[TowerTalk] Tuning a four square

[Dana Roode <droode@cox.net>]

Towertalkans -

I am retuning my 40m 4 square - for some reason it started tuning high in 
the band after I found a faulty dummy load (gory details below).  The basic 
procedure is to tune a single vertical to about 100khz lower then the 
frequency you are after, then duplicate the resulting length in the other 3 
identical verticals.

Question: Should I tune the first vertical fed with the quarter-wave 75 ohm 
coax used in the array, or with a random length 50 ohm coax?  For some 
reason my vertical tunes to 7.1 mhz with the 50 ohm random length coax, but 
to 6.5 and 7.4 mhz (double dip!) when fed with the quarter-wave length of 75 
ohm.  Does this make sense?

  Dana, K6NR


Background:

I've had the array working for quite sometime but when I went to use it this
weekend the SWR was bad (2:1+) except at the top end of 40, about 7.290.  I
don't recall what the SWR was previously, but it was under 2:1 on most of
the band.  I originally tuned it for 7050 as Im a CW op.  I did not check
the power dumped to the dummy load this weekend, but when I first setup the
array it was very low across the 40m band.

I went out to check on the array and found the dummy load, a very old one,
was intermittently open.  I replaced it with a good dummy, and the SWR was
better, but still skewed high.  I checked it with an antenna analyzer and
saw two SWR dips - one above 7.3 and a deeper one below 7.0.  Seemed like
the verticals were adjusted too low in frequency.

I decided to verify the tuning of the verticals and took all but one down.
It had the lowest SWR at 6.950, which was I what I recalled tuning it to
(your manual says to tune individual elements 100khz lower than the desired
frequency).  However, when I checked the 1:1 frequency at the end of
feedline to the shack (about 200' long) it was lower, 6.820.

I shortened the vertical to resonate at 7.080 when measured at its base,
which was 6.950 at the end of the feedline.  I reset all the other verticals
to the same length and put it all back together.

Now the system has 2 1:1 SWR dips when measured at the end of the feedline -
6.589 and 7.475.  The array has about a 2:1 SWR across most of the 40m band.

I feed the bases of the vertical through a base tuning coils to keep the
elements at ground potential.




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