W1MJ 5-Watt Fox V1.0


Eliot Mayer W1MJ

January 15, 2022

Updated March 19, 2022



Introduction. 1

Main Diagram.. 3

Top-Level Bill of Material 4

Temperature Considerations. 4

Antenna. 5

Design, Interface Circuit. 6

Assembly, MP3 Player & Interface Circuit. 9

Test Results. 10

Bench Measurements. 11

Final Assembly Tests. 12

Antenna Measurements. 12

MP3 Scripts. 12

Appendix 1:  UV-5R Interfacing Information. 15

Appendix 2:  MP3 Player Information. 17

Description from Amazon. 17

MP3 Player Measurements & Observations. 18




This document describes the design of the W1MJ 5-Watt Fox, Version 1.0.  This type of fox is an automated radio transmitter used by the amateur radio community in hidden transmitter hunts, AKA fox hunts.


This fox uses an mp3 player to send voice messages to a Baofeng UV-5R handheld transceiver via a homebrew interface. 


The UV-5R has a VOX feature, but it exhibited undesired dropouts even at maximum sensitivity.  The dropouts could be eliminated by adding a strong “sub-audible” tone to the recording.  However, the interface in this design contains its own VOX circuit, which controls the radio’s PTT (transmit control) input.  The sub-audible tone is therefore not needed.








MP3 Player

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Main Diagram




·                     The PTT return is via the power supply returns (GND).

·                     After 5VDC supply current was measured, it appears that a simpler linear regulator could have been added to the Prototyping Board in instead of using the separate DC-DC Converter.  Impact on battery life would be negligible.

·                     In the past, and in one instance with this fox, I have found the reliability of cigarette lighter plug / socket mating to be so-so.  While the part of the Battery Eliminator that mates with the radio is required, the regulator may be replaced in the future to improve reliability.



Top-Level Bill of Material



Baofeng UV-5R

Transceiver Battery Eliminator

Baofeng Tech BL-5

Battery Eliminator Adapter Cable

Cigarette Lighter Socket

+ 15A Powerpole connector

Battery, LiFePO4 12.8V 30AH (Note 1)

Bioenno Power BLF-1230A

Power-Pole Distribution Block, 4-Position

Quicksilver PWR-BLOK 4 or equivalent

DC-DC Converter, 5V 3A Output

Biznet / Tobsun EA15-5V

Prototyping Board, 3.2” x 2.6”

Proto Advantage SBB2805-1

MP3 Player, 5VDC Power


Interface Circuit

See BOM in Interface Circuit section.


Twinlead J-Pole; see Antenna section.

Adapter, SMA-Female to UHF-Female

From Quicksilver Radio

Double Female SO-239 Bulkhead Mount, 2 inches long

From Quicksilver Radio

Washers for Bulkhead Mount

From Quicksilver Radio

Antenna Pigtail Cable

RG-58 w/ PL-259 connectors

Shielded Cable w/ 3.5 mm TRS plug

(1/2 junk box cable)

Shielded Cable w/ 2.5 mm TRS plug

(apparently not needed for PTT return)

From Amazon if needed (cut in half)


Pelican 1200, Black (available on Amazon)


Schlage Weatherproof Key Padlock with Flexible 3/8" Steel Looped Security Cable

(padlock too small; see next item)


Master Lock 1KALJ


Note 1:  A smaller 12V or 12.8V battery may be used with reduced operating time per charge.  The specified battery will run the fox for an entire weekend and had already been purchased for other ham radio purposes.


Temperature Considerations

·         The UV-5R transceiver has a specified operating temperature range of -20˚C (-4˚F) to +60˚C (140˚F).

·         Bionenno Battery:  According to Kevin at Bionenno, the battery operates down to -10˚C (14˚F), but must be charged at greater than 0˚C (32˚F)

·         DC-DC Converter:  80˚C (176˚F) maximum operating temperature; no minimum specified.

·         MP3 Player:  Unfortunately, has no specifications.

·         Interface:  Using only through-hole parts for ease of construction results in limitations.  The comparator IC is only rated for 0˚C (32˚F) to 70˚C (158˚F), but I am taking an intelligent guess that it will be functional and performing adequately below its rated range.


The overall operating temperature range has to be determined by experimentation.  See Final Assembly Tests.





Twinlead J-Pole antenna:

·                     Per this article:  https://www.qsl.net/wb3gck/jpole.htm

·                     Feedline = RG58, wrapped 3X around an Amidon FT140-43 ferrite core.


