Re: LF: Re: VLF dj8wx on 8970.022Hz

[uwe-jannsen@kabelmail.de]

Dear Jim,

thanks for estimating the ERP.

 

no buildings in vicinity, only some walnut trees, willow trees and oaks. the topload of the aerial is some metres beyond those trees (still).

 

the diameter of the naked wire: 2,5mm . the isolation: 025mm of unknown material (no PVC).

the spacing of the triple wire: 30cm.

 

I cant let it go with the 3A radial current. the fence and even wett pols of wood get under tension.

 

1A radial current is the max Ill work with.

 

best regards

Uwe/dj8wx

 

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Von: rsgb_lf_group@blacksheep.org
Gesendet: 08.03.2011 21:00
An: rsgb_lf_group@blacksheep.org
Betreff: LF: Re: VLF dj8wx on 8970.022Hz

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Dear Uwe,

One can make a very approximate, order-of-magnitude estimate of ERP and
voltage from the info you have:-

The effective height of a heavily top-loaded vertical such as your antenna
will be a bit less than the average physical height of the conductors -
assume about 12m. The radiation resistance Rrad = 160 * pi^2 *
(Heff/lambda)^2 at 9kHz is therefore 200 micro-ohm - it will be reduced if
there are trees, buildings etc. close to the antenna. The antenna current
will be slightly less than the current in the radials, due to the
distributed capacitance of the loading coil, etc., at max power let us say
2.7A. ERP will then be 1.8 * (2.7)^2 * 200u = 2.7mW.

The exact capacitance will depend on the spacing and diameter of the antenna
wires, etc., but I estimate roughly 1200pF based on your drawing. The
reactance at 9k is therefore 15kohm. With 2.7A antenna current, the antenna
voltage would be 40kV RMS.

There are major possible sources of error when trying to measure the antenna
voltage in the way you describe - even at 9kHz, the effect of stray
capacitance will be very serious - a 10M resistor has the same magnitude of
impedance as 1.7pF of stray capacitance, so only a short length of exposed
wire will cause a large change in voltage at the scope input. Also, as you
have seen, the strong E field can couple significantly to the internal
wiring of the measuring equipment unless it is very well screened. Any
potential divider must also be well screened, and take into account the time
constants due to resistance and stray capacitance.

Cheers, Jim Moritz
73 de M0BMU