[Elecraft] More on doublet and feedline issues

[dw]

Prior to using the SGC auto-tuner at the end of the coax I used a home
made balanced tuner.

This was simply two 30uH tapped coils ( one for each side of the ladder
line )
And then a variable cap switched in with a DPDT for either step-up or
step-down config.

As mentioned before, putting the tuner at the end of the coax helps to
keep the coax usage for what it is designed for (unbalanced 50 ohms). 
This minimizes SWR losses on the coax and heat losses and saturation
within a balun, which can in some cases eat up a percentage of the
transmitters output power.
This is especially critical in QRP usage.

In order to find the taps for this I built a simple qrp tapped
L-network.
I used my MFJ antenna analyzer to find the correct combinations of L + C
on the L-network for each band.
Then divide the L between the two parallel coils and you have a balanced
tuner.
In this case: rig to coax to balun to balanced tuner to ladderline.

Roy McCoy indicates that if one uses a tuner prior to the coax, one
should use a Kwatt rated balun at the ladder line end of the coax to
minimize flux saturation and possible toroid destruction. 

I use a Kwatt balun anyway.
Just get the best in the first place and then I won't be fishing for it
afterwords.

Prior to this, I used a standard MFJ versa tuner sitting between the rig
and coax, and there were always bands or band-segments that I could not
operate in due to the inability to find a match.
This is because every design has compromise built into it and as such,
most T type tuners have just enough component reactance to meet AVERAGE
matching requirements.
And so, when your antenna system matching requirement exceeds average,
your out of luck.
This is especially true with auto tuners built into rigs.
Meeting ROI (return on investment) demands requires minimizing matching
capability.

L.B. Cebik (http://www.cebik.com) was probably one of the most
knowledgeable Hams I've ever had the privilege to correspond with, on
the topic of feed-line lenghts.  He is the first one to remind me of the
(1/8 wL @ lowest operating Frq) rule for optimizing ladder-line length.
 
Use plenty of antenna connector gum outside to prevent moisture from
seeping into the coax, and keep all of the wire/connector points dry.
-- 
 Bw_dw at fastmail.net