73 john m0hem
In reply to M0HEM: I’ve had a bit of a delve into mag loops and built a 1 metre diameter 6-wire cage unit similar to that in RADCOM June 2005. Very good as a Rx antenna, but no QSOs when tried with the 817 using ssb. Mind you, I only gave it one try, and chasers were listening for me. I’ve not tried it with cw - mine isn’t good enough. Further reading indicated that losses in the tuning capacitor were critical, and most articles suggest vacuum capacitors. There’s no indication what type of capacitor G4TPH uses.
My other thought is that it must have quite a large wind-loading and could be a problem to keep up on a hill.
And - wing nuts with cold fingers?
An alternative method of making a mag loop for portable operation is to cut a quarter wave of thick wire for the band of interest. This is supported by two fishing rod blanks crossed to form an X. You need to fix the two fishing rod blanks to the ground with guy pegs or tied to bags filled with rock/sand. The two guy lines attach to the centre of the X at right angles. A variable capacitor connects the wires at the bottom or offset and attached to one leg of the loop. I normally find a 100pf variable is about right for the band of interest. Feed it with a ferrite ring and experiment with the number of turns required and moving the position. This structure is extremely strong.
To increase efficiency increase the length of the fishing rod blanks and get the loop higher off the ground, also you can add additional wires in parallel.
In reply to G6SFP:
Mag Loops perform differently from resonant loops, and are typically
less than (and sometimes very much less than) a quarterwave in
circumference. The tuning capacitor needs to be able to withstand the
very high voltages present on the ends of the loop, - hence the common
use of vacuum caps in this application. Safety should be a prime
consideration when setting up and tuning a Mag Loop, especially in the
less than ideal conditions working portable, as voltages of several kV
can develop across the Capacitor even with low power operation.
In reply to M0HEM:
Alex PY1AHD is specialised to small magnetic loops http://hps.infolink.com.br/py1ahd/ .
He has developed also some light weight ones for portable use. For example a new 690 g loop for 10 - 40 m bands.
My large loop tuned with T1 ATU gives about the same weight: 320 g of 4 m fishing rod, 220 g of flexible wire and 140 g of T1 ATU. Of course also short coax is needed between the radio and tuner, so there will be some additional weight.
73, Jaakko OH7BF/F5VGL
In reply to G0ELJ:
Yes care needs to be taken in construction and the variable capacitor needs to be insulated correctly, however there is no need for vacuum caps unless the loop Q is very high or its being designed for high power operation.
With the loop I have described a broadcast receiver air variable will handle about 30 watts or so. Use both sides of a split stator type e.g. 2 x 100pf to make a 50pf capacitor with higher voltage handling and no wiper contacts.
In reply to G6SFP:
Yes, I think that Vacuum Caps would be a unnecessarily expensive (and weighty!)
way of tuning a loop designed for a specific band. My experience was with
a multiband loop which was well below a quarterwave on 40 metres and tuned with
a Command Transmitter Variable Cap. The arcing was very pronounced with only 20 watts!! I haven’t heard of anyone using one of these on 5 MHz yet, but the Americans designed the ‘Vietnam Loop’ for this band back in the 1960’s specifically for NVIS use.