From: "Clemens Paul" <cpaul@gmx.net>
To: <rsgb_lf_group@blacksheep.org>
References: <686787990.1269761.1548450438946.ref@mail.yahoo.com> <686787990.1269761.1548450438946@mail.yahoo.com>
In-Reply-To: <686787990.1269761.1548450438946@mail.yahoo.com>
Date: Sun, 27 Jan 2019 17:12:00 +0100
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Subject: RE: LF: Re: TXing WSPR/MF on an indoor loop
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Hi Markus,

>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.

I've done some simulations with EZNEC 6+ which can simulate also small loops
with reasonable accuracy due to its built in double precision engine which can optionally been engaged.
For free space the numbers for Rrad and XL coincide nicely with the numbers from
the analytical formulas.
For perfect ground the radiation resistance doubles as you have predicted.
For lossy ground (average medium) I have to lift the loop up to 20m to get reasonable results.
Then Rrad remains about the same as in free space,i.e. it does not double.
I'm aware that NEC2 asumes a homogeneous ground to an infinite depth.
There is the possibility to model ground with two media, each with its own characteristics
which I haven't tried yet.

73
Clemens
DL4RAJ 


>-----Original Message-----
>From: owner-rsgb_lf_group@blacksheep.org 
>[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Markus Vester
>Sent: Friday, January 25, 2019 10:07 PM
>To: rsgb_lf_group@blacksheep.org
>Subject: Re: LF: Re: TXing WSPR/MF on an indoor loop
>
>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
>
>
>
> 
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