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Old 19th Sep 2017, 3:47 pm   #121
Bazz4CQJ
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

The only reference I’ve found on the web so far about quantifying the performance of a magloop system is this comment from LZ1AQ http://www.lz1aq.signacor.com/

“Appendix IV Wideband loop amplifier gain
The amplifier used for wideband loops is a current to voltage converter - so called trans-impedance amplifier. It is convenient to express its gain as a ratio of output voltage to input current.

Gain = Uout / I inp [ohms] (13)

The dimension is in ohms. The AAA-1 amplifier gain is 850 ohms. That means that 1 uA input loop current will give 850uV output voltage. It is now easy to compare different loop amplifiers irrespective of their input resistance. For example if we have two trans-impedance amplifiers with the same gain and different input resistances e.g. 6 ohms and 9 ohms their voltage gain differs substantially (with 30% ) but their gain as a wideband loop amplifiers will be almost the same.”


It would be useful to define some kind of arbitary and relatively simple test for any loop amplifier where the loop was replaced with a small RF transformer (probably using torroid) which is then fed from a sig gen, and the output of the amp assessed using either a receiver or a scope? Then, for example, you could say for sure whether the 5109 or the 3866 is better, or the same, in Gary’s circuit? More generally, one amp design could be compared against another. Of course, that's only going to look at gain, not noise factor, but it's a start.

At the moment I'm still in the 'sawing and drilling' stage of my project

B
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Old 19th Sep 2017, 4:07 pm   #122
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

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Does the loop have a null?
Yes indeed, probably the second most useful outcome of using one (the first is lower manmade noise). Mine is on a rotator, and for listening to stations on the same frequency it is very handy to "null out" the other one. The effect is smaller at higher frequencies (greater than a few MHz),as my main interest is low frequency stuff that is OK.
 
Old 20th Sep 2017, 8:20 am   #123
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

You'll find that with any loop antenna, the peaks are rather broad and the nulls are very sharp. So if it's rotatable you'll find yourself turning it to minimise your worst noise source rather han peaking the wanted signal.

For reception, a loop made out of whatever wire you have kicking around will be fine.The amplifiers used have much higher input impedances than the radiation resistance of the loop, so there is no attempt at matching.

For transmitting you'd like to match into the radiation resistance, but this is spoiled by the losses in the copper and whatever joints you have. The loops are usually resonated, multiplying the circulating current to get the best radiated mag field, but this increases the dissipation in the losses. So thick copper with no joints is needed, and something special is needed as a low loss resonating/tuning capacitor. Transmitting is much more difficult and the best care will get you up to merely poor efficiency. Lack of care gets abysmal efficiency!


David
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Old 20th Sep 2017, 8:33 am   #124
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Yes, two coat hangers and a broom stick....

https://www.youtube.com/watch?v=ALaVfM71jQ4

Then there's an untuned loop and a Beverage

https://www.youtube.com/watch?v=4VHnTvARRUA

Lawrence.

Last edited by ms660; 20th Sep 2017 at 8:41 am.
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Old 23rd Oct 2017, 10:51 am   #125
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Finally, I have completed construction of my magloop (Gary Tempest Design). I’ve actually built two, but one is proving hard to sort out. The first used a PCB which Dave (4EBT) very kindly supplied (and using Motorola 2N3866) kindly supplied by Graham (Station X). I wanted to use transistor holders to enable swapping out of alternative transistor types. This meant I had to do a bit of ‘surgery’ on the board, and long story short, that is not yet working. As a fallback option, I also assembled a version on a piece of Veroboard and that is now working.

The suitability of Veroboard for this job might appear debatable, but of course it is now in the public domain that the Wellbrook amp uses Veroboard.

I’m still checking a few things out before I go and put the loop in the garden. I noted that when I connected the +DC lead the 12V psu (an old linear bench unit), even at 0 volts, the noise level out of the Rx increased audibly, so there is some RF / noise getting in to the amp down the DC line, and that needs improving. The 3866’s seem to be taking a little less current than Gary reported for the 5109’s; the whole unit runs at just under 100mA.

