Can I use a ZM-2 QRP antenna tuner to tune a 1/4 wavelength loop?

[KC9YKI]

I'm intrigued by what I've read about the MFJ Loop Antenna Tuner and wonder if I can use my Emtech ZM-2 QRP antenna tuner in a similar fashion into a quarter-wavelength loop (40 meters? 30 meters?).  Still working on my CW skills, so not on the air yet.  Is there any reason I couldn't use this tuner for such a loop?  Severely restricted on anntenna possibilities here.  Wondering if I should give this a try.

Thanks for any input. 

Jim Carpenter

[K3VV]

Looking at the ZM-2 schematic, you have a polyvaricon capacitor (C1) in series with the loop.  Any capacitor in that location needs to have high Q and a high voltage rating  A polyvaricon has neither, unfortunately.

Look at the loop calculator here:

http://www.66pacific.com/calculators/small_tx_loop_calc.aspx

Plug in your desired length, frequency, and transmitter power.  Run the calculation and look what happens to the capacitor voltage.   Depending on band and loop size, it can be hundreds of volts even for a 1-watt transmitter.

I would say using the ZM-2 as a small loop tuner can't be recommended.

[KC9YKI]

aha! Just as I suspected/feared.  You've probably saved me the cost of a new tuner, and for that I at least owe you a beer.  If you're ever in Milwaukee...

[N3LCW]

Jim,

You could build a very simple and effective 1/4 wave loop with little cost:

http://g0cwt.co.uk/magloops/new_page_2.htm
 
and

http://g0cwt.co.uk/magloops/new_page_6.htm

It took me less than 2 hours to build and tune my first loop and its results were very exceptional for its size.
You can use a type 43 ferrite toroid if you don't have those ferrite tubes as Ben suggests.  I use both.

I also built an 80M version, 10ft vertical by 22ft horizontal.  It was the most effective way for me to get on 80M from a postage stamp size yard.

You won't need a vacuum variable capacitor at QRP, but something better than a polyvaricon.

73...

Andy
N3LCW

aha! Just as I suspected/feared.  You've probably saved me the cost of a new tuner, and for that I at least owe you a beer.  If you're ever in Milwaukee...

[KC9YKI]

Thanks Andy...

Clearly, I shouldn't have thrown away the dozens of variable capacitors I accumulated as a kid 50 years ago.

This is neat stuff. 

Thanks to you both.

Jim

[JAHAM2BE]

You can build a small loop tuner yourself fairly easily:

http://homepage.ntlworld.com/g4kki.william/army_loop_tuner.htm

Note that this kind of tuner may have problems with common mode currents:

http://www.eham.net/ehamforum/smf/index.php?topic=87076.0

[AA4PB]

One source of loss (due to the high currents) is the wiper contact on the capacitor rotor. MFJ uses a split stator variable so that no wiper contact is required to carry current. Current flow is from one fixed stator, through the rotor, to the other fixed stator.

The ZM2 is definately not a reasonable candidate for a small loop tuner.