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End Fed Tuner Success… sort of
July 10th, 2010 by kmack

Had a good day in the park with Jim K4AHO and Wally KG4LAL. Spent a good bit of time testing a couple tuners for End Fed Half Wave antennas using Jim’s AIM 4170. Wow is that thing a great tool for tweaking antennas! Info overload!

I built an antenna tuner based on AA5TB’s design for an end fed half wave antenna. I am using a 3 ft or so counterpoise on the ground as Steve suggests. On the analyzer in a test lashup it was a bit touchy to hand capacitance but tuned well even up to 21Mhz. Since I am not thinking of backpack size I used a pretty good sized enclosure for it. I am using an air variable 6-160pf cap instead of a polyvaricon like Steve used since space is not a big issue. I also used a T68-6 toroid instead of the T50-2 Steve used.

Test lashup of EFHW TunerWhen I mounted it in a plastic box the sensitivity seemed to increase. I have not put a LED SWR bridge in the box yet, as I was waiting to see how it worked before adding more variables. Today I was able to put an AIM 4170 analyzer on it and it did tune the antenna… seems that the air variable I used is perhaps a tad small. It is almost fully meshed on 40m cw and on 20m it acts like even at minimum capacitance the sweet spot is very narrow and hard to tune.

In the box details

Here's the innards

My question(s) are:
1) Is the hand/body capacitance normal? If not, what might cause it to
be so touchy?
2) Would my parts layout be part of the issue?
3) Does the DPDT switch (mini toggle) I added for later use with the SWR
bridge add significant capacitance to the circuit? I was able to match a
21Mhz load on the raw test setup, but not once it is in the box.
4) I have a small bus wire for a ground, do I need to increase that?
5) Is the plastic box the problem? Would it be better in a metal enclosure?
6) Am I asking too many questions? Sorry, this is how I learn. Build,
test, ask… :-)

The SWR Bridge I want to use

SWR bridge I want to use

Thanks for your wisdom and experience on this one.

72,
Kelly K4UPG


One Response  
  • Alex writes:
    July 12th, 20108:59 pmat

    Kelly,
    As you already know I have my issues with tuners and getting them to work the first time around. I will say that my first EFHW tuner is in a metal box, my second is in a plastic box, see pix at Polar Bear yahoo. Either one will tune my wires just fine and pretty near,but not quite, identical. The plastic box is a Non-issue.
    I respect Steve’s work,AA5TB, met him at the qrp gathering at Hamcom, he’s a nice guy and interesting to talk to. I tried the short counterpoise wire as his web site suggested and I had a good bit of Hand capacity effect. So now my counterpoise is anything I can toss out between 10 and 20feet in length.
    DPDT switch is Non-issue.
    I use whatever wire I am wiring the circuit up with for a ground wire inside the box, usually 22ga insulated.
    Your SWR bridge is the same as I use in both of my SLT and my newly minted ZM-2 Clone.
    My suggestion to Hand effect and tuning is a longer counterpoise, but hey I am no expert just a tinkerer!! Someone else will chime in here and may give us both a lesson!!!!
    ANd OH, check all your solder joints once more!
    73 Alex K5UNY


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