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eHam Forums => SWL (Shortwave Listening) => Topic started by: N5RWJ on November 22, 2011, 10:20:02 AM



Title: Gen Loop Antenna for SWL?
Post by: N5RWJ on November 22, 2011, 10:20:02 AM
What are your opinion ?


Title: RE: Gen Loop Antenna for SWL?
Post by: KE5JPP on November 22, 2011, 02:57:42 PM
What are your opention ?

What is an "opention"?

Gene


Title: RE: Gen Loop Antenna for SWL?
Post by: AA4HA on November 22, 2011, 03:45:54 PM
What are your opention ?

What is an "opention"?

Gene

I am perplexed by that word too. I thought an opention was one of the time-lords from Dr. Who.


Title: RE: Gen Loop Antenna for SWL?
Post by: W5LZ on November 22, 2011, 08:13:16 PM
My opinion is that loop antennas work just fine.  It helps if the loop is a full wave on the lowest frequency of interest.  Usually, they will be usable on higher frequencies.
 - 'Doc


Title: RE: Gen Loop Antenna for SWL?
Post by: W1AEX on December 21, 2011, 09:48:00 AM
I use two triangular un-tuned wire loops for reception from the bottom of the BCB to 30 meters. One loop is oriented to favor north-south reception and the other is oriented for east-west reception. They are fairly deaf above that range but with a preamp at the feedpoint they would probably be very usable at higher frequencies. The loops are +100 feet from the house, fed with RG-6, and offer much better SNR than my dipoles. They're cheap and easy to build:    

http://www.w1aex.com/loop/loop.html

Rob W1AEX


Title: RE: Gen Loop Antenna for SWL?
Post by: N5RWJ on December 21, 2011, 02:44:51 PM
Check out this Pixel Rf PRO-1B SW loop at $399.00, and the AM-2 MW loop at $240.00,www.pixelsatradio.com (http://www.pixelsatradio.com)


Title: RE: Gen Loop Antenna for SWL?
Post by: WB6BYU on December 24, 2011, 10:42:02 AM
That's a whole lot of money for a simple loop.  Granted, they are nicely made, and may include
preamps, but you can build your own for much less than that.  (And their description of how a
"magnetic loop" works is rather out of date with modern thinking.)

I don't know why loops seem to be so difficult to comprehend.  For a tuned loop, you just make
it big enough to pick up lots of signal without being so large that it is self resonant, or otherwise
outside the range of your tuning capacitor.  One ham I visited used three turns of wire around
the house (over the roof and back through the basement) for AM BC receptions with a simple TRF
receiver.  Another ham had a large loop on the back of his closet door, allowing him to easily
rotate it while keeping it out of sight when not in use.

I've built some shielded loops, but the stray capacitance to the shield lowers the self-resonant
frequency and reduces the tuning range.  I've had better luck building the main loop just
shielding the coupling loop rather than the whole assembly - an easy way to do this is to use
coax cable.  For example, my standard loop assembly has 4 or 6 turns of wire inside a loop of
plastic tubing - an even number of turns allows me to ground the center tap in the same housing
as the tuning capacitor.  I then add an extra turn of coax for the coupling loop to the receiver
or preamp.  Ideally the shield of the coax would be grounded at each end and open in the
center, but often I get lazy and just ground the shield at the opposite end to where the
center conductor is grounded.

For an untuned loop the common method is to run the ends of a loop (sized for the desired
frequency range) into a balanced amplifier.  There was a design in RadCom using a pair
of 2N5109 transistors for a low noise, high dynamic range amplifier with the loop connected
between the bases of the push-pull transistors.