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 - it is easily measured by other stations.
--
Regards, Peter, G3LDO (VQ4HX, VQ3HX, VQ1HX, 9L1HX, PA9APV, ZK1XE)
<[email protected]>
|
