This document describes the construction of a very simple diode power meter
for low signal levels (between -30 and +20 dBm). It is used in combination with
a cheap chinese wideband 'Cavity Coupler' to measure output powers on 2.4 GHz, and
reverse power without having to carry around a heavy professional power meter.
The entire 'RF head', including the 50 Ohm termination, is soldered directly
on an SMA connector. The terminator consists of two 100 Ohm SMD resistors,
directly soldered between the SMA center connector and the ground plate.
There are no wires or PCB tracks at all - all components are 'concentrated'
around the SMA center pin to minimize lead inductance.
Photo of the diode power meter (detector head)
on an SMA Jack Panel Mounting (SMA Flanschbuchse)
There are no active parts on the 'detector head'. To measure signal voltages
way below the Schottky diode's forward voltage, the load resistance
for the rectified DC voltage must be high. Instead of building yet another
microcontroller-based display, I simply use a digital voltmeter with at least
1 MOhm input resistance. Then, with the aid of a 'calibrated' signal source,
the detector output voltages were measured for signal levels between -30 and +20
dBm (100 mW is the maximum, limited by the tiny SMD resistors), for 144 MHz,
435 MHz, and 2.4 GHz. The results were entered in a LibreOffice 'Calc' table,
and plotted into a LibreOffice X/Y diagram. The X axis is the power in dBm,
the Y axis (with logarithmic scale) shows the detector voltage in mV (DC).
Note that the 1 pF coupling capacitor, together with the diode's capacitance,
will form a capacitive voltage divider - thus your mileage may vary.
Diode power meter circuit (detector head)
Since the author's last BAT62-03W decided to drop onto the carpet (where it
vanished into dust), an old BAT 45 with up to 1.1 pF at zero bias was used instead.
The LibreOffice 'Calc' file can be downloaded from here,
so you can replace the measured detector voltages in columns 'Udet 144 MHz', 'Udet 435 MHz', 'Udet 2.4 GHz'
(the column titles should speak for themselves). When printed, the entire spreadsheet
looks like this (PDF), including the
diagram of detector voltage ("Udet" in millivolts) versus RF power (in dBm):
When connected to a DC voltmeter and one of those low-cost Chinese directional
couplers (used to be available for 6 or 7 Euros in 2020), the detector allows
measuring forward- and reverse powers of up to 50 dBm = 100 watts (with a
"30 dB" coupler and 20 dBm maximum for the detector itself).
Diode power meter detector curves
(detector voltage versus RF power level)
For moderate power levels (QO-100 narrow band transponder), a "20 dB" coupler
would be better, but it was not available. Anyway, a digital voltmeter in the
200 mV range measures with 0.1 mV resolution, so even with only 1 Watt forward
power through the coupler (= +30 dBm), the voltmeter showed 140 mV DC, so the
resolution is sufficient to check output power and antenna matching.
Diode power meter connected to a Chinese directional 'cavity' coupler.
TX output on the left ("IN"), forward power attenuated by 30 dB on top connector.
To measure the reflected power, reverse "IN" and "OUT" port.
On this occasion: The Chinese directional coupler shown above, with a specified
frequency range of 800 to 2500 MHz, can also be used on lower frequencies (with
lower coupling of course, but you can take this into account since it's easy
to measure). The author's 'WINHAP WHDSCPQ-30' (= coupler with nominal 30 dB
'forward' coupling between 800 and 2500 MHz) delivered on the "30 dB" port...
On 2.4 GHz, the Chinese '8 Watt' WiFi booster
delivered a detector voltage of 330 mV "forward" (= ca. +5 dBm on the coupler's
"30 dB" output), i.e. approximately 35 dBm = 10^(35/10) mW = 3.16 Watts. This is
in the right ballpark for this kind of amplifier when not driven too hard.
+12 dBm "forward" with 50 watts (=+47 dBm) fed to the "IN" port on 435 MHz,
thus 35 dB "forward" power attenuation on 435 MHz;
-18 dBm "reverse" with 50 watts (=+47 dBm) fed to the "OUT" port on 435 MHz,
thus roughly 12 dB - (-18 dB) = 30 dB directivity on 435 MHz;
+4 dBm "forward" with 50 watts (=+47 dBm) fed to the "IN" port on 144 MHz,
thus 43 dB "forward" power attenuation on 144 MHz;
-24 dBm "reverse" with 50 watts (=+47 dBm) fed to the "OUT" port on 144 MHz,
thus roughly 4 dB - (-24 dB) = 28 dB directivity on 144 MHz;
not bad if you consider that this is far outside
the coupler's specified frequency range.
With the coupler reversed (3.16 Watts into the "OUT" port, and a questionable 50 Ohm
dummyload on the "IN" port), the diode power meter indicated 2 mV "reverse" (= ca. -20 dBm)
on the coupler's "30 dB" output. On 2.4 GHz, the "power dummyload" possibly
wasn't good enough to check the coupler's directivity (which should have been
greater than the measured 25 dB).