Its really difficult to have a wire antenna that is truly balanced. One
end nearly always has a different exposure to dielectric and grounded
objects than the other leading to differing capacitance to ground at the
high impedance open end of the doublet. Then even if the doublet is
decently balanced its often difficult to run the feedline exactly
perpendicular to the doublet, so there are antenna currents coupled to
the feedline. So the feedline currents are often inherently unbalanced.
Its not enough to measure current amplitudes to determine balance, its
necessary to measure current phase and have equal currents exactly out
of phase to eliminate feedline radiation. One way to find that condition
is to use an inductively coupled ammeter and to run both feedline
conductors through it to measure the unbalance current. The success of
that depends on the shielding of the inductive coupler from capacitive
voltages of the lines. E.g. measure the vector sum of the two currents,
when equal AND out of phase that sum will be zero. And that the
inductive coupling is not dependent on the relative positions of the
conductors and the inductive coupler.
Current baluns don't force balance, they accept all loads from balanced
to completely unbalanced by inserting a large common mode impedance.
They won't make an inherently unbalanced antenna with feed line not
perpendicular to radiate best broadside (on the fundamental resonance)
and they might not prevent feedline radiation and reception. Neither
will a voltage balun. There will be capacitance to earth from the
feedline near the balun to still carry unbalanced current.
73, Jerry, K0CQ
On 11/14/2010 12:44 PM, Steve Hunt wrote:
> Jim,
>
> I agree!
>
> I've been doing some balance measurements on a 100ft doublet I have fed
> with ladderline. Because of its local environment it's a pretty
> unbalanced antenna system.
>
> I measured ladderline leg current balance using a couple of clip-on
> meters; I also ran the ladderline through a single ferrite core wound
> with pick-off turns to overcome the phase imbalance "uncertainty
> problem". I tried the following arrangements:
>
> Palstar BT1500 balanced tuner
> Ten Tec tuner equipped with internal Ruthroff 4:1 Voltage Balun
> Ten Tec tuner equipped with home-made external balun configured as 1:1
> Guanella
> Ten Tec tuner equipped with home-made external balun configured as a 4:1
> Ruthroff.
>
> The Ten Tec tuner plus external 1:1 Guanella produced noticeably better
> current balance than any of the other arrangements - particularly at the
> lower frequencies. The 4:1 Voltage baluns were especially bad.
>
> I was unimpressed with the Palstar BT1500; the 1:1 balun it uses at its
> input has pretty low CM impedance on the lower bands.
>
> 73,
> Steve G3TXQ
>
>
> On 14/11/2010 18:30, Jim Brown wrote:
>> The primary concern with balance in an antenna system, INCLUDING THE
>> LINE, is minimizing common mode current on the line. We care about this
>> because we do not want the line to radiate to our neighbor's living room
>> or receive noise from his computers, and because we do not want that
>> common mode current in our shack. Indded, the primary function of a GOOD
>> choke at the feedpoint (or down the line below a matching section) is
>> to disconnect the feedline from the antenna from a common mode point of
>> view.
>>
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>
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