Re: LF: Re: Re: 500kHz Portable

[lawrence mayhead <laurie.mayhead@googlemail.com>]

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 <andy.g4jnt@googlemail.com>

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 <g3kevmal@talktalk.net>

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" <mike.dennison@ntlworld.com>
To: <rsgb_lf_group@blacksheep.org>
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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