[Elecraft] OT: High RF currents in small TX'ing lopps

[Ken G Kopp]

Small transmitting loops by their very nature involve high levels of
current flow and this can/will cause "ordinary" variable caps to heat,
which results in tuning drift.  Most home builders use vacuum caps
and relatively large copper pipe in their designs.  Mechanical joints
(clamps, wiping contacts, etc.) are to be avoided.

This tuning drift problem plagued the AEA loops of some years ago.
(I had the prototype here for early testing.)  Does anyone know how
well MFJ's loop performs in this respect?

On Tue, Jan 1, 2013 at 10:03 PM, Ron D'Eau Claire <ron at cobi.biz> wrote:

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> "Eliminating every stray milli-Ohm counts towards achieving highest
> efficiency!"****
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> http://www.ahars.com.au/documents/the_underestimated_magnetic_loop_hf_antenna_vers%201.1.pdf
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> wunder****
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> K6WRU****
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> On Jan 1, 2013, at 12:17 PM, Jim Beard wrote:****
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> I've been playing around with A Magnetic Loop I built and have found
> Several good Air Variable capacitors in old radios or test equipment at
> antique stores. Most of the old stuff can be bought for 15.00-30.00 and you
> get some great spare parts. I've found singles and double air variable
> capacitors and 20-80 pf up to 56-560 pf air variables and so far the
> antenna has worked quite well. I believe The Alex loop Walkham is the best
> by far. But I spent very little using old spare parts to make a loop. My
> goal is to get a Alex loop but have had fun playing with mine. There are
> some good math formulas on the web on what size versus what pf for what
> frequency. Happy New Year to all and good DX in 13. ****
>
> Jim Beard****
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> Sent from my iPhone****
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> AE5ZZ****
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> On Jan 1, 2013, at 11:15 AM, Walter Underwood <wunder at wunderwood.org>
> wrote:****
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> On Jan 1, 2013, at 4:25 AM, David Wilcox wrote:****
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> Also, not "fiddly" to build if you are handy. I am some but no time.  I
> have a neighbor who whips up stuff like that in an afternoon.  Coax, a
> varicap with 10:1 reduction gear off eBay (with a simple search he found
> the same one Alex uses with the same reduction drive..... looks just like
> the one in my AlexLoop), PVC pipe from Ace Hardware, some RG-174 and a RS
> box is all they are.****
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> You can't use just any capacitor. The resistance in the wipers will kill
> the performance of the loop. You need a butterfly capacitor with no
> rotating contacts. Those aren't easy to find, but MFJ does sell two models.
> ****
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> You also need really low resistance material for the loop. For a home
> setup, I'd start with the flexible copper tubing used to hook up
> refrigerators with icemakers. After you get the details worked out, you
> could switch to something better.****
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> Loops are usually limited to a 2:1 frequency range, so you need multiple
> loops to go from 80-10.****
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> This article is really useful:
> http://www.mfjenterprises.com/antennatalk6.php****
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> The instructions for the MFJ-935 have good info on loop sizes and RF
> exposure limits: http://www.mfjenterprises.com/pdffiles/MFJ-935B.pdf****
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> wunder****
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> --****
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> Walter Underwood****
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> K6WRU****
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> --****
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> Walter Underwood****
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> wunder at wunderwood.org****
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> ****
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