Related article:




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Design, Interface Circuit


First schematic from Digi-Key Scheme-It:



The following simple circuit would suffice but requires a strong sub-audible tone in the mp3 file in order to prevent UV-5R VOX drop-outs.  The trimpot may not be needed, based in on initial testing with an iPad as the mp3 player.  The transformer is probably needed to prevent undesired PTT keying if the mp3 player and radio battery eliminator share a common ground (TBD).


To avoid the need for the strong sub-audible tone, the circuit shown below is used.  It is intended to be a better VOX.  It keys the UV-5R PTT.  The PTT return via the battery eliminator GND appears to suffice, but J3 is shown in the diagram just in case.


For reference, see the following:

·                     Appendix 1:  UV-5R Interfacing Information

·                     Appendix 2:  MP3 Player Information


Schematic, draw with Digi-Key Scheme-It:




Audio peaks from the MP3 player charge capacitor C2.  R3 limits the loading on the player to prevent distortion.  When the voltage on C2 exceeds the reference voltage at U1-3, comparator output U1-1 pulls the radio PTT line (P1-Sleeve) to GND to key the transmitter.  Hysteresis is provided by R7 to ensure clean PTT switching.  When the audio pauses, C2 discharges through R4, ending transmission after approximately 2 seconds.  LED D2 indicates that PTT is active.


Potentiometer R2 is used to set the audio level to the radio for proper modulation.  Audio transformer T1 is needed to prevent the audio circuit from keying the PTT.  


Design calculations (Excel file embedded in Word document):





·         LM393 Comparator data sheet:  https://www.ti.com/lit/ds/symlink/lm193.pdf

·         If an audio amp is needed for the vox, consider LM324:  https://www.ti.com/lit/ds/symlink/lm324.pdf


Interface BOM, downloaded from Digi-Key schematic project then edited:




Assembly, MP3 Player & Interface Circuit




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The assembly drawing is maintained in this Excel file embedded in the Word document:




Test Results


A successful test with a solderless breadboard was done on 1/9/2022.

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Breadboard is from REXQualis Electronics Component Fun Kit.


Bench Measurements

12V supply:  1.1A (TX=5W) / 0.6A (TX=1W), 0.0A (not transmitting)

12V supply current w/o radio:  36 mA

5V supply current:  62 mA during transmit, 60 mA when not transmitting.


The comparator threshold measured close to expected levels per Fox_Calculations.xlsx:


Vt, Rising (VOX Not Tripped)



Vt, Falling (VOX Tripped)




PTT voltage:

·         Receive:  +3.3 V

·         Transmit:  +80 mV


Comparator input, solderless breadboard w/o transformer:


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Final Assembly Tests


When the components were transferred to the prototyping board, U1B was somehow damaged.  Pin 6 had about 5 kohms resistance to ground, which dragged down the comparator threshold voltage.  The connection to this pin was then removed, leaving the fox functional except for the Transmit LED (D2).  The LED was just a nice-to-have feature so changing U1 didn’t seem worth the trouble.  In hindsight, I should have used an IC socket.


Operating Temperature:  The fox ran continuously from 10 AM to 10 PM on 1/14/2022, during which time the temperature dropped down to -7˚C (20˚F).  Battery voltage 13.26.


Antenna Measurements

·         SWR, away from other objects: Approximately 1.5

·         SWR, right next to a tree trunk:  Approximately 3.0


MP3 Scripts


The audio recordings were made with Reaper DAW.  Each message is preceded with a hand clap about ˝ second before the message proper.  This gives the VOX time to trigger and the transmitter time to start transmitting, ensuring that the start of the message is heard.  The claps themselves are not heard.  Only one file is stored on the MP3 player USB drive, and the player repeats the file.





fox_20s_60s.mp3:  File for 20 second transmission once every 60 seconds.  This file was only used for initial testing.


Welcome to the hidden transmitter hunt.  This is the W1MJ 5-Watt fox.  Please email your fox hunting report to w1mj@arrl.net.  Good luck from W1MJ.


fox_12min_v1.mp3:  This 12-minute file is normally used with the fox.  The following messages are typically 20 seconds long and start roughly at 1 minute intervals.


(1)  Welcome to the hidden transmitter hunt.  This is the W1MJ 5-Watt fox.  Please email your fox hunting report to fox@w1mj.com.  Good luck from W1MJ.


