Something to contemplate, in line with the comment about whether a kilobuck
is appropriate for a high power tuner...
The tuner's a pretty important part of the "system". If you've spent $3K on
the rig, $2 or 3K on the tower, another kilobuck on the antenna, a few
hundred on decent coax, etc. $1K on the tuner might not be all that bad an
investment.
>From a performance standpoint, one might want a remotely adjustable tuner
that can be put at the antenna feedpoint. Why have all that reactive power
circulating in the feedline. Yes, there's a potential(!) reliability
problem with the tuner remoted, but, if you're setting out to do that from
the get-go, perhaps it's something that can be appropriately designed in.
I don't know that I'd try to do something like modifying some ancient
hamfest special (high quality as it may have originally been) with steppers
or servo motors. Too many potential problems, mostly mechanical in nature.
A suitable remotely adjustable network would be quite the benefit for those
running stacks, too,because it would allow you to adjust the relative
phasing (assuming you put two networks out there...).
Two basic approaches here... one is the mass of relays and lumped
components, a'la LDG or SGC autotuners (which work quite well, but have some
limitations in the matching topology, as well as potential component Q
issues (I say potential, because I've not gone through the calcs to tell if
it's a significant thing or not)).
The other is basically a remote operation of a conventional variable cap
(vacuum for low loss) and a switched or continously variable inductor.
Given the difficulty in making a good variable inductor with wide range, a
suitable tapped inductor, with series or parallel C (which can be made very
high performance) might be better. Or, a goniometer (rotating coil in
another) or sliding nested inductors might be fairly high performance
particularly at high powers.
Tuning speed might be a consideration. Any of the motor operating
approaches have that limitation, while the "Capacitor/Inductor relay ladder"
can switch in milliseconds. A halfway decent controller could remember the
tuning settings and be controlled by the radio (or the computer that's
controlling the radio), unless you're in the habit of sending QSK CW while
spinning the dial. Even if you do carrier sensing to measure frequency, a
halfway decent circuit (fast reciprocal counter, lots of SNR to work with)
should only take a millisecond to measure the Tx frequency to a few Hz and
figure out how to switch the relays, then 10ms for the relays to settle. A
really clever controller would essentially figure out the feedpoint
impedance as a function of frequency and then calculate how to set the
tuner, even for frequencies that haven't been tuned before.
Alas, there aren't sufficient folks lining up to buy such a clever tuner
controller, so even though the parts cost would be low, the development cost
would be high (say, 40K-50K, if you paid someone to do it). Are there 500
potential sales out there? I thought not.
Jim, W6RMK
----- Original Message -----
From: "Rob Atkinson, K5UJ" <k5uj@hotmail.com>
To: <towertalk@contesting.com>
Cc: <k5uj@hotmail.com>
Sent: Friday, November 28, 2003 5:07 PM
Subject: [TowerTalk] Best tuner value? (I think that was the topic)
> Right, the best tuner depends on what you want to do. For balanced feed
> lines I use the Bliss Z Matchmaster. It uses two synchronized 30 mH
roller
> inductors and a 1000 pf vac. variable. One of the things I like about it
is
> that the roller inductors are in-line on a threaded shaft with fixed taps
so
> the coils move back and forth. I think this is a bit more rugged and
stable
> than two rollers turned together from a single shaft with rubber belts.
> This is a transmatch for low band QRO. It would not be necessary for
lower
> power and/or (obviously) unbalanced lines.
>
> Rob Atkinson
> K5UJ
>
> _________________________________________________________________
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