2 Dec 2012

Magnetic loop HF TX antennas

Although I have successfully used a large 80m sq vertical wire loop (made with 32 x 0.2mm PVC covered wire) on both 137 and 500kHz, I have never tried a hi-Q small loop on HF transmit. There is a very useful calculator available at http://www.66pacific.com/calculators/small_tx_loop_calc.aspx which suggests a 3m circumference loop using a 5mm diameter conductor - for example small bore copper pipe or a thick coax cable - would have a loss of only 0.9dBd and a bandwidth of 352kHz at 28MHz.  This would comfortably cover both 28MHz CW, data, WSPR and beacons without retuning.

An advantage of magnetic loop antennas is reduced detuning from nearby objects so this antenna, if mounted in the loft space, might make a neat and efficient stealth antenna for 10m. I guess one could even arrange to switch the tuning capacitor remotely so that several sub-bands could be covered. When using QRP power levels, as I do, the high voltages and currents that are encountered in small loops are less problematic. For example, choice of variable or fixed tuning capacitors is less onerous than if using 100W.

KR1ST has some nice information on magnetic loop antennas on his website.  I hope he doesn't mind me linking to the image of his magnetic loop on his site. There is also some useful loop information on the Wikipedia site.

In all, I think I'm missing out (so far) on a whole range of antenna experiments!


gj7rwt said...

Evening Roger. The problem with a loop in the loft is one of directivity as there are deep nuls at 90 and 270 degrees. Mounting in the horizontal plane may overcome this but i've heard they dont perform as well. They are great for the lower hf bands when space is a problem. The lower noise from these loops counteracts the loss in efficiency on receive and they work quite well when mounted close to the ground. For 10m i've used an inverted v in a loft which worked well.
73's Andy

TiƩgui said...

a magnetic loop over ground have really small efficiency when is mounted in the horizontal plane. At least in the HF bands. It's even worst than an halfwave dipole at the same heigth !

I use one small loop on VHF. Look at this video : http://www.youtube.com/watch?v=G9m1JMh82GA