Capco Magnetic Loop Antenna

[Sparky67]

Hi all,

Much to my wife’s delight I have just bought a s/h dismantled Capco AMA5 mag loop antenna, which came with a box of various brackets and leads. Does anyone have a diagram for these antennas showing how they are put together correctly so I can test it? There is no central pole with it, a photo I saw indicates there should be one, so may have to repurpose a broom handle or two temporarily…!

I’m also interested in ‘best practice’ installation methods for these and any experience of using them.

This one is green. We’re these ever used by the military?

Thanks

Martin
G4NCE

[Bazz4CQJ]

Interesting, I've not heard of that make before. Over the years, there have been quite a few threads on the forum about magloops of various kinds, but I don't think there's any consensus on mounting arrangements.

David (G4EBT) has one mounted on the back of his garden shed, mine (a homebuilt Wellgood amp and homebrew loop, which I'm quite pleased with) is suspended under a tree limb. Of course the ideal option would be using a rotator.

Most reports suggest that height is not critical.

B

[David G4EBT]

I may be mistaken, but I think the Capco one will be a transmitting loop.

Back maybe in the 80s and early 90s, as I recall, Capco ('Capacitor Co') was more or less a 'one man band' making wide-spaced variable capacitors, 'roller coasters' and high power ATUs. He used to have a stand at events such as the Leicester Show. I guess he'll be long gone.

Someone will know.

[Sparky67]

Here is the loop. It’s about 1.7m across, the motor-driven variable capacitor section is about 45cm long. The central mounting pole is currently missing but can easily be reproduced.

Where I live I am inundated with man-made noise on the lower HF bands so I’m hoping this will reduce that somewhat.

There is the option of mounting it on the lawn or on a flat roof above the second-storey extension, directly above my shack. Need to work out which will be best for NVIS, with an eye on the OfCom EMF requirements of course!

Martin
G4NCE

[Bazz4CQJ]

Yes, I too think that is a transmitting loop. Tx magloops are well known for their seriously high voltages and the need for tuning caps with good spacing. There lot's of info about them on the web. Restrictions on erecting aerials in some US cities has been one driver for interest in them, with people sometimes making their own tuning caps.

I live out in the sticks and the noise level here on 80m is S8. The magloop (when first erected) was good wrt noise, and got signal strengths which compared well with an 80m inverted V I had at the time. The last time I used the magloop, it seemed less good, and I took it down and gave it an overhaul, but not sure if something could be wrong.

I need to make an effort to understand the noise issue more; I have an overhead power cable and an overhead telephone cable come in to the house over the front garden. There's a mains substation less than 100m away. My own UPS is also a concern.

B

[Radio Wrangler]

I remember them from when they were current and heavily advertised. They are indeed transmitting loops. They were priced too high to sell many. I have a set of Capco high power variable capacitors and roller coaster in one of my ATUs They were OK, but lacked a couple of little details which didn't affect them with transmitter power on the go, but which spoiled them on receive.

The quality was better than a lot of amateur stuff, but not good enough for the military.

Transmitting loops have very low effective radiation resistance and the resistances of joints, both in the loop and the resonating capacitors can easily turn most of your power into heat. There have been several better designs since the Capco in these respects.

David

[Bazz4CQJ]

Google is saying the the AMA5 was useable for both Rx and Tx (where is the Rx pre-amp located?), so you could use the magloop for Rx to avoid the noise and use whatever wire antenna you have for Tx.

I modified my TS530 with an additional 'receive only' aerial socket to work that way.

Something I've never thought about before is the use of an RF choke in a magloop coax feeder https://ham.stackexchange.com/questi...0your%20house.. Looks like another job for the "to do list".

B

[G6Tanuki]

Magloops like the Capco design [and similar ones from AEA and MFJ in the 'states] are extremely-high-Q-resonant loops, so unlike non-resonant designs [Wellbrook/Wellgood] they don't need a preamp/impedance-transforming amplifier, any more than the ferrite rod antenna in a typical transistor-radio needs a preamp.

The usual design of the antenna coupling of a magloop is a smaller loop of coax that lives inside the tuned loop, usually directly opposite the tuning-capacitor; this can be configured as a screened loop, Faraday-shield style, which reduces pickup of electrostatic fields [noise] but doesn't affect the electromagnetic component [signal]. I wouldn't be bothering with chokes on the feeder.

One magnetic-loop I once played with used a DC motor (the type with commutator and brushes) geared down, to operate the tuning capacitor. The commutator noise as the motor operated was actually a good thing - it provided a nice noise-indication in the receiver to tell you when you'd achieved optimum resonance!

[Bazz4CQJ]

Originally Posted by G6Tanuki

Magloops like the Capco design [and similar ones from AEA and MFJ in the 'states] are extremely-high-Q-resonant loops, so unlike non-resonant designs [Wellbrook/Wellgood] they don't need a preamp/impedance-transforming amplifier, any more than the ferrite rod antenna in a typical transistor-radio needs a preamp.

Yes, blurred thinking on my part. So, on say 80m, do you know what the bandwidth might be? Especially if you only wanted to work 80m AM, I guess you could get away with using a pre-set cap on the loop, and not need motors?

Especially with a feeder from a magloop that runs through the length of the roof-space (close to mains cables) like mine, I guess the best location for an RF choke is as close to the Rx as possible?

I seem to use the expression "I guess" a lot, don't I ?

B

[G6Tanuki]

The practical bandwidth varies with frequency - it's a percentage of the frequency not an absolute value. I experimented a while back with a 14MHz magloop made out of 22mm copper water-pipe and found that shifting frequency by about 25KHz caused a significant rise in SWR. The lower the frequency the narrower the bandwidth so on 80M you would expect to need tweaking the tuning with smaller frequency-excursions.

