|[AMRadio] Open Wire - was AMRadio Digest, Vol 46, Issue 24|
quixote2 at ix.netcom.com
Wed Nov 14 12:34:29 EST 2007
>Ben Dover wrote:
>> I personally love the stuff. Coax has it's place, but often it's not the
>> best solution to the problem.
>I've always used coax, but wanted to give open wire a try, and so far
>I'm very pleased with it. But let's talk tuners for a minute, assuming
>that we're not directly coupling to a balanced plate tank. I've been
>running 1500 watts PEP into a Heathkit SA-2060 with it's 4:1 balun
>without incident, but I've heard all the stories about core saturation,
>etc. How much advantage would a "real" balanced line tuner (e.g. a
>link-coupled tuner or Johnson Matchbox) have? Then antenna by the way
>is 130 ft, center fed, and primarily used on 75 meters.
>Also, having read comments by Cebik and others - any thoughts on
>homebrewing high-power balanced tuners?
>Thanks and 73,
For all but very low power, I'm very skeptical about using a balun or an RF
transformer at all for any matching scheme. Besides the core saturation thing,
my biggest fear is high voltages flashing over.
Even tho I wind (and evaluate) my own RF transformers as I need them, and do
so with the largest cores I can lay my hands on, I still impose an arbitrary
power limit of 250 watts maximum on my ferrite and powdered iron creations;
in most cases, I go with an even more conservative 100 watt limit (what my
little Kenwood TS-140S is rated for!).
I mostly use the transformers on my pet antenna design, the T2FD broadband. I
would NOT use them for something like a multiband dipole fed with open wire.
My reasoning is simple.
With a T2FD, the antenna is resistively terminated. That has the effect of
limiting SWR excursions at the feedpoint. Coincidentally, that ALSO limits the
VOLTAGE EXCURSIONS at the feedpoint. In a nutshell, the feedpoint impedance is
pretty much unchanged as frequency changes.
With a multiband dipole fed with open wire, the voltages at the feedpoint are
swinging wildly as frequency varies; on some bands it's voltage fed, and on
others it's current fed. This dipole (Hell, let's call it what it IS... a
"Center Fed Zepp"!) will vary at the feedpoint from below 75 ohms to 1500 or
higher, depending on frequency.
Besides the possibility of flashover or arcing to the core on high Z bands,
you have another problem on low Z bands; relatively large currents that are
presenting the possiblity of core saturation.
As far as tuners are concerned, I tend to be a traditionalist... you may have
seen me at the various hamfests we both hit, making panic dives for big, dual
section variable caps as soon as I spot them!!! <<GRIN>> Right now I'm just
sitting on a nice old Johnson; 250 pf per section @ 9000 volts spacing! That
critter is being saved for a high power 160 monoband tuner. The coil will most
likely be 3/16" or 1/4" soft copper tubing, with a link coil made of the same
stuff... either a swinging link (mechanically complex), or tuned to resonance
with a big variable cap (a lot easier).
I tend to use the Old School tuner designs like those from early 1950s vintage
ARRL Handbooks. To keep things mechanically simple I prefer parallel tuned
tank designs over series, and cut my transmission lines accordingly.
My basic design rule of thumb is to design the L/C tank around a coil with an
Xl of about 350 - 400 ohms at the operating frequency, and naturally make the
Xc correspond to that to achieve resonance. The line gets tapped up and down
the coil with clips as necessary. Tuning and setup can be done 2 ways; either
max line current as seen by RF ammeters, or (easier these days) best SWR as
seen at the link coil input.
A hint on positioning the line clips on the tank coil; try to use points that
give good power transfer that are as close to the coil ENDS as possible. As
the taps get closer to the coil center, the circulating RF current in the
center goes up, increasing tank coil heating there. Thge optimum tap points
can be moved around the coil by adjusting the variable cap; the resonance
setting is slightly different for every tap position on the coil.
As for the link coil... it's design depends on if it's a swinging link or a
fixed, capacitively tuned link.
For a swinger, it's Xl at operating frequency is equal to the characteristic
impedance of the coax that connects it to the transmitter (i.e., usually 50
For a fixed link, the link Xl is TWICE the coax characteristic impedance (i.e.,
100 ohms, or half the inductance of a swinging link), and the variable cap has
an Xc of twice the coax Z (again, 100 ohms) at link coil/capacitor resonance.
I personally don't like bandswitching antenna tuner designs. I've flashed over
bandswitches too many times. If multiband operation is necessary, I try to use
plug-in coils on a jack bar.
Hope that helps out some.
Mr. T., W9LBB
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