Sorry for the blank response. Blame it on late night 'fumble fingers'.
While 'common logic' will say to match the antenna to the line
impedance, that 'common logic' is flawed.
It certainly will be that a good common mode choke is all that is needed
at the antenna feedpoint, and that is a 1:1 impedance transformation.
Its purpose is to keep common mode currents off the feedline and to keep
the real radiation confined to the antenna. That aspect is unchanged no
matter whether the feedline is 50 ohms or 600 ohms.
The feedline will act as an impedance transformer no matter what (unless
the feedline is exactly matched to the antenna feedpoint impedance).
Keep in mind that open wire transmission lines are low loss (that is
important).
If you connect a 600 ohm open wire transmission line to a 'resonant'
dipole having a nominal impedance between 50 and 75 ohms, that
transmission line (even thought it has an SWR between 8 and 12) will
have extremely low loss. 450 ohm ladder line and 300 ohm ladder line
are similar, but will have higher loss than open wire line.
The real problem is to match the impedance of the shack end of the
feedline to 50 ohms. The transmission line will act as an impedance
transformer, and depending on the length of the feedline and the
frequency, the shack end may be vary between a very low impedance and a
very high impedance.
In other words, attempting to use a 4:1 balun or a 1:1 balun (or any
other ratio) at the antenna feedpoint may be an exercise in futility.
It all depends on the feedline impedance transformation at the frequency
of interest. And that depends on the feedline length and the frequency.
One would use a 4:1 balun to match coax to an antenna feedpoint
impedance of 200 to 300 ohms (folded dipole) or to a properly tuned OCF
antenna, but for all other conditions, all bets are off unless you know
the antenna feedpoint impedance of the antenna at all frequencies where
you want to use it.
A study of the ARRL Handbook or ARRL Antenna Book section on
transmission lines should provide you with more information.
73,
Don W3FPR
73,
Don W3FPR
On 9/28/2015 8:02 PM, Byron Servies wrote:
On Mon, Sep 28, 2015 at 4:37 PM, Douglas Hagerman
<douglas.hager...@me.com> wrote:
With 450 or 300 ohm ladder line, a 4:1 would be more appropriate to
match the feedline to the expected impedance of the K1 input.
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