Re: LF: Re: TXing WSPR/MF on an indoor loop

[Markus Vester <markusvester@aol.com>]

Hi Rik, Stefan,

I think we need to coinsider the fact that a loop above ground has twice the radiation resitance, and also 3 dB more gain because it illuminates only the upper half space. The combined 6 dB effect is consistent with the intuitive notion that the image-loop should equally contribute to far-field fieldstrength. I have been wondering if that is also true for moderately conducting ground. But I believe this is true because the field decay in the ground requires the existence of shielding surface currents, and the skin depth is small against lambda/2pi.

So my calculation would be
EMRP = Rrad * i^2 = 62199 ohm * 4^2/630^4 * (14A)^2 = 6.33 microohm * (14A)^2 = 1.24 mW,
ERP = 1.83 EMRP = 2.27 mW.

Regarding the missing dB's versus Stefan's "normal" signal, note that due to the well-known Radio-Eriwan-Effect, 1 Watt may not always be equal to 1 Watt ;-)

Proximity of the antenna to the bed - magnetic fields penetrating the human body could possibly be stimulating, so the absence of an (X)YL may be a lost opportunity ;-)

73 and have fun,
Markus (DF6NM)

-----Ursprüngliche Mitteilung-----
Von: Rik Strobbe <rik.strobbe@kuleuven.be>
An: rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>
Verschickt: Fr, 25. Jan. 2019 16:26
Betreff: LF: Re: TXing WSPR/MF on an indoor loop

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