[TenTec] [Fwd: Line Isolator Balun (sorta) question.]

[Dr. Gerald N. Johnson]

One has to take care that the added reactive impedance doesn't become 
series resonant with the available wire length. You wish to shift to a 
wire plus reactance that makes an odd multiple of a quarter wave to 
minimize the current in that wire. If you slip up and the combination 
makes any multiple of a half wave, you have maximized the current in 
that wire.

73, Jerry, K0CQ

On 8/7/2010 4:26 AM, Steve Hunt wrote:
> Just to expand my previous posting:
>
> With a 20ohm ground connection at the bottom of the braid, if the
> reactance seen at the dipole feedpoint looking into the braid path were
> cancelled by the CM choke, the resulting CM path resistance would be
> less than 100 ohms for any braid length between 0-12ft and 22ft-35ft
> (and "repeats" thereafter). In other words, this effect is not
> constrained to braid lengths close to an electrical half-wavelength -
> over 70% of all possible braid lengths could exhibit significantly
> increased braid current from the insertion of an Reactive choke.
>
> Steve G3TXQ
>
> Steve Hunt wrote:
>> It's actually short of an electrical half-wavelength, which gives it
>> about 300 ohms of series capacitive reactance. That reactance is then
>> cancelled by the inductive CM choke, leading to a lower CM path
>> impedance and increased braid current. The effect is most noticeable,
>> not when the braid is an electrical half-wavelength and the CM path
>> impedance is lowest, but when it has a significant reactive component
>> which may be offset by the choke impedance.
>>
>> Even with 16ft of coax a Reactive choke will increase the braid current;
>> albeit the current is low to begin with.
>>
>> The point is that most operators will never know what their braid
>> impedance will be; and it will likely be very different on different
>> bands. So the safe option is a CM choke with a high RESISTIVE impedance.
>>
>> 73,
>> Steve G3TXQ
>>
>>
>> Dr. Gerald N. Johnson wrote:
>>
>>> Of course there's lots of current in the braid, you made it nearly a
>>> half wave long. Try the same experiment with that braid 16 feet long and
>>> you will find much less current because then its a quarter wave long
>>> terminated in a low impedance.
>>>
>>> 73, Jerry, K0CQ
>>>
>>>
>>
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>
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