ve2pid wrote in news:e7536acc-6fec-4836-8aaf-
:

I have a question about OCF dipoles; it refers to the choice of the
type of balun transformer (voltage or current) to be used with that
type of antenna:

Since an OCF has by nature an asymmetrical distribution of currents in
its two legs, the use of a current balun seems a kind of 'brute
force' imposing a symmetrization of the currents in the legs.

For the transformation of impedances, would it be more appropriate to
use a voltage balun instead?

Pierre,

An interesting question, especially in the light of some versions of OCF
dipole that are attributed magic properties.

If you were to feed a half wave dipole in free space at the centre
directly using a minute transmitter, the currents in the legs must be
equal an opposite by Kirchoff's Current Law (KCL)... there are no other
connections to the nodes, the tx is small wrt wavelength (deals with the
standing wave issue), easy. It is an ideal antenna, not necessarily
practical, but current distribution is ideal.

Now, if the development of the off resonance OCF dipole logic is to find
a feedpoint with a more suitable Z, then feeding it with the same
physically small tx would give the same 'ideal' performance. No matter
how you achieve it, the objective is equal but opposite currents each
side of the feedpoint.

However, as you know, using a remote tx with feedline doesn't compare
with the above ideal case. It is very challenging to reduce common mode
feedline current to the point that it is insignificant on all
frequencies, even with one or more current baluns.

So, if you want to market such an antenna, how do you deal with the nay-
sayers?

You get creative. Don't hide the disadvantage, call the effect out as a
positive feature. Yes, feedline radiation (in the right place) is one of
the advantages! There, that has neutralised the nay-sayers, and
'enlightened' buyers will go beyond conventional thinking and embrace
this new concept.

Now, how do you maximise this fill-in vertical radiation? Use a voltage
balun and explain it to people using DC circuit theory that the voltage
balun (extremely low common mode impedance) will not impede common mode
current flow at this all important circuit node.

Ah, but now you will have the dreaded 'RF-in-the-shack syndrome'. Ok,
lets fix that with a magic component, an 'isolator' that is explained
again using DC circuit theory as a switch, that blocks the common mode
current at the bottom of the designate vertical radiator. Of course, this
explanation is not consistent with KCL.

Brilliant! No wonder the Carolina Windom is so popular. With the trend to
'rebalance' the amateur population from a bunch of self taught OFs with
knowlege related to experience and age with infusion of a growing
proportion of six hour hams, the Carolina Windom's future is assured. A
solution to the ham's quest for the Holy Grail, a low cost antenna for
all HF bands with most importantly good VSWR, no need for a lossy tuner.

It was explained to me by many people as we created our structure for six
hour hams that modern hams have no need for circuit theory. I counter
with the example that a common component of HF stations is a simple
device with three passive components, two capacitors and an inductor, and
it is capable of consuming the majority of a tranmitter's output power
when used without appropriate knowledge, and it is not well understood.
Yes, it is the humble T-Match ATU. If you are interested, have a look at
my article "Is a 4:1 balun a good choice for use with an ATU on HF?" at
http://vk1od.net/blog/?p=987 . There are a bunch of recent articles in
the blog on ATU and feedline matters that might prompt thinking about
traditional ATU designs and advice on feedlines and balun types and
ratios.

I am unusally sarcastic this morning, but the above is not really written
in humour, and not really a lament.

A current balun at the feedpoint will not of itself reliably eliminate
common mode current on the OCF feedline, nor will an 'isolator' of itself
at another point reliably eliminate common mode current in the shack.

The same can be said for the same components in a symmetric centre fed
dipole system, just that the common mode current is (much?) lower by
virtue of antenna symmetry.

Owen