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Date: Thu, 26 Feb 2009 21:16:01 +0100
From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: LF Antennas - back to basics
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Andy,


Quoting Andy Talbot <andy.g4jnt@googlemail.com>:

> 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?

Raising one end of the antenna should increase the radiation =20
resistance, even if the tophat is drooping.
G4CNN (SK) did extensive calculations on this matter, see =20
http://www.strobbe.eu/on7yd/136ant/#Umbrella

> 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.

If found out that on 500kHz (ground) losses are much lower than on 136 =20
or even 73kHz. On 136kHz I measured a loss resistance of 130-160 Ohm =20
(winter - summer), on 500kHz it is 35-60 Ohm. At these values I =20
reached a point where the loss is the loading coils (abt. 7 Ohm) is no =20
longer neglectable.

Another thing that might improve teh antenna is placing (a part of) =20
the loading coil at the top of the vertical part, see =20
http://www.strobbe.eu/on7yd/136ant/#CombTop

73, Rik  ON7YD - OR7T







>
> Any ideas please ...?
>
>
> Andy  G4JNT
> www.g4jnt.com
>
>



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