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Date: Thu, 09 Mar 2000 13:19:59
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: Re: LF antennas
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At 10:16 9/03/00 -0000, G3XDV wrote:
>> Due to its directivity, a short vertical monopole has a gain of 2.6dB over
>> a dipole (4.77dBi versus 2.15dBi for a dipole).
>
>Careful! Surely this assumes perfect ground, doesn't it? Even at 
>HF, I think most amateurs would prefer to use a vertical dipole than 
>a ground-plane antenna.

Good point. I have been breaking my mind about that for some while. But it
seems to me that we take the inperfect ground into account already with the
ground-loss resistance, so do we have to take it into account a second time ?

eg : assume a 10m high vertical (radiation resistance 8.2 milli-Ohm)

If we had a perfect ground (and loadingcoil) only the radiation resistance
will limit the antennacurrent, so if we put 100W into the system we will
have an antennacurrent of 110A (!) and radiate 100W, what due to the
directivity of the antenna results in 183W ERP.

Now we take the real-world case and assume a loss of 100 Ohm (environement
+ coil). If we put 100W into this system we will have an antennacurrent of
1A and radiate 8.2mW (-42dB compared to the 'perfect case'). 
The question is if we can take the directivity of a perfect vertical
(2.6dB) into account ?
My opinion is that we have to take this gain into account, as the
enviromental losses already played their part by limiting the
antennacurrent. But the only way to find out what really happens is to
measure the radiation pattern of the antenna, is the anyone with acces to a
small airplane or helicopter ?

73, Rik  ON7YD