lOOP cONSTRUCTION


Russell Manhire
 

I'm about to build a loop for a CCW HF/VHF Loop amplifier and have come across a couple of designs for a Shielded Loop and also the Mobius Loop. 
How would these designs work with the CCW Amplifier and what would be the best way to implement the "Earth" connection?

Thoughts gratefully received 

regards
Russ Manhire


Chris Moulding
 

Russ,

I'm in the early stages of planning a YouTube video about loop design including the shielded loop and the Mobius loop.  I want to make detailed measurements of signal to noise performance to compare different loops and other antennas in a real situation.

There is an awful lot of incorrect information about shielded loops on the internet including amateur radio text books and the only reference I would recommend with a correct description of how they work is this link by Charles T Rauch Jr, W8JI:

https://www.w8ji.com/magnetic_receiving_loops.htm

For my own use I'm using a split shielded loop 1m diameter made out of coax with the split in the outer shield at the top of the loop. The coax inner conductors come out at the bottom of the loop to feed the Loop Antenna Amplifier. The coax shields at the bottom of the loop are connected by a short wire to clamp onto the BNC shield on the Loop Antenna Amplifier.

If the coax is correctly measured so that the lengths to the split are identical local noise is cancelled out very effectively and subjectively it's the best loop configuration I've used so far. I want to spend some time taking accurate measurements but I need a clear weather window for safe mast work.

The Mobius loop was originally designed as a magnetic probe to measure electromagnetic pulse (EMP) fields during nuclear testing in 1965. It probably works well for that but it's not as good for radio listening as the split screen loop.

If you need a drawing to see what I'm using let me know.

Regards,

Chris


Paul Sayer
 

Hello Chris  

I would be interested in your drawings as shielded loops is something I've been reading about and thinking of trying. 

I should also add a belated thanks for the upgrade kit. Not long had the chance to make the mod, but saw an immediate improvement on 20m. I don't have another loop to compare or a second original to try a simultaneous testing which would have been interesting. 

Paul G0VKT 


Helge Kyndbo <kyndbo@...>
 

Hi Chris,
I wold like to see your drawing of the loop too,
helge

Den tor. 27. feb. 2020 kl. 06.54 skrev Chris Moulding <chrism@...>:

Russ,

I'm in the early stages of planning a YouTube video about loop design including the shielded loop and the Mobius loop.  I want to make detailed measurements of signal to noise performance to compare different loops and other antennas in a real situation.

There is an awful lot of incorrect information about shielded loops on the internet including amateur radio text books and the only reference I would recommend with a correct description of how they work is this link by Charles T Rauch Jr, W8JI:

https://www.w8ji.com/magnetic_receiving_loops.htm

For my own use I'm using a split shielded loop 1m diameter made out of coax with the split in the outer shield at the top of the loop. The coax inner conductors come out at the bottom of the loop to feed the Loop Antenna Amplifier. The coax shields at the bottom of the loop are connected by a short wire to clamp onto the BNC shield on the Loop Antenna Amplifier.

If the coax is correctly measured so that the lengths to the split are identical local noise is cancelled out very effectively and subjectively it's the best loop configuration I've used so far. I want to spend some time taking accurate measurements but I need a clear weather window for safe mast work.

The Mobius loop was originally designed as a magnetic probe to measure electromagnetic pulse (EMP) fields during nuclear testing in 1965. It probably works well for that but it's not as good for radio listening as the split screen loop.

If you need a drawing to see what I'm using let me know.

Regards,

Chris


Chris Moulding
 

Regarding preparing a drawing it's probably going to be quicker if I take photographs of one of the test loops in the workshop tomorrow.

I'll post the photos tomorrow.

Regards,

Chris


Russell Manhire
 

Thanks Chris - I look forward to your Photographs and the video......


Roger Need
 

Russ,

Chris Trask is a well respected for his work with Active and Loop Antennas.  He has published numerous articles on the subject and holds 6 patents in this area.  He has published an excellent paper called "Mastering the Art of Shielded Loop Aerials"

Here is a excerpt....
"The doubling of the output signal voltage together with the same S/N performance makes
a Moebius Strip loop aerial comparable to adding a low-noise amplifier (LNA) having a gain
of 6dB and a NF of 0dB (not attainable) to a single-turn shielded loop aerial having the same
diameter, and this is readily attainable without the burden and cost of any additional material.
At the same time, one may use a Moebius Strip loop aerial that is 70.7% the diameter of a
shielded loop aerial and still have the same amount of signal power delivered to the load."

You can download the paper here: 
Chris Trask Paper (PDF)


Russell Manhire
 

Hi Roger
Most of the Mathematics went straight over my head, but certainly the conclusions are clear. I'm not sure Chris is convinced though.....

I will have an attempt to construct both the split loop and the Mobius, Both, however, require accuracy in construction that I'm not sure I can achieve......

