Re: NEC-LIST: CFA

-------------Forwarded Message-----------------

From: Brian G Stewart, INTERNET:b.stewart_at_gcal.ac.uk
To: Grant Bingeman, DrBingo
        
CC: Jack Stone, INTERNET:jack_at_antennex.com
        Ted Hart, INTERNET:W5QJR_at_hom.com
        
Date: 2/12/99 9:08 AM

RE: Re: CFA

Dear Grant

Thank you for your e-mail. I noticed your paper in the same session as
ours at the NAB conference so it will be nice to meet you in Las
Vegas. I look forward to hearing your paper and am sure it will be a
stimulating and helpful presentation.

Thank you for your comments regarding the CFA. Unfortunately the CFA
has not had a good press - even at its inception. Looking back we
could have tackled the presentation of it a little better - but
experience and hindsight are wonderful things. People just couldn't
get it to work - our fault I think - we should have been more open at
the very beginning (see later) regarding what the phasing and
electrical field requirements were.

Just to keep you in the picture - I have recently returned to academia
and also to CFA work, about a year ago, after a spell as a Christian
Pastor. Now that I'm involved in it again, over the past year I have
started to push forward, along with Kabbary in Egypt, CFA work and
attempt to get it back into experimental and theoretical dissussion on
a wider basis. Ted Hart has been a tremendous encouragement in the
US. He believed in the basic concept, but wished to try it out
experimentally as all radio engineers would. To our delight he has had
many CFAs operating with good performance evaluation of bandwidth and
signal strength. The phasing circuit in AntenneX is a "Ted
modification" of one which has been in CFAs in the past.

I'm afraid Kabbary has always been hesitant to release anything
related to CFA work for fear of not being rewarded or for people
stealing insights. Things have started to change in this respect since
I've returned, and now with the NAB conference on the horizon we
intend to share more about CFA operation. Hopefully, at the NAB
conference, information will be given to anyone who wishes to build a
MW low power (<100W) CFA for the purpose of testing, fiddling,
experimenting etc.

2I'm sure there is more for everyone to learn about CFAs - including
ourselves. However, let me outline a few concepts of CFAs in relation
to "conventional" theory.

The D plate of the CFA is a parallel capacitor which with an ac signal
creates a surrounding H field. This is well known and is even used to
prove continuity of the fourth Maxwell equation i.e. curl H = J +
D'. Feynam's lectures on physics even evaluates how to calculate the H
field surrounding a capacitor. If anyone is not convinced, they can
simply measure the existence of the H field around a capacitor and
prove that it is 90 degrees phase advanced from the applied voltage
and also the E field lines between the plates.

The E plate produces E-lines since it is also a capacitor. This there
can be no question of this either.

If both the D and E plate structures were fed together with voltage
signals which were in phase, then, I'm afraid we would have E and H
fields which are 90 degrees out of phase close to the D and E
plates. This is similar in some ways to a conventional antenna. There
should be little radiation and the behaviour would mimic an extremely
short antenna. Radiation would be produced from conventional theory in
the very far field and have little power. Indeed some publications
have presented this feature - ignoring the phase details of the
feeders and also the phase properties of the field distributions
surrounding the antenna which actually produce the radiation.

Now phase shift the D plate voltage by 90 degrees. Now we have E and H
in time phase close to the structure. However, a point of note which
has unfortunately not been communicated by us very well in the
past. If the phasing is 90 degrees one way, the Poynting vector S = E
x H is inward. No power radiation is possible, and the antenna input
has a high impedance. I believe many people have dismissed the CFA
because they have attained the 90 degrees but no or extremely small
levels of radiation. When the correct 90 degrees is achieved,
i.e. phase shifted the other way, S = E x H is outward and power
radiation occurs.

This is the ultimate test of the CFA. Where does the power radiation
start. If it is conventional theory then close to the antenna will be
inductive reactive field, which has a well established relationship
with distance from the antenna. If it is power radiation and most of
the fields participate in this radiation, then the distribution with
distance will be different showing the CFA is a different type of
antenna system. In addition, because the fields close to the CFA are
"strong" in comparison to fields in the far field which produce
radiation in conventional theory, then we may expect strong radiation.

I have not suggested anything which is unusual or breaks any
electromagnetic laws. Simply, E and H in time phase produce radiated
power - the Poynting theorem. The D and E structures produce E and H
fields, easily experimentally proved, and these are simply made to be
in time phase.

A further proof is also the voltage levels on the antenna
structures. The CFA appears to have about 1/6 the voltage level of
conventional antennas. In conventional theory this should result in
much less radiated power than is measured, even for the "worst" CFAs!!

In all of this, one must also get the ratio of E/H to match space
impedance for maximum power transfer, so you can appreciate that 90
phase and little power is actually a space impedance matching
problem. The voltages on the E and D plates must be altered to change
the field strengths for maximum radiation. This has been how Kabbary
has maximised radiation on the Egyptian CFAs.

Now, in terms of conventional theory, well the CFA is fundamentally
different and standard antenna formulae and techniques cannot be
invoked when E and H exist close to the structure in time
phase. Conventional theory cannot accomodate this fact and thus should
not be applied. I suppose it is obvious from another
viewpoint. Conventional monopoles have one signal feed to the antenna
structure and an earth. The GP CFA has two signal feeds to two
structures and an earth. There is no way conventional theory of
monopole structures can be transferred directly without alteration to
another type of antenna.

A further thought relates to phasing circuits. One remarkable feature
is that the phasing circuit to provide 90 phase shift appears to
compensate frequency movements when radiating. The E and D plates go
"off frequency" together as they are both capacitive thus introducing
"wider" band radiation as E and H are still then in time phase.

In regard to the FCC 73.186 et al. Let me mention a few details. The
purpose of the NAB paper is to present broadcasting CFAs to the
US. For that reason, and because no-one has actually built a broadcast
CFA in the US, we have not been motivated to satisfy every demand of
the FCC yet. Give us time! However, in our paper relative normalised
radiation field strength patterns of a broadcast CFA (at reduced
power) taken at about 600m (not the 1km for the FCC 73.186) are
presented. We have the measurement values and will bring these to the
NAB conference. We also hope that prior to NAB, some more information
regarding FCC criteria will be available as further measurements are
taken in this respect.

One need only take a field strength meter and measure the shear signal
strength levels of a CFA. Having done this for the CFAs in Egypt,
there is no comparison. The levels are so strong.

In this respect, last year I asked the BBC to make reception checks on
a 30kW 1161kHz CFA in Cyprus - a distance of about 500km from the
CFA. The BBC reported fair-strong signal strengths during daytime and
evening with no fading characteristics. No other distant broadcast
programmes using even higher power levels compare. This is reported in
the NAB conference.

Grant, thank you for lending me your ear. I hope some of the above
will at least get you to think through some of the issues relating to
CFA work. I'm sure you may even bring some additional insights which
would be helpful for us to hear and to discuss.

I realise the CFA will still face opposition and still come in for
criticism, some perhaps justified, alot of it not. However, what
concerns me is that those who have been most vocal in dismissing it
have never actually built one and properly phased it! If I truly felt
that they didn't work I'd pack it in - since I'd be wasting my time.

In relation to NAB papers I'd be glad to exchange papers. What format
would you like it to be in? Let me know and I'll e-mail it off to you
next week. I'm able to take Word 6/Win 96, or rtf or any other (I
think!)

Kind regards

Brian

Dr Brian G Stewart
Dept of Engineering
Glasgow Caledonian University
City Campus
Cowcaddens Road
Glasgow G4 0BA
Received on Mon Feb 15 1999 - 19:33:01 EST