[Lf] LF antenna losses]

[Andre' Kesteloot]

Peter Dodd wrote:

> I found the recent discussion on LF antennas, coils and losses most
> interesting. My contribution is as  follows:
>
> Early this year I cut back the large conifer hedge at the bottom of
> the garden to tidy it all up and found that I had a bonus of around
> 0.8amps antenna current.
> When I returned from holiday I found the garden had gone wild with
> masses of tree and bush growth. Also the maximum current I could get
> into the antenna system was back down to 2.5 amps.
> Reducing most of this foilage in not practical until later in the year.
>
> I have now built a new loading coil with some of the Litz wire
> obtained from Crawley. The original one used 1mm wire with plastic
> insulation, which has a DC resistance of 3.8 ohms. The new coil is
> wound with 1.6mm Litz with a plastic insulation giving it a diameter
> of 2.8mm. The coil is close wound on a 360mm former made from six
> 30mm plastic pipes fixed in a circular manner. The new coil has a DC
> resistance of 1.9 ohms. Both coils have an inductance of 2.2mH. The
> increase in antenna current, 2.6amps, was disapointing. However this
> current could be achieved at a much lower power of 250W rather that
> 340W and increasing the power does not increase the current and some
> other factor appears to be limiting the antenna current.
> I use a multi-tapped transformer to match the antenna to the PA and
> there is no sign of overheating anywhere.
>
> The simple model of the ground loss and radiation resistance in
> series does not explain the inconsistencies that we have had reported
> on the LF reflector. Most articles on LF antenna systems, such as
> 'VLF, LF and MF Antennas' by John Belrose in the Handbook of Antenna
> Design consider (naturally) only well engineered commercial sites
> rather than suburban gardens!
>
> Of all my antenna books the Admiralty Handbook of Wireless Telegraphy
> (my copy is 1925) is the only one that discusses losses on LF
> antennas. At the time LF was the main communication band and the
> antenna size was size was restricted.
> In this publication the following losses are identified in addition
> to  ground connection and antenna/loading coil conductor losses:
>
> Loss in inferior dielectrics - due to hysteresis phenomina in
> inferior dielectrics such as wood, concrete, masonry and trees near
> the antenna.
>
> Brushing losses - bluish discharge from sharp points and angles
>
> Loss by leakage over insulators - dirt and salt build up on insulators.
>
>  Loss by eddy currents in neighbouring conductors caused by
> transformer action. Antenna supports should ideally be insulated from
> ground or well grounded. Supports connected to earth via a high
> resistance path should be avoided.
>
>
> Regards, Peter, G3LDO
>
> <g3ldo at zetnet.co.uk>