Jerry,
You obviously didn't read what I wrote about running: "the ladderline
through a single ferrite core wound
with pick-off turns to overcome the phase imbalance "uncertainty problem" !
73,
Steve G3TXQ
On 14/11/2010 19:11, Dr. Gerald N. Johnson wrote:
> 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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