B
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Old 23rd Oct 2017, 2:38 pm   #126
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Hi Bazz.

I too built two amplifier boards but so far haven't been able to test them since I've not decided upon the final version of the loop aerial. I've had a huge amount of work on and many setbacks this year but would like to complete this project soon.
In the attached pic, the first board is built to Gary's original design ie Manhattan style construction and using 2N5109 transistors. The second board is built to David Taylor's PCB design. I used turned pin sockets extracted from an IC holder so that various RF transistors can be compared. Once I have sorted out the final loop then I'll test the two boards.

Regards
Symon
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Old 23rd Oct 2017, 2:50 pm   #127
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

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The suitability of Veroboard for this job might appear debatable, but of course it is now in the public domain that the Wellbrook amp uses Veroboard.
Well done Barrie on designing a strip-board version and getting it to work.

Basically, when I designed the PCB I just used the same layout, passive components and transistor as Gary used. I figured that as Gary had built several to the design and had put them out on test, the closer I stuck to the original, the greater the prospect of success, but if the thread can - as it has - encourage others to get their soldering irons out, that's all to the good!

As to strip-board, in the mid '70s I built a 200 MHz nixie tube frequency counter designed by David Pratt, G3DMP, (who later became the City and Guilds Chief Examiner when the C&G Radio Amateurs’ Exam was still in being), which featured in Radcom. It used strip-board extensively, with all the redundant strips removed, and worked fine, albeit it had a divide by 40 pre-scaler at the input, so the maximum frequency that the strip-board had to handle was 5 MHz.

The Wellbrook is of course only 2-transistor amp, (ZTX 327s?), so I guess that the strip-board will be quite compact, and looking at the X-Ray of a Wellbrook board at the 'Wellgood' link at post #25 of this thread, areas of unused strip appear to have been removed, so that the strip-board takes on the appearance of a printed circuit board, sort of, to minimise the risk of stary capacitances at higher frequencies. However, the picture of an example of a board which had failed (thought to be either due to water ingress or transmitting through what is only a receive antenna), seems to show quite a lot of redundant strip in place.

I'm not knocking the Wellbrook in any way - it has countless satisfied customers including on this forum, and there's much more to it than just the amp.

Whilst the strip-board design obviously works well, I wonder why - for such a simple amp - Wellbrook didn't knock up a little PCB, as George Smart did?

Happy listening!
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Old 23rd Oct 2017, 2:55 pm   #128
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Hi Symon

I heard from Gary that he has used Motorola 5109's with gains typically around 120. He made a comment that he has some RCA 5109's with gains of 40-50, but I wasn't sure whether he had decided not to even try them, or had tried them and got poor results.

@ David, I've used Veroboard on lots of small projects in the past, but I think this is the first time I've used it for HF work. I kept things pretty tight around the first stage and the second stage, but I may have lost the plot with the way I positioned Tr3 in between them; sure I could achieve a better layout were I to do it again.

I'd like to have some method of 'quantifying' the sensitivity of these amps (even if only one build to another); I guess you could remove the loop and connect a small torroidal transformer in its place and feed an RF sig gen in to that?

B
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Old 23rd Oct 2017, 7:44 pm   #129
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

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Originally Posted by Bazz4CQJ View Post
I heard from Gary that he has used Motorola 5109's with gains typically around 120. He made a comment that he has some RCA 5109's with gains of 40-50, but I wasn't sure whether he had decided not to even try them, or had tried them and got poor results.
Hi Bazz.

On the first board I used RCA 2N5109s and these had a typical HFE of about 45 to 50. There was one which tested at 78. I tried to closely match the gains of the transistors particularly in the first stage amplifier. I paid good money for those RCA transistors which aren't exactly high gain.

I purchased some 'cheap' Motorola 2N5109s from China via ebay and these have a range of HFE of 120 to 170.

The Motorola 4-247 (2N3866) transistors kindly sent by Graham, StationX, have an HFE range of 180 to 205.

