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[TowerTalk] Rising VSWR

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Subject: [TowerTalk] Rising VSWR
From: (Chris BONDE)
Date: Thu Jan 30 14:38:52 2003
I know that this is admitting lack of knowledge but ....
When is the capacitor at highest capacitance?  (I shall leave that question 
open to see what answers I receive, as I could put in a few qualifiers.)
Chris opr VE7HCB

At 11:37 AM 2003-01-30 -0500, you wrote:
>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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