[TowerTalk] 4 Square question

[Tom Rauch]

>   When I built the 40M 4 square, the then owner of
> Comtek told me to use a multiple of 1/4 wavelength
> 75 ohm coax (I think I have 3/4) so it would reach
> between the box and base. A lot of coax, but it works
> great.

OK if it's an odd multiple of 1/4 wl, and assuming the phasing 
system is designed for the impedances presented by 75 ohm 
cables and the various impedances from each element.
  
>   When I built the 80M 4 square, the owner of DX
> Engineering told me to just use equal lengths of 50
> ohm coax to drop down from the elevated bases to the 
> ground, then hook up the 1/4 wavelength 75 ohm coax.
> This also works great.

I'm surprised it works "great", although that can mean anything.  
Four squares are forgiving of many design errors, but that is a big 
one unless the 50 ohm to 75 ohm transition was planned in the 
system.

I can't imagine anyone who understands phasing and current 
distribution in arrays like this telling you that **random** but equal 
lengths of 50 ohm cable will work! It is just pure blind luck if it did! 

That is very bad advice.

>   Questions: Are these two feed methods equivilant?

No, not at all.

>   I plan to feed the 40M like the 80M, all I would 
>   have to do is cut the good 75 ohm into 1/4 wave
>   pieces, cut up some 50 ohm coax and solder a few
>   plugs. I guess I could elevate the 40M phasebox on
>   a pole and string 1/4 wave phasing lines to the 
>   elevated bases, but the 50 ohm drop down line 
>   method sure would be quicker.

If you get very lucky, it will work. I guess it mostly depends on 
what we call "work", and how your luck runs.

The reason you have to use odd 1/4 wl lengths of coax is so the 
equal voltages supplied by the phasing system are translated into 
equal currents at each element. When you mix 50 and 75 ohm 
cables, some very wild things can happen to the system's current 
and phase.

The best "mix length" of 50 and 75 ohm cables is 1/2 wl electrical 
of 50 ohm into 1/4 wl of 75 ohm, assuming the system was 
designed for 75 ohm cables.  

The worse possible length would be 1/4 wl of 50 ohm into the 75 
ohm 1/4 section.

All of this assumes the system was designed to use 75 ohm feed 
cables.
73, Tom W8JI
W8JI@contesting.com 

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