Dear LF Group,

Thanks for the comments...

K2ORS wrote:

> Bob has an open field with no vegatation. Recently Bob has made field
> strength measurements at 500 and confirmed that the calculations are
indeed
> very close.

I have just re-calculated the ERP based on values of Rrad obtained using a
NEC-based antenna simulator (MMANA-GAL). This should give a more accurate
value of Rrad than the simple formulas based on capacitance, because a more
complicated geometry can be modelled, and the interaction of the fields of
the horizontal and vertical parts of the antenna will be accounted for. This
resulted in slightly higher values of Rrad, and slightly higher expected
ERP. This gave even better agreement between calculated and measured ERP at
the open-field site - at 136k the measured ERP is now +0.3dB on the
calculate value, and at 503k it is -0.1dB. This is probably well within the
likely measurement uncertainties.

G3NYK wrote:

> I suppose you could see the Rrad reduction due to nearby structures and
the
> environment as being due to some of the feed current being diverted by
> capacitive coupling into the lossy surroundings and so not traversing all
of
> the distributed Rrad.

I think this ties up with the "tree current" experiment I did some months
back - it was clear then that, especially on 136k, a substantial proportion
of the antenna current was returning to ground through the trees near the
antenna. In the far field, the total radiation would be a function of the
superposition of the currents flowing "up" the antenna and "down" the
trees - the bigger the trees, and the more current flowing in them, the more
they would tend to cancel the radiation from the antenna. The measurements
of the "tree current" were very rough, but at 500kHz it appeared that the
currents were a smaller fraction of the antenna current, which would tie up
with the smaller observed reduction in Rrad at 500kHz.

G3YMC wrote:

> What would be interesting is if you could quantify any differences in
> the type of ground at the two QTHs. The much lower earth loss at the
> open site may well be due to totally different soil characteristics
> rather than solely due to the surrounding trees.

The ground was probably as similar as it is possible to be at two separate
locations - the open field site was only about 4km from my home QTH, and
geographically part of the same landscape. The soil in both places is a
heavy clay that is waterlogged at this time of year. If anything, the ground
was wetter at the /P location, because my QTH is on a slope and relatively
well drained, but I generally find that wetter conditions lead to higher
loss resistance. It would certainly be interesting to see how the type of
ground afffects loss resistance, but it would probably be quite hard to find
a suitable selection of sites.

G3LDO wrote:

> At one time there were a row of conifer trees along the bottom of my
> garden, which caused a reduction of antenna current when they were wet.

At my QTH, over the period I have been making antenna measurements, the
antenna has stayed basically the same, but the surrounding trees have grown
quite a bit larger. During this time, the apparent Rrad has decreased to
about half of its initial value, and Rloss has increased by around 50%.

Cheers, Jim Moritz
73 de M0BMU