Shortwave Loop Kit

[K5MOW]

Hi all

What do all of you think of this antena for my apartment?

http://www.mtmscientific.com/swloop.html

Thanks Roger

[jFarley]

Roger;

It really depends on what your expectations are.  Loop designs along these lines are to be found all over the internet, and I have built a couple similar loops in the past.  They are simple, and for some listening scenarios they may be appropriate.

Making any recommendation on this particular loop requires guesstimating what its' performance characteristics are from a couple of images.  All evidence seems to point to the fact this is a relatively low Q loop with a low Effective Height (ability to convert the magnetic field component of the RF into a voltage which the receiver can use) and thus may suffer with regards to weak signal performance.  It should have a good ability to null or depress local QRN in your apartment, but only from two collinear directions at once; a typical residence tends to have a ton of junk running at once.

What I see:

This loop uses link coupling to "match" the main winding impedance at resonance to the receiver.  This forms a voltage step down transformer.  While both windings have one turn, it is not a 1:1 transformer.  The step down ratio is closer to the ratio of the loop areas, and in this case is closer to 16:1 at best.  This means that the loop is taking a -12dB -24dB voltage hit in matching impedances.  At some point weak signals will not be able to beat the receiver noise floor.

One can get away with link coupling AND preserve weak signal performance if the loop Q is high enough, which does not appear to be true here.  I have never been able to build a loop which shows respectable Q at shortwave using the garden variety aluminum plate tuning cap which is used here; they tend to be a lot lossier than you would think.  Here's a very valuable web page:

http://www.bentongue.com/xtalset/24Cmnts/24Cmnts.html

Bottom line: if you are looking for a reasonably priced loop which would offer good performance for stronger SW stations indoors, with the ability to fight off some QRN by loop rotation, then this looks like a good build.  If you are looking for good performance for weaker DX indoors, I am far less sure about that.  

Loops like this do work, but gauging your expectations is important.

[rdla4]

I actually have built one of these. They are OK for "super signals". This type of loop will let you "null" out nearby sources of noise. But it is not a "DX-Magnate". (See JFarley's comments regarding design above)Mine tuned from around 4mHz to around 15mhz, if I recall correctly. You may hear some of the stronger pirates on this antenna....but you will not hear the marginal or sub marginal signals that are fairly common.

I actually ended up using this loop design lightly coupled to a longwire antenna to suppress a terrible noise issue I had last summer. (In effect phasing the two antenna's together to produce nulls directed toward my neighbor's house---It worked very nicely for that.