Re: whip antenna coupling prediction

Larry,

We have carried out detailed studies (using MININEC which is a very good
model for a ship on the ocean) for standard marine whips on fibre-glass
ships and sailboats. It requires quite a detailed wire grid structure to
model metal ships (several thousand elements) and so that is a very
different problem, but it has been done by Stan Kubina, Concordia
University, by Ed Miller and others -- not only for machine capacity but
for the job of setting up the model.

For whips or wire antennas on ships we were concerned with coupling,
because of the problem of burning out the antenna tuner connected to one
antenna when transmitting on an adjacent antenna.

We assumed that both antennas were being used in a transceive
configuration, i.e. an antenna tuner was used whether the antenna was used
for receive or transmit. This requires quite a bit of iteration if the
antennas are rather closely coupled -- tuned to closely the same
frequencies. We put the source first on one antenna and determin the
antenna's impedance. We then tuned out the reactance. We put the source
on the other antenna and determine its impedance. We tuned out the
reactance. We keep repeating until both antennas were resonant each at its
used frequency.

We then put transmitter power into one antenna and determine the current
induced into the other tuned antenna (tuned albeit to a different
frequiency) -- but certainly a significant power flows in the so-called
receive antenna. You have to use current sources and determine induced
current in the receive antenna, to calculate voltage across the tuning
reactance in the receive antenna.

For antennas on sail-boats we were interested in the current induced into
the rigging. The so-call insulated back-stay antenna introduces so much
current into the rigging you might as well throw away the back-stay antenna
and shunt feed the rigging!!! While you may not realize the radiation
pattern you want at least this eliminates the problem of insulating the
back-stay -- and you may not realize the pattern you think you have if you
drive a back-stay antenna directly!!!

This in my view should be a problem of general interest, and so I am
posting my reply to all on the NEC-Users List.

Good luck with your modelling.

73, Jack, VE2CV

------------------reply separator----------------------------------------------

>3/6/96
>
>I would like to use NEC to determine the coupling between 2 30' whip
>antennas on a small ship. I'm planning to model the ship using a wire
>grid and then add some vertical segments for the whip antennas. I
>will drive one of them with a unit voltage source at the base. For the other
>antenna, I will insert an impedance (equal to the corresponding receiver's
>input impedance) at the base and note the current into this "load" (to
>determine received power).
>
>I understand the NEC wire grid modeling guidelines and am not looking for
>advice here. Rather, what I am questioning is whether the approach I have
>outlined will give me accurate coupling information. I have heard that NEC
>is not particularly good at estimating antenna input impedances and it
>seems like I am trying to do something similar (i.e. I'll have to quantify
>output power of the driven antenna as part of my coupling calculation which
>will use the same current that is used to estimate antenna input impedances
>and which has a tarnished reputation therein).
>
>Does anyone know of a published validation study for a problem similar to
>mine or have an alternate idea for computing coupling?
>
>Thanks for any and all help.
>
>Larry Gray (gray_at_engr05.comsys.rockwell.com)

John S. (Jack) Belrose, VE2CV
Director, Radio Sciences
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
Received on Wed Mar 06 1996 - 22:08:00 EST