Hello
Do not be tempted to droop
the ends of any top wires.
... I challenge that. My umbrella antenna 18m high with 5 pieces of top-load
radials sloping downwards (each about 7 m long) works fine (see sketch at
http://www.qru.de/station.htm), with an antenna "gain" of roughly - 30 dB.
One of the key factors that had increased my ERP was to put up my loading
coil 1/3 way up the fiberglas mast so that any losses occuring due to high
voltages due to surrounding trees and walls are reduced.
Best 73
Geri, DK8KW (W1KW)
----- Original Message -----
From: "mike.dennison" <[email protected]>
To: <[email protected]>
Sent: Saturday, October 19, 2002 5:50 PM
Subject: LF: Re: Ant Questions
G0CZD wrote:
>I'm about to put up a 136kHz Antenna and have a couple of questions.
>I don't have much space in the garden, approx 20m from the apex of the
roof to a 20m high poplar tree in the opposite corner. I am considering
an offset 'T' with a multi-wire top.
>I am assuming (within practical limits) that as much capacitance as
possible in the antenna is beneficial. It will resonate with lower
inductance and therefore lower resistive losses.
Yes, that is correct.
>The RSGB LF experimenters h/b (p52) suggests that 2 wires separated by
>100mm will provide 39% increase in capacitance. Is it safe to assume
>that putting multiple wires up at 100mm will provide a compound
>percentage increase? I am considering 6 wires at 100mm separation. Are
>there any better solutions - I have neighbours I wish to remain on
>speaking terms with...
The capacity hat should be as wide as possible. This is more important
that
many wires. In fact, you quickly get into the law of diminshing returns
with
extra wires, plus the weight increases much faster than the capacitance. I
use three wires over a total width of 1m. Making it wider would be better
than having extra wires.
>What effect does height have on the antenna? I can fairly safely get the
>antenna up to 10m above ground level without too much difficulty. Will
>there be any significant benefit to raising the tree end of the antenna
>to say 15m.
Yes. Height is all important as the ERP is directly related to the
=square=
of effective height. Effective height varies from 50% to 100% of the
actual
height depending on how good the top section is. Do not be tempted to
droop
the ends of any top wires.
Good luck, Martyn. If you need to see a sketch of my antenna, which is 14m
high by <18m long, it can be found at my web site.
73
Mike, G3XDV
http://www.lf.thersgb.net
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