Re: Bitx40 - receiver works great, but do not transmit #bitx40help

Arv Evans


It is possible to forward bias the detector diode to get much higher sensitivity.

Of course this does require DC power to the RF detector probe, or a small battery.


On Sun, Feb 11, 2018 at 8:30 PM, Jack Purdum via Groups.Io <jjpurdum@...> wrote: has the 1N34a for $.24. Combine with some other stuff you need/want to cut down on shipping costs.

Jack, W8TEE

From: Jerry Gaffke via Groups.Io <>
Sent: Sunday, February 11, 2018 5:08 PM
Subject: Re: [BITX20] Bitx40 - receiver works great, but do not transmit #bitx40help

Good advice, build a diode RF probe.
A germanium diode such as the 1n34a or 1n60 works well, but they are expensive now and hard to get.
A schottky diode (such as the Bat54s used on the Bitx40 diode mixers) works reasonably well.
A standard silicon diode such as a 1n4148 might be good enough for measuring fairly high RF power nodes 
such as at the antenna port of a working transmitter, but definitely will not see the expected RF voltage at the base of Q13.
The issue here is the forward voltage drop for small currents, perhaps 0.3v for the Schottky and 0.6v for the 1n4148.

The germanium diodes will be less than the Schottky but dependent on temperature.
The datasheet punts, and says 1 volt max at 5ma, should be good down to 100mv or so with the currents involved in an RF probe at 10megaohms:

N5ESE tries to explain the probe, but only goes so far.
That RF probe design assumes a voltmeter with an input resistance of 10megaohms,
some DVM's will have an input resistance that is considerably less.
So don't count on that probe reading RMS voltages accurately until somehow calibrated.
As the RF voltages approach the forward voltage drop across the diode it will become less and less accurate,
though can still be surprisingly useful (perhaps with a calibration chart) down to a few tens of millivolts if you have the right diode.
Also, N5ESE fails to mention that the capacitance between the probes into the DVM is an integral part of the design,
that capacitance along with his 4.7megaohm resistor forms a low pass filter so that the meter sees DC instead of rectified RF.

We should have a spare Bat54s plus cap and resistor into an analog Nano pin, 
so we can all have a common way of measuring RF voltages when debugging.
With different diodes and different voltmeters, we will all get different results and cannot usefully compare our measurements.

Jerry, KE7ER

On Sun, Feb 11, 2018 at 01:14 pm, M Garza wrote:
Since you do not have a watt meter, and you have a multimeter, you could build a rf probe.  You can see the rf voltage and calculate the wattage.
Here is a link to one you can build.  It is very easy:

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