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Re: LF: Loop discussion

To: <[email protected]>
Subject: Re: LF: Loop discussion
From: [email protected]
Date: 23 Jul 2009 20:20 GMT
In-reply-to: <[email protected]>
References: <[email protected]> <[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
Dear James and Alberto,

I must confess that my considerations were much simpler. I wanted to
find a solution for an outside wire (normal litz wire for electric home
wiring) loop for broadband reception and to transfer its received energy
into the shack via a cable. Especially at VLF to MF cable losses are
rather low. Furthermore I wanted to locate all electronic equipment in
the shack only, no remote controls or similars. My solution is simple
and is working according to my expectations.

73 Ha-Jo, DJ1ZB


"James Moritz" <[email protected]> schrieb:
> Dear Alberto, LF Group,
> 
> To give a sensible answer, you have to look at the numbers specific to your 
> antenna and requirements. For example, suppose you want to use a broadband 
> loop as you have described on 136kHz. I reckon the noise floor under quiet 
> band conditions is about 1uV/m in 300Hz bandwidth at 136k. The figures I 
> have to hand are for a 1m x 1m loop element made of 15mm copper water pipe, 
> so let us use that as a basis.
> 
> The EMF induced in the loop is 2.1e-8 x fNAE, where f is Hz, N is number of 
> turns, A is m^2, and E is field strength in V/m. The 1uV/m band noise will 
> therefore produce an EMF of approx 2.9nV in the single-turn loop. Let us 
> follow Roelof's example and use a 1:30 step-up transformer. The EMF at the 
> secondary will therefore be 86nV. This is quite small, but we can use a low 
> noise amplifier - a J310 as used in the high impedance "Mini-whip" design 
> has an input noise voltage density of about 1.2nV/sqrt Hz, so contributes 
> 21nV noise in 300Hz BW, about 12dB below the band noise level, so there is 
> some margin to allow additional noise from other sources.
> 
> The other thing we need to worry about is the thermal noise contributed by 
> the loss resistance of the loop and transformer. Suppose we want this to be 
> at least 6dB below the band noise level, i.e. 43nV at the transformer 
> secondary. The thermal noise EMF of a resistance is sqrt(4kTBR), so the 
> maximum resistance we can have is Vn^2 / 4kTB, where k is Boltzmann's 
> constant 1.38E-23, T is absolute temperature (say 300K), B is bandwidth of 
> 300Hz - R works out to 372ohms. The impedance at the transformer secondary 
> will be the overall loss resistance of loop and transformer, in series with 
> the transformed inductive reactance of the loop (assuming that the 
> transformer winding inductance is high enough to have negligible effect). 
> The 1m^2 single turn loop has L of about 3.4uH, so with a 1:30 transformer, 
> the inductance at the secondary will be 900 times this, 3.06mH. This has a 
> reactance at 136k of 2.6kohms, so the minimum permissible Q of the 
> loop/transformer combination is 2.6k/372, i.e. about 7.
> 
> This is not very high. My prototype loop was made of 4 x 1m lengths of tube 
> with 4 x 90 degree elbows, so 8 solder joints, plus various hardware and 
> bits of wire connecting the resonating capacitors. The unloaded Q at 136k 
> was about 120 (and it seems the capacitors actually contributed 
> significantly to the loss), so the loop itself is probably not an issue, or 
> the addition of a few turns of wire as the transformer primary. I suspect 
> the transformer would be a greater contributor to the resistance due to 
> losses in the ferrite core , but provided the inductance of the primary was 
> several times larger than the loop inductance, it should be quite easy to 
> achieve an overall Q greater than 7. You also want to make the transformer 
> inductance high in order to minimise its shunting effect on the loop - this 
> would further reduce the already low voltage at the transformer secondary.
> 
> The  impedance at the transformer secondary is kilohms, so a high input 
> impedance preamp is needed, whose noise figure is not seriously degraded by 
> the high source impedance, such as a JFET. A mini-whip-type buffer meets 
> this requirement, but with unity voltage gain, the output voltage is still 
> very low, so a sensitive receiver or further gain would be needed. Most 
> amateur type rigs used for receive on 136k would need 20 - 40dB additional 
> gain due to their poor sensitivity. An issue with a wideband design like 
> this is that the transformer would probably resonate somewhere in the 100s 
> of kilohertz range, which would put a sharp peak in the loop response at 
> that frequency - if this is in the broadcast band, it would probably be a 
> good idea to add a damping R-C combination to reduce the chances of strong 
> overloading broadcast signals. Some selectivity would be desirable in any 
> case, if the loop was used with a high gain preamp. Of course, if you 
> resonate the loop at the operating frequency, you can get quite a lot of 
> voltage gain "for free", but the bandwidth will be much smaller.
> 
> So it looks like this particular design is feasible and the loop 
> construction not very critical. I have not tried this particular 
> configuration (my loops need to incorporate some selectivity to be viable, 
> due to the 10s of volts/metre from the broadcast stations), but I used a 
> similar approach with the bandpass loops and several other designs, and got 
> more or less what was expected, so it should work OK.
> 
> Cheers, Jim Moritz
> 73 de M0BMU
> 
> 
> 



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