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

[Markus Vester <markusvester@aol.com>]

Hi Clemens,

calculating efficiency is easy, I'd make that
Rrad+Rloss = 50 ohm/9^2 = 0.62 ohm,
Rrad/(Rrad+Rloss) ~ Rrad/Rloss = 6.33 uohm / 0.62 ohm ~ 1e-5 = -50 dB.

The hard part is to find out how much of that 0.62 ohm loss resistance is from the wire (and capacitor) itself, and how much from the surroundings. Note that the relatively thick Decca litzwire has been optimized for applicatioin around 100 kHz. So eddy current losses (which scale with f^2) will already be significant at 475 kHz, making it hard to directly calculate RF resistance. One way would be to measure the excess temperature of the wire with 14 A RF applied, and later replace RF with DC and measure DC power for same temperature increase. Then we can calculate how much power has been lost to the nearfield environment, i.e. walls, wires therein, neighbour's electronic infrastructure, and persons in bed... 

Best 73,
Markus 
 

-----Ursprüngliche Mitteilung-----
Von: Clemens Paul <cpaul@gmx.net>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Fr, 25. Jan. 2019 22:12
Betreff: RE: LF: Re: TXing WSPR/MF on an indoor loop

Hi Stefan,

it would be interesting if you could measure the
upper and lower frequency where the antenna current drops by 3dB,
Then the efficiency of the loop including the influence of its environment
could be calculated.

73
Clemens
DL4RAJ 

>-----Original Message-----
>From: owner-rsgb_lf_group@blacksheep.org
>[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of DK7FC
>Sent: Friday, January 25, 2019 7:02 PM
>To: rsgb_lf_group@blacksheep.org
>Subject: Re: LF: Re: TXing WSPR/MF on an indoor loop

>
>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@blackshe
ep.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

>>
>>
>>   
>
>
>---
>Diese E-Mail wurde von AVG auf Viren geprüft.
>http://www.avg.com

>