertical. You can 
scale it from there to your power.

Element impedances are not all the same in a directional array 
with a unidirectional pattern. I'd have to think about how that would 
affect end-voltage. My gut reaction is voltage would still be the 
same on all elements because the rule of thumb when voltage 
feeding elements is they all need the same voltage.

Because of the high voltage, you'd have to be sure you minimized 
"leakage capacitance" across the insulator. That would require 
some form of compensating inductor.

I'd opt for a decoupling cage around the vertical, or separate 
elements and detune or sectionalize the 160 meter elements when 
on 80. 

73, Tom W8JI
w8ji@contesting.com


--
FAQ on WWW:               http://www.contesting.com/FAQ/topband
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com