I did a check on my RF transistor tester and by far the best performance was from the 2N3866s.
The cheap Motorola 2N5109s didn't do well in the RF test. Perhaps these are fakes.

I also checked some 2N2219As, these had an HFE range of 100 to 420 but had the worst performance on the RF transistor tester.

I have a gut feeling that the 2N3866s will turn out to be the best performers. I'd like to be getting on with this project which I'm looking forward to seeing the results.

Regards
Symon

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Old 29th Oct 2017, 1:43 pm   #130
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Hi.

I am now looking to start construction of the loop aerial and have a few possible options where to locate the 15mm copper pipe loop.
I was wondering about the feasibilty of mounting the loop in the loft. I thought about fitting it to the sloping part of the roof trusses keeping it as far away as possible from mains wiring. For a large loop, it would obviously need to be made in two or more sections in order to be able to get it through the loft hatch. I wonder if the sloping plane orientation could be both an advantage and disadvantage from a signal nulling point of view?

Has anyone gone down this route of a loft mount which would appear to have some advantages re weather protection and easy cable access?

Can anyone advise me the minimum distance the loop should be mounted away from mains wiring to avoid interference?

Or, should I abandon the idea of a loft mount and stick to mounting it outside say on a fence?

I look forward to your replies.

Regards
Symon
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Old 29th Oct 2017, 2:01 pm   #131
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Just intuitively, I think outside must be best and mine will eventually get out there.

Weather proofing does seem to be a "consideration", but a very simple "solution" I saw on one guy's site was to put the amp in a small box and put that box inside a bigger box. I think he was packing in some silica gel absorbent too. The Military seem to cope!

As for loop construction, I've used the blue polythene tubing that they use to supply water underground; think I bought (by the metre ?) it very cheaply at Jewsons and it comes in coils of somewhere between 1 and 1.5m diameter, so it is a willing servant for a round loop. Old TV coax braid squashed flat is pushed round inside to form the 'element'.
B
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Old 29th Oct 2017, 5:47 pm   #132
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Hi.

I quite like the idea of using the blue alkathene water pipe which is effectively pre shaped into a loop when it comes off the drum.

My main difficulty will be in routing the cable if the loop aerial is to be mounted outside. A loft mount would be ideal for cable routing and access.
Regarding the copper pipe loop, do you think there would be any disadvantage in using compression fittings rather than solder ring fittings? I can successfully solder plumbing joints but thought about the use of compression fittings for easy assembly and the option of making adaptions such as increasing the overall length of the loop. Any thoughts on this?

Regards
Symon
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Old 29th Oct 2017, 7:49 pm   #133
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Hi.

In view of the lack of response to my request for help, I'll abort the loft option and go for the outside option. It's a pity really as it would have been ideal for routing the coax cable. With the outside option, I will have difficulties with the cable in that it will have to go under a pathway, something I'm not relishing.

Symon.
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Old 29th Oct 2017, 8:40 pm   #134
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Bury a length of pipe so you can pull the cables through and change them later if you need.
With an antenna with two sharp nulls, I assume there's a rotator cable needed as well. Nulls are good for removing your worst interference source.

David
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Old 29th Oct 2017, 9:29 pm   #135
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

@Symon, I'm sure the compression fittings (copper ferrules?) will be as good as soldered, at least to start with. When the design is final, probably soldered would be better, if only from a corrosion standpoint.

@David, I keep reading that the nulls on a simple loop depend a lot on the opn frequency; quite good at LF, but on the HF bands, the nulls are not so useful?

My Magloop is now working but is on the subs' bench at the moment, as an executive order has come down that my HRO rebuild, started in ...~2010 , MUST now be complete before Xmas 2017. The Old Girl has not seen action since the mid-1970's .

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Old 29th Oct 2017, 11:14 pm   #136
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

If you build a reasonable HF loop, the nulls will still be there and be quite sharp enough for direction-finding. With an open-circuit gap in a loop, you may get a reduction in depth, but any attempt at matching makes it every bit as good as at LF.

