>On Tue,6/21/2016 9:31 PM, Roger (K8RI) on TT wrote:
>> Didn't you once say "there is no such thing as a balanced antenna"?
>
>Yep, but I was speaking more in terms of perfection -- i.e., that in
the
>real world, most practical antennas are unbalanced by their
>surroundings, even if we attempt to make them a balanced antenna and
>feed them with 2-wire line and a so-called balance tuner. It is that
>imbalance that causes common mode current. If the imbalance is
>relatively small, and if the antenna is resonant and fed with coax, we
>can choke it effectively.
>
>73, Jim
The degree of "natural balance" that one might expect in a center-fed
antenna will also depend on the frequency.
Jim's description is very true at low frequencies, where antennas are a
typically only a fraction of a wavelength above their surroundings. It
is quite rare that those surroundings are truly symmetrical, and any
asymmetry in the surroundings *will* lead to unblalance - and more
importantly, to common-mode currents on the feedline. Even when the
antenna appears symmetrical to the naked eye, the presence of
common-mode currents on the feedline will often prove that "a balanced
antenna" and "a balanced feedline" were merely wishful thinking.
At low frequencies in particular, a much better starting-point is to
assume that your antenna and feedline will *not* be balanced unless
*you* have done something to make them so.
At higher frequencies, up into VHF, antennas are more likely to be
several wavelengths above their surroundings, and a center-fed dipole is
more likely to be the driven element of a Yagi. At VHF, it is much more
reasonable to assume that a Yagi driven element is "born balanced"...
but there are still many ways to spoil that situation when you connect a
feedline.
But why are we discussing balance at all? The harder we try to grasp
what "balance" really means when applied to an antenna way up in the
air, the more the whole concept slips through our fingers - and the less
useful it becomes, in terms of showing us what practical steps to take.
A much more useful approach, I find, is to "FOLLOW THE CURRENT". In
other words, understand that the real problem is not "unbalance" but the
RFI that is caused by unwanted common-mode currents. If we understand
how and where those currents arise, consistently follow (and measure)
where they might be flowing, we can then take practical steps to choke
them off.
There are obviously many more points to be made about this, but that's
enough for one email.
73 from Ian GM3SEK
>-----Original Message-----
>From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
>Jim Brown
>Sent: 22 June 2016 07:22
>To: towertalk@contesting.com
>Subject: Re: [TowerTalk] To get a truly balanced antenna feed
>
>
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