[Elecraft] Experiences using a portable HF loop

[Michael Babineau]

I agree with almost everything that Ron says in the post, except I would substitute the 
statement  "The very BEST magnetic loops are incredibly inefficient” with “The very BEST magnetic loops are incredibly inefficient on the lower bands”.

Pretty much all of the Magnetic Loops on the market are around 1 meter in diameter, which means that circumference of 
the radiator is about 10 feet.  This size of loop if well constructed can have efficiencies approaching 90% on 10m and something
in the range of 30 to 40 % on 20m, so they will work reasonably well on these higher bands. 
The wheels start to fall off on 30m and especially 40m, where typically you would be looking at single digit efficiencies.  
This is really not surprising, if you put a 10 foot vertical on 40m you are going to find that it is not very efficient either ! 
If you had a 2 meter diameter loop then efficiencies on 40m and 30m would be significantly improved, but you would lose the
ability to resonate it on 10m/12m and likely on 15m and 17m too. 


What a 1 meter diameter Magnetic Loop has going for it is extreme portability (at least for those with a foldable radiator), 
quick deployment (typically < 5 minutes for something like the Alex Loop), ground independence (no need for radials), some bi-directionality
in the plane of the loop but more significantly a very deep null broadside to the loop which can be effective at killing a noise source.
It also typically provides continuous band coverage from 10m through 40m with 2:1 SWR bandwidths for a well
constructed loop around 100+ Khz on 10m dropping to something around 10 Khz on 40m, all in a very small package. It is both a reasonable DX antenna
on the higher bands as well as a usable NVIS antenna on 40m. As with any antenna they work better if higher, but as a minimum they must be 1 loop radius above ground, so 
mounting on a lightweight camera tripod works ok. 

It is possible to make NVIS QSOs on 40m using only a few hundred mW of power, often with surprising signal strength, so putting 5 or 10 watts into an 
antenna with  5% efficiency can still yield a usable signal. 


Cheers

Michael VE3WMB 

P.S. The secret to squeezing out the best performance when building a loop is having a solid conductor that is welded/soldered to 
the tuning capacitor (you need to try to avoid mechanicalconnections as they add resistance) and using either a split stator or butterfly air variable 
capacitor or a vacuum variable capacitor. Cheap capacitors with wiper contacts on the rotor will kill your efficiency. 

The radiation resistance of a Magnetic Loop is low (typically less than a few ohms) and it drops as you press it into service on lower frequencies. Even small additional
resistances add up and become significant when compared to the radiation resistance on the lower bands and this is what causes the Loop efficiency to go down the toilet. 


>From: "Ron D'Eau Claire" <ron at cobi.biz <mailto:ron at cobi.biz>>
>Subject: Re: [Elecraft] Experiences using a portable HF loop
>Date: April 21, 2017 at 11:00:09 PM GMT-4
>To: "'Phil Hystad'" <phystad at mac.com <mailto:phystad at mac.com>>, "'Eddy Avila'" <k6sdw at hotmail.com <mailto:k6sdw at hotmail.com>>
>Cc: "'Elecraft'" <elecraft at mailman.qth.net <mailto:elecraft at mailman.qth.net>>


>The very BEST magnetic loops are incredibly inefficient, yet like any
>"incredibly inefficient" antenna one can make amazing contacts on them when
conditions are right. 

>The problem is with resistive losses. The day we have room-temperature
>superconductors, we will have efficient small transmitting loops. Until
>then, only a few percent of the power applied is radiated. 

>For now, they are great when simplicity of setup allows operation where
>otherwise nothing could be done. 

>73, Ron AC7AC