On 11/13/2010 8:31 PM, Rick - NJ0IP / DJ0IP wrote:
Jerry, we all prefer a link coupler tuner.
Unfortunately there are no commercial units available anymore, at least not
in the amateur radio market.
Most of the old Johnson Viking matchboxes on the market are really grubby
these days, and even if you find a good one, they are very limited in their
matching range.
True for their tuning range. It never was great and hasn't included WARC
bands ever.
Maybe I'm an old pack rat and should put a few things up for sale, but
the last tuner I built (wasn't balanced) I built from parts on hand. My
big tuner that I built about 1964 uses alligator clips for band
switching, so I can tap the output coil at any turn of the coil. Its
kind of overbuilt, that output coil is from a 5 KW broadcast antenna
match box wound of quarter inch wide flat strap, wound on edge, so the
alligator clips fit on nicely. And I can connect it for series or
parallel tuning, for the two output capacitors that are mounted on
ceramic standoffs and coupled with insulate shaft couplings. There's
probably enough coil to get down to 500 KHz which is a another problem
at higher frequencies. Might wish for more capacitance too at 500 KHz.
Sure you "could" build one but unless you are only interested in one or two
bands, the challenge would be finding an appropriate bandswitch. These are
really quite complex.
RF parts has bandswitches. Most often figured for amplifier PAs but if
two section for a Pi-L tank there would be enough switch for a balanced
tuner.
I think you would have to build it yourself, which is beyond the ability of
most of us.
I've lost track of the number of link couple tuners I've built. My big
tuner has too much coil for 20 meters and higher, so I have built
individual tuners for 15, 10, 6, and 2m, maybe even 220 and 432 though
I've not used the HF center fed wire on 220 or 432. I may have built
more than one tuner per band. I used split stator capacitors for those
and coils wound of #12 bare stripped building wire with a gap in the
middle for a variable link. Places like Dan's Small Parts still has
small air variables suitable for the higher bands. Its quicker to swap
tuners than to flip a bandswitch and retune and these tuners are often
no more than a 4" cube in volume, mostly exposed. In these smaller
tuners I tuned the whole coil and adjusted the antenna taps for the
handiest match, and soldered them together.
Or, as an alternative you could use a Z-match, which requires no bandswitch,
but they have reduced efficiency at some impedances. There may still be a
high power version of the Z-Match available from UK Linears.
Never used a Z match.
The MFJ and Palstar designs for symmetrical matchboxes are not as good as
the link coupled, but they get the job done in good fashion. You need some
pennies for those boxes though, but they'll handle a kilowatt.
The unbalance of a coaxial tuner feeding a balanced load while being
driven through a balun isn't enough to upset any antenna. There's no
need for the post balun tuner to be physically balanced in my opinion.
Sure it looks nice but its not necessary. Then the balun on the matched
side always sees its rated impedance, without the core or insulation
smoking wild impedance variations of the tuned antenna and can work
well. Even handier one or two the the MFJ automatic tuners get their
power through the coax when set up for remote location. So that power
can pass through the balun.
For 100w, I have found the MFJ-974B to be a great little box.
Previously, at field day, my club used an old Viking 275w version, link
coupled matchbox to match our openwire fed doublet.
It had great difficulties finding a match across all bands.
We had to play with feedline lengths.
True, the Johnson Matchbox has a limited tuning range, but its handy
when it works.
The last two years we replaced it with the 974B and were able to obtain a
good match on all bands.
We have retired the Johnson Viking.
Here for high power, I use a home-brew coax balun and a Model 238 tuner.
Works great on ALL bands. As with any openwire fed antenna, I sometimes
have to "adjust" the feedline length when I change the QTH.
In the February or March edition of QST there was a really great article on
d-i-y 4:1 current baluns.
I suggest building one of these and placing it external to the matchbox,
with a very short length of coax between the balun and the matchbox. Any of
the solutions proposed in that article will have less loss than any of the
suggestions previously made in this thread (except for the link coupled
matchbox, of course).
The balun loss on the antenna side of the tuner can be severe because of
it being operated at impedances way outside the range it was designed
for. I got a rude awakening one time at a club station tuned up a tuner,
couldn't hear signals though, unhooked the antenna and the impedance
match didn't change. I had tuned to the toroid balun, instead of the
antenna probably because the balun had a much lower impedance than the
antenna at that frequency. That has made me unwilling to operate a balun
at arbitrary impedances.
73
Rick (running openwire since 1963)
I've run open wire, twin lead, and Ma Bell shielded direct burial cable
on my long center fed wire. Twin lead flopped about too much in the wind
for the strength of the 20 guage copper conductors, e.g. too good an
airfoil. 1" space #18 copperweld didn't fly in the wind, but it
shattered when a slight breeze moved it when covered in a quarter or
half inch of ice. The bronze strip wrapped direct burial cable did well
for years, until the antenna took a hit by lighting, then the cable went
open at numerous spots. I'm figuring on using what RS calls heavy duty
twinlead on my next centerfed wire, but I plan on punching holes in the
dielectric every 6" or foot and threading polyester line or parachute
cord through them making a half turn or a full turn around the line
between the holes to have a much greater tensile strength. The turn
around may also upset the airfoil effect. So far this year, moving has
occupied too much of my time to get that up and now its started to snow
nearby. I do have a box of 3" ceramic insulators that would make a fine
balanced line with some #10 or 12 copper weld. I think I have a bit of
#10 copperweld that hasn't been in the weather that would work good for
that feed line. I still have my double extended 75 meter zepp that has
outlived two supports and three feed lines and a new QTH without room
for the full double extended zepp where I'd wish to put it with the
center handy to the house, but there is room for a 160 meter dipole
between the trees with the feed point at the flag pole.
73, Jerry, K0CQ
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