Message-ID: <38852697.2012A382@netscapeonline.co.uk>
Date: Tue, 18 Jan 2000 18:51:03 -0800
From: "g3kev" <g3kev@netscapeonline.co.uk>
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Re: Big LF Antennas
References: <2000011813032668199@zetnet.co.uk>
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Peter Dodd wrote:

> BIG ANTENNA MEGALOMANIA
> >From the heading you might suppose that I am anti-large antennas.
> This is not true. My first introduction to Amateur Radio was G3JKV on
> Top Band from RAF Shawbury, with a 140ft antenna supported at one end
> by a 100ft tower  (Its all your fault Walter!).
>
> Later I operated on Top Band from Sierra Leone, West Africa, with an
> antenna fixed to a 140ft mast on an 1100 ft high location a mile away
> from the sea. And worked the world with 8watts input to the PA of the
> transmitter. All good fun.
>
> I am currently experimenting with a large antenna at Amberley
> Industrial Museum for 136kHz
>
> However, G3KEV makes the following assertion:
>
> "From statistics available the only amateur stations active on 137
> khz in the UK capable of making a two way qso across the atlantic are
> MM0ALM/G3KEV/GI3KEV/GI3PDN. The statistics are based on
> 1: Antennas MM0ALM - 130 Ft towers
>                    G3KEV     -  100 and 80ft towers
>                    GI3KEV    -  120 Ft tower ( another tower same
> height to be installed soon to accommodate better antennas for the
> transatlantic test)
>                    GI3PDN    -   80 Ft tower ( upgrade soon to 120 ft)"
>
> Another e-mail from G3KEV ran as follows:
>
> "There is a big difference in the radiation pattern and angle of take
> off between a high vertical antenna and a very low antenna given the
> same rf power to the antenna. 1 Watt erp from a dummy load or low
> horizontal wire is hardly the same as 1 Watt erp from a vertical
> antenna at 120 ft. There is also the RX consideration especially on
> long haul signals. At LF the delivery system ie ANTENNA is the most
> important factor. Even with large towers, and high elaborate
> antennas, which are still physically small relative to 137 khz and a
> limit of 1W erp, one has to realise the limitations,...."
>
> which gives the impression Mal might be a little hazy as to what
> e.r.p. actually means.
>
> So lets take a look at a couple of antennas and see what they are
> capable of. They are modelled using EZNEC, which gives a good
> correlation with observed performance. First of all the monster
> antenna a G3KEV. His description of it is as follows:
>
> "I now have 3  inv L's in parallel, each one is 110 ft vertical and
> 250 ft top section. These all feed to a common point and connected to
> my loading inductor....."
>
> I modelled this antenna using just one inverted L (putting the three
> inverted Ls in parallel gave a 1.5 dB improvement) A single inverted
> L antenna has a gain of -15dBi on good ground and a radiation
> resistance of 0.3ohms. The elevation angle of maximum radiation is 15
> degrees and an omni-directional azimuth pattern (ignoring the effect
> of the sea). An antenna with this gain gives 1w erp with a
> transmitter power of 96W (say 100W). I am not sure what power Mal is
> using but it was quoted in an early e-mail.
>
> Now lets have a look at my antenna, contemptuously referred to by Mal
> in an e-mail as "a piece of wet string". It has a gain of -25dB with
> an elevation angle of maximum radiation of 15 degrees. With my 300W
> transmitter it gives an erp of around 300mW. I can raise my erp to 1W
> by using a 1KW output amplifier.
>
> The polar diagram of an electrically short antenna fed against ground
> (if you are using a loading coil then your antenna is electrically
> short) is the same whatever shape you make it (sort of half
> doughnut), with a deep vertical null. An inverted L antenna that is
> approaching a quarter wavelength long, such as OH1TN and  MM0ALM is
> different and radiates much of its energy skywards.
>
> >From this it is obvious that you too can radiate a 1w erp without
> having to buy a farm, provided that you have enough room to put up an
> 80m dipole. G3KAU has developed a 1.2KW amplifier on a G0MRF board
> and G3YXM and G3XTZ also use similar power amplifiers.
>
> As regards receive G3KEV asserts:
> "On RX a small antenna does not have the capture area to produce a
> good strong signal...".
>
> Capture area depends as much on antenna Q as on size. In fact if the
> antenna is too large then you finish up having to switch in
> attenuation to maintain the receiver system gain distribution and
> avoid non-linearity. The AMRAD group in the USA found this out when
> they used some ex-navy big antennas at 300ft high. A large antenna
> should have the advantage having less electrical QRM although G4GVC
> seems to have very good 'ears' from a modest site in suburbia.
>
> So, try to disregard this big antenna hype and smaller station
> discouragement; concentrate on general experimenting and improving
> your station. Above all, have fun - after all its only a hobby.
>
> Finally, some countries have not yet been granted permission to use
> the 136kHz band. You can be sure that organisations, such as the FCC,
> will be monitoring our behaviour and activity before going ahead with
> this allocation.
> If you have a large antenna farm then by all means use it, but watch
> the erp

Watch the ERP it is easy to measure.
What he is really saying is, if he gets his calculator out and your are
British and he thinks that you might be a fraction over 1W erp he would
drop you in the Proverbial. This means for the Continental audience that he
would report you to the Establishment.- Pathetic

> - it is easily measured by other stations.
>
> --
> Regards, Peter, G3LDO (VQ4HX, VQ3HX, VQ1HX, 9L1HX, PA9APV, ZK1XE)
>
> <g3ldo@zetnet.co.uk>