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

[DK7FC <selberdenken@posteo.de>]

Hi MF,

Thanks to the many spotters and the discussion. Indeed, there were 53 stations detecting the signal. Those are the ones above 1000 km distance

Timestamp	Call	MHz	SNR	Drift	Grid	Pwr	Reporter	RGrid	km	az	# Spots
2019-01-25 18:46	DK7FC	0.475684	-26	0	JN49ik	0.01	LA9IY	JP53	1567	4	1
2019-01-25 19:16	DK7FC	0.475683	-25	0	JN49ik	0.01	LA2XPA	JP33	1565	357	4
2019-01-25 21:22	DK7FC	0.475683	-28	0	JN49ik	0.01	GM4OAS	IO76cx	1274	317	8
2019-01-25 22:40	DK7FC	0.475683	-30	0	JN49ik	0.01	EI0CF	IO65ih	1263	307	18
2019-01-26 00:52	DK7FC	0.475660	-23	0	JN49ik	0.01	LA8AV	JO59cs	1153	4	1
2019-01-25 20:34	DK7FC	0.475683	-28	0	JN49ik	0.01	GM3YXM	IO75xc	1070	311	9
2019-01-25 18:10	DK7FC	0.475683	-24	0	JN49ik	0.01	LA3EQ	JO28xj	1012	351	22

So far the transmissions are still decoded, so the time offset seems to be small enough.

I had to hurry building up the system last afternoon because i left the QTH a few minutes later. So i just wanted to get some antenna current and moderate TX power but i didn't match it accurately to 50 Ohm. The PA losses are very small, maybe we can assume 40 W RF power. With the measured 1.6 A we seem to have 15.6 Ohm on the primary side and then 0.2 Ohm on the secondary side, just as Rik estimated.
The antenna is in the 4th floor and it is an old house, with wooden stairs and not to massive walls. I am quite sure that the wall which carries the loop does not have internal steel grids. The loop is quite close to the outside wall in north-west direction. So there is more stuff causing some attenuation into the south-east direction. But i would expect an overall attenuation by all the lossy stuff arround the loop rather than a directivity given by the lossy stuff. Anyway, it seems the loop performs is better into the north-west direction. Or is it just because the QRN levels are likely to be higher in Italy than in Scotland? And the Alps making an effect?

All in all we see that anyone can be QRV on MF, even with very poor antenna possibilities. That is the good message :-) Oh and that system is totally immune against rain, storm, and even thunderstorms! You can even use the losses to heat the home, so is it a loss at all? ;-)

The limiting factor in that system is the capacitor! I use 2.2 nF +6.8 nF + 1090 pF variable C. Maybe it can be optimised by using 10 x 1 nF WIMA-FKP-1-2000. The voltage is about 450 V, which is well below the 700 V limit.
Maybe two turns of litz (keeping the loop size) would make a better antenna, with lower currents and lower losses in the capacitors. The thermal capacity and surface of the loop is high so that will not limit the system....

73, Stefan


Am 26.01.2019 08:49, schrieb marcocadeddu@tin.it:

Hi Stefan,

I just checked on WSPRnet and red you got plenty of reports from your QRP activity so my 3 spots are a drop in the sea.. but

3 spots:

Timestamp	Call	MHz	SNR	Drift	Grid	Pwr	Reporter	RGrid	km	az
2019-01-26 03:34	DK7FC	0.475683	-27	0	JN49ik	0.01	IK1HSS	JN35tb	493	190
2019-01-26 03:28	DK7FC	0.475683	-25	0	JN49ik	0.01	IK1HSS	JN35tb	493	190
2019-01-25 16:10	DK7FC	0.475683	-27	0	JN49ik	0.01	IK1HSS	JN35tb	493	190

the rx is a direct conversion rx with a simple bandpass input filter.. it is sensitive but lacks of course of image rejection and other features that make a real receiver another thing :-)
The strange point is how the 3 spots are spreaded in the period: 16:10 03:28 03:34..

Some years ago I made some trials on 137 kHz using an (outside) 2 turns loop (5x5m) and it was funny

73, Marco IK1HSS


----Messaggio originale----
Da: selberdenken@posteo.de
Data: 25-gen-2019 15.23
A: <rsgb_lf_group@blacksheep.org>
Ogg: 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