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Old 2nd Feb 2019, 8:46 pm   #1
Bazz4CQJ
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Default Magloop Antenna Using ebay LNA

Sometime ago I built a magloop antenna using the Gary Tempest amplifier design. That worked well on LW/MW, but seemed to fall off at higher frequencies.

Looking for options for a magloop which would work on 5MHz, I stumbled upon this project, which tries out the use of one of the ebay 30dB LNA modules https://pa0fri.home.xs4all.nl/Ant/Ac...enna%20eng.htm. These particular LNA modules were discussed in a previous thread on the forum https://www.vintage-radio.net/forum/...d.php?t=150649

This is not working at all well, with just one or two very weak signals being heard.

Can anyone suggest whether there is an issue here with the match (or lack of matching) between the loop and the input of these modules, or is there any other factor(s) which could account for the lack of performance?

Thanks
B
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Old 3rd Feb 2019, 12:29 am   #2
David G4EBT
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Default Re: Magloop Antenna Using ebay LNA

Don't know if you are aware Bazz, but Gary Tempest learnt that someone had X-Rayed a faulty Wellbrook loop antenna and had reverse engineered it, referring to it as the 'Wellgood loop'. Gary decided to carry out further experiments after his successful initial design, which resulted in a 'MK2' version, which has the merit of being simplified, yet with enhanced performance.

The updated design featured in the Spring 2018 BVWS Bulletin.

(It's on my round tuit list!).

Unfortunately, Gary isn't a member of this forum, but wrote about his updated design at post No 11 in the thread at the link below. He also owns a Wellbrook loop so was able to make comparisons with both his earlier design and the Wellbrook. His views on the performance of the second version on the short-wave broadcast bands were favourable:

https://golbornevintageradio.co.uk/f...id=5944&page=2

This doesn't of course answer your query, but nevertheless, I thought it might be of interest.
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Old 3rd Feb 2019, 1:23 pm   #3
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Default Re: Magloop Antenna Using ebay LNA

Thanks for that reminder David. I've been trying to eliminate factors like the coax line (the loop is outside) but there's nothing wrong with the installation as far as I can see, so I'm out of (good) ideas now.

I've just taken a look at Gary's follow up feature and ordered some of the ferrite cores for the transformers of the WG design and will try that out. IIRC, it turned out that Wellbrook had used Veroboard for their amps, and I may do the same, paying due regard to a layout for RF use at <30MHz

B
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Old 3rd Feb 2019, 1:45 pm   #4
David G4EBT
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Default Re: Magloop Antenna Using ebay LNA

The 'Wellgood Loop' was conceived by Dr George Smart, M1GEO who had owned and used a Wellbrook Loop for some time, but which - to use his words 'in a moment of madness, he transmitted into it and blew it up'. It aroused his curiosity as to what was inside the Wellbrook, so got a chum to X-Ray it, and as you say, he discovered that the WB was built on stripboard.

He remarked that 'since this webpage went online, I have been contacted by many amateurs regarding failed loops. Primarily the cause seems to be either transmitting into the receive-only loop, or, water ingress'.

He designed and constructed his own version including a PCB, which he can supply.

He's written it all up, with X-Ray pictures, full constructional details of the 'Wellgood' loop and showing why water ingress has been a problem with failed WB loops. The link is here:

https://www.george-smart.co.uk/projects/wellgood_loop/

Hope that's of interest.
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Old 3rd Feb 2019, 3:39 pm   #5
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Default Re: Magloop Antenna Using ebay LNA

Chris Trask's papers are generally worth seeking and reading. He seems to have taken over transformer feedback amplifiers where David Norton left off. There are some papers and articles also by Anzac and the Q-bit corporation.

Trask's application of transformer feedback around an active mixer is particularly wonderful.

2N5109, 2N3886 were rather suitable devices in hermetic cans. Wrobably well suite to exposure to cold and condensation.

David
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Old 3rd Feb 2019, 3:52 pm   #6
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Default Re: Magloop Antenna Using ebay LNA

Mmm, I've just mailed George to ask about the PCB.

Re Trask's papers, I'm sure that I have browsed those at some stage, but some (much/most?) of what's in there may have gone over my head.

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Old 3rd Feb 2019, 6:25 pm   #7
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Default Re: Magloop Antenna Using ebay LNA

David, did you mean 2N3866?

