Ron asked
> >Are there ways of achieving balanced feed at tuner output without utilizing
> >a front end balun?
>
>
The classic way is the one that's been in the ARRL handbook for at least 50
years. A parallel or series (depending) tuned circuit, link coupled to the coax
line. Historically, you used a swinging link, but these days, it tends to be a
fixed link and a series variable capacitor. Needs to be a pretty big cap to
avoid too high a voltage e.g 1500pF on 3.5 MHz.
Whether the tuned circuit is series or parallel depends upon the feed impedance
offered by the transmission line. High impedance for parallel. Typical tuning
cap values are 1.5pF of tuning cap for each metre of wavelength. So for 80
metres, you need a 150 + 150pF split stator variable cap. I think there are
advantages in grounding the coil centre tap, and floating the cap, as it
provides a static discharge path for the antenna. Depending upon the reactive
component of the transmission line input impedance, you may need to fir shunt L
or C to bring it within range. Or trim the length of line to get an acceptable
feed impedance. There's more ways than one of skinning a cat, as they say! (A
statement that my two cats don't find at all PC!)
I find this works very well, although on 40m, I can get marginally more
bandwidth for a given setting from a ferrite bead balun driving a balanced L
network. This is purely because of the way the feed point impedance of my
particular antenna (80m dipole fed with open wire line) varies with frequency.
Variants on this put the coil in two halves with caps to ground, so it becomes a
variable capacitive tap. Similar to the Johnson matchbox. Another one popular
over here is called the Z match. Two controls, working Q goes all over the place
with varying antennas, and can easily arc or catch fire.
Hope this helps, Ron.
73
Peter G3RZP
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