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Message-ID: <D5pAzCAh0gx4EwSG@dennison.demon.co.uk>
Date: Wed, 8 Mar 2000 08:19:45 +0000
To: rsgb_lf_group@blacksheep.org
From: "Mike Dennison" <mike@dennison.demon.co.uk>
Subject: Re: LF: antennas for 136kHz
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G4CNN wrote:
>If to the top of a vertical an equal vertical descender is added, insulated
>from Earth at the bottom, but otherwise the same length as the original
>vertical and coming straight down (like an old-fashioned hair-pin), and
>assuming that this is resonated and matched to 50 ohms, will the new antenna
>radiate more power, less power or the same power for the same input and why?
>This might for example be the other wire of a sufficiently well spaced
>feeder (no voltage breakdown), effectively short-circuited at the top and of
>low RF resistance.

If the end of the downwards vertical is insulated from earth, it won't
work as the current in it will partly cancel out the current in the
upward part.

However, =in theory= you can earth the end of the downward wire, and
resonate both wires separately as quarter waves, giving effectively half
of a folded vertical dipole. I said =in theory= because it would be very
difficult to equalise the two wires such that the current distribution
was identical, and you have twice the coil losses. If you could get it
to work, there would be an improvement because the radiation resistance
would be quadrupled and hence earth losses would be much less. BUT if it
was easy to do there would be lots of topband operators using it! Good
luck.


-- 
Mike, G3XDV
IO91VT
http://www.dennison.demon.co.uk/activity.htm