Message-ID: <4D25D875.2090509@iup.uni-heidelberg.de>
Date: Thu, 06 Jan 2011 15:57:57 +0100
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: Re: Vertical antenna and connection line
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Hi Jim, Fausto, LF,

It seems i have misunderstood what Fausto wants to do. Of course, having 
the complete coil into the shack does not allow using RG213 ;-) I 
thought Fausto only wants to TUNE the system into the shack and wants to 
place the major part of the coil (say 90% or above) at the feeding point 
of the antenna.

Placing the whole coil into the shack is no good idea in my opinion. But 
placing a small fraction in the shack (e.g. to adjust when QSY from 
137.7 to 136.177)  is no problem...

73,Stefan

Am 06.01.2011 15:42, schrieb James Moritz:
> Dear Fausto, LF Group,
>
> Beware of sweeping generalisations - I think the answer is that "it 
> depends on the antenna". You need to think about your particular 
> antenna carefully.
>
> At 136k, 20m of transmission line feeding a high impedance load (such 
> as an electrically short vertical antenna) will behave essentially as 
> a shunt capacitor. The capacitance of a high Zo open wire line is of  
> the order of several pF per metre, which may be increased by being 
> close to the ground. So your feeder would have capacitance of a few 
> 100s of pF. This will be in parallel with the antenna impedance, which 
> will be equivalent to a capacitor in series with a resistor. If the 
> antenna capacitance is large compared to the feeder capacitance, the 
> feeder will make little difference, while if the antenna capacitance 
> is small compared to the feeder, the feeder current at the TX end will 
> be much higher than the antenna current, leading to increased losses 
> in the antenna tuner and feeder. So for small antennas, having the 
> tuner close to the antenna is very desireable.
>
> So what is the capacitance of the antenna? According to your info on 
> QRZ.com, the mast is 25m high, and you will be using it to suspend an 
> inverted L for 137k - unless you have another 25m mast, I guess the 
> inverted L top loading wire will actually be sloping... Your QTH seems 
> to have plenty of space, so I guess it would be no problem to have at 
> least 100m of wire in the top load. As a rough estimate, an isolated 
> wire has about 6pF/m, so with a downlead, the capacitance might be 
> 700pF. This would be about right for the variometer in the picture. So 
> this would be substantially more than the capacitance of the feeder, 
> which is good.
>
> But as Stefan and Rik say, another problem wil be the high voltage on 
> the feeder. The voltage depends on the required antenna current. At 
> your good QTH, with a sloping wire 25m high at one end, it should not 
> be difficult to get an effective height >10m. This would have 
> radiation resistance of about 30milliohms, and to achieve 1W ERP, an 
> antenna current less than 4A would be needed. The reactance of 700pF 
> at 137k is about 1700ohms, so 4A would result in a voltage of about 
> 6.6kV. This is quite high, but should not be a problem for the antenna 
> - many amateurs have managed to operate LF antennas at 20kV or so with 
> reasonable reliability. However, it would require wide spacing, and 
> careful design of insulators, for the transmission line.
>
> But why do you need the ladder line? If you want to have the tuner in 
> the shack, from your photograph it looks like you could simply run a 
> downlead from the top of the mast directly to the shack... This would 
> have the minimum of loss and insulation problems.
>
> Hope this is helpful,
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
> ----- Original Message ----- From: "Fausto Coletti" 
> <faustocoletti@alice.it>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Thursday, January 06, 2011 11:28 AM
> Subject: LF: Vertical antenna and connection line
>
>
> Hello,
> A question for RF expert:
> you think that i can connect a Marconi antenna and relative gound 
> system to the variometer with a high impedance ladder line?
>