Envelope-to: dave@picks.force9.co.uk
Delivery-date: Wed, 15 Feb 2006 16:52:55 +0000
From: "Mike Dennison" <mike.dennison@ntlworld.com>
To: rsgb_lf_group@blacksheep.org
Date: Wed, 15 Feb 2006 16:47:55 -0000
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Subject: Re: LF: Antenna question..
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Dave, 

The slope downwards on the top section will reduce your effective 
height and hence the ERP. Note that with a short Marconi the ERP is 
proportional to the square of the effective height, so this factor is 
=extremely= important. If there is any way to increase the 5m end, 
even by a little, then do so. 

The loss will be reduced by placing a loading coil higher up the 
vertical section, preferably at the top but the middle will do if 
it's more converient mechanically. This has the effect of reducing 
the voltage on the up wire and hence reducing local environmental 
losses, such as those to the brickwork on your house. It should also 
reduce your interference and pick-up problems. So it is well worth 
doing.  I can advise on light-weight inductors (or see my book: LF 
Today).

If you have a mast at the house end, you may find it helpful to 
ground it (yes, I know it ought to 'short circuit' the vertical, but 
in fact it provides a relatively low loss capacitor to earth, instead 
of a resistor of variable loss depending on the weather).

If you want to do a 'before and after' test, let me know. I am 
usually at my desk/shack during the day.

Good luck.

Mike, G3XDV
==========


> I am planning to re-engineer my antenna during the Spring, and I have
> a design decision to make.
> I use an "L" antenna with a vertical section and a horizontal top
> section that is 20m long. The "horizontal" top section is 10m high at
> the house end, and slopes down to 5m at the far end. The vertical
> section is a single wire, with a large loading coil/variometer at the
> bottom, in the usual time-honoured fashion.
> My choice is:- do I feed the top section at the house end, in which
> case the vertical section will be 10m high, or at the far end, where
> the vertical section will be only 5m high? At first glance, it might
> be thought that the 10m vertical would be preferable, but the fact
> that the top section slopes confuses the situation.
> 
> Another problem with feeding at the house end is that the vertical
> section is very near the house and causes all sorts of RFI problems.
> It also picks up a load of mains-borne rubbish!
> Second question:- Has anyone experienced any tangible benefits by
> including an extra loading coil part-way up the vertical section?
> 73, Dave G3WCB IO91RM
>