Dear LF Group,
I have now finished the antenna experiments that I started earlier this
week - this is a summary of the results.
The idea of the tests was to measure the effects on LF/MF antenna
performance of the environment around the antenna. It is generally
expected
that when a small amateur-type antenna is surrounded by trees, buildings
etc. that loss resistance will be increased. Also, the ERP achieved using
such an antenna is usually lower than expected from calculations using the
antenna geometry and antenna current. But with measurements on a single
antenna it is difficult to know how much of the observed losses are due to
these effects, or maybe some sort of error or omission in the calculations
and measurements. So the idea of these experiments was to compare two
antennas that were as nearly identical as possible, except that one was
located at the M0BMU home QTH, surrounded by trees and buildings that are
in
some cases within metres of the antenna, while the other was located in a
relatively ideal flat, open field, with only a few bushes and fences
within
a 50m radius of the antenna. Both antennas were inverted-L configurations,
with a single top wire about 40m long at a height of around 10m max.
Actual
measurements of the antennas, and using handbook formulas to calculate
effective height gave Heff of 8.3m for the home antenna, while the open
field antenna was slightly lower at Heff = 7.9m. Both antennas used ground
systems of 4 x 1m long ground rods, within a 1m radius of the antenna feed
point, and the ground under both antennas was a waterlogged clay soil,
which
should have quite high conductivity.
I measured the antenna loss resistance over the range 10kHz - 600kHz using
a
RF bridge. The home antenna has a resistance that decreases steadily with
frequency, from 395ohm at 9.5kHz to 56ohm at 136kHz, and 25.5ohm at
503kHz.
The open field antenna had radically lower resistance; about 50ohms at
10kHz, reducing to 8.5ohms at 136k, showing a broad minimum of around 8
ohms
at 200k, and then increasing slightly to 8.5ohms at 503k and 10 ohms at
600k.
Multiple field strength measurements were used to determine ERP. The
average
measured ERP and calculated ERP are calculated below, along with the
efficiency calculated as (radiated power)/(power to antenna). The
calculated
ERP assumes that the antenna has 2.62dB directive gain over a dipole.
Home QTH, 503.8k: Iant = 400mA, Calculated ERP = 88mW, Measured ERP =
43mW,
difference -3.1dB, Efficiency = 0.58%
Open field, 503.8k: Iant = 380mA, Calculated ERP = 74mW, Measured ERP =
82mW,
difference +0.5dB, Efficiency =3.7%
Home QTH, 136.0k: Iant = 3.9A, Calculated ERP = 0.62W, Measured ERP =
0.18W,
difference - 5.4dB, Efficiency = 0.012%
Open field, 136.0k: Iant = 3A, Calculated ERP = 0.34W, Measured ERP =
0.40W,
difference +0.8dB, Efficiency 0.29%
So the open field ERP values are quite close to those calculated using
simple text book formulas, while the home QTH figures are substantially
lower. This could be interpreted as a reduction in Heff and radiation
resistance Rrad of the home QTH antenna, caused by the screening effect of
surrounding trees and buildings.
The combined effect of increased Rloss and reduced Rrad of the home QTH
antenna lead to a surprisingly large reduction in efficiency compared to
the
open field antenna. At 503k, the open field antenna is about 6 times as
efficient, while at 136k it is a massive 24 times more efficient!
The big difference in Rloss also has implications for loading coil design.
For these antennas, the required inductance is roughly 4mH at 136k. For
the
home QTH antenna, a modest loading coil with Q of a couple of hundred will
cause a negligible reduction in radiated signal, due to the relatively
high
loss resistance of the antenna. But for the open field antenna, even a
coil
with a Q of 1000 would dissipate about 1/3 of the TX power, so a much
better
loading coil is needed to get the full benefits of increased antenna
efficiency. The situation at 500k wouild be much easier due to the lower
inductance needed. Of course, if you have a big field to put the antenna
in,
a better approach would be to increase the amount of top loading, which
would also reduce the required inductance, and probably the loss
resistance
too. The fact that Rloss of 8.5ohms was achieved with only a few ground
rods shows that, for most amateur antennas with higher Rloss than this,
the
ground system is not a very critical factor, at least when the soil has
reasonably high conductivity.
So the results show that the open field antenna behaves quite closely to
the
text-book expectation, which if nothing else gives a degree of confidence
in
the calculation and measurement methods. The loss resistance has the
characteristic shown in some texts on LF/VLF antennas, where the
resistance
is a minimum at some frequency, and increases at higher frequencies due to
increased skin effect loss, and at lower frequencies due to increased
dielectric loss. The home QTH has increased losses and reduced radiation
resistance due to its environment. Unfortunately, most of us are stuck
with
this, unless operating /P. Clearly, in these kinds of circumstances, it is
not very meaningful to think of a LF/MF antenna just in terms of lengths
of
wire and a ground system, but the nature of the surroundings must be
considered too.
Cheers, Jim Moritz
73 de M0BMU
