Re: NEC-LIST: NEC2 versus NEC4

> Hello John,
>
> You have mentioned about the modeling of the FAN antenna using NEC2 or
> Mininec. As we are trying to model the FAN antenna to find out the HERP and
> HERO at the low HF range, at 2-7MHz, we are very interested to know more on
> your approach to the modeling.
>
> We have attached a simple figure of the Fan Antenna. As you know, the Fan
> Antenna is make up of wires and it is fed with a matching unit. How you
> model the matching unit? As Fan Antenna has multiple slanted vertical wires,
> how you connect the horizontal wires with all the vertical wires?
>
> Does the antenna mask have the impact on the near field strength?
>
> We have obtain some results. The near field strength just below the Fan
> Antenna wires is very strong. Do you have similar results as us?
>
> Our Fan Antenna suppliers inform us that, generally, we could not obtain the
> Fan Antenna input impedance by modeling. The Fan Antenna has to be tuned at
> site. It is true?
>
> Thank you.
>
> Best Regards,
>
> Chong, KwanMeng

We have numerically modelled 3-, 5-, 7-,12- and 54-wire fan antennas,
and compared with impedance measured (a MF antenna scaled to VHF/UHF
frequencies). Unfortunately we found out that the support structure
(although teflon) influenced the response at frequencies above the
self resonant frequency, and we did not redo the experimental models
for the complete set of frequencies. The measurements were made on an
elevated GP 30m diameter, and so the GP has an influence on some
frequencies (reflections from the edges of the screen being in phase
with the source (impedance analyzer) on the antenna).

The agreement for frequencies beyond the first resonance deteriorates
as the number of wires in the fan increases.

Three wire fans (in particular) give you a very unexpected impedance
vs frequency response --- very very sharp resonances --- and this is
real since I find very good agreement between numerical simulation and
experimental measurement [1].

The problem with fan antennas with closed ends is that we have
monopole type and transmission line like currents on the wires
comprising fan. Transmission line currents are currents circulating
around the triangles formed by closed end fans. These problems
disappear if the ends of the fan wires are open.

I see you have two 3-wire fan monopoles. What are you trying to
simulate?

I have spent a great deal of time modelling closely spaced ship born
antennas (fibre glass ships so I was NOT concerned in this study with
the effects of the metal ship) --- considering the case of using the
antennas on different frequencies. I resonated each antenna by trial
and error, putting the source on one and then the source on the other
--- back and forth until both antennas were resonant in the presence
of the other, at their respective frequencies. This assumes that I am
not transmitting on both antennas at the same time. I can then put
the source on one antenna and deduce the current induced on the other.
Since both antennas have base loads (resonated), I can deduce the
currents induced in the "receive" antenna --- and find out that it no
wonder that the antenna system tuning network for the "receive"
antenna gets fried.

This work was never published, since the results are very very model
sensitive, and difficult to present in a compact way.

Regards, Jack

1) Belrose, J.S., "Revisiting Wire Aerials at the Turn of the
Century", Proceedings of the Fourth Symposium on Antennas and EM
Theory, Xi'an, China, August 1997, pp. 73-78.

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
Communications Research Centre
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2779
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Thu Mar 02 2000 - 13:21:19 EST