Ah, sorry, i didn't read your mail until the end.
OK about your argumentation and of course i know that from the VLF
times :-) But you need to consider the changing angle of the wire while
driving. If C is just 10 pF, this can vary by +20 % i expect. And this
will change the resonance. If you are out of resonance, the voltage
drops and then there is unwanted fading. If the limiting factor is the
voltage, not the TX power (within certain limits of course :-) ), then
a parallel C will help here i find.
I have been active on 40m CW /m and i know the SWR can vary quite
strong.
OK, if i go down from 470 pF to 100 pF, L will be 4.7 times higher and
so the current needs to be 4.7 times smaller to achieve 10 kV again.
OK, 3 A sounds easier than 14 A. But i would need about 1 mH for the
coil. That is a better decision.
To many projects now here! What about you?!
73, Stefan
Am 28.05.2015 01:33, schrieb DK7FC:
Sorry Markus, can you do all the calculations again for 475 kHz ? :-)
73, Stefan
Am 28.05.2015 01:23, schrieb Markus Vester:
>
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
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
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.
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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