Hi guys,
After Don Kirk's posting re the NAB99 CFA article earlier this month, I
decided to investigate this concept a little further. If it works, it has
obvious application for Topband, and I managed to persuade my boss that it
could have possibilities in military communications as well.
I therefore bought the series of articles from Antennex.
I decided to build one of these devices using the info given in the series,
but scaled it to 70 MHz so that tests could be done in a small lab. The
articles indicate that the principle should be independent of frequency. I
tuned my model up by feeding it with a signal from a generator, and with a
small piece of wire connected to a spectrum analyser as a field strength
meter. I was quite excited when the radiation from the CFA compared
favourably with a quarter wave piece of wire, and even more excited when I
swept it on a network analyser and obtained S11
<-25 dB.
I was a bit concerned when the radiation between two CFA's in the lab seemed
to be highly dependent on position, and on the presence of observers. I
therefore wound a few turns of the feed coax through a balun core to prevent
radiation from the coax, and then came the first disappointment. The signal
dropped about 15 dB! I then mounted a commercial vertical dipole on the
roof of the building and monitored the signal strengths from some of the
local community repeaters around 70 MHz. I removed the dipole and replaced
it with the CFA. Again, disappointingly, the signals from the repeaters
were 20-30 dB lower on the CFA.
So then I wondered where all the power was going. The match was so good,
and I had used low loss components in the phasing network. Only the balun
used to split the signal had any ferrite, and its Q was around 50. I then
measured the capacitance between the cylinder and the plate, removed both of
these, and replaced them with a capacitor of around the same value (about
2.7 pF). Sure enough, with very minor re-tweaking, I could repeat the
excellent match with no part of the antenna connected at all. The
arrangement of phasing components just happens to provide a good match to
the losses in the splitter balun.
My conclusions-
1) I suspect that those amateurs who have had apparent success with the CFA
would not be able to repeat their results if they took care to prevent
radiation from the coax feed.
2) The good match they have observed results from the good match to the
loss resistances.
3) I am not yet ready to spend a lot of time and effort in constructing a
3-4 m tall version of this to try on Topband!
One question still remains. What is wrong with the theory that the Poynting
vector can be synthesised from independent E- and H-fields?
Replies to the latter direct please, and if appropriate, I will summarise
for the Reflector.
73, Greg, ZS5K.
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