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Message-ID: <88184BDCCE244700B62ACEBF39F34582@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <fc7eccec0902260816u68aa7394lc2adc64e2440b62e@mail.gmail.com>
In-Reply-To: <fc7eccec0902260816u68aa7394lc2adc64e2440b62e@mail.gmail.com>
Date: Thu, 26 Feb 2009 23:43:26 -0000
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Subject: LF: Re: LF Antennas - back to basics
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Dear Andy, Dave, LF Group,

My experience is that improvements to the ground system soon reach a point 
of diminishing returns where the ground connection losses are small compared 
to other losses - I would expect you have reached that point already. In my 
case, literally filling the garden with wire made at best about 0.5dB 
improvement by reducing loss resistance, compared to having about 5 ground 
rods spaced a few metres around the downlead and loading coil. However 
increasing the height of the top loading, even if only in the middle, does 
lead to substantial improvements. One benefit of doing this is that the 
effective height and radiation resistance is increased by increasing the 
average height of the antenna wires. If you are in a fairly built-up 
environment, the improvement is probably more than you would expect from 
calculating the effective height from the dimensions of the antenna itself, 
because increasing the physical height of the antenna puts it further above, 
or nearer the top of, screening buildings and trees etc., that are 
surrounding it. Also, moving the antenna further above loss-causing objects 
leads to a modest reduction in the loss resistance. In my case, with an 
inverted L about 10m high at the ends and sagging to 9m in the middle, 
propping up the middle of the span with a fibreglass pole to about 14m 
increases ERP by 3 - 4dB.

As to possible advantages and disadvantages of a loop compared to a 
vertical, in general the loop ought to benefit from lower dielectric losses 
due to the generally lower voltages. The directional pattern is often 
helpful on receive. But the figure-of-eight directional pattern could also 
be a drawback for a transmit antenna where it isn't practical to rotate the 
antenna to avoid having nulls in awkward directions. Also, one has to think 
about the scale. AA1A's loop is quite big in overall dimensions compared to 
G4JNT's vertical. The radiation resistance of a loop is proportional to the 
square of the area, which is proportional to the square of the linear 
dimensions of the loop conductor - so when scaling down a loop, one would 
expect Rrad to reduce much faster than Rloss due to reduction in perimeter, 
and so efficiency of relatively small loops to be poor.

Cheers, Jim Moritz
73 de M0BMU

----- Original Message ----- 
From: "Andy Talbot" <andy.g4jnt@googlemail.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Thursday, February 26, 2009 4:16 PM
Subject: LF: LF Antennas - back to basics


> Firstly, bearing in mind there is no scope whatsoever to raise one end
> of the capacity hat which is defined by the apex of the house roof,
> and the other end would also need some major sugery to raise its heigh
> substantially, that only leaves the middle, ie the height of the
> actual radiator.    It wouldn't be too difficult to raise this to 10m
> or even more with a fibreglass pole, but will I be throwing away all
> the advantage by having the top-hat drooping down to 7m?
>
> Secondly, pictures of other peoples antennas aften show a substantial
> grounding plate immediately under and around the antenna base.  Just
> how far out is is worth going with a really substantial base.   I
> could cut more conductors into the ground; while the ground is still
> is still soggy in March may be a good time to do this.
>
> Any ideas please ...?
>
>
> Andy  G4JNT
> www.g4jnt.com
>