CT2GXW said:
Hi Mark,
Do you have a commercial mag loop, or a homemade one? I've been tempted to build one a couple of times, but the constant need for retuning keeps me away from it, as well as the price of the capacitor, in order to handle 100W.
I have two, both are commercially made. The one I was using tonight is my main (for main read only!) station antenna which is an MFJ-1788X - 15-40M loop. The capacitor is quite large in that one, and can handle 100w easily.
It has a control box that allows it to find bands/frequencies semi-automatically. It 'sweeps' the tuning of the loop until it detects a dip in the SWR and then halts the tuning. As it does this it sounds a buzzer and also puts an indicator LED on to show whether you need to tune up or down to get the final fine tuning of the loop.
The down side is that you still need to be aware of what frequency the loop is tuned to so that you can start the sweep in the correct direction. If you get it wrong it means waiting maybe a minute or so while the loop sweeps back down or up to where you needed it to be in the first place.
You get used to it after a short while, plus I find that with the amount of electrical noise around this QTH I can peak the loop by ear, and it only requires a few pulses in the fine tuning mode to get the correct SWR while TXing.
The other loop is a collapsible portable loop made by Chameleon Antennas in the USA. There's a post about it in the Antennas and Masts section of the forum >>>
https://hamradioforum.com/threads/portable-loop-antenna-by-chameleon.38/
Normally that one would be used with a QRP rig, I don't think the tuning capacitor is rated for high powers any way, and I really don't fancy sitting next to a loop radiating a lot of energy anyhow. I once say next to a loop for the 10m amateur band that was being fed with about 5W. After about half an hour of use I started to get a really bad headache, and after maybe another half an hour had elapsed I thought my head would explode! I was in real pain for maybe two to four hours after transmitted for maybe only 10-15 minutes tops. So these days I make a point of sitting in the null of any loops while TXing, and even then only at QRP levels.
The Chameleon loop is manually tuned, and has a slow motion drive to the capacitor, so it's very easy to peak the tuning. There is an add-on kit now though that allows remote tuning, but last time I looked it was more expensive than the loop itself! Apart from which, I would normally be near to the loop anyhow if I was portable with it, so it's easy enough to just reach over and tune it by hand.
The Chameleon Loop covers a wider range than the MFJ loop, and there is an add-on section that allows it to tune down to 80m. I could make an even longer add-on section, but I doubt it would be very efficient on Top Band unless the diameter of the loop was increased.
Both loops are reasonably quick to retune within a band, but it can take longer if moving from a band at one end of the loop coverage to the other. I like them though because they are surprisingly efficient for their size, and because I only normally run QRP I need all the help I can get.
There are some newer auto-tuning loops that are very fast. There was one designed by G4WIM station last year I think it was, and that design was published in Radcom magazine. He used a stepper motor and a microprocessor at the heart of his design. I've seen videos of the loop in action, and even the simple act of him moving his hand towards the loop causes it to retune.
Hope that lot makes sense!
73, Mark...
