LF: 9kHz

[Rik Strobbe <rik.strobbe@fys.kuleuven.be>]

Stefan,

did you have a look at the graph Jim (M0BMU) showed yesterday ? This may
indicate that in an open area you might be better off than 1/f, maybe in
the 50..100 Ohm range.
Last night I was mulling about Alexanders remark that a parallel C would
increase the losses. At first though I would say that the parallel C just
performs some kind of impedance transformation without inducing
additional losses. And this is true if the loading coil would be perfect
(no losses).
But for a real world situation (ie. assuming a constant Q = 250 for the
loading coil) Alexander is right: the transformation caused by a large
parallel C causes extreme high currents in the loading coil, and thus
higher losses.
Simulation (with SIMetrix) confirms this.
Taking your antenna (580pF / 50 Ohm)
1. Without parallel C you would need a 550mH loading coil. At Q = 250 the
coil loss would be 130 Ohm. Putting 100W into this system would result in
an antenna current of about 750mA and antenna voltage of 23kV. 750mA
trough the loading coil means a dissipation of 73W.
2. With a 3.2nF parallel C you would need a 85mH loading coil. At Q = 250
the coil loss would be 20 Ohm. But putting 100W into this system would
result in only 330mA antenna current, while the current through the
loading coil would be 2.1A (= 88W dissipation). The reduction in antenna
current (750mA to 330mA) would lower the ERP by 7dB.
In addition the parallel C will decrease the antenna bandwidth
significantly, making it difficult to tune.

73, Rik  ON7YD - OR7T

At 01:14 25/02/2010, you wrote:
> Here i agree. And here i
> understand "dreamers" in a positive and optimistic way, just
> like children are dreamers!
> We have written so many mails (including me, of course ;-) ) und could
> start now just to try what is possible to reach more than 10km if not
> done yet. 
>
> On the LF pages of DK8KW i found a notice of a successful 2 way QSO over
> 10km @ <9kHz. Does anybody know about more than 10km by
> amateurs?
>
> Yesterday in the night i talked more than 1 hour to Markus/DF6NM and he
> persuaded me to try it with the 100m vertical in a first test. Maybe i
> get that temp licence for a 300m kite vertical but the public autoritys
> are still discussing if i need a special casualty insurance...
>
> Do you, the group, think that it makes sense to try it if the next
> listening station is >50km or even more away? What could be possible
> if this would be the arrangement with the 100m vertical:
>  
> The wire has abt 580pF
> I can get a air coil with L=85mH out of a 20kV field. DC resistance abt
> 150mOhm, continous current abt 30A, voltage over coil abt 30kV rms (if it
> remains dry(!)), measured self resonance frequency abt 50kHz.
> So, if f=8,9kHz, a parallel C of abt 3,2nF is needed (i can get some
> 470pF/40kV Cs, no problem)
> The voltage across my thin wire should be limited to abt 20kV rms, that
> means, antenna current should not exceed 650mA.
> If Rik's 1/f dependency is valid, then my ground losses will be in the
> region of 250 Ohm (no houses and trees there on the hill in a radius of
> abt 100m; 15Ohm @ 137 kHz).
> So, 100W TX power would radiate 1,4mW and thus the efficiency would be
> 0,0014%. In that case, i mustn't apply more than 100W since the voltage
> across the wire would be to high, so no need for a QRO PA...
>  
> If that calculation is reasonably correct, what distance could be reached
> with 1,4mW @ 8,9 kHz in e.g. QRSS10 and the noise level that can be
> expected on a sunday morning? :-)
>  
> 73 es wish gl to all those, who are trying in these days ;-)
>  
> Stefan/DK7FC