On 10/28/13 12:31 AM, Michael Tope wrote:
On 10/27/2013 11:01 AM, Jim Lux wrote:
Whether it will take the SWR to 1:1 is a function of the step size in
the tuner design (if it's a switched L and C) and the control algorithm.
Most autotuners stop when the SWR is below, say, 1.3:1 or 1.2:1.
What you might want is a way to manually configure the L and C, and
then store that for the frequency (most tuners can do this now.. the
AT200PC can, for sure), or have a computer that knows what frequency
you're tuned to, and then set it up.
(this is what I was doing with my active phased array.. I used AT200s
as essentially computer controlled LC networks)
If you are shooting for really close to 1:1 VSWR to keep an super
finicky SSPA happy, you probably want something with continuously
variable matching components in it. RF Concepts (aka Alpha Power) is
working on one to handle a wide impedance range:
http://www.rfconcepts.com/PRODUCTS/New-Products/Alpha4040
I think they are still struggling with making it work reliably over what
amounts to a fairly ambitious impedance range, but I am sure it as it
stands now it would pull in the VSWR of a Tennadyne LPY from 2:1 down to
1:1 without breaking a sweat. While it's not exactly the same, the Alpha
87A and 9500 as well as the Acom 2000 already do this sort of thing -
that is - automatically bring in a <3:1 VSWR (<2:1 for the 87A) into
something close enough to make the finicky ceramic tubes with delicate
grids happy. The auto tuning of the stepper-motor driven variable caps
on these products is very fast.
In this case of more forgiving PAs that don't need super low VSWR, the
discrete tuning step approach is as you suggest perfectly fine.
That's what I was thinking..
I think you could probably adjust the L and C under computer control
with stepper motors as fast as you can spin the VFO dial. They do this
sort of thing on big RF plasma systems (industrial processing) to keep
the match optimized.
Big band switches might take a bit longer if you have to make big L
steps. OTOH, for a "purpose designed" tuner, you could use relays to
switch big L in steps, and have a small variable L.
Also, it's really more a matter of how small the LSB is on the relay
approach. 0.001 microhenry steps (0.37 ohms at 30 MHz) are doable.
It's just a cost thing: how many relays do you want to buy. Narrow band
is easier (you probably don't need 0.001 uH steps for top band)
The challenge is in calibrating. I've measured 5 AT200PC tuners and
a) they are not the same as each other
b) the steps are not linearly combined. That is, if you have a 1,2,4
sequence, turning on the 2 does not always add the same inductance or
capacitance. It's, I suspect, coupling between the components, and
interaction with the wiring.
This isn't a problem in the "autotuner" because the search algorithm can
always find a minimum, and as long as the steps are always getting
smaller (i.e. it's monotonic) then it works.
As a result, though, you can't set a particular L and C in an a-priori
sense. For the LPDA/Yagi matching scenario, you'd have to make a cal
run, and store the actual values you need for each frequency.
This is really no different than the SteppIR thing, if you tweak the
factory default settings.
For the phased array scheme, it's a WHOLE lot more complex because of
the mutual impedances between elements (that is, changing the match at
the feedpoint of element A changes the Z at feedpoints of elements B, C,
and D). It starts to get very complex, because you now need to measure
the element current mag and phase in real time. (A pain in the rear 20
years ago, not so much today, though)
This is why I'm thinking that synthesizing the reactive component with a
suitable PA, rather than using switchable passive components for "power
factor correction", is a nice idea. (it also lends itself to very rapid
frequency changes)
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|