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From: "Richard Newstead" <g3cwi@btconnect.com>
Sent: Sunday, May 24, 2009 12:45 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: Re: Re: First Portable Trip
mal hamilton wrote:
Jim
I did not say an inv L virtually lying on the ground as in Richards
case(1 METRE ABOVE GROUND), this is more like a beverage antenna.
This is all very interesting. I have spent an hour in a darkened room
contemplating suitable antennas (well not quite). Comments from experts
welcome:
1) T antenna. Am I right in thinking that in the classical T
configuration, the horizontal part of the aerial is only there to add
capacity - i.e. it makes no contribution to the groundwave?
..........Yes
If so, I
assume that I can use very thin wire for the T?
.........No , depends on thin, but I sqr R losses and skin losses will
be high
2) In the more likely configuration with "horizontal" wires actually
acting as an inverted V can I still use very thin wire without reducing
performance?
...... As above
3) Is there any benefit to be gained by having an inductor at the top of
the vertical section of the T?
........ I would say Yes , I use a top loading coil on my inverted L with
2 wire horizontal capacity section , A centre mounted coil , when
modelled gives slightly more gain
If so, will the loss of the inductor be
relevant if the part of the antenna above it (the T wires) don't
contribute to useful radiation?
......... would depend on the coil , I use 40 turns of insulated 35 amp
mains cable on a 4 inch former .. no idea of the losses but the system
seems to radiate ,,,,,, assume the only losses could be dielectric
losses or I sqr R , but with a heavy cable and polycarbonate former , I
don't expect much ...... you are making a series acceptor cct , max I
should be a the coil/cap section junction , which will be the highest point
.
73, Graham .,.
NB I looked into the portable antenna (for use with a handy fence) and
found that a 'arrow shape' or drooping T , with 10 mtr vertical section
(fishing pole) with sloping top section looked to give a reasonable
performance, however , once the ends of the arrow , dropped to 6 feet or
less , the radiation pattern and the performance became much reduced .. can
only assume it was the capacity of the end wires to earth
73
Richard
G3CWI
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