Tom, et al:
With regard to T-networks, this is why its important to
record your tuner settings instead of searching for minimum
VSWR each time. At the local club, one of my friends
was using an MFJ-9 series KW tuner to match an 80
meter delta loop on 75 meters. He was complaining that
the tuner wasn't big enough and was trying to find money
in his budget for a larger tuner. One day last winter I
dropped by to work a little longpath on 75 meters. Not
wanting to miss an opening, I pulled the top cover off
the tuner and fiddled with the settings a bit. One matching
solution resulted in arcing at a few hundred watts. Another
matching solution caused the shunt inductor to overheat
(smoke plume). I then found a matching solution that
was in between the two (modest shunt inductance and
modest series capacitane). The matching solution hums
along nicely at 1500 watts. I then marked the front
panel of the tuner with a template so I could easily find
those settings again. We can now spend the tuner money
on something else.
I have the same problem at home with my Dentron T-match
tuner (Super Super Tuner). If I used the largest inductor
setting to match my Marconi on 160, I get good VSWR
but the tuner arcs at around 700 to 1000 watts. If I use
the next lower inductor setting, I can only match to about
1.5:1 but the tuner will handle the full output of my L-7
without arcing (~1200 watts). When my 80 meter dipole
was only 10 feet off the ground (I wasn't ready to face the
bumble bee in the walnut tree yet), the input impedance
transformed through the feedline was really low. With
this load, that same tuner that routinely handles 1200 to1300
watts would arc with only the 100 watts of input power
from my exciter.
Low VSWR definetly doesn't tell the whole story with T
networks. All low VSWR settings aren't created equal!
73 de Mike, W4EF......................................
P.S. A useful trick is to take the cover off your tuner, and
then mark the mininum and maximum capacitor and
inductor settings on the tuner's front panel (instead of
meaningless letters or numbers). Of course make sure
your knobs aren't slipping :):)
----- Original Message -----
From: "Tom Rauch" <w8ji@akorn.net>
To: <towertalk@contesting.com>; "Pete Smith" <n4zr@contesting.com>; "alex"
<alex@sandlabs.com>
Sent: Tuesday, December 11, 2001 9:48 AM
Subject: Re: [TowerTalk] ++ High SWR and Modern Transceivers
>
> > less drive from the transceiver is available. As all other form of
> > energy, SWR must go somewhere ... As it is not allowed to reach the
> > transmitter output it will dissipate in the antenna tuner components
> > as heat. If there is enough heat it might melt some components like my
> > roller inductor in the MFJ -989C :)
>
> Actually transmitters are energy conversion devices. Maximum
> available power capability that does not reach the antenna does not
> necessarily turn to heat. As a matter of fact a mismatch just as
> often reduces heat in the transmitter PA....but it does so at the
> expense of increased voltages in the PA.
>
> If the mismatch is such that load impedance at the output device
> increases, heat is generally reduced. If the mismatch is in a
> direction that reduces load impedance seen by the PA output
> device, the dissipation generally increases.
>
> Despite common wisdom that professes SWR adjustment is
> important for improved tuner power ratings, tuners have a similar
> effect as PA stages. Heating generally relates to current through a
> component, and certainly does NOT depend on a match at the
> antenna. As a matter of fact, a mismatch can greatly reduce
> heating!
>
> > Manufacturers of Antenna Tuners are specifying them for certain power
> > levels ... 100W to more than 4KW. However, this is valid in a perfect
> > resistive load, where there is no reactance. Once some reactance is
> > added, like a real life antenna will do, the tuner specifications must
> > be derated.
>
> Not so.
>
> Consider the case of a conventional T network matching an
> impedance lower than the lowest obtainable surge impedance of
> the network. With 220pF capacitors, that would be about 3000
> ohms on 160 (1500 ohms on 80 meters, 750 ohms on 40 meters,
> and so on).
>
> If I add inductive reactance to the load on a T-network, the power
> handling of the network actually increases. In some cases it can
> be a significant increase in power handling.
>
> If I add series capacitive reactance, the power handling generally
> decreases.
>
> With no change at all in load impedance, just by changing how I
> adjust the capacitors in a tuner, I can make a tuner that melts
> down or arcs at 100 watts safely handle a kilowatt or more! This is
> the reason it is VERY important to ALWAYS adjust your tuner so
> the capacitors are as far meshed as possible while still allowing a
> match, and why a better tuner design generally uses a large value
> of capacitance (lots of plates) over having wide plate spacing (and
> fewer plates) in a similar physical size capacitor.
>
> If you are buying a tuner by looking for wide plate spacing and high
> voltage rating, you are making a poor choice (unless the
> capacitance is also the same or higher)!
>
> The same tuner into exactly the same load can "melt down" if you
> use less than maximum available capacitance match the load.
>
> T-network tuners (on lower bands) also handle significantly more
> power into a higher impedance load than they do into low
> impedance loads, so the power rating at 3000 ohms load
> impedance (a 60:1 50-ohm normalized VSWR) can be many times
> better than the same tuner feeding a 50 ohm load!
>
> How much "wasted power" does it take to melt a conventional
> inductor with #14 wire and plastic supports? About 10 to 50 watts
> of heat will do it. An insignificant amount of power loss, when
> running a kilowatt, will destroy most tank coils.
>
>
> 73, Tom W8JI
> W8JI@contesting.com
>
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>
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