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Message-ID: <6BA400053F4E4A45935BEA7FF2FC10EC@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4A01AEA0.1030308@btconnect.com> <4A01B25C.2040404@btconnect.com> <00a201c9ce65$850fe540$0301a8c0@mal769a60aa920>
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Date: Wed, 6 May 2009 19:26:42 +0100
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Subject: LF: Re: 500kHz Portable
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Dear Richard, LF Group,

I would suggest using an "umbrella" top-loaded vertical using a telescopic 
fibreglass mast. From experience, antennas with horizontal sections like 
inverted Ls or Ts have the disadvantage of requiring at least two masts 
(unless there is a suitable support already on your hilltop), and are a real 
handful when the wind starts blowing due to the masts flexing, requiring 
several guys to keep the antenna in shape. The umbrella is somewhat less 
efficient for the same overall height, but only needs a single mast, and the 
sloping top loading wires double as guys to keep the mast straight. The 
height is the most important dimension. A 10m mast would make a reasonable 
500kHz antenna.

For a reasonable estimate of antenna capacitance, assume 6pF per meter of 
wire. For antennas with a number of wires running close together, such as an 
umbrella, the actual capacitance will be a bit lower due to proximity 
effects, but this is a good first estimate. You can use NEC antenna 
simulators to calculate the capacitance quite accurately, although the 
resistance will probably be way off. Small antennas will have very sharp 
resonance, so you will want to add a variometer to your loading coil, or 
some other means of fine tuning. G4FGQ's DOS software runs fine in a DOS box 
under Windows 98 and Vista here.

For insulated ground radials to work well, their capacitance to ground needs 
to be much larger than that of the antenna, so you probably want a few 
hundred metres of wire in total. If the ground is soft, a few 1m ground rods 
connected together works well as a ground.

There is a lot of handy info on LF antennas at 
http://www.wireless.org.uk/on7yd/index.htm


Hope to work you /P soon...

Cheers, Jim Moritz
73 de M0BMU



>
>> Dear all
>>
>> I am hoping to be granted an NoV for portable operating on 500kHz. I
>> have parts on order for the GW3UEP Economy transmitter which will be run
>> on 12 Volts. Suggestions for portable aerials that can be set-up by one
>> person on remote and windy hilltops will be most welcome.
>>
>> 73
>>
>> Richard
>> G3CWI
>>