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Date: Fri, 25 Jan 2019 19:02:18 +0100
From: DK7FC <selberdenken@posteo.de>
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Subject: Re: LF: Re: TXing WSPR/MF on an indoor loop
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Hello Rik,

Thanks for your calculations. Looks like the 1000 km distance is already 
done.
Of course i also matched the loop. It uses a small blue ferrite core 
with just 20mm diameter. The big RF litz (DECCA litz with 729 strands 
(3*3*3*3*3*3) passes just one time through the core, and then 9 turns on 
the primary side, so this is much closer to 50 Ohm ;-)
I can measure the antenna current on the primary side, 1.6 A. So it must 
be 9 times higher in the loop...

73, Stefan

Am 25.01.2019 16:23, schrieb Rik Strobbe:
> Hi Stefan,
>
> the radiation resistance of a small loop: Ra = (320*Pi^4*A^2)/lambda^4 where A is the loop area in m^2
> So for lambda = 630 m and A = 4 m^2 the radiation restistance Ra = 3.17 µOhm (micro Ohm).
> At least in theory, an indoor antenna will probably have some extra losses.
> The gain of a small loop is -0.39dBd = 0.9
>
> So the theoretical ERP = 0.9*14^2*3.17e-6 = 0.56 mW = -32.5dBW
>
> Will I be able to copy you?
> Your regular 1W ERP signal is often up to +15dB here in WSPR, so the 0.56 mW should peak 15-32= -17dB. That's a solid copy in WSPR!
> Even with 10dB additional loss I should be able to copy you.
>
> Did you just resonate the loop or also matched it?
> Based on 40W RF power (50W DC at 80% efficiency) and 14A antenna current the antenna impedance is only 0.2 Ohm. That would result in a VSWR of 250 (!) for 50 Ohm TX output impedance.
>
> 73, Rik  ON7YD - OR7T
>
> PS= I hope you have a wooden bed (and bedsprings)
>
> ________________________________________
> Van: owner-rsgb_lf_group@blacksheep.org<owner-rsgb_lf_group@blacksheep.org>  namens DK7FC<selberdenken@posteo.de>
> Verzonden: vrijdag 25 januari 2019 15:23
> Aan: rsgb_lf_group@blacksheep.org
> Onderwerp: LF: TXing WSPR/MF on an indoor loop
>
> Hi MF,
>
> A few days ago i took some RF litz wire and built a loop that is now
> hanging on the wall inside my home, just 1m besides the bed :-) (no XYL
> here spoiling the fun). The loop is beaming to about 150/330 deg.
>
> After playing some days on 160m (an unusual high frequency, witch feels
> very strange. And the band is full of unknown callsigns!!) i did now
> move down to more or less low frequencies, the 630m band!
>
> About 10 nF is needed to series resonate the loop. It has about 2m x 2m
> of size. The antenna current is not stable because the WIMA FKP-1
> capacitor becomes warm during the 2 minute WSPR sequence. But the
> maximum current in the loop is 14 A. That explains why the cap becomes
> warm. A small parallel variable capacitor tunes to resonance. It is
> tuned so that the upheating capacitor runs into the resonance point in
> the middle of the WSPR sequence, then it runs out of the resonance by
> upheating further. It can cool down during the following quiet periods..
>
> 14A into a 4 square meter loop at 475.7 kHz, how much ERP is that? The
> PA consumes about 3.5 A at 13.8V, i.e. about 50 W.
>
> The system will run over the weekend, even during daylight. The signal
> generator is a Raspi which will have no internet connectivity from now
> on, so let's hope the time offset stays inside the accepted range for
> this experiment.
>
> What will the best distance be? So far the band is closed but i'm
> optimistic to crack the 1000 km distance with this QRP setup.
>
> 73, Stefan
>
>
>