Steve,
If the topic is filtering and maintaining a good match - then why can
we not transform up to the input impedance of an inexpensive
multiband matching system?
The problem is imperfect transformers. Introducing delays, and having
poor coupling. We don't even need "high" impedance. 50 ohm would be fine.
What would be the input impedance of a multiband matching and
filtering system that was half the price and complexity of the
currently used per-band filter?
I would say, it's convenient to design it for anything between about 30
and 100 ohm input impedance. This would even allow to relay-switch
complete per-band matching/filtering networks. Each of them might use
just three parts. Or more, if more bandwidth is wanted.
But this ends up very close to the usual switched low pass filter approach!
Would the loss of that transformer be a magnitude worse than a 50ohm
output transformer?
It would be a 50 ohm transformer! :-) Designers came up with this in
the early 1970s...
Bill,
Harmonic filters do need to be switched in and out depending on the
band, but they don't need "tuning".
I can conceive one truly broadband approach, that would need no badn
switching of any kind: A class D amplifier with pulse width modulation,
having a carrier frequency around 200MHz or so. It would need a single
but good 30MHz low pass filter at the output, used on all frequencies
from zero to 30MHz.
The little problem with this approach is the lack of suitable power
devices... But maybe in the near future we will get them!
Cathy,
Tuning is a fact of life,
You are right, and that brief sentence has almost philosophical weight!
I try to set up my antennas so that I can have an SWR below 1.5:1 over
all band segments I use, but still occasionally I need to move out of
those segments, or have ice on the antennas, and then I need a tuner.
Which makes an SS amp with tunable output network an attractive option.
The tuning can be automatic or manual.
Instead of the current approach of using a broadband transformer to step
up the impedance, then a switchable low pass filter to reduce harmonics,
and then a tuner to adapt to practical antennas, it should certainly be
less expensive and more efficient to use a single tunable tank network
that does all three things in one. But to make it band-switchable, we
need high voltage transistors that can run at a few hundred volts.
That's the single problem. Unless somebody can think of a way to
band-switch at a 3 ohm impedance level. I can't.
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|