I just bought a 238B and noticed that the schematic shows two 220pF in
parallel for each stage of added capacitance to be switched in with the Low
Z - High Z switch. In the parts list, they show what translates to two 180
pF capacitors in parallel at each switch terminal. So I had to look
inside...and looking inside in the actual 238B, I see three 150 pF ceramic
capacitors in parallel at each switch terminal.
It looks like they are increasing the current carrying ability of the
capacitors. An upgrade for the older tuner may well be paralleling more disc
ceramics, using lower values to add up to the required total for each
capacitance stage for increased current carrying ability.
Chuck, KE9UW
> -----Original Message-----
> From: tentec-bounces@contesting.com
> [mailto:tentec-bounces@contesting.com]On Behalf Of denton
> Sent: Sunday, November 14, 2004 18:16 PM
> To: tentec@contesting.com
> Subject: Re: [TenTec] Hi power Ten Tec ant tuner swr drift?
>
>
> Very good Al..I am the originial poster..
> I was merely inquireing as to why one L network would have swr
> drift while
> another later model L network would not under some circumstances.
> I will query Ten Tec next week and see if I can either purchase or secure
> the hf fixed caps they use in their current L network.
> Whenever I am experimenting with antennas that are fed with twin lead, I
> will dig up my old Johnson Matchboxes to use for a reference...that and a
> field strength meter, some on the air reports and just plain feeling the
> components after transmitting a bit. That and an antenna modeling
> program is
> what I have to work with.
>
> ----- Original Message -----
> From: <al_lorona@agilent.com>
> To: <tentec@contesting.com>
> Sent: Sunday, November 14, 2004 3:30 PM
> Subject: RE: [TenTec] Hi power Ten Tec ant tuner swr drift?
>
>
> > The feed impedance of that short antenna is much too low. You
> > are going to
> > melt down your tuner. Any tuner will get hot and consume a
> > large percentage
> > of your RF power when attempting to match such a low Z.
> > Steve N4LQ
>
> No, not necessarily true. Normally a blanket statement like this wouldn't
> bother me at all, but to say that "Any tuner will get hot and consume a
> large percentage of your RF power" demands a little bit more
> engineering and
> mathematical rigor in its defense.
>
> We can, for example, take a link-coupled tuner with large components of
> sufficiently high Q (air variables and large diameter, large gauge coil)
> that would have very little loss, simply because there are predominantly
> reactances in the circuit, which dissipate no heat. Many such
> tuners are in
> use all over the place, happily matching short antennas at low
> frequencies
> with very little power loss.
>
> Of course, for the vast majority of tuners out there, including and
> especially the automatic tuners that are especially in vogue (re:
> November
> QST Short Takes product review) nowadays this statement is,
> unfortunately,
> absolutely correct.
>
> Eliminate the baluns and toroidal coils and small capacitors and
> insufficient inductors, and the problem of matching a very low impedance
> goes away completely.
>
> The efficiency of the antenna itself (as compared to, say, a half-wave
> dipole) and ground losses are other topics that we can deal with
> separately.
>
> To the original poster of the question: The SWR drift you are seeing is a
> result of the components in your tuner heating up. You need a different
> tuner architecture to handle the low impedance presented by your antenna.
> Discontinue use of any tuners with toroidal baluns immediately.
>
> Al W6LX
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