[Lowfer] Amplifiers for 136 kHz

[Jay Rusgrove]

John & Bill

I like the idea of the driver sending a sine wave out to the final box where you
square it up for the switching final. That makes a lot of sense.

When running in a linear mode, an amplifier with 50 ohm output impedance in the
shack probably makes the most sense.

Two coaxial transfer relays could be used to select or switch around either the
indoor linear amplifier or the outdoor switching amplifier. With this system,
when the outdoor switching amplifier is bypassed by the transfer relay, the loop
antenna is available for use as a receive antenna at the shack. Another single
pole double throw relay would be required in the shack to separate the
transmit/receive lines out.

Jay



John Andrews wrote:

> Bill,
>
> > Haven't figured out a way to drive a 100ft coax with a square wave having
> an
> > impedance near 0 ohms, unless I use a tuned circuit between the final and
> > the coax to convert the output Z to 50 or 70 ohms. But that makes it two
> > circuits to tune rather than one each time I want to change frequency. Any
> > ideas?
>
> You see, I ducked that here. I feed 10 milliwatts from the house into a 50
> ohm resistor at the transmitter in the woods. That's 2 volts peak to peak,
> which I run through a Schmitt trigger arrangement done with a 74HCT14
> package that is the driver for the PA transistors. I'll dig up a diagram.
>
> > Don't see too much of a handicap with an untuned final in the woods (well,
> > not counting the loop tuning cap). Takes < 2 minutes each way to run out
> and
> > bring in the final box to the bench to make alterations and then return
> it.
> > I have a loop simulator coil and exciter simulator on the bench so there's
> > no surprises when I re-connect it back to the loop and shack cables.
>
> Of course, you'll be feeding a little more DC power out there, as well.
>
> But it does provide good exercise to keep walking out there!
>
> John A.
>
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