[Elecraft] Antenna matching

[W3FPR - Don Wilhelm]

Paul,

If your "impedance matching ladder line" is of the correct length, the 
impedance looking into it can be close to 50 ohms on ONE band!  That may 
well be 40 meters in your particular case.

Your 'double balun' idea MAY work, but you will still be running the coax at 
a high SWR and that can cause significant loss, especially with the smaller 
diameter coax (OTOH, if you are using VERY low loss coax - like hardline - 
it will do just fine)

For multiband operation, you will need an ATU, and it could be mounted at 
the current junction of your coax and ladder line (assuming that is 
physically possible).  If that is not possible, scrounge some more of the 
ladderline material and run it all the way from the antenna to the tuner 
location.  It should work FB, although you may have difficulty with some 
lengths of feedline on some bands (which lengths and which bands depends on 
the length and the feedpoint impedance of the antenna).

There are several resources that you can use to predict the impedance seen 
at the shack end of your transmission line.  For the G5RV antenna length, 
there are several instances of published antenna feedpoint impedance charts, 
and I believe there is one on L B Cebik's website www.cebik.com.  Then to 
estimate the transformation effect of your transmission line, you can use 
several computer programs to do the calculation for you - TLW is available 
from the ARRL website, and it will do the job nicely.

If you must use your current feedline configuration, use a balun at the 
junction of the ladder line and the coax.  Whether you want to use a 1:1 
balun or a 4:1 balun depends again on the antenna feedpoint impedance and 
the length of the ladderline, so you still need to work through the 
calculations to make an informed decision.

Should you desire a bit of information on the transformation properties of 
antenna feedlines without delving into the math, take a look at my website 
(www.qsl.net/w3fpr) article on 'Antennas, Transmission Lines and Tuners'.

73,
Don W3FPR

----- Original Message ----- 
>
> I'm still learning about this so apologies for any gaffs.
>
> I have an Elecraft K2 (with which I'm extremely happy) running into a full
> size G5RV.
>
> The G5RV consists of a horizontal dipole with an impedance matching ladder
> line dropped from the centre.
>
> It currently terminates in a BNC connector and thence to a length of 50 
> ohm
> co-ax to the transceiver.
>
> It works tolerably well on 40 but is hopeless on other bands.
>
> I'm uncomfortable about going from the unbalanced co-ax straight into the
> ladder line without a balun though it's more intuition than having done 
> the
> sums.
>
> I'm mean and impecunious so I want to use what I have to hand. I have an
> ex-military ATU built by Redifon. This takes a 50 ohm unbalanced co-ax 
> feed
> and matches to an arbitrary dipole. BNC connector on the front, two
> terminals on the back for the antennae and a further terminal for a ground
> stake.
>
> Of course the best place for the ATU would be at the base of the antenna 
> but
> this is impracticable so the ATU must reside in the shack.
>
> I'm only using 10W so I want as much of the transmitter power to be 
> coupled
> to the antenna and waste as little as possible in the feeds.
>
> My guess at the moment is that what I need to do is to connect the 50 ohm
> output of theK2 into the ATU, take the ATU outputs and feed into a balun 
> to
> drive the coax feed and then have a further balun at the base of the G5RV 
> to
> match to the antenna.
>
> Does this sound sensible or am I completely barmy?
>