[Elecraft] High power tuner

[Paul Christensen]

Looking at the RF Concepts simplified schematic diagram, the directional 
coupler appears to remain active when the tuner is in "bypass."  So, as long 
as the Smith chart feature works in bypass, it would definitely provide some 
useful information about the line input Z within the limits of the 
directional coupler accuracy.

Paul, W9AC

----- Original Message ----- 
From: "Paul Christensen" <w9ac at arrl.net>
To: <elecraft at mailman.qth.net>
Sent: Saturday, November 26, 2011 11:04 AM
Subject: Re: [Elecraft] High power tuner


>> An example of how the price of a tuner can grow astronomically with
>> power level.
>> At what point does it make more sense to spend the same $$ for antennas
>> which don't require a tuner?
>
> For some of us where only one HF antenna is feasible, it may make sense to
> put all the money in the tuner and *not* the antenna.  For example, I've
> gotten away with an 80m dipole at 60-ft in my gated community, only due to
> the natural antenna supports: pine trees.  For me, the optimal solution 
> for
> 80m-10m coverage, consists of a remote-controlled, symmetrical tuner built
> in a WX-proof enclosure and located at the base of an open feeder line.
> This offers me minimum system loss, minimum RFI leakage to my transmission
> line, and all band coverage.  Sure, I have no control over maximum lobes 
> and
> minimum nulls at higher frequencies, but given the QTH constrains, it made
> sense to put all the money into a truly balanced tuner and nearly zero 
> cost
> in the antenna.  Through 4Nec2 modeling and TLD, my antenna system losses
> are very low and in all cases, exceeds the performance of resonant 
> mono-band
> dipoles fed with LMR-400.  My 600-ohm line length is optimized for all 
> bands
> using an N2PK VNA.
>
> The new RF Concepts tuner would not work well for me.  It's an unbalanced
> C-L-C design with a CM choke input, similar in design with several Palstar
> tuners, only with much more rugged components than what Palstar provides.
> W9CF, W7EL, and W8JI have analyzed a current choke placed at the input to 
> a
> C-L-C tuner and have concluded that placement at the input is not as
> effective for maintaining line balance as a balun placed at the tuner's
> output.  The exception being if the CM choke is placed at the input of a
> symmetrical, balanced tuner (e.g., AG6K type).  My tuner is of this type 
> and
> can be seen on my QRZ.com page.
>
> Read through the eHam reviews and see just how gullible we are when it 
> comes
> to tuner evaluation.  Nearly all accolades are based on: (1) the ability 
> of
> the tuner to achieve an input VSWR of 1:1; (2) pretty layout; and (3)
> component size.  None of these factors tell us about the tuner's 
> efficiency.
> When a tuner needs it own cooling and ventilation system to function, that
> should throw up red warning flags.
>
> The new RF Concepts tuner has switched, 8pF to 800 pF output C.  Although
> better than most commercial tuners, it would have been even better to at
> least double that amount so long as minimum C is maintained through high
> isolation switched C.  The real *big* unknown with the new tuner is coil Q
> over its entire range.  I would like to see a Q plot of the coil mounted 
> in
> the metal enclosure.  That's a critical piece of evidence in order to
> evaluate the tuner's efficiency, especially with low-Z line terminations.
> With both input and out C being vacuum types, I would expect high-Q for 
> the
> C components.  I'm not too concerned about a tuner's ability to tune and
> match low-Z loads.  For base station operation, there are few combinations
> of full size wire antennas that place a low-Z at the line input, no matter
> the line length.  If I'm dealing with low-Z transmission line inputs, then
> its an antenna I probably don't want in the first place. For portable and
> mobile installations,  that a wholly different matter.
>
> Finally, the new RF Concepts tuner has a slick Smith chart display on the
> front panel.  Only, the charting shows the Z at the input of the tuner 
> which
> is of very limited value.  Since the input is always tuned and matched for
> 50+j0, even the simple Monimatch circuit is sufficient for that purpose.
> So, it's nice "eye candy" but I see nearly no value in the display.
>
> Paul, W9AC
>
>
>
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