[Lf] comments Big and Small Antennas at Puckeridge]

[Andre' Kesteloot]

vernall wrote:

> To all reflector readers:
>
> Thanks to James M0BMU for carrying out the practical experiments at
> Puckeridge and writing a report on the results.  Well done.
>
> Consideration of why the "small" antenna was better by some 4 dB (using
> the normalised information) is a challenge.  It may be speculation, but
> a suggestion is that the effective height of the "small" antenna may
> have been more than the physical height.  Noting from the report:
>
> > ....  a small, amateur-style inverted 'L' antenna.
> >
> > The vertical part of the L was 9m high and about 130m to the south of
> > the main mast. The single 24/0.2 top wire (about 1mm diameter) ran
> > directly away from the main mast, and was 42m long. The whole of the
> > small antenna was off the main ground system, which extended over a
> > circular area for about 120m from the base of the mast. The small
> > antenna ground system had 4 ground rods, 1m long, spaced a couple of
> > meters around the antenna tuner. The fencing wires that ran under the
> > antenna, about 50m long, were also bonded to the small antenna ground,
> > and were not connected to any other fences.
>
> The ground system of the "big" antenna would have many long radials and
> establishes "reference RF ground" for the wider environment.  If I read
> the report correctly, the "ground" from the small antenna was separate,
> and at LF the skin depth in soil can be many metres, so there exists a
> possibility that the small antenna system has a greater effective height
> (relative to the big antenna ground) than physical height (9 m).
> Radiation resistance is a function of effective height squared, so only
> a few "extra metres" would be worth a few dB.  As the radiation
> resistance figure used in the experiment was estimated on physical
> height, then it could have a lower value than actual and possibly
> explain some of the observed discrepancy?
>
> Testing the small antenna in a "backyard" may have some reverse issues
> for effective height, where houses, sheds and trees raise the height of
> the ground plane.  Testing on a genuinely clear site would give less
> uncertainty to an experiment.
>
> 73, Bob ZL2CA