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Re: LF: MF mobile ?

To: <[email protected]>
Subject: Re: LF: MF mobile ?
From: "Markus Vester" <[email protected]>
Date: Thu, 28 May 2015 01:23:50 +0200
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> I bet you have all the formulas in your mind :-)

Yes Stefan, sure do... this is the kind of stuff I sometimes like to think about during my bike ride to work - about 70 minutes, twice a day ;-)
Ok, no pocket calculator allowed then...
 
A 1.5 m stick would have about 10 pF or -j 120 kohm at 137 kHz. If you're keen you could probably go up to 20 kV, giving 0.16 A. Effective height will be around 0.7m, so radiation resistance is 1579ohms*(.7/2200)^2 = 160 microohms. Thus radiated power = 0.0256 * 160 = 0.4 microwatt or -34 dBm.
 
Assuming a coil Q of 400, required TX power would be only 0.16^2 * 120k / 400 = 8 watts. If you're not afraid of non-ionizing E-fields you could probably do that from a bicycle (sic), or walking around with a backpack ;-)
 
At a range of 180 km between us, we'd get a groundwave fieldstrength of 50-34-45-4 dBuV/m = -33 dBuV/m. Assuming a quiet day with of -26 dBuV/m/Hz background noise, and watching in a "QRSS-60" spectrogram with 16 mHz (-18 dBHz) noise bandwidth., the noise floor would be -44 dBuV/m, and we'd achieve 11 dB SNR. Phew, we've made it....
 
Scaling from LF to MF while maintaining same antenna voltage, current will be 3.4 times higher, ERP 144 times (ie. 60 uW), and fieldstrength 12 times. 
 
BTW Forget about that parallel capacitor to reduce tuning variation - that would be just cheating yourself, similar to adding an attenuator after the TX to improve matching. Only the current going into the actual antenna is the one that's radiating.
 
All the best,
Markus
 

From: DK7FC
Sent: Thursday, May 28, 2015 12:10 AM
Subject: LF: MF mobile ?

Hmm, i find someone should do a real mobile (mobile-mobile, with a velocity > 0) experiment. A 1.5 m long CB antenna with a special preparated feed point should work. And i think that the wire works better with a fixed C in parallel, even if this reduces the efficiency. But it stabilises the SWR or better said, the voltage on the wire. Corona? Where is the problem with corona? :-)
Markus, could you calculate the ERP when 10 kV rms is applied to a 1.5m high antenna on a car roof? I bet you have all the formulas in your mind :-)

Maybe the antenna has 10 pF. I remember i have a 470pF/16kV capacitor at home. So if C = 470 pF and f = 475 kHz, L = 239 uH With 470 pF parallel to the antenna, a moving wire (= changing C) does not make a significant effect i think.
10 kV at 239 uH at 10 kV is 14A. If P = 200 W, the losses must be 1 Ohm !
With a good RF litz wire, this is possible :-)

What would be the ERP and possible distance?

It would be interesting to try that in WSPR / QRSS-60 :-) I would also drive to someone for making a CW QSO but most likely there is a LOT of QRM when driving...

More ideas?

73, Stefan


Am 27.05.2015 15:58, schrieb John Langridge:
Andy,

>Perhaps a 2 meters whip is a bit too short ?

The smallest antenna that I have experimented with was a base loaded vertical that was almost 3 meters long and I observed reasonable success with about 100w TPO.  The radiator was a 2 cm diameter aluminum antenna tube so charge was distributed pretty well and no corona was observed.  There was no additional top loading in the test.   I was using a fair radial system at the time but perhaps its time to try this same arrangement in a fixed mobile setting as you have done here.  

I look forward to hearing about your further experimentation.  I hope you will conduct this test again in the winter.

73!

John KB5NJD / WG2XIQ


From: Guyé André <[email protected]>
To: RSGB LF Group <[email protected]>
Sent: Wednesday, May 27, 2015 7:05 AM
Subject: LF: Fw: De F6CNI : WSPR Test using mobile on MF to night.

Hello Markus.
 
I am sad about my experiment, only one station copied my very loud signal last night : F5WK at 72 km. Perhaps a 2 meters whip is a bit too short ?
 
For a next experiment, F6GEX just give me an other mobile antenna shape idea. Intesresting indeed ! Be sure, I’ll give to you also the results of the doppler effect. HI.
 
Best 73 de Andy F6CNI.
 
image
 
 


Sent: Tuesday, May 26, 2015 7:06 PM
Subject: Re: LF: De F6CNI : WSPR Test using mobile on MF to night.
 
Great Andy, this is a wonderful experiment!
 
If you could persuade yourself to drive on a highway while transmitting, would we be able to measure your speed using Doppler effect? Probably not using WSPR, but surely with QRSS-60...
 
73, Markus (DF6NM)


 
-----Ursprüngliche Mitteilung-----
Von: Guyé André <[email protected]>
An: RSGB LF Group <[email protected]>
Verschickt: Di, 26 Mai 2015 1:47 pm
Betreff: LF: De F6CNI : WSPR Test using mobile on MF to night.

Hi to all.
 
I’ll be testing this evening WSPR 2 and at times WSPR 15 on MF using a short mobile antenna. My call be F6CNI/M.
 
Of course, I don’t drive my car, it is resting in my garden…
 
  • TX power 2 Watts ;
  • Antenna is a 2 meters long whip ;
  • Total Antenna Z = 85 -16610 j. Using base L tuner to 50 Ohms ;
  • Antenna base current = 145 mA, base voltage = 2400 Volts RMS ;
  • Average expected soil up to the Fraunhofer zone : 0.003 S/m and 13 for dielectric ;
  • Effeciency about – 43 dB. Maxi lobe is -40,5 dBi at 22° site ;
  • Apparent radiated power is about 150 micro Watt, and the E.I.R.P. is about 270 micro Watt.
 
Reports be appreciated.
 
Begining the experiment this evening at 2000 TU to next morning, but I think the best period is around 2300 to 0200 TU.
 
Best 73.
 
Andy - F6CNI – JN19QB
 
.



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