I would say this assumption is basically correct. The most sensitive
area of the cable is that adjacent and parallel to the antenna. In the
case of a bicon this is about the top 1m or so. Ferrite on the top
horizontal section would only be of benefit if the antenna is not
perfect, so a couple close to the antenna might be well be in order,
assuming that everything is not perfect.
For a tuned dipole at 30MHz you would need just over 2.5m of choked
feed. The ferrites need to be spaced so that they break up the induced
currents at the highest frequency of concern. Practically this is
around 200MHz as the directivity of a log minimizes this effect. So
ferrites every 150mm for the top 1m or will probably suffice for a
bicon and log set up in my opinion. Since the effect is present for
both transmit and receive I would leave the ferrites installed for
measurements also.
A full length feed with ferrite cores can get to be quite heavy too!
I used the example of a 30MHz dipole with various feed locations in a
recent book [EMI/EMC Computational Modeling Handbook]. If I remember
the details correctly, for the geometry chosen a 2dB variation was
demonstrated moving the location of the feed drop, ferrites should
eliminate both the error and uncertainty due to feed line movement.
Colin..
Compaq
Maynard, MA.
-----Original Message-----
From: Flinders, Randall
[mailto:[email protected]]
Sent: Friday, November 13, 1998 12:29 PM
To: 'EMC PSTC mail list'; Colin Brench
Subject: RE: ANSI C63.4
When attaching the Ferrites, do they need only be
attached to the Vertical portion of the cable? Assuming the common mode
currents force the cable to become an active element in the "system",
then it would stand to reason that the portion of the cable running
across the Mast mounting bar (perpendicular to the antenna elements) and
the portion running along the ground plane would not contribute
significant effects on the NSA results.
Is this assumption correct? Also, what spacing of the
ferrites has been found to be required in order to eliminate this
effect? I am also assuming that Ferrites are then left on during all
product testing as well.
Regards,
Randy Flinders
EMC Engineer
Emulex Network Systems
(714) 513-8012
[email protected]
----------
From: Colin Brench
Sent: Friday, November 13, 1998 6:04 AM
To: 'EMC PSTC mail list'
Subject: RE: ANSI C63.4
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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