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?
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