(2)  This fox uses a Baofeng UV-5R transceiver.  The output power will normally be set to 5 watts but might be set to 1 watt on some hunts.  W1MJ


(3)  This fox uses an MP3 player with a homebrew interface to the transceiver.  Thanks go to K1MJC and K1PJW for their contributions to the design.  Photos and technical details of this fox are available online at w1mj.com/fox.


(4)  I’m considering a fancier fox design based around a Raspberry Pi.  Features under consideration include:

·         limiting transmissions to daylight hours, and

·         reporting battery voltage.

If you have any suggestions, please let me know.  Thanks.  W1MJ


(5)  This fox has no logbook.  Please email your fox hunting report, your suggestions, or your complaints to fox@w1mj.com.  Thank you from W1MJ.


(6)  Deployments of this fox are announced on two groups.io groups:  Northeast Massachusetts Fox Hunters and Waltham Amateur Radio Association.  W1MJ.


(7)  CW (18 WPM):  I hope you can find me DE W1MJ


(8)  Many years ago, when most foxes were live humans, I put together an automated fox with a reel-to-reel tape recorder.  I made the 2-hour tape sound like I was a live fox, and successfully fooled many hunters.  I openly admit that this fox is automated.  W1MJ


(9)  According to the Transmitter Hunting article on Wikipedia, Transmitter Hunting is also known as T-hunting, fox hunting, bunny hunting, and bunny chasing.  Very interesting, but then again, one cannot trust everything that one reads online.  W1MJ


(10)               Fox hunting equipment can be very sophisticated but does not have to be.  I hunt with only a walkie-talkie, a whip antenna, and a 30 dB attenuator that I can put inline with the antenna.  I use the “body fade” method and pay attention to signal strength.  W1MJ


(11)               I usually fox hunt along with my dog Noah.  Once, Noah and I were just out for a hike in the Middlesex Fells Reservation, and Noah suddenly broke into a run.  I looked and saw that he was doing a more traditional fox hunt.  That fox got away.  W1MJ


(12)               CW (18 WPM):  Good luck finding the fox DE W1MJ



Appendix 1:  UV-5R Interfacing Information


From  http://www.miklor.com/COM/UV_Technical.php


Diagram, schematic

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From https://k0rx.com/blog/2017/11/baofeng.html


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1.    On UV-5R:

a.    All 3 connections to the 3.5 mm jack measure +3.3VDC against sleeve of 2.5 mm jack.

b.    All connections to the 3.5 mm jack measure 0 VDC against each other.

c.    The sleeve of the 2.5 mm jack is connected to Battery Eliminator (-) contact.

d.    Keying radio with DMM from 3.5 mm jack sleeve to 2.5 jack sleeve, current measured about -7 mA (unstable, but always negative {?}).  See next item…

e.    Keying radio with DMM from 3.5 mm jack sleeve to battery eliminator – (cigarette lighter plug), current measurement depended upon setting of Radio Shack DMM; it was either +0.5 mA or -1.5 mA.

2.    With direct connection of iPad Ring/Sleeve to radio Ring/Sleeve:

a.    The audio level seems reasonable (without attenuation).

b.    VOX dropped out unexpectedly a few times with voice recording.

c.    VOX stays on with a strong sub-audible tone added to the voice recording.


Appendix 2:  MP3 Player Information


Description from Amazon



Use this DIY MP3 player/audio decoder module to build your own MP3 player, Boom Box or portable audio player. Connect directly to headphones, or pair the stereo output of this player with an audio amplifier board to power your speakers.


Product Overview:

Product information

Package Dimensions

6.5 x 4 x 1 inches

Item Weight

1.58 ounces





Customer Reviews

3.9 out of 5 stars 4 ratings

3.9 out of 5 stars

Best Sellers Rank

#45,416 in Electronics (See Top 100 in Electronics)
#291 in MP3 & MP4 Players

Is Discontinued By Manufacturer


Date First Available

January 23, 2019

MP3 Player Measurements & Observations


·         Neither Speaker terminal is connected to BAT- (GND).

·         The sleeve of the 3.5 mm jack is connected to BAT- (GND).

·         When playing an mp3, there is a blinking red LED.

·         Output:

o   Headphone jack:  Approximately 200 mV peak AC at maximum volume, 0 VDC offset.

o   Speaker+ terminal:  +2.5 VDC offset.  Approximately 2V peak at maximum volume:


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§  No output if plug inserted in headphone jack.