<color><param>0100,0100,0100</param>Hi Press,
Having built, measured, and used dozens and dozens of tuners
with baluns, I disagree totally with the conclusions McCoy
reached.
<italic><color><param>0000,0000,0000</param>> run is suggested, and the balun
is one Lew McCoy tested and wrote up in CQ
> Mag a few years ago after testing it to a 10:1 SWR at 3Kw. (HF) The
> rationale is typically explained as follows:
</italic></color>It is virtually impossible to construct a balun that
transforms
impedance and balances all in one section that will handle 10 : 1
VSWR at 3 KW from 1.8 to 30 MHz, unless it is a simple choke
balun.
For example, I designed and use a 9:1 balun that will handle 1500
watts at a 10:1 SWR over that range, and it uses a total of eight 2
inch diameter 65 material cores with 1/4 inch thick teflon insulation
on the wire!
Peak voltage between any two wires is 3700 volts at 4500 ohms,
and maximum RMS current is 5.8 amperes in any given wire at 45
ohms.
<italic><color><param>0000,0000,0000</param>> wound any way one wants it - a
transformer of 1:1 to 16:1 is possible. On
> the other hand, 4:1 is what you want, if your use is the typical one, that
> of allowing coax to finish the last few feet of a balanced line run from
> antenna to tuner. The function here is to extend the tuner's field of
> operation, so to speak, from where it sits, to balanced line terminals
> outside the house. It does two things: - first, it makes the match from
> balanced to unbalanced(coax or unbalanced output from the tuner) to
> balanced - the ladderline. Second, it gives the tuner more latitude. Most
> tuners do their best work when handling loads of impedance in a range from
> less than 25 ohms to 300 to 1000 ohms, depending on the tuner's design and
> quality. A typical dipole of any given length fed with ladder line (300 to
> 600 ohms) will present a load of, say 20 to 2000 ohms as a system - the
> characteristic impedance of the LL is not relevant. The 4:1 balun, either
> remote or in the tuner as an option or design. centers that device at a
> point 4 times greater, and expands its capability tremendously.
</italic></color>Actually not.
A 4:1 balun causes the tuner to operate at lower impedances. That
is where typical T network tuners have the lowest efficiency and
lowest power handling capability.
Not only will the balun handle less power safely over a wide
impedance range by being a 4:1 ratio or higher, so will the tuner!
With 250 pF of capacitance in a T network tuner, maximum power
handing and efficiency occurs when the load impedance is about
3000 ohms on 1.8 MHz...the exact values depend on the particular
components used. The higher the frequency, the lower the
maximum power impedance becomes for a given amount of
available capacitance.
<italic><color><param>0000,0000,0000</param>> been proven over the years that
4:1 is the most effective effective ratio
> for the job, in or remote. Just remember - you are not matching the
> characteristic impedance of a piece of ladderline - you are matching a
> system at its feedpoint to a signal at any given frequency. Hope this
> makes it easier to deal with! ~~~
</italic></color>Lew McCoy is wrong. It does not work that way in theory, and
it
certainly does not work that way in practice.
<nofill>
73, Tom W8JI
w8ji@contesting.com
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