RE: NEC-LIST: CFA modeling results - 2

>Hi,
>
> I have a question about CFA modeling.
>
> The phase relation between the disk and the cylinder is clear, but
>how about the amplitude? I read the NAB99CFA paper (
>http://www.antennex.com/preview/cfa/nab99cfa.htm ), it seems that CFA
>not only require E and H in-phase but also |E|/|H|=377ohm. So it seems
>that the amplitude of the feeding to the cylinder and disk may not be
>arbitrary.
>
> Another page( http://www.antennex.com/preview/Folder02/cfa_5.htm )
>gives an instruction about the feeding network. How to model it with
>NEC?
>
> I do not know whether my understanding is correct.
>
>Yaxun

Yaxun,

For the CFA (according to NEC-4D) the near field ratio |E|/|H| cannot
be made equal to 377 ohm no matter what source currents or voltages
you apply.

I have used voltage sources and current sources, and keeping the phase
difference equal to 90-degrees, I have varied the ratio of
Icylinder/Idisc and Vcylinder/Vdisc over the ratio one to ten (or ten
to one).

This produced a set of data which I have yet to summarize in a compact
way.

All I can say at present is that the port impedances change very
significantly as one changes the current or voltage ratio. But the
CFA is a poor radiator however you try to feed it!

To realize performance claimed one has to have the magic hand of the
inventors.

Regards, Jack

Post Script

In the table below I show computed values(according to NEC-4D) of
impedance, and field strength at 200 m (for a transmitter power of
1000 watts, antenna resonated, coil Q = 300), for the MF CFA over a
PEC ground fed by current sources (phase difference 90 degrees), for
various Current Ratios Icylinder/Idisc.

___________________________________________________________________________
Current Zcylinder Zdisc FS(200m)*
Ratio V/m
_____________________________________________________________________________

0.1 1869 - j 715 -17.9 - j 466.6 0.1132

0.2 934.3 - j 715 -35.8 - j 466.7 0.1088

0.4 467.2 - j 715.2 -71.6 - j 466.7 0.1169

0.6 311.5 - j 715.2 -107.4 - j 466.7 0.1305

0.8 233.7 - j 715.2 -143.2 - j 466.7 0.144

1.0 187 - j 715.3 -179 - j 466.7 0.1561

1.25 149.6 - j 715.3 -223.8 - j 466.7 0.1693

1.667 112.3 - j 715.3 -298.3 - j 466.7 0.187

2.5 74.9 - j 715.3 -447.7 - j 466.7 0.213

5 37.5 - j 715.3 -895.4 - j 466.7 0.2523

10 18.8 - j 715.3 -1791 - j 467.1 0.2817
______________________________________________________________________________

* Recall, previously calculated, that the FS(200m) for a 75 m monopole
over a PEC ground is 1.3555 V/m.

Notice that the reactive component of the port impedances is
independent of the Current Ratio, and that the cylinder is the better
radiator. Resonating the port impedances should be straight forward,
inductive reactances equal to +j715.2 and +j466.7 ohms in series
with the current sources feeding the cylinder and the disc resonates
the antenna.

If one employs voltage rather than current sources the impedance
characteristics of the antenna appear at first sight to be very very
different. The first point to note is that I have to reverse the
phase relationships, cylinder +45 degrees/disc -45 degrees (we now
have the phase relationship shown in the ICAP'91 paper). A second
point is that the apparent port impedances depends not only on the
Voltage Ratio, but on the value of the inductances inserted in series
with the sources (a strong dependance), the purpose of which is to
resonate the port impedances.

As an example for a Voltage Ratio = 1, if one tries to resonate the
port impedances by "change-and-try", the apparent resistive component
can have any value from 200-100 ohms (one value negative), to a few
ohms (both values positive), but it is not possible to exactly
resonate the antenna. However, if one inserts reactances having the
same values as computed above (for current sources), the port
impedances for voltage sources are resonant, and the resistive
component and field strengths are almost identical with the values
computed for current sources.

It appears therefore that one can fool oneself (using voltage sources)
into thinking that you can adjust the port impedances to a value close
to 50-ohms (for example), but the port impedances are not resonant.
Kabbary et.al. (NAB'99 paper) state "a facinating feature of CFAs is
that the input impedance can always be adjusted to match any desired
impedance at the broadcast frequency." In principle my modelling
tells me that by luck-and-chance this might be possible, since for
certain values of series inductances it might be possible to find port
impedances where the resistive components are nearly equal and equal
to 50-ohms (for example), and the reactive components are also nearly
equal but of opposite sign. So when driven by a single source
(transmitter) the antenna system is resonant.

But all this is just a question of matching. It appears to me that it
would be better to employ CURRENT SOURCES (current baluns), since the
resistive components do not change as the reactances to resonant are
adjusted.

Recall (practically), a current balun forces equal currents into
unequal impedances (the two arms of a dipole or a loop, or with
centre-tap to ground, the two ports of the CFA). A voltage balun
applies equal voltages to unequal impedances.

John S. Belrose
16 July 1999

_____________________________________________
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 Mon Jul 19 1999 - 02:40:42 EDT