Regards
Russ 


Roger Need
 

Russ,

Everett Sharp N4CY has done extensive tests comparing different types of loops including the Moebius.  He has written a practical
  article which you should find interesting.

https://www.george-smart.co.uk/wordpress/wp-content/uploads/2017/02/EWS-Article-in-MW-News.pdf

Roger


Chris Elmquist
 

Chris,

I too would like to build a shielded loop with your Loop Antenna Amplifier. Wondering if the photos or drawings of what yours is like have made it up somewhere since this posting?

Thanks so much,

73,
Another Chris N0JCF


Chris Moulding
 

Hi Chris,

After a lot more testing of the shielded loop and Mobius loop designs it became clear that they gave no improvement in noise rejection over an unshielded loop.

The design of the Loop Antenna Amplifier in the later "+" and "++" versions gives a tightly balanced loop input that helps reject common mode reception from the loop element. The early HF/VHF version also had a similar input design.

In all versions of the Loop Antenna Amplifier there is a common mode filter on the coax output to reject common mode noise coming up the feeder to the loop element.

I suspect that it was the common mode noise coming up the feeder that made the shielded and Mobius designs appear to have lower noise in amplifiers that didn't have good common mode rejection.

It's far better to stop the noise before it can reach the loop antenna element.

Regards,

Chris


Chris Elmquist
 

OK. Very interesting Chris. I was going to build the loop out of some scrap 1/2" heliax, with the center conductor as the element so I guess instead, I'll just use the shield as the element :-)

Have a lot of the usual noise sources here-- LED bulbs, neighbor's computers, switch mode wall-warts everywhere so was hoping the shielded loop would offer some better immunity to that
junk.

73, Chris N0JCF


Simon
 

Hi all

I would agree with Chris here..
I have tried all.. no difference at all. So just use a good non shielded loop..

Snake oil comes to mind..

Simon


Chris Moulding
 

The problem with a lot of "improved" loop designs like the shielded and Mobius designs is that they don't work as you would expect at first glance.

The "shielded" loop actually works as two halves of a short dipole transformer coupled to the inner wire.

The Mobius design is similar. It's original purpose was to protect an RF measuring instrument from the EMP spike of a nearby nuclear explosion. It is similar in operation to the shielded loop.

I think it may be a bit harsh to describe them as snake oil. More a case of enthusiasts looking for something better than a simple loop without thinking it through.

Chris, if you want to try a different design try the cardioid loop designs on this web page:

http://www.crosscountrywireless.net/cardioid_loop_antenna.htm

Rather than having two sharp nulls the cardioid loop has a wide (120 degree) null off the back. If you can mount it so that it faces the area you want to cover and null noise off the back then it works really well.

Regards,

Chris


Simon
 

Hi Chris

Ok point taken.. but I put money on a good simple loop will be as good as anything as long as it has a good preamp like yours or wellbrock etc.

I tried sooo many..in the end cardioid’s where for me here in high qrm land..

Simon


Chris Elmquist
 

Yes. Thanks Chris. I have studied your cardioid write-up as well and that looks encouraging. I can use with a rotor so that would give me ability to point where I need it (or opposite of where I don't need it!)

Will pursue. Thanks again, 73 Chris N0JCF


David Cutter
 

I'm guessing a bit that amateurs copied ship-board DF loops and the Bellini-Tosi variety all made to reject the ship's magnetic field, thinking this was the way to do it.  W8JI wrote a bit about it and rejected the idea for amateur situations, so, you are in good company, Chris. I still like your bicycle wheel loop, it does it all. 

David G3UNA

On 14 June 2021 at 19:39 Chris Moulding <chrism@...> wrote:

Hi Chris,

After a lot more testing of the shielded loop and Mobius loop designs it became clear that they gave no improvement in noise rejection over an unshielded loop.

The design of the Loop Antenna Amplifier in the later "+" and "++" versions gives a tightly balanced loop input that helps reject common mode reception from the loop element. The early HF/VHF version also had a similar input design.

In all versions of the Loop Antenna Amplifier there is a common mode filter on the coax output to reject common mode noise coming up the feeder to the loop element.

I suspect that it was the common mode noise coming up the feeder that made the shielded and Mobius designs appear to have lower noise in amplifiers that didn't have good common mode rejection.

It's far better to stop the noise before it can reach the loop antenna element.

Regards,

Chris


Martin - G8JNJ
 

On Mon, Jun 14, 2021 at 07:52 PM, Chris Moulding wrote:
I think it may be a bit harsh to describe them as snake oil. More a case of enthusiasts looking for something better than a simple loop without thinking it through.
I'd agree, many of the more complicated loop designs are actually worse performers, and the novelty value exploited as a sales feature to help make them stand out from the rest of the crowd.

e.g. More metal must be better :-)

https://www.g8jnj.net/moebius-loop-antenna

Regards,

Martin