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[TowerTalk] Rising VSWR

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Subject: [TowerTalk] Rising VSWR
From: (Joe Reisert)
Date: Thu Jan 30 11:54:09 2003
Michael, Chuck etal,

The most popular ham antenna tuners use the so called "T" network because 
it is the most versatile network (matches the greatest range of 
impedances). For lowest loss, always start with the output capacitor at 
maximum capacitance and tune the inductor next. This is all explained in 
great detail in "Getting the Most Out of Your T-Network Antenna Tuner" by 
W4ULD in QST.January 1995.

Frank Witt, AI1H also wrote a two-part classical article on how to evaluate 
your antenna tuner, especially on loss (April and May 1995 QST). The latest 
QST (Feb. 2003) has more info and the results of tests on many of the 
popular high power antenna tuners. I suggest you get a copy and read up on it.


Joe, W1JR

At 11:30 PM 1/28/2003 -0800, Michael Tope wrote:
> > QST very sanctimoniously proclaims that they won't allow antenna
> > advertising that includes gain figures; yet every month QST carries
> > ads for so-called "legal limit" and "high power" antenna tuners that
> > -- ARRL's own lab testing reveals -- dissipate 30 to 40 percent of
> > the applied RF power if the antenna is a G5RV driven at 3.5 MHz.
>Hi Chuck,
>The problem with antenna tuner specs is that they don't
>include qualifications about the power rating versus degree
>of mismatch. Another problem that was not addressed in
>the ARRL comparison article has to do with the nature of
>"T" network tuners. Most of these networks don't provide
>a unique matching "solution". There are in fact a number of
>different combinations of inductor and capacitor settings
>that produce a 1:1 VSWR at the tuner input. These
>multiple "solutions" don't, however, produce the same
>efficiency or power ratings. A case in point is the MFJ
>Tuner we have at the local club station here (I think its
>the 989C). On 75 meters we use it to match an 80 meter
>delta loop. With that tuner I can use a number of different
>roller inductor settings to get a 1:1 VSWR. Unfortunately,
>depending on the settings, the capacitors in the MFJ tuner
>will arc (high voltage) or the roller inductor will overheat
>(high shunt RF current). If I pick the right "in-between"
>settings, the roller inductor doesn't overheat and the
>capacitors don't arc. With that combination of tuner settings,
>the tuner handles the 1500 Watt output of our AL-82
>without any trouble.
>If you take the cover off the MFJ tuner and observe the
>settings of the capacitors and the roller inductor for the
>various tuning combinations, it becomes clear what is going
>on. When you use a combination of a small capacitance and
>large inductance, the voltage drop across the capacitor
>becomes large (V=I*Xc), and the capacitor arcs. The shunt
>inductor current on the other hand is given by I = V/Xl.
>Since Xl is large in this case, the shunt RF inductor current
>is small. This tuning combination probably results in the best
>overall efficiency since most of the losses in a "T" network
>are due to the finite Q of the shunt inductor. This combination
>is power limited, however, by the breakdown voltage of the
>series capacitors. If I retune the network for larger capacitor
>settings (low Xc), the RF voltage across the capacitors
>drops, so the arcing goes away. Unfortunately, to get a 1:1
>VSWR with the larger capacitor settings I need to lower
>the shunt L. This causes the RF current in the inductor to
>increase, thereby increasing the losses in the tuner. If the
>inductor setting is too low,  the inductor losses will power
>limit the tuner (e.g. the coil will smoke).
>I am wondering if the folks who did the ARRL comparison
>test went thru this exercise of optimizing the tuner settings
>for efficiency, or if they just the spun the knobs until
>they got 1:1 VSWR and then measured efficiency. If this
>is the case, some of the tuners in the test may have been
>unfairly stigmatized as "low efficiency" when in fact the test
>operator just happened to randomly hit upon a bad
>combination of components settings. This is what happened
>to me the first time I tried to tune up the MFJ989C on
>75 meters. I concluded it was a piece of junk, when in
>fact it was quite capable of handling 1500 watts if
>adjusted properly.
>If you look at the ARRL data, all of the low efficiency
>numbers correspond to very narrow VSWR bandwidths.
>This suggests a very high loaded Q for those tuner settings,
>which would be consistent with higher circulating current
>in the inductor and the resulting poor efficiency. It will
>be interesting to find out if the ARRL controlled for this
>in their testing, or if the tuner knobs were treated as
>roulette wheels.
>BTW, I for one think that an "inductor temperature"
>readout would be a useful feature to have on an antenna
>73 de Mike, W4EF................................................
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