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Author Topic: How to calculate the antenna input impedance of a receiver?  (Read 7548 times)
W8JI
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« Reply #15 on: October 29, 2011, 03:44:26 AM »

And, I see what you mean about the "omission" of any
bypass capacitor on that audio amp.  That capacitor
also sets up the AC-Load-Line, no? 

No. The impedance presented by the speaker or headphones through the transformer sets up a "loadline".

Quote
And, maybe changes
the frequency response to add more "lows" to the audio
output?

Any impedance in the cathode develops a voltage in phase with grid voltage. This has the effect of reducing the varying G-K voltage, and reducing the available voltage across the plate load resistance. The effect it has depends on tube characteristics and the load impedance presented to the plate by the audio transformer or headphones. The DC path also sets bias, of course, which determines the operating point of the tube.

In short form, the cathode resistance adds negative feedback and also wastes a little output power by decreasing the effective plate voltage. It also sets the bias. The negative feedback reduces distortion and gain. It will mostly pick up the volume if you bypass the 100 ohm resistor.

I'd worry more about how they connected the detector diodes in the grid circuit. That's very strange.

73 Tom
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WB6THE
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« Reply #16 on: October 29, 2011, 08:42:36 AM »

Here we go again but I had a brainstorm. I think you can measure the
receiver's Zin like this:

Disconnect the ground side of L1.
Between the disconnected side of L1 and ground install a non-inductive
plain old carbon resistor of a value you'll determine by whatever works best.
Set your signal generator to a frequency of interest and connect it to the
receiver input terminal and ground. Use your 'scope to set and measure
the amplitude of the generator. Remember what amplitude you set it to.
Now use the 'scope to measure the voltage across that resistor. If need
be adjust and remeasure the generator to get an easily measured reading
across the resistor. And-or try a different value resistor. But ensure that
you know the generator's amplitude at the time of measurement. Write all
that down. Restore the L1 connection to ground and place the same
resistor in series with the generator and antenna terminal. Ensure the generator
amplitude is set to the same value as you wrote down. Keep the generator
frequency constant.Use the 'scope and measure the voltage across L1.
Knowing the resistor value and the ratio of Ecoil to Eresistor the impedance
of the receiver input can be calculated.

That won't give you the complex vector but it should give you Z.

Z should change with generator frequency and the settings of the two tuning
capacitors.

I did that once to find the reactance of an inductor when I didn't have an LC meter.

Yeah... that *might* work.

Alan
WB6THE
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G3RZP
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« Reply #17 on: October 29, 2011, 08:44:49 AM »

>In short form, the cathode resistance adds negative feedback and also wastes a little output power by decreasing the effective plate voltage. It also sets the bias. The negative feedback reduces distortion and gain. It will mostly pick up the volume if you bypass the 100 ohm resistor.<

A purist would perhaps worry about the fact it effectively increases the plate resistance of the tube, thus reducing damping of the loudspeaker. As this isn't hi-fi, it doesn't matter - for communciations purposes, you usually win more than you lose.

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W8JI
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« Reply #18 on: October 29, 2011, 04:05:32 PM »

Peter,

Did you look at the diode detector in that receiver? Check it out.

Tom
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G3RZP
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« Reply #19 on: October 30, 2011, 03:31:06 AM »

Tom,

Interesting. Quite a high DC load, and lower AC load. It would appear to rely on the reverse leakage of the 1N34 for the DC load.

I suspect it will distort with high modulation percentages and it is not good design practice.

The unbypassed cathode resistor in the audio won't matter  with the low plate resistance of the triode.

That's the 1969 Handbook. The 1970 is all solid state in the receivers section.
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W8JI
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« Reply #20 on: October 30, 2011, 05:19:06 AM »

Tom,

Interesting. Quite a high DC load, and lower AC load. It would appear to rely on the reverse leakage of the 1N34 for the DC load.

I suspect it will distort with high modulation percentages and it is not good design practice.

The unbypassed cathode resistor in the audio won't matter  with the low plate resistance of the triode.

That's the 1969 Handbook. The 1970 is all solid state in the receivers section.

I wonder if it detects in the grid with good diodes. :-)
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G3RZP
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« Reply #21 on: October 30, 2011, 09:41:43 AM »

Quite possibly doesn't work with good diodes. Not that it's anything some gamma rays and nichrome won't fix.
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N6AJR
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« Reply #22 on: October 30, 2011, 11:19:47 AM »

I don't know how to figure the  impedance, but in the past when I did some SWL I used a small tuner box and tuned for the lioudest noise in the reciever.  It may have been an MFJ product or perhaps a QRP tuner. It did work well with a random wire.
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KC9KEP
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« Reply #23 on: October 30, 2011, 01:14:59 PM »

2 additional comments ..

There were two versions of the Mighty Midget published by the ARRL.  The first was in QST magazine.
It indicated the two 1N34A type diodes in the detection circuit.

If memory serves me, the author stated that the diodes were arranged as a "voltage doubler" (as is
commonly done in DC power supplies.)

With no fan-fare, these two diodes dissappeared in the ARRL Handbook version of the same receiver
a couple years later, being replaced with a single 1N34A detector diode.

Also, just one more comment/question about the resistor/capacitor in the cathode of the audio amp ..

I thought that the plate resistance, cathode resistor, etc. set up the DC load line.  And, because
the paralleled capacitor "passes" the AC to ground, that the capacitor set up the AC load line?

Just curious :-)

73

--KC9KEP
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G3RZP
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« Reply #24 on: October 30, 2011, 02:43:16 PM »

For a given plate voltage, the cathode resistor sets up one end of the load line, since the transformer has minimal effect. The Load resistance in shunt with the plate resistance affects the slope, althoug usually, you use the Vp/Ip curves and figure on the grid swing to give the desired plate swing for the output.

The cathode capacitor reduces the effective plate impedance. For a full description, see M. G.Scroggie 'Second thoughts on Radio theory'. Or maybe even 'Radiotron Designer's Handbook' by Langford-Smith.

The single diode detector as per the 1969 Handbook is a very poor way to go.
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W8JI
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« Reply #25 on: October 30, 2011, 06:00:51 PM »

2 additional comments ..

There were two versions of the Mighty Midget published by the ARRL.  The first was in QST magazine.
It indicated the two 1N34A type diodes in the detection circuit.

If memory serves me, the author stated that the diodes were arranged as a "voltage doubler" (as is
commonly done in DC power supplies.)

With no fan-fare, these two diodes dissappeared in the ARRL Handbook version of the same receiver
a couple years later, being replaced with a single 1N34A detector diode.

Thomas,

That detector circuit is not a doubler, and actually will not work at all with good diodes. It lacks a load resistance to pull the diodes back down.

The output of the series diode must have a resistance to ground somehow or the detector can't work at all.

73 Tom
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