With a loop, you can have it inside a screen all the way round except for a gap to avoid a shorted turn. This screens it from electric field components and can usefully reduce QRM.

David
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Old 29th Oct 2017, 11:37 pm   #137
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

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With a loop, you can have it inside a screen all the way round except for a gap to avoid a shorted turn. This screens it from electric field components and can usefully reduce QRM.
Hi.

When I get around to trying out the water pipe idea of having a thick stranded wire inside for the loop, it therefore seems a good idea to apply an electrostatic screen around the plastic pipe. I thought about using self adhesive copper slug tape. I guess an overall outer layer of insulation can then be applied to give protection from the elements.

Regards
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Old 30th Oct 2017, 1:03 am   #138
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Although it won't last for ever (if it's outside), you can buy quite cheaply, 50mm wide aluminium adhesive tape which is extensively used in the air con and ventillation industry. That would provide an easy over-wrap, maybe spray it with varnish.

Regarding the active "element", am I correct in saying it should have the lowest possible inductance? I left the dielectric material inside my coax braid in the MkI, so it's still circular in section. Not sure if the MkII should have that polythene removed and the braid squashed flat?

At the time this thread was hot, I don't think I fully absorbed the significance of the one post about the Wellbrook having been "unravelled"; two (small signal) transistors and some Verboard, but I guess some effort went in to the detail. Of course, one of the 'commonish' failure modes appears to be corrosion of the Veroboard. Curiously, the design mentioned earlier in the thread by the Bulgarian amateur also used just two PN2222 transistors. There's one design on the web where someone (a Brit) has stuck on a makeshift heatsink to a 2222 using epoxy adhesive. 2222's are so very cheap, and very linear.

But.... so far as I can see, nobody has yet published a method (viable with amateurs) of getting a performance figure from these various designs.

B
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Old 30th Oct 2017, 8:19 am   #139
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

For a receiving loop, you don't need a hefty conductor. The inner of a piece of coax will do nicely and its braid makes your screen. Something like UR67 or RG213 is easy to handle, or Heliax if you want something self supporting.

With coax, take great care to keep moisture out of the braid. If it tarnishes it makes it VERY lossy.

This electrostatic screen is a real Faraday Screen. It stops electric fields and passes magnetic fields. Too many people stick the name 'Faraday' in when they mean something to stop all waves, electric and magnetic.

As you go up in frequency, you eventually hit frequencies where the loop resonates and the behaviour changes rapidly. For broad, untuned coverage, you want a loop small enough that its first resonance lies well above the highest frequency you want to cover.

Inductance is OK. Inductance is defined as the voltage produced by the rate of change of the strength of a magnetic field. This is what the loop antenna is intended to do. It only becomes a problem if capacitance resonates with it and puts a resonance in the frequency range you wanted to cover. The resonance would make the antenna a lot more effective, but then you'd need a variable capacitor to tune it.

If you are transmitting, or if you want to receive very small signals, then resonance is necessary, and antennae either have tuning capacitors, or else are made for spot frequencies. There are plenty of articles on transmitting loops, there are also yagi-derivatives where each element is not a dipole but is a loop.

The broadcast receiving loops are operated well below resonance and are inefficient, but that doesn't matter with large signals and cheap amplification. They give the benefits of lack of needing tuning and rejection of near field noise which can have strong electric field components.

David
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Old 30th Oct 2017, 10:32 am   #140
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Hi.

It looks like my loop aerial is going to be fitted to the reverse side of a fence panel. The approximate dimensions will be about 1.5m wide by 1.5m tall and built from 15mm copper pipe.
As I say the difficulty will be going under a pathway but I can re-route the cable (a longer run) under a second pathway which has better access. The possible issue with this second option is there's a mains armoured cable running through the access channel under the concrete path. Do you think there will be sufficient mains electromagnetic fields to cause interference? The cable is armoured and the steel reinforcement wires are of course earthed so I assume there shouldn't be too much radiated fields from this mains cable?
If this seems reasonable, then I'll start making the copper loop.

Regards
Symon.
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