The 2N3886 seems to be a thyristor, but the 2N3866 is a 500Mhz NPN transistor in a TO39 metal can.
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Old 3rd Feb 2019, 7:58 pm   #8
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Default Re: Magloop Antenna Using ebay LNA

Yes, sorry, 2N3866. Bad typing!. There's a load of them set loose in amateur radio QRP circles, carrying the marking 4-247 and a motorola batwings logo in case you come across any. Lots of people took loads, but they all seem to have gone into stashes rather than getting used. About 10,000 transistors in the UK vanished like pouring water into a desert. I'd expected them to be popping up all over the place in projects etc.

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Old 5th Feb 2019, 2:45 pm   #9
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Default Re: Magloop Antenna Using ebay LNA

I'm one of those with a stash of 4-247's. I'd be happy to supply some FOC to anyone who wants them for a project.
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Old 5th Feb 2019, 8:33 pm   #10
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Default Re: Magloop Antenna Using ebay LNA

Graham, I do think you ought to charge a nominal "stockholding and administration" fee at least!- I feel that free things often get hoovered up simply because they are free. At least a few shekels, even directed towards a charity, means that it's the folk who really want/need them who will show interest,

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Old 5th Feb 2019, 8:42 pm   #11
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Default Re: Magloop Antenna Using ebay LNA

I've put the ebay LNA to one side for the moment and one of the PCB's for the Wellgood should be here very soon and we'll see how that works out.

I might mention that the route to where I am now started with me putting up a 5MHz dipole just before Xmas, only to find I had a noise level of S8-9 coming out of it. Switching off everything in the house did not change this. I suspect that the overhead power cables, which feed all the local houses including mine, are not helping; there's no way for me to put a dipole up without one leg being uncomfortably close (~8m at the closes point) to the power cable that comes in over the front garden. So, onwards to the magloop.

The Wellgood PCB design is quite compact and was designed for use with PN2222 (TO92) transistors. Others have squeezed 2N5109's and 2N3866 on to the board and reported improved performance.

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Old 6th Feb 2019, 1:00 am   #12
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Default Re: Magloop Antenna Using ebay LNA

I still have part of a small box of 4-247s if anyone fancies building a loop.

A total of >40,000 transistors were handed out, free, around the world. That's not a misprint. And the world swallowed them up without a trace. Nary a burp! I've kept an eye open for any surfacing anywhere just out of curiosity. All I kept myself was a couple of hundred, and yes, I've actually used them in real projects. They were lifetime buy parts for the calibrator boards of RF power meters, and when the plant was closed down there was no plan to transfer old products out East, so a boss asked me if I could find a better home for them than landfill which was their only alternate fate.

The free aspect was morally necessary, but a psychological mistake, I fear.

David
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Old 6th Feb 2019, 11:15 am   #13
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Default Re: Magloop Antenna Using ebay LNA

It's a difficult one- this is a very good-natured forum, I've recently been passed a couple of nice items by members and in return I asked what they felt was a worthy charity to donate to. Also, I've responded to specific requests on the odd occasion and passed on things that were just occupying space.

The various aspects of loop projects have attracted a fair amount of interest here and it was very generous of Graham and David to offer to give suitable transistors of good provenance on to other members. Unfortunately, I (and no doubt many others here!) have come across a small contingent in any area of life who almost obsessively jump and collar all of anything free, then either refuse to pass things on to someone with genuine application at any price, or make a small killing on the auction sites. In a way, that's an aspect of capitalism and entrepreneurship and many famous names in the world of business probably started in this way but with an informal and friendly hobby forum such as this it leaves a bit of a bad taste when, say, radiogram parts given in good faith to someone then end up for bid in a familiar place!

I'd be happy to put my money where my mouth is and offer either Graham or David a fiver for 10 of the 4-247s- at least that represents a "thankyou" for bothering to keep a useful electronic component and not throw them out at some point over the years. It also saves buying apparently good value 2N3866s from an auction site and then finding out that they're actually diligently re-marked 2N3053s.....,

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Old 6th Feb 2019, 1:18 pm   #14
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Default Re: Magloop Antenna Using ebay LNA

Personally, I agree with the view donations to charity are appropriate for items gifted on the forum. I've passed things on via the Offered section stating that the money would go to my own favourite charity (orphaned elephants) and people seemed delighted with that choice. However, I'd appreciate any further discussion being the subject of another thread.

Thanks
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Old 7th Feb 2019, 8:50 pm   #15
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Default Re: Magloop Antenna Using ebay LNA

I may have found the reason why the MMIC LNA pre-amp did not work well on the magloop. I’ve been reading an old paper by Chavdar Levkov LZ1AQ, who has been very active in this area for quite some time, and has an interesting website.

