On Thu, 21 Mar 2013, m II wrote:


On 13-03-20 01:55 PM, Brenda Dyer wrote:

Actually, most receiver inputs are nominally 50 ohms, though some
do have high impedance inputs as well.


It looks as though I screwed up. I mistakenly thought that a field
effect transistor would automatically mean a high impedance input.
Back to the books..

No.

Brenda's right, unlike decades ago when many receivers had 300ohm input
impedance (or something very vague), virtually any tabletop receiver for
decades has 50ohm input impedance.

But, portables are another matter. I have no idea if the antenna jacks
are intended for some specific impedance, I suspect not, the mini phone
jacks suggest nothing too serious, and you're basically sticking some
length of wire into those jacks.

And of course, your original thought wsa right, most of those portables
use 'active antennas" where the whip feeds a very high impedance point,
and the FET transforms that down to a more reasonable impedance. That
first stage is not amplifying voltage, it's amplifying current. The high
input impedance means the whip antenna is acting like a voltage probe,
rather than some sort of resonant antenna.

It's not that different from car radios in the past, where the whip
antenna was fed to a very high impedance point, in their case the first
tuned circuit. That's why special low capacitance cable was needed to
hook the radio to the antenna, and why they all had trimmers, to
compensate for the capacitance of the cable and the whip across the tuned
circuit. But there it was a tuned circuit that helped boost the impeance
point. The whip was there only to act as a probe to get the signal onto
that high impedance point.

I don't know whether portable shortwave receivers have their antenna jack
after the active antenna stage or not. Well, I'm sure I've seen some
where the antenna jack doesnt' feed the active antenna stage, but I don't
know if that's the case for all.

One thing worth remembering is that the multi-receiver couplers of the the
old days were about isolating the receivers from each other, not just
about the impedance. Since each had a tuned circuit at the front (though
fed with a low impedance link) tuning one might affect the other(s), so
you want isolation between receivers. The "active antenna" stages with
their high impedance don't have tuned circuits at the input, but I don't
know if they interact in a way you don't want.

Michael