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 situation
and this test provides very welcome information.
Paul-Henrik / OH1LSQ
Quoting James Moritz <[email protected]>:
> 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
>
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