Sat, 12 Aug 2000 16:05:30 -0400
James Moritz wrote:
> Dear LF Group,
> All the professional texts say that ground losses are the most
> significant factor that lead to reduced efficiency of antennas at LF
> and VLF, and grounding improvements have been the most
> effective way of increasing the efficiency of these antennas. But
> except for a lucky few, amateur antennas start off with increased
> sources of loss that are not possible to eliminate, such as the
> proximity to buildings and trees. Also, the height of amateur
> antennas is restricted for various reasons.
> Most amateur antennas are less than 20m high; this puts the scale
> of them more on a par with professional VLF antennas than LF
> antennas - for example a 100m high antenna at 17.2kHz has the
> same radiation resistance as a 12.6m high antenna at 136kHz. But
> I doubt if any such VLF antennas are supported by 150m trees, or
> surrounded by 100m high buildings! For these reasons it is not
> suprising that amateur LF antennas behave in a different way to
> commercial LF and VLF antennas, and so different rules can be
> expected to apply.
> It's worth getting things into perspective - for example:
> The Puckeridge Decca station had a 99m high mast, with 36 buried
> ground radials about 120m long (quite modest for a commercial LF
> antenna). This had a total loss resistance of about 3 ohms at
> 136kHz (not including the loading coil), plus about 0.6 ohms
> radiation resistance. Nominal capacitance was 3750pF.
> The SAQ antenna has an antenna current of 600A with 200kW
> input, implying a total loss resistance (including the loading coils) of
> about 0.5ohms, whilst with an effective height of roughly 100m, it's
> radiation resistance should be about 0.05ohms at 17.2kHz. Other
> VLF antennas have even lower loss, some less than 0.1ohm.
> Typically, capacitances are in the range 10's of nanofarads.
> The 'small' antenna that we put up at Puckeridge (in an open field)
> was typical of a modest amateur LF antenna. This was a 42m long,
> 9m high inverted L with a single top wire. The earth system was 4
> ground rods in a 2m radius around the antenna downlead. Loss
> resistance (not including the loading coil) was about 25ohms,
> radiation resistance about 0.02 ohms. Capacitance was about
> 280pF. When moved to my home QTH, using an identical ground
> system, capacitance was unchanged, but the loss resistance
> increased to 40ohms.
> As far as grounds go, my experience has been similar to others.
> increasing the extent of my ground system reduced the loss
> resistance of my main antenna from about 80ohms to about
> 50ohms, but further increases have had minimal effect. It is
> interesting to note that the 42m inverted L had a slightly lower
> resistance than the main antenna, in spite of having a much
> smaller ground system. This suggests that other factors are now
> dominant in the case of the main antenna. Indeed, the 42m wire
> was further from the trees and buildings, which might account for
> the difference.
> It is also worth noting that amateur LF antennas are much higher
> impedance devices than their commercial counterparts - their
> impedance is mostly a few kilohms of capacitive reactance, of the
> order of 10 times higher than the commercial antennas - which
> combined with lower heights means the voltage gradient they
> produce will be much greater for a given antenna current. With 10
> times the voltage on the top section of the antenna, and one tenth
> of the height, the electric field strength produced for the same input
> current would be 100 times as great. This suggests that dielectric
> losses would be much more significant in the case of amateur
> antennas than the professional ones.
> The grounds used by professional antennas are usually in the form
> of a continuous mat extending quite a way beyond the antenna.
> This means that almost all the electric field produced by the
> antenna will be screened by the ground mat, and so will not
> penetrate into the ground far. Most amateurs cannot reproduce
> this, and have a much more sparse system of ground radials and
> rods. This would enable the field produced by the antenna to
> penetrate much further into the ground; the text books say that the
> 'skin depth' at LF should be many metres. This could have 2 effects
> - one the one hand, desireable in that the height of the antenna
> may effectively be increased. On the other hand, undesireably,
> earth has rather high dielectric losses, so would be expected to
> increase the loss resistance of the antenna. Since amateur
> antennas are lower height, a greater proportion of the field will be
> in the ground, and therefore subject to losses of this type, so again
> these effects would be more significant with amateur antennas.
> The difficulty of trying to prove any of this, or trying to put numbers
> to the different losses, is in trying to isolate different effects. Short
> of running amok with a chain saw, it isn't practical to eliminate the
> losses due to trees and buildings. Changing the capacitance of the
> antenna means the loading coil must be altered, and the change of
> losses in the coil must be taken into account. Moving the antenna
> will change the distribution of ground currents, and so the ground
> resistance. It isn't easy to find an open field where people don't
> mind you burying miles of wire and so on. Likewise, since
> everybody's QTH is different, what works for one station may be
> no good for another. The best thing we can all do is to try as many
> experiments as possible, and hope that useful conclusions can be
> drawn from the collected results.
> Cheers, Jim Moritz
> 73 de M0BMU