TopBand: MFJ Tuners, etc

[W8JITom@aol.com]

Hi Carl,

My goal was to improve the understanding of power ratings, not to make you or
any other manufacturer "happy or unhappy". My main point was the tuner rating
"system" used BY ALL MANUFACTURERS has little to do with the power a tuner
will really handle. Tuners are the most unclearly rated products on the
amateur market today, and will remain so until everyone demands a change in
the meaningless transmitter input power rating system.  

My secondary point is the roller for the 989 has been greatly improved, and
has higher Q than the Cardwell roller AND no series resonances at the upper
end of HF. 

Since only part of your reply has technical content of interest to 160 ops,
and since this is a top band group, I'll reply to those areas. You have not
presented my employment status (both past and present) correctly, but let's
stick to technical issues. What I do is no more of your business than what
you do is to me. I certainly wouldn't claim because I think you own a little
business selling parts that you are dishonest, I expect the same treatment
from you. Technical facts stand on their own.

In a message dated 97-01-01 14:56:37 EST, you write:
<snip>
>Radiokit (me again) has worked with Cardwell to produce a higher power
>version of their 229-203-1  28uh roller inductor. It uses #10 silver
>plated wire in lieu of the original #12.  <snip>

Here are a few important facts to consider when designing or buying a T
network (or a derivative of a T network) tuner for 160.

1.) If you want to use a tuner on 160, the amount of capacitance is likely
MUCH more important than the wire gauge in the roller, the voltage rating of
the capacitor, or the current and voltage rating of the switches. 

The amount of C establishes network Q, and network Q determines circulating
currents and the voltages appearing in the components. It is the amount of
VAR power (related directly to operating or loaded Q) that mainly determines
power loss, arcing, and overheating. 

Tuners usually need more capacitance on 160, not "bigger parts". With more
capacitance we obtain more efficiency and more power capability (with given
wire and capacitor voltage rating) until a certain maximum capacitance limit
is reached. The maximum capacitance required is determined by load impedance
and frequency. I've yet to see a tuner with optimum C for 160 into a 50 ohm
load (let alone a lower Z load). 

This is why when we use T network tuners we should use as much capacitance as
we can. Mesh those capacitors as much as possible.

2.) A change from # 12 to # 10 wire in an inductor results in a minor (almost
undetectable) change in Q. Especially when the inductor is less than optimum
in L/D ratio and all other physical design attributes. 

In an optimized air core inductor, maximum Q generally occurs with wire
between number eight and number 14 AWG. Make the wire too big and Q drops,
make the wire too small and Q drops. Q is more radically affected by the
turns spacing, L/D ratio, insulation in the areas of concentrated electric
fields, and spacing to other metalic objects (in particular to end plates)
than by wire size.

This "coil stuff" applies to loading coils also.

3.)  Silver plating does almost nothing to change loss in a copper HF
inductor, except as it reduces contact loss due to oxidation.

There would be almost no measurable change in HF Q because the skin depth is
much deeper than the practical plating depth.

4.) A tuner can only be optimized only for one impedance range and frequency
range.

There is a certain optimum value of components and load impedance that allows
maximum power handling and efficiency. What works best for a low Z 160 load
will be unworkable at a high Z load, especially on higher bands. What works
best on high bands and/or with high Z loads won't work well in the opposite
conditions. 

All tuners are compromises, there is no magic solution. It certainly isn't as
simple as changing two wire guages or silver plating, especially since form
factor affects Q much more than wire size and silver plating offers almost no
improvement at all over a clean copper or tinned copper surface at HF. 

I had to use a four inch diameter AIR INSULATED # 8 roller coil and a pair of
1600 pF variable capacitors to handle 1500 watts normal CW use with a 25 ohm
-25 j load to 100 ohm +j10 in my antenna system. These components won't allow
matching above 15 MHz or at impedances above a few thousand ohms, and would
cost someone $1000 in production units. 

73, Tom

--
FAQ on WWW:               http://www.contesting.com/topband.html
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Sponsored by Akorn Access, Inc & KM9P