Jim
Thanks for the very informative investigation.
So...if you make accurate FS measurements and accurately characterise your
antenna system (and
document it for the authorities!) can one can just add more coal to the fire to
achieve the
allowable ERP? Or do the NOVs (for 500 kHz) or your regular license (for 137
kHz) specify a maximum
transmitter power as well?
Jay W1VD WD2XNS WE2XGR/2
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Sunday, December 09, 2007 8:14 AM
Subject: LF: Antenna tests on 136k and 503k - Results
> 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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