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Old 19th Sep 2017, 2:47 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, 3:07 pm   #122
merlinmaxwell
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Default Re: Magnetic Loop Receiving Aerial (Gary Tempest)

Quote:
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.
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Old Yesterday, 7:20 am   #123
Radio Wrangler
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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 Yesterday, 7:33 am   #124
ms660
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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.

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