North Texas QRP Kits



   Circuit design  by Joe, N2CX,  the Accuprobe is a descendant of the common diode RF detector.  It uses a special operational amplifier with a compensation diode to cancel out non-linear diode characteristics at low RF input levels.  This allows accurate measurements down to about 50 mV rms as compared to several volts for ordinary detectors.  This translates to a very low QRpp level of about 50 microwatts.  The upper end of the range is 5 V rms or 1/2 watt.  A second uncompensated range is provided to allow measurements up to 35 V rms or 24-1/2 watts.  Compensation is not needed for the higher voltage range since the detector is linear for high input levels.

     The Accuprobe is used as an accessory to a reasonable quality digital multimeter.  It provides a DC output voltage that gives a DMM reading calibrated to the RMS value of the Accuprobe RF input.  On the LOW range the DC is read directly while the DMM reading is 1/10 the RF input on the HIGH range.

     The Accuprobe is AC coupled so it can be connected directly to biased circuits with voltages up to 50V without upsetting circuit bias or inaccurate readings due to presence of DC.  Its high input impedance and low capacitance allow it to be used with minimum loading on high impedance low-level circuits.

     A unique housing method will be described to allow the builder to construct a self-contained shielded detector with a built-in probe and convenient ground lead.  Hints for improving performance and tailoring the detector for other uses will be given later for the advanced homebrewer.



     The Accuprobe is useful for measuring RF signals with predictable results from 100 kHz to at least 30 MHz at levels ranging from 10s of millivolts up to 35 V rms.  It provides minimal circuit loading so that it can be used for signal tracing in oscillator and mixer circuits as well as multistage QRP transmitters.  Several suggested applications are:

               Amplifier input and output voltages to determine gain

         Filter input and output voltages to determine loss

         Measurement of resonant circuit or filter circuits across a frequency band to check bandwidth or Q

         RF power levels across a dummy load from microwatts to beyond QRP.  Note that a very accurate 50 ohm dummy load is needed to retain accuracy.

         Attenuator calibration by accurate loss measurements.

         Signal generator output calibration

The Accuprobe circuit board can also be incorporated into other projects wherever accurate repeatable RF voltage measurements are needed


- Printed circuit board dimensions: 1.8X1.9 inches
- Power: 9 VDC at approx 1 ma using a standard 9-volt battery
- Input levels: LOW range - 50 mV rms to 5V rms (usable to 20 mV)
                     HIGH range 5 V rms to 35 V rms
- Frequency range: 100 kHz to 30 MHz (Upper end not tested but should extend to VHF)
- Outputs : DC-compensated to rms input
     - LOW range 50 mV DC to 5 VDC (Uncalibrated readings to 20 mV)
     - HIGH range 5 VDC to 35 VDC (1/10 RMS input)
- Output accuracy: approx. 10% of reading decreasing to 25% at 50mv

What you get: You will receive the PC board, all board mounted parts and link to the instruction / operation manual. The builder will need to provide the enclosure and  battery.

Price: $ 21.00 post paid to any location in the USA.  $25.00 post paid to Canada and DX locations. MULTIPLE ORDERS (i.e. Group/Club Elmer, building projects, etc.) See Below

Availability:  Now taking orders!!

Ordering Instructions :

We accept PayPal to KK5NA @, or check, M.O. or cash to Joe Spencer 3618 Montridge Ct, Arlington, TX 76016  (Note: Remove the spaces in the email address [they are there to defeat spam robots])


Be sure to include:
Your Name
and email address (If you have one)

   email: kk5na @

WE OFFER SPECIAL PRICES FOR MULTIPLE ORDERS (i.e. Group/Club Elmer, building projects, etc.) 

5 kits  for  $90.00        plus $4 shipping         (Total= $94.00)

 10 Kits for $140.00     plus $5 shipping          (Total= $145.00)

 15 Kits for $185.00     plus $6 shipping          (Total= $181.00)

20 Kits for $250.00     plus $8.50 shipping     (Total= $258.50)

Documentation : The latest revision of the kits'  Instruction and Operation Manual are available here. Windows users can right click on the manual and choose "Save Target As..." to download. 


 The Accuprobe project:  Printed circuit layout has been redone by Rex, W1REX.  Circuit design was by Joe, N2CX.