K8RI wrote:
>a truly balanced, center fed dipole on 75
>would be rare.
Truly balanced antennas are beyond "rare". In the real world, they don't
exist at all.
There is a strong ham myth that center-fed dipole antennas in some way
"want" to be balanced; but that is almost completely untrue. Very much
like a pencil balanced on its point, a balanced antenna is a highly
UNnatural state - deceptively easy to imagine, but never seen in
reality.
So please let's stop imagining. All real-world antennas will have some
degree of unbalance caused by asymmetry in either the antenna or its
environment; or both. It follows that there will also be *some* level of
common-mode current on the feedline; it won't ever be zero. The
challenge for hams is to stop imagining an idealized state of balance
that cannot exist, and to engage with the real-world engineering
questions:
1. How much common-mode current is flowing? Is it enough to cause RFI
problems, either to my own receiver or to others?
2. How strongly unbalanced is this antenna? In other words, how hard
will we need to work to squeeze the common-mode current down to an
acceptably low value?
A Yagi with a center-fed driven element and installed on a tower is
probably as close as any antenna can ever come to being symmetrical and
balanced. Therefore common-mode current on the feedline is likely to be
small, and also quite easily suppressed. Even quite a poor common-mode
choke will often seem to work for situations like this - not because the
choke was much good, but because this was a "soft" problem, easy to
solve.
But over at the other extreme are some much tougher common-mode problems
which are going to make us work a lot harder...
>Any sloping center fed dipole is likely to be unbalance, possibly
highly
>unbalanced.
>
ALL sloping or vertical antennas are unbalanced <categorically and by
definition>. This is simply because one side is always closer to ground
than the other, so the environment is always asymmetrical - sometimes
very strongly so. The same applies to strongly asymmetrical antennas
like the OCFD, regardless of how they are installed. Once again the
real-world questions are: How much common-mode current do we have; and
How easy will it be to suppress?
The answers for these strongly asymmetrical antennas are not good. They
are all likely to be major sources of common-mode current which will
also be difficult to suppress - as witnessed by K8RI's experience with
his sloping dipole, and by countless posts about OCFDs.
>I've mentioned the 75 meter, center fed, sloping, fan dipole I have
here
>a number of times. I don't remember the impedance as it currently is
>connected only to the station in the house. What I do remember is that
>antenna is so highly unbalanced that it took two choke baluns to keep
>the RF out of the shack.
>
Yup. If we really think what's happening with the antenna and feedline,
there are no surprises here.
The key to all this is to understand that "balanced" antennas are a ham
myth that doesn't exist in the real world. Only then can we get to grips
with the real-world problems.
73 from Ian GM3SEK
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