I had a similar experience to Jon's with trying to tune an 80 meter delta
on 75 meters using an MFJ989C. The tuner arced very badly at even
moderate power levels, so I pulled the top cover off to see what was going
on. Turns out that one of the caps was barely meshed (I don't remember
if was the input or output cap) and the roller inductor setting was near
maximum L. When I retuned using a different inductor setting, the arcing
stopped (more capacitance), but the roller inductor started to smoke at
1500 watts (shunt L was too small). I ended up with a compromise
setting between these two extremes that seems work okay (no smoking
or arcing at legal limit).
As Tom points out, T networks can provide a number of tuning solutions
at a given frequency and load impedance. Some of them having better
power handling characteristics than others (I have been able to make my
Dentron Super Super Tuner arc with 100 watts at the right settings even
though it happily handles 1300 watts with other settings).
73 de Mike, W4EF.....................
----- Original Message -----
From: "Tom Rauch" <w8ji@akorn.net>
To: "Amps Reflector" <amps@contesting.com>; "Jon Ogden" <na9d@speakeasy.net>
Sent: Wednesday, June 20, 2001 9:03 AM
Subject: Re: [AMPS] Review of Antenna Tuner from Dayton
>
>
> > My first stop was to look at the Ameritron/MFJ/Vectronics units. After
> > all, they are the most prolific tuners out there right now. Upon looking
> > at all these units, I found that the plate spacing on their caps were no
> > better than what my Heathkit had. Even their highest power units had caps
> > with the same spacing. It is interesting to note that later I noticed
> > that the Ameritron tuner rated for 1200 Watts has larger cap spacing than
> > the ones that are more expensive and rated for higher power!
>
> Well Jon, it might be advisable to spend time looking at voltages in
> T network tuners. If you run the numbers, you'll see the largest
> problems causing arcing are excessive Q on the low bands.
> Excessive Q is caused by not having enough capacitance.
>
> many people operate tuners incorrectly. They don't use maximum
> capacitance possible when matching loads. That will make any
> tuner arc or fail.
>
> That's why better tuners for the low bands and for high power have
> more plates in the capacitors, even if it means reduced spacing.
> There is absolutely no reason to have the plate spacing larger than
> the air gap in connectors and switches, providing you have enough
> capacitance on the lowest bands. It's a design balancing act.
>
> For example, the ATR30 will handle over six kilowatts into most
> loads on 80 meters yet the wider spaced MFJ 989C tuner will arc
> in the capacitors and fail at several hundred watts under the same
> load conditions.
>
> > Well, upon getting the tuner home, I immediately went to 80M, tuned up and
> > VIOLA! - NO ARCING!! So, I was like, COOL! I've used the tuner on just
> > about all bands now at full legal limit and it works just great. It will
> > even tune my 80 m dipole on 160 m which is something my Heathkit wouldn't
> > do.
> >
> > My only complaint about the unit is that the roller inductor is a little
> > stiff and hard to turn in some spots, but that will probably get better
> > over time.
>
> Maybe. Maybe not. Look at the roller construction.
>
> When MFJ bought Paul's last company (vectronics), I was
> assigned the task of looking at the rollers to see what could be
> done. Ameritron inherited roller parts from that sale. The roller used
> a chrome plated bushing riding on an aluminum shaft in the roller,
> and had other mechanical connection problems such as a lack of
> wiping contacts to insure solid connections. There were steel wave
> washers in the path of RF current, and steel on aluminum hardware
> current carrying points.
>
> Eventually all those parts were scrapped.
>
> What materials are used in the roller contact area?
>
> > Also, Palstar makes power meters. They have a very reasonably priced unit
> > that is a 3KW unit that is a pseudo peak power reader. It is about $75.00
> > or so. A much better price than similar units from MFJ/Diawa, etc who may
> > not even make a 3KW meter or would charge well over $100 for it. Sure,
> > it's not as good as a Bird, but it is a very nice unit.
>
> Ameritron makes a TRUE peak reading 3 kW meter. I don't know
> how much it costs, but I do know it can handle 6 kW.
> 73, Tom W8JI
> W8JI@contesting.com
>
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