It also varied with things moving nearby - parking a Land Rover 20 feet away was noticeable! If it was mounted in an attic or somewhere then there could be issues with wet vs dry roof slates?

So even if it's only going to be used on a spot frequency you may still need a tuning mechanism to 'tweak' it.

[Bazz4CQJ]

I've seen designs for receive-only magloops where the tuning is done by a varicap diode. Cannot recall how they cabled it. I guess that a varcap is a low-Q component, so not sure if that works well?

B

[frsimen]

Attached is a PDF of the instructions for the Capco magnetic loop aerials. The original wasn't a very clean copy, I'm afraid.

When new, they work very well. When used outside, the variable capacitor tends to develop high resistance at the sliding contacts and corrosion between the vanes and the mounting rods A good clean up is required every few years, particularly if it doesn't get much use. You will know when you need to do this as the tuning becomes very erratic.

The variable capacitor is driven by a cheap motor/gearbox arrangement via a Jackson ball drive. The ball drive will exhibit all the usual problems of that type of drive as it ages. The tuning on 80m is very sharp, with a bandwidth of only a few kHz. The motor drive can't be slowed down sufficiently to make tuning easy as it will stall. The trick is to set it to a slightly higher speed and pulse the supply to it with the up/down buttons for fine tuning.

Tuning within a band isn't too difficult, but swapping from 80m to 40m takes quite a while. Make sure you keep the end caps of the cylinder painted as otherwise they degrade very quickly in sunlight.

Paula

[Radio Wrangler]

Corrosion on those iffy connections is a problem on receive long before on transmit as transmit power tends to blast the crud out of the way. Eventually poor connections spoit transmit efficiency enough to get noticed.

Receive loops tuned with varactors? Yes I've read of them. BUT varactors are non-linear, almost instantaneous voltage controlled capacitors. Once used in UHF power freq multipliers and microwave parametric amplifiers. You don't want any of these intermod generators seeing all the signals in your environment at once.

David

[G6Tanuki]

Originally Posted by Radio Wrangler

Corrosion on those iffy connections is a problem on receive long before on transmit as transmit power tends to blast the crud out of the way. Eventually poor connections spoit transmit efficiency enough to get noticed.

Yes, my experiments with HF transmitting-loops always avoided any sort of wiping contacts in the capacitor; either I used a two-gang traditional variable capacitor so I could essentially connect the two gangs in series with the RF current then going through the spindle-between-the-gangs, or I used a 'butterfly' type VHF variable capacitor for similar reasons.

If you have to use 'wiping' contacts, I'd suggest bypassing them with a length of flexible coax braid, but that assumes your capacitors have rotational end-stops.

A properly designed HF transmitting magloop should aim for high Q - so can get plenty of Amps circulating, and plenty of Kilovolts appearing across the capacitor even with surprisingly low transmit powers. Push the power to a Kilowatt and poor contacts can get explosive!

[Bazz4CQJ]

Originally Posted by Radio Wrangler

Receive loops tuned with varactors? Yes I've read of them. BUT varactors are non-linear, almost instantaneous voltage controlled capacitors. You don't want any of these intermod generators seeing all the signals in your environment at once.
David

Thanks for that. I'll stick with my Wellgood for time being.

B

[Sparky67]

Thanks for the assembly details and all the other information Paula, that is very useful. Looks like it has a couple of parts missing, easily replaced.

Bazz, I read somewhere the loop has a 2:1 VSWR bandwidth of 3kHz, I think at its lowest freq, 3.5MHz. Maybe it’s best tuned in the centre of the 3kHz sideband it’s going to be used on…

Capco appear to have also made receiving mag loops covering 1.6 to 10MHz and 7-30MHz. I presume physically smaller variable caps with tighter vane spacing were used in those. They also advertised 200W loops covering down to 3.5MHz and 100W loops down to 1.8MHz, both of those were 3.4m in diameter.

I have found a local ali stockist where I can probably buy a suitable central pole to mount the variable cap which joins the top halves of the loop in its waterproof housing plus the bottom bracket on, so will get the longest I can fit in the car. When the weather has improved a bit and I have the missing parts it will be cleaned up, motor lubricated if it needs it, assembled in the garden - and compared to my 5MHz quarter-wave long wire with its counterpoise, on Tx and Rx. Should be interesting.

Cheers,

Martin
G4NCE

[Sparky67]

Just further to this...

From the same source as the loop I have now received a 48mm fibreglass pole with a spigot on one end which fits both top and bottom loop tuning cap container and driven element bracket support mount U-bolts snugly. We thought this was the correct pole, but it's a few cm too short. It has now been identified as an American military 4' tent pole / cam net support pole.

The U-bolts on the two mounts are different sizes, so I'm guessing a 'special-to-type' pole was originally provided? It does run along the length of the tuning cap up to where the loop halves are conneceted to the cap at the top. Am I right in thinking a glassfibre pole is best for this application, or is an ali pole suitable providing is isn't attached to the loop (or gets too close to the loop?) at both ends? The documentation does say metallic objects close to the loop can affect its performance...

Thanks.

Martin

[m0cemdave]

A metal mounting pole is definitely to be avoided. The usual support method for these loops is either a fibreglass pole or something made out of plastic drainpipe or similar.

[frsimen]

Hello Martin,

When new, Capco supplied an aluminium pole with the aerial and that doesn't seem to cause any problems.

Paula

[Sparky67]

Paula, the instructions you kindly sent mention “half tube clamps”. I presume these increase the ~38mm pole diameter up to ~48mm for the bottom bracket U-bolts. Do you know whether these clamps are insulated or conductive? Both sides of the bottom of the loop are connected to that bracket, as are the 48mm pole U-bolts they fit into.

Thanks.

Martin
G4NCE