> 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
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|