Re: NEC-LIST: cross field antennas

Dear readers,

I have constructed a small CFA in order to determine validity of
claims at the time. My model was excited in the 160m band
(1.825MHz). By rather rough observations (little test equipment) I
determined that is was 26-30dB down compared with a half-wave dipole
that was mounted at a slightly higher height. This rough experimental
result bares some correlation with the gain expected from just the
length of E plates (fat cylinders) - which was arrived at by NEC2
simulations.

This model was constructed after I had the occasion to witness the
operation of another amateur's model approximately the same size as
mine but operating on 3.5MHz. The other amateur claims that it is
reliable and consistantly works within an S-point or so of his
horizontal full-wave loop mounted slightly higher. He also
transmitted A/B comparisons to me between the CFA and a full-wave
loop. He lives about 26km away over the otherside of a hill from
me. The CFA seemed to work but who knows where the nulls in his
horizontal loop pointed. I can't recall the signal differences but I
heard him (it was certainly less) but it was enough to stir interest
though and a quick visit (with a witness) was arranged.

He used a 4 port LC quadrature phasing network (with the 4th port
terminated into 50 ohms) that was suggested in the ARRL Handbook. I
used substantial lengths of coax to obtain the quadrature phasing -
rather than using an adjustable reactive network. I can't believe that
the other amateur's fixed network will realise a match. I was about to
draw the conclusion that I had the wrong phase relationship between
the E and D plates and that perhaps excessive coaxial losses was the
reason that "mine" didn't work. However, I did some NEC simulation for
the E-plates alone. After some time my wife determined that the
"garbage-can" antenna was an unsuitable adornment on the top of the
garage, and my interest was lost so it was decommissioned.

I too am skeptical but I want to keep an open mind.

Hately does mention that the phasing is very critical at the lower
frequencies and I feel I should have persisted a bit longer and
constructed proper adjustable phasing networks so I could completely
dismiss the situation. Those that have done this claim that the
feepoint becomes resistive (but I don't know whether this accounts for
loss in their matching circuits or radiation).

My dilema is exacerbated because:

1. I have not seen an independant paper or satisfactory attempt at
simulating/debunking the claimants effect (null result included), and

2. I haven't seen (first-hand) or experienced a conclusive pratical
application of the effect (null result included).

If an untainted simulation/model can be performed/constructed along
with results for the near-field showing generation, cancellations,
power storage etc and far-field results - then perhaps it can be laid
to rest.

I am yet to pay for open access to the site below - but they have run
articles on the CFA - along with construction details. Some
article/parts are on their free pages.

The url is http://www.antennex.com/index.htm I have also been informed
that they are considering producing kits under licence.

Cautions: On HF you can operate on wet piece of string and the guy on
the other end says you have a pretty reasonable signal. Most amateur
operators (me included) don't have HP analysers etc at their disposal
and are at the mercy of the Radio equipment manufacturer's scant
calibration of the S-meter. Also substantial radiation can be had from
mismatched feeders, powerlines and other inopertune (often covert)
sources. Unless a carefully controlled setup is employed constructors
and inventors may be leading themselves up the proverbial garden path.

Regards,

Ralph Holland, VK1BRH
http://www2.dynamite.com.au/vk1brh
Received on Wed Feb 10 1999 - 12:13:42 EST