[TowerTalk] Choking on chokes

[Roger (K8RI) on TT]

Take for instance the 75 meter, center fed dipole whose impedance can 
vary widely from well below 50 ohms to above 50 ohms depending on 
height.   You are feeding a supposedly balanced antenna with unbalanced 
line.  (Good place for a choke).

I said supposedly because a truly balanced, center fed dipole on 75 
would be rare.
Any sloping center fed dipole is likely to be unbalance, possibly highly 
unbalanced.

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.

Wound with RG-400, or even BuryFlex on 6 cores, the choke is not unduly 
heavy and can be supported by any dipole using good wire without a 
noticeable increase in sag even when the feedline is LMR-400, or 
BuryFlex(TM)

Just use the "cook book" portion of Jim's (K9YC) tutorial and you need 
to learn little, if any magnetic theory.

73,

Roger (K8RI)


On 12/3/2014 3:45 PM, Steve Hunt wrote:
> The level of CM current on a transmission line is *not* directly 
> related to how well it is matched. You can have a badly mismatched 
> line with no CM current, and you can have a perfectly-matched line 
> with lots of CM current.
>
> If what you say is true, you would not need a CM choke when feeding a 
> 50 Ohm balanced antenna with 50 Ohm coax.
>
> Steve G3TXQ
>
>
>
> On 03/12/2014 20:21, Earl Morse wrote:
>> If you have an antenna that is well matched to the feed line then the 
>> CM currents are going to be minimal.  However, that match is 
>> frequency dependent so while you may have minimal CM currents on the 
>> feedline at the resonant point of the antenna they will change as you 
>> move away from resonance and the antenna impedance is no longer 
>> matched perfectly to the feedline.
>
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-- 

73

Roger (K8RI)


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