When its plates are fully meshed.
"Success is the ability to go from failure to failure with no loss of
enthusiasm." -Winston Churchill
> -----Original Message-----
> From: Chris BONDE [SMTP:email@example.com]
> Sent: Thursday, January 30, 2003 11:31 AM
> To: Joe Reisert
> Cc: firstname.lastname@example.org
> Subject: Re: [TowerTalk] Rising VSWR
> I know that this is admitting lack of knowledge but ....
> When is the capacitor at highest capacitance? (I shall leave that
> 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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