----- Original Message -----
From: "Ashlock,William" <[email protected]>
To: <[email protected]>
Sent: Thursday, November 14, 2002 4:50 AM
Subject: LF: RE: Loop vs Marconi
Peter,
>When we started on 73kHz some years ago G2AJV was the first to put out a
reasonably
>strong signal and he was using a large loop. G4JNT and G3XDV also put out
reasonable
>signals from very restricted sites and later found that a Marconi from
the
same site
>radiated a stronger signal for the same transmitter power. As a result it
>has become part of UK LF collective wisdom that the Marconi is superior
to
a
>loop of the same size.
That was also the 'collective wisdom' amongst Lowfers in the US, until 3
years ago. The Wisdom was based on 'cut and try' and "how am I coming in?"
types of scientific study. The net result in the US was ZERO loop
antennas.
It is also the 'collective wisdom' of most Low frequency textbooks in the
MIT Engineering library dating back to the 1920s that loops have no
practical use for transmitting at low frequencies.
I have spent 100s of hours experimenting with different configurations
making field strength measurements, comparing them with the classic
radiation equations, determining the effect of soil loss, and conductor
loss, determining if a ground screen is needed. My data and mathematical
studies indicate that a simple 50'x50' loop with a .37" conductor is ~6db
inferior to a 50' tall vertical,
A loop only 6db down is a BIG DOWN ie signal 4 times weaker than a vertical
having a 50' diameter top hat and with a
elaborate ground system located in an open field. On my property, because
of
the canopy of trees,
A canopy of trees will also effect a loop antenna, try a 50 ft vertical and
gain 6dbs and you will probably still be on the plus side even with some
attenuation from the trees.
there is absolutely no hope for a vertical antenna and
I spent 2 years with the same diligence with verticals as I have with my
loop antenna and it ended in failure - as is the case for, no doubt, for
many other vertical users that we don't hear from any more.
I'm not surprised at all about the mediocre reputation of loop antennas
when
I read about other loop antenna descriptions. I often find conductor Rac
inadequate and loop-to-ground spacing too small, poor matching, and a
general lack of knowing what math to apply to these variables. There is
one
'expert' that doesn't even know how to match a coax to a large loop and
boasts how poorly his 'best effort' loop compares to his vertical on
receive.
Bill.
I have tried fairly large loops, matching correctly at the loop/coax end
both series and parallel systems for comparison and on Receive they work but
not as well as the vertical.
On the question of Transmitting, some here in the UK are aiming to run the
max power permitted ie 1W erp. This means running power levels of up to 2000
Watts to small verticals to achieve the object. RF current at these power
levels is manageable at SEVERAL amps using a vertical antenna but using a so
called large loop the RF current could be up to 100 amps and virtually
impossible to operate.
In the USA as you have said the permitted power level is so low that you do
not have a problem with a loop with RF current of a few milliamps. Your
circumstances over there are totally different to the UK and some parts of
EU.
Recent mail posted here by Jim about loops and RF amperage and by Peter
about those that started with loops and then switched to verticals, find the
vertical superior given the same circumstances.
I always thought a vertical took up less space than a large loop and has the
advantage of being omni directional as opposed to a loop limitations being
only bi-directional.
Some UK/EU stations are using both loop and vertical antennas and their
signal are much superior when using the vertical.
The only loop with an advantage is one used on a frequency with its natural
resonance on that frequency ie a 20 metre loop vertically orientated used on
the 20 metre band. Horizontal loops also work well but need to be at least
1/2 wavelength high at the required frequency.
These are my observations from over 50 years experience both as a radio
amateur and professional in the business.
The above criteria works well for me and last years transatlantic tests to
W4DEX and other USA and Canadian stations proves the point.
73 de Mal/G3KEV
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