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Date: Sun, 09 Dec 2007 19:08:16 +0200
From: Paul-Henrik <phl@netikka.fi>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Antenna tests on 136k and 503k - Results
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Very interesting and enlightening reading Jim!

Thank you for putting all the effort into this test and sharing your results
with us. There is no "schoolbook" for the average city/suburban lot situatio=
n
and this test provides very welcome information.

Paul-Henrik / OH1LSQ


Quoting James Moritz <james.moritz@btopenworld.com>:

> 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 expecte=
d
> 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=20=
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 withi=
n
> 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. Actua=
l
> 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 =3D 7.9m. Both antennas used grou=
nd
> 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, whi=
ch
> 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 oh=
ms
> 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 avera=
ge
> measured ERP and calculated ERP are calculated below, along with the
> efficiency calculated as (radiated power)/(power to antenna). The calculat=
ed
> ERP assumes that the antenna has 2.62dB directive gain over a dipole.
>
> Home QTH, 503.8k: Iant =3D 400mA, Calculated ERP =3D  88mW, Measured ERP=20=
=3D 43mW,
> difference -3.1dB, Efficiency =3D 0.58%
>
> Open field, 503.8k: Iant =3D 380mA, Calculated ERP =3D 74mW, Measured ERP=20=
=3D
> 82mW,
> difference +0.5dB, Efficiency =3D3.7%
>
> Home QTH, 136.0k: Iant =3D 3.9A, Calculated ERP =3D 0.62W, Measured ERP=20=
=3D 0.18W,
> difference - 5.4dB, Efficiency =3D 0.012%
>
> Open field, 136.0k: Iant =3D 3A, Calculated ERP =3D 0.34W, Measured ERP=20=
=3D 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 t=
he
> 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 th=
e
> 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 hig=
h
> loss resistance of the antenna. But for the open field antenna, even a coi=
l
> with a Q of 1000 would dissipate about 1/3 of the TX power, so a much bett=
er
> 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 i=
n,
> a better approach would be to increase the amount of top loading, which
> would also reduce the required inductance, and probably the loss resistanc=
e
> 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, th=
e
> 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 t=
he
> text-book expectation, which if nothing else gives a degree of confidence=20=
in
> the calculation and measurement methods. The loss resistance has the
> characteristic shown in some texts on LF/VLF antennas, where the resistanc=
e
> 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 wit=
h
> 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 o=
f
> wire and a ground system, but the nature of the surroundings must be
> considered too.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
>
>
>
>