|Subject:||[TowerTalk] Tuning a four square|
|From:||Dana Roode <firstname.lastname@example.org>|
|Date:||Sun, 24 Aug 2003 17:40:29 -0700|
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?
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.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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