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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