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| To: | [email protected], [email protected] |
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| Subject: | Re: LF: MF mobile ? |
| From: | Markus Vester <[email protected]> |
| Date: | Mon, 29 Jun 2015 07:22:09 -0400 |
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Revisiting the topic... My ERP calculation was erroneous by a factor of ten (looks like I can no longer do without a calculator). It should be
radiated power = 0.0256 * 160 = 4 microwatts or -24 dBm
... which could let me produce a comfortable 21 dB SNR in Stefan's LF QRSS-60 window - and even mobile QRSS-10 comes within reach.
Scaling from 137 to 475 kHz, we'd be able to achieve 0.6 mW EMRP, which would probably be good for aural CW up to 50 km over land.
When driving around with a mobile antenna it will probably anyway need to be guyed. So we might use the strings partly as a topload, forming a miniature umbrella antenna with higher current and effective height.
I'm only a bit worried about unexpected behavior of modern vehicle electronics. In particular, unappropriate firing of the airbags could potentially become a nasty surprise. I guess this may be less of a concern in an old Golf Diesel ;-)
All the best,
Markus (DF6NM) Von: Markus Vester <[email protected]> An: rsgb_lf_group <[email protected]> Verschickt: Do, 28 Mai 2015 1:23 am Betreff: Re: LF: MF mobile ? > 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
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 ...
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