433 mhz rf module experiments

•June 10, 2020 • Leave a Comment

I had these 433 mhz rf modules in my parts inventory for a copule of years and finally have gotten around to hooking them up to arduinos to experiment with them. 433 mhz is in the 70cm ham band making it legal for ham radio license holders to operate these devices.

transmitter: tx       receiver: receiver

The transmitter puts out it’s maximum signal at 433.85 mhz.  The power level is a question and I’ve seen figures from 1 mw to 15 mw.  To measure would require a voltage reading on a dummy load.


Theory of op: Receiver: The receiver antenna feeds a tank circuit which tunes the received signal.  This signal is mixed with the output of an oscillator, apparently  down to baseband.   This baseband signal goes to an opamp circuit to generate the digital signal.  What is it? – a variant of a superhet but without an IF, the output goes to baseband.  This resembles the front end of an SDR without the quadrature detector.

The receiver s/n can be subject to noise – a training preamble in the packet data train greatly increases the range of the system.

Even at this low power the signal can be heard on my mobile radio (FT7900) from a point 400m away.  In an open field I expect further than that.  The modules are designed for Amplitude Shift keying (ASK).

By using this kind of modulation,  digital signals transmitted at 9600 baud are possible over short distances.  In my testing short ranges of 10m are quite reliable.  In the open field, 100 meters and more are possible.

The first experiments directly applied digital serial pulses of 0’s and 1’s to transmit byte streams over an RF link layer.  Raw signalling produced ranges of no more than about 10m.  Use of the ‘RadioHead’ library which employed a packet frame structure and the transmission of a ‘training preamble’ preceeding the digital data transmission, and a CRC field greatly improved the link level signaling.  Depending on baud rate,  range over 100m may be practical with clear line of site.

Trees, the body, rain, walls all reduce 433 mhz signal strength greatly.  Clear line of site is optimal for signal strength.

1/4 wave antennas produce good short distance performance.  High gain antennas have yet to e tried.

Programming technique is also challenging.   I have just succeeded in using memset() to copy the received data frame buffer from it’s raw form as a uint8_t  byte array, to a C struct of floating point numbers, long and short integer data types, and even raw ascii.


They are primitive devices, extremely low power,  to see if I there are possible applications, including entertainment or education.  , e.g. for the weather station and a remote sensor… something like that where running data cables is a huge problem.  The Radiohead library http://www.airspayce.com/mikem/arduino/RadioHead/  for the arduino that can be leveraged for data, and a good deal of basic code is available online so it’s easy to get started.

At this point, I’ve breadboarded a test project, and assembled a receiving breadboard with an 8×2 LCD screen to function as a receiver data display.

Range. Range between the transmitter and receiver with simple whip antennas is disappointing,  less than 10m in the vicinity of the house.  ( Others report ranges of 15 to 30m).

But the guy in the open field near Berlin got 300+ m.  Now I think he probably used the RadioHead library to get that range.  Without the library on raw IO on the TX/RX  pins, 10m is about the limit.

Testing on 6/5/2020 with the RadioHead library, and using my car to receive the pulses
I could hear the signals on Iho Pl near the turnaround – 415m or .26miles.  Did not bring the arduino receiver… that next time.  As far as an audible signal, I’m sure it can go farther.

The ham radio tranceiver can tune 433.85 as it is in the middle of the 70cm ham band. The Yaesu FT-7900 has a good receiver.

The RX led on the arduino pulses with the signal from the receiver.  Data detection however requires that the arduino digital pins detect the  This limitation is likely due to limits of the receiver design.  The receiver seems is a simple superheterodyne  design with a tank circuit tuner that renders it susceptible to noise and other interference.  This greatly reduces it’s ability to detect a useable signal using amplitude shift keying (ASK).   The UHF ham radio receiver is vastly superior and with it I have been able to hear the transmitter long after the digital receiver stops picking up data packets from it – even though the RX led still pulses weakly along with the transmitted signal.

The Radiohead library employs technique of sending a training sequence 36 bits called a “training preamble” that allows the receiver AGC to settle down, before hitting it with the data package of bits.  The RH library employs a packet format, with a 16 bit CRC to enable the receiver to determine if the packet was received intact.  I

There is surely a way to process the AFK signal from the FT-857, say with a raspberry pi.  The USB signal


How 433MHz RF Tx-Rx Modules Work & Interface with Arduino

This is a good starting place using the RadioHead library.

markfickett kb3icy, github arduino-morse from kb3jcy


May 2020 – Month 3 of the Pandemic – Radio Activity

•May 24, 2020 • Leave a Comment

It’s May in the year of the COVID 19 Pandemic.  It’s been a while since my last post here and I’m significantly behind, despite being holed up and sheltering in place. A strange time of Zoom virtual meetings online like our EARC club meetings, increased ham net activity – no brainer that one.


