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Re: Texas Potato Masher(s)



Jack,
I copied the BB in case others had the same questions (Al, XE2YVW, who is
building a pair of TPMs asked the same question as your no. 2).

>  I had several questions:    
>  1) In Table 1 you
>  refer to a test procedure using a coax tee and dummy load.   I should
>  know what this is but I don't!  Would you mind explaining?

This procedure involves using a coax tee (center) on the ANT jack of your SWR
meter.  On one side of the tee, place a 50 Ohm dummy load (I use a short piece
of coax and two 100 Ohm, 10 Watt resistors).  On the opposite side, place the
1/4 wavelength piece of coax, with the end shield shorted to the center
conductor.  Trim the coax for  minimum SWR.  At resonant frequency, the
shorted coax looks like a very high Z to the signal and is ignored and all the
power is fed into the dummy load.  I find this is a little "broad' at 70 cm,
so you might try using 1/2 wl of coax and leaving the end open--then cut in
half after trimming.  Either one works OK.

> 2) Where the two loops cross at the far end of the
>  antenna, I presume they are not electrically connected.

Correct.  I used insulated #10 wire and applied a drop of glue to the point
where they crossed.    BTW, modeling shows the shape of the rectangles affects
the resonant frequency almost more than the length of wire (when the length is
close to resonsant).  If you have trouble getting a low SWR using the lengths
in the article, adjust the length-to-width a little and see if that helps.

>  I am glad to see someone using the 1/2 wavelengh cable technique.  I use
>  it a lot even on HF.

If all SWR meters were really accurate, that step wouldn't be necessary.  But,
if you are using an impedance bridge (or an MFJ/Autek Ant Analyzer), then it
is absolutely necessary to avoid the transformer affect in the testing line.

>  Thanks for your article, I'm going to build these.  I need to find some
>  92 ohm coax.  I had hundreds of feet at one time but that was a long
>  time ago!  Some one in our area has a piece.
>  73,
>  W3TMZ

Good luck.  I'm sure somebody near you has some.  I use my TPMs every day
(although I cheat and use my Arrow sometimes for low AO-27 passes).

73, Jerry

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