[Elecraft] Antenna Feedline Question

[Don Wilhelm]

Tom,

In any antenna system where the feedline operates with an SWR other than
1:1, the transmission line acts as an impedance transformer, and the
impedance seen at the feedpoint of the transmission line will depend on the
feedline length.  The feedpoint impedance is not to be confused with the
characteristic impedance of the line.  A 450 ohm ladder line may have a
feedpoint impedance ranging from very low to very high.

You 40 meter flattop antenna should have an feed impedance of about 70 ohms,
and if your feedline is an electrical halfwave (or a multiple) on 40 meters,
the shack end will also have a 70 ohm feedpoint impedance, but if it is an
odd multiple of a quarter wavelength, the shack feedpoint impedance will be
quite high.

If you want a bit more information on the transformation properties of a
transmission line (in plain language with no math), take a look at the
antenna, feedline, tuner article on my website http://w3fpr.qrpradio.com

This same length antenna on 20 and 10 meters will have a high impedance at
its midpoint because each side is a half wavelength or full wavelength
long - again, this will be transformed to a low impedance at the shack end
if the feedline length is an odd multiple of a quarter wavelength at these
frequencies.

Be aware that the K2 autotuner has a bit of trouble loading a very high
impedance, so I recommend that you extend the length of your antenna a bit
to keep away from the high impedance points and make the transmission line
some multiple of a half wavelength for all the bands of interest.

The use of a balun is optional, but if you do use one, the current type
(like the BL1) is preferred over a voltage balun and whether or not a 1:1
balun or 4:1 balun will handle the line better depends on the feedpoint
impedance at the shack end - some experimentation may be in order (or
transmission line measurements and analysis).

73,
Don W3FPR



> -----Original Message-----
>
> Hi all,
>   I am running a barefoot K2 with various options, including the
> auto-tuner
> and am getting ready to put up a new antenna.  I've decided on a 40 meter
> dipole that I will feed with 450 ohm 'window' line.  This will be my only
> antenna, at least for a while.  I operate mostly on 40 meters
> (CW) but also
> a bit on 20 (psk31) and 15/10 (also CW) when they are open and
> hope to tune
> the 40 meter dipole to those bands with the auto-tuner.
>
>  I am planning to run the 'window' line all the way into the
> shack to the K2
> (which is about 5 or ten feet from the outside wall).  My
> question is should
> I also use the BL1 at the K2 and will I have any trouble tuning
> to the other
> bands.  I guess I don't understand what 4:1 means, in this case
> as I'll be
> using 450 ohm feedline to the K2.
>
> Thanks in advance for your comments.
>
> Tom
> WB2QDG
> K2 1103
>