No dispute this time.
Laurie
2009/5/12 mal hamilton <[email protected]>
Lawerence
No one is disputing downward SLOPE reduces the
effective height. We are discussing 500 Khz PORTABLE and the most convenient way
to erect a suitable antenna with minimum effort and you agree at 500 Khz it will
not be a serious disadvantage to have such a slope.
therefore a single mast and some slope is
acceptable. You are AGREEING wiith me so I am confused by your email !!!!!! The
topic is 500 Khz PORTABLE.
G3KEV
----- Original Message -----
Sent: Tuesday, May 12, 2009 5:02 PM
Subject: Re: LF: Re: Re: 500kHz
Portable
I fully agree with Mike and Andy, any slope downward will
reduce the effective height of the antenna, and hence its radiation
resistance. This may not be too much of a problem on 500kHz where enough RF
can be generated but on 136 not so good and on 73 useless. I have
proved this in practice as well as by simulation. So there you are
Mal. Laurie G3AQC
2009/5/12 Andy Talbot <[email protected]>
A full quarter wave inverted L with sloping end is starting to become a
loop with the ground path forming part of the loop conductor, rather than a
vertical. ANY antenna that approaches more than about a tenth of
a wavelength cannot be treated as small any more, and must be considered in
its entirety. Such is obvious even before resorting to
modelling. Mike was referring to electrically short antennas where the
downward sloping part simply opposes the current flow in the upward
path.
A small amount of slope on a large capacity hat might not be too
noticeable in practice, as a lot of extra capacitance at the top could do
more good than its slope undoes.
Please stop and actually think, and perhaps before being so rude
and treating all why don't own millions of hectares of farmland as
second-class radio-ams, you stop and read the exam question.
Andy G4JNT
wobbleU cubed
.g4jnt.kom
2009/5/12 mal hamilton <[email protected]>
You
are certainly NO EXPERT at amateur radio never mind LF/MF and especially
antennas.
I have worked the world on LF/MF using Inv L antennas with
the far end just above ground , and in normal CW mode.
Some time ago it
was Comics, elevated lemonade bottles as coil formers, thought an
AR88D was a diversity fly catcher.
What is coming next? The
diversity UMBRELLA!!!!
My current MF antenna is a full 1/4 wave inv L,
100 ft high and performs well but for some this might be difficult,
therefore use what you can get on your property and let the far end tilt
down if necessary.
G3KEV
----- Original Message ----- From:
"Mike Dennison" <[email protected]>
To: <[email protected]>
Sent: Tuesday, May
12, 2009 10:29 AM
Subject: Re: LF: Re: Re: 500kHz Portable
You
do not need 2 masts for an inv L or T antenna.
The so called
horizontal wire(s) can slope down and the far end tied
to a bush or
tree, or a stake in the ground via an insulator, as far
away as
possible from the vert section, the longer the better. Keep
it
simple. mal/g3kev
This is quite wrong. Any
slope downwards will reduce the effective
height of a Marconi. Since
ERP is a function of the square of the
effective height this is
extremely important. An umbrella is far
preferable to an inverted-L
in a single mast situation because a
large amount of capacitance to
ground can be achieved without much
reduction of effective
height.
Based on my experience of LF portable (both on 73 and
136kHz), my
advice is to make very sure that the remote ends of a
Marconi are
very well insulated as they carry large voltages. Keep
well away from
wet trees. This is not just a safety issue; any
voltage leaked is
power lost.
Mike,
G3XDV
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