The paper is at http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm, but I’ve put in to Word, and done a bit of highlighting, and that’s attached. On page 11, the question of balance is addressed and point arises that you need to have a transformer in the output of the pre-amp, and the LNA (as supplied) does not have that. So, it looks like it’s not matching problem but a balancing one .

B
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File Type: doc LZ1aq 2009 paper.doc (794.5 KB, 26 views)
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Old 27th Feb 2019, 12:38 pm   #16
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Default Re: Magloop Antenna Using ebay LNA

Quote:
Originally Posted by Station X View Post
I... [have] a stash of 4-247's. I'd be happy to supply some FOC to anyone who wants them for a project.
Hello, yes, please, I’m interested in A couple of projects using these and would love to take up your kind offer. Thank you
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Old 27th Feb 2019, 1:50 pm   #17
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Default Re: Magloop Antenna Using ebay LNA

Quote:
Originally Posted by Bazz4CQJ View Post

This is not working at all well, with just one or two very weak signals being heard.
........ or is there any other factor(s) which could account for the lack of performance?

Thanks
B
Just a glance at the schematic, unless I'm missing something really obvious, I wouldn't expect this setup to work very well at all.

I have experimented with loops for many years as transmitting antennas and through the principle of reciprocity a good transmitting antenna is likely a good receiving antenna. With loops, for any reasonable efficiency they have to be resonant at the receiving or transmitting frequency. Just as the loop-stick in your transistor radio has to be, or the frame antenna in your valve radio.

The schematic just shows the loop connected into an LNA. It ideally needs a capacitance (varicap) or V/C to tune it and a loop tap and/or impedance matching transformer to feed the LNA and be peaked on the frequency of interest to get the best out of it.

I appreciate the idea of an untuned broadband antenna, it will be self resonant somewhere though, but I would not expect it would be a great performer at all at frequencies where it was not resonant.

So apart from the electrostatic shielding, which would be good at eliminating interference, I don't think that loop would be much chop, except at a frequency it was resonant and then it is not ideally matched to the LNA input.

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Old 27th Feb 2019, 3:33 pm   #18
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Default Re: Magloop Antenna Using ebay LNA

Quote:
I don't think that loop would be much chop
Non resonant wideband loops only have to be a bit better than the noise floor to work well, I have a Wellbrook and it is very good 10kHz to 30+ MHz. For transmitting you want it to be resonant to get out as much as possible, a non resonant TX aerial will work but require an inordinate amount of power, most of which will be dissipated as heat.
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Old 27th Feb 2019, 6:31 pm   #19
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Default Re: Magloop Antenna Using ebay LNA

The more I read about magloops, the more uncertain I become about what is true. Whenever I need a some absolute reference point I go back to the fact that Wellbrook have had lots of people paying what seems like lots of money to own their kit, and pretty much all owners like it. Other than that fact, finding consistent, dependable info on magloops is hard.

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Old 28th Feb 2019, 2:02 am   #20
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Default Re: Magloop Antenna Using ebay LNA

I'm nervous to talk about antenna types on any forum these days because it is such a touchy subject...

However, I think your loop antenna is equivalent to a scaled up H field probe. i.e. similar to the classic near field probes used for EMC work. I have a suite of these here.

I also have a large version in the loft and this was intended for use with my 198kHz offair standard. This was made with some RG174 coax and I think it is something like 2m in diameter. However it might be a bit bigger. I found that it wasn't any better than a ferrite rod antenna for the purposes of receiving 198kHz and getting a reliable lock so I don't use it any more. At work we have a similar loop antenna provided by Quartzlock for use with our old 198kHz offair standard boxes. The loop is supposed to give better phase stability according to Quartzlock. I think I made mine the same diameter as theirs and it is like hanging up a floppy skipping rope when deploying this type of antenna here or at work. I'm not certain the QL antenna is the same as mine but it looks like my antenna and it is also made from what looks like RG174 coax.

I don't remember having any problems receiving signals on it across LF through HF but the signals were obviously much lower than I'd expect on a full sized dipole. I'd expect it to be 30 or 40dB down on a dipole but this would still deliver lots of signals across a fair chunk of the lower half of the HF band. So at night on the 7MHz broadcast band I'd expect to see signals at the equivalent of S9 or so. Note that this is all without any form of LNA after the loop.

Putting an INA2186 (dual) MMIC with 30dB gain after this type of antenna seems a tad reckless from a stability point of view. Could your system be deaf because you have something like a UHF oscillator instead of an LNA?
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