Magnetic loop antennas–a follow up

As I have mentioned previously I have a desire to build a magnetic loop antenna for HF use, primarily with my CRT SS9900 on 10 metres, though something that could cover 12 through to 10 metres meaning will work on CB as well as I still have a keen interest in that side of things despite holding an amateur radio foundation licence and hoping to go for my intermediate exam very soon, as alluded to in my last post.

My antenna situation as you know is very poor right now, made worse due to the fact I live in a first-floor flat, though I have a CB antenna in the attic it functions very poorly with high SWR readings and the need of a tuner to prevent damage to rigs connected to it, though this antenna works on 10-meters as well the tuner does not and to get the SWR to safe levels requires it be tuned with both sides turned all the way up to 10 (the unit is a Euro-CB EM-110 and is really suited to 11 metres rather than 10 metres), 2 metres is currently done with an antenna on a biscuit tin, also poor but it does work.

This is where the magnetic loop comes in, and I saw a YouTube video showing a design made out of copper tubing, with a power meter on the larger loop itself, a variant of this design was also on YouTube by an English member of Charlie Tango (of which I am a member with the callsign 26CT730) for CB use, with the right capacitor and correct dimensions I should be able to build a loop to this design (without a power meter fitted to the loop itself as I intend to fit the loop with a remote tuning system utilising a stepper motor and gearbox) and be able to start using 10 metres, despite the poor conditions at this time on that band I am eager to give it a go, also as I have not played with 11 meters since I scrapped my old car this would be a good opportunity as I understand CB is alive and well and there are regular nets held now, even on the midblock using AM and SSB since those modes became available to UK CB operators in 2014, but naturally I am keen to give 10 metres a go, whether I get a working magloop before or after I get my intermediate remains to be seen however I intend to make sure I get on 10 metres regardless, I have invested in equipment for that band I may as well use it.

I know I can get copper and plastic tube locally but for a suitable tuning capacitor I may have to look to rallies to do so, the requirements are that the tuning capacitor will work happily up to 4kV (though I suspect 50 watts is far below this but I like the idea of a safety margin).

The antenna will be connected with a suitable SWR meter in line almost permanently, my Zetagi Mod. 203W is perfect for this as it covers a frequency range of 26-30MHz for TX power measurement and 3-200MHz for SWR measurement.

I will post more on this subject as it progresses

 

73 de M6RSQ

Possible solution to my antenna problems

Now as you are most probably aware by now, operating from home is difficult for me because of the antenna arrangements, at the moment the 2-meter antenna for home is the old one from my car stuck on top of a biscuit tin (and it doesn't work brilliantly), for 70cm it's the same antenna but I rarely work 70cm and stand no chance with an indoor antenna anyway, and 11-meters (that's CB radio to the not-so technically minded) makes use of an Excalibur loft antenna into an ATU specifically for that band as otherwise I have SWR readings in excess of 10:1 but this could also be the dodgy PL259 on the other end of the coax feeding the antenna but time has not been on my side to get into the loft.

I have a CRT SS9900 as you know, which has seen nil use since removed from my old car last year, it has had it's 2 fuseholders reinstated and powers up fine from an ATX PSU (as I don't have any other PSU for it and the one powering my CB and 2-meter gear is rated at 7A whereas the current draw of the SS9900 is higher than that at full power, this radio is not connected to an antenna and for the sake of legality is in "HF mode" so I can use 10-meters if I choose.

An antenna to operate 10-meters is needed, a CB antenna could be used and cut down here as some of these are cheap, however as I plan to only operate 10-meters from home for the main a different approach is needed where I don't have to mess about in the loft as it's full of dust and nasty fibreglass insulation and put up and remove temporary antennas.

Enter the magnetic loop

The magnetic loop antenna is not obviously an antenna to the untrained eye, it has a smaller "Faraday loop" to couple it and a variable capacitor of sorts to tune it, usually a large one as when transmitting even at QRP the capacitor can and will exhibit high voltages, they are narrow banded and the rating of the tuning capacitor gives the range of what the loop should operate at, my interest is one that can potentially operate between 40-meters and 10-meters as I do plan to upgrade my shack for other HF bands as and when I can, probably with a second-hand HF rig with the power wound down to at most what I am able to use.

Commercially built magnetic loop antennas are very expensive to buy, about the cost of a radio in some cases, the vast majority of operators build these themselves at a fraction of the cost of a commercial one, usually sourcing materials to build the loop itself is generally easy as they can be made of copper tube (preferably painted to disguise it and protect it from copper thieves) which is available from either auto parts suppliers as brake lines or DIY stores as water pipes, refrigeration line would work too, though most suggest diameter of the tube is important, I won't go into this in depth as it's beyond the scope of this blog and there's plenty of information on the internet.

It is the capacitor to build one that is the difficulty to obtain, you could build this yourself or you could find the appropriate capacitor on the Internet, important is that it needs to handle very high voltages without flash over, as running at even a few watts creates very high voltages in the capacitor, also a remote tuning arrangement should be built here to tune the antenna to your operating frequency, again all this is outside the scope of this blog and Google can be helpful here.

One of the pros of these antennas is the amount of interference received on the bands is very very low if not nil, quite useful in the modern age as modern electronic devices generate more and more interference which blights the Amateur HF operator to no end, the other pro is that it is a small antenna compared to other HF antennas which normally result in complaints by people who have no understanding of the hobby.

I plan to put one of these antennas together and put it outside in a very discreet manner, to get it connected I require a jumper through the window, Comet Antennas make these and they are sold by Nevada Radio for a reasonable price, and I have plenty of self-amalgamating tape left, I'm still on the first roll, so waterproofing the plugs is not a problem.

Another reason I want to go onto HF is to operate CW, as I am planning on learning morse code again when I have time to sit and do it, for me to do it on 10-meters I'd need an Anytone AT-5555 or one of it's many variants as the CRT-SS9900 (Anytone AT-6666) does not have CW on it, but that's something for another day

I'll update this in due course.

73 de M6RSQ