|To:||"[email protected]" <[email protected]>|
|Subject:||Re: LF: MF mobile ?|
|From:||John Langridge <[email protected]>|
|Date:||Thu, 28 May 2015 13:55:59 +0000 (UTC)|
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>But you need to consider the changing angle of the wire while driving.
I've always felt like the match change due to antenna angle could be regulated in realtime with a motorized vario or other matching elements controlled with an arduino or comparable platform, possibly using scopematch-style sensing... So as the match changes due to speed, crosswinds, etc, the match could be touched up. Thats a lot going on on the roof of a moving vehicle but seems reasonable enough for many adventerous souls.
The NPRM for 630m here in the US does not allow for true mobile operation as it currently stands but its nice to be able to live vicariously through colleagues around the world.
John KB5NJD / WG2XIQ..
From: DK7FC <[email protected]>
To: [email protected]
Sent: Wednesday, May 27, 2015 6:46 PM
Subject: Re: LF: MF mobile ?
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?!
Am 28.05.2015 01:33, schrieb DK7FC:
Sorry Markus, can you do all the calculations again for 475 kHz ? :-)
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,
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...
Am 27.05.2015 15:58, schrieb John Langridge:
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