Concerning the discussions on the use of ferrite loaded feed cables
during NSA (or any other measurements). There have been a number of
useful replies already but I would like to add a bit more information
for your consideration.
Using a feed cable which is co-polarized with the antenna will result in
that cable becoming a second element in an antenna array due to the
currents induced on to it. While the position of the feed is fixed at
the top and sometimes the bottom also, it is free to move around
somewhat which can effect the degree of coupling especially where the
movement is a significant part of a wavelength. The ferrites will
greatly minimize this induced current and so practically eliminate this
effect.
As far as having perfectly balanced antennas, this is possible in free
space, but if you use any antenna vertically polarized and close to the
ground plane, the loading of the ground plane does imbalance the
antenna. This is most noticed at lower frequencies where the height
above the ground plane is minimal. Horizontally the balance should be
preserved, but, the source impedance of the antenna will be altered due
to the loading effect of the ground plane, resulting is a change in
match (or rather degree of mismatch :-)).
No one would think of using a metal antenna mast for vertical
measurements, but few worry about the feed cable. These effects are
easier to see and understand in terms of the transmit antenna, but they
apply equally to the receive antenna.
There have been a number of papers presented at the IEEE EMC symposia
over the years, from about 1990 on if I remember correctly. One this
year looked at the errors in using the free space (real) antenna factor
with for a biconnical being used for anechoic chamber NSA measurements -
unfortunately I don't have the reference to hand.
Regards,
Colin Brench.
Compaq
Maynard, MA
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