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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