Continue to build matchbox antennas but the field day rush is off – field day 2020 is going to be a ‘field day alone’ or ‘field day at home’ event.

I revisited my 2M moxon today (5/24/2020) and checked it out using Winlink Express and VARA-FM. At 145.07 mhz measured the front back receive signal difference of 1-2 S units (6-12 dB) and SN ratios of between 5 and 9 dB – at least it is not omnidirectional.

Most significantly I’ve upgraded my digital software suite for WSJT-X. I get much better situational awareness than before. In addition, it’s much easier now to set goals and go after WAS, for example. The work factor is much less and helps interest overcome apathy. We’ll see.

| -- Ham Radio Software - 
|       |-- WSJT-X  2.2.0 -rc1  May 2020
|       |-- JT-Alert 2.16.5
|       |-- DxLab Launcher
|       |-- DxLab Keeper 156.4.9
|       |-- DxLab View Map 4.6.4
|       |-- pskreporter
|       |-- QRZ.com

JT-Alert allows a qso callsign  lookup on QRZ - nice to have, and provides a significant "w/b" worked before

20 meter opening to Europe on FT8

•May 10, 2019 • Leave a Comment

For the first time I caught a 20m opening with FT8 the last couple of nights and worked stations in the UK, Finland, and France.  9may2109z ~0600-0700

8 may,  the sunspot number was 25.   The sunspots went from 11 on may 4 to 25 by the 8 may( today on 10may N = 25).  I wonder.   Probably unrelated, earlier in the day 8 may the 0900W 40m net was a bust on 40m 80m and 60m – very poor conditions.


Working conditions: 20m ft8/wsjt-x, 84′ doublet, linear amplifier – 30/400 W (-13 db), 4:1 #61 balun temp : 43°C

Other:  heard only 25% or fewer stations than could hear me.  reception is 75% of the  problem!  Power probably needed and I used it to make contacts: 400w -13db: calc- 200 -16 db, 100w -19 db.

80 Meter Dipole upgrade

•February 21, 2019 • Leave a Comment

19 Feb 2019.  upgraded the 40m dipole to 80m by adding two 30’4″ lengths of #30ga stranded wire.  It took a couple of stages but the job is done.

SWR at rig end from the 4:1 balun
80 3.88  4.5
60 5.35 11.1
40 7.04  24
40 7.18  24
30 10.1  5.3
20 14.0  6.3
20 14.2  7.0
17 18.1  1.6
15 21.2  4.0
12 24.9  4.9
10 28.2  2.7

SWR at rig end from the twinlead before balun
80 3.88  28
60 5.35  2.4
40 7.04  > 30
40 7.18  > 30
30 10.1  18
20 14.0  2
20 14.2  2
17 18.1  3.5
15 21.2  3.4
12 24.9  9.3
10 28.2  2.7

With the 4:1 Balun
The Kenwood TS590SG can tune  80, 30, 17, 15, 12, and 10  meter bands.
The MFJ-901B tuner is needed for 60, 40, and 20 meters.

Materials: 30awg wire – white teflon coated  for the west leg to blend with the building, black silicone (200 F) for the E which is more invisible in the air.  U shape, first 30′ on both sides then just folded/dropped to fit in.

Construction: original PC board T support with two hole strain relief – this design has held up well since the beginning.  The original 30 ga single copper conductor wire became brittle over time and had to be replaced.  The stranded should be a bit stronger and more flexible.  I made a splice board with copper clad board – two hole strain relief design, 1/2″ by 2.5″ copper sheet jumper taped in place on the west side, antenna wire  soldered to the copper plate.  The jumper is soldered on the E … originally the idea was to use the copper sheet as a switch conductor to easily remove the 80m stub – this is impractical for permanent installation.

Performance : so far so good, definitely louder on 80m, suspect 40m is compromised.  The balun observed to heat up to 140F after operating for a while long 100w FT8, runs cooler on other bands so need to be careful on 40m.

Is the end fed now better on 40m ?   2/21/19  –  it works pretty well and may be outperforming the dipole … several factors including EF wire orientation (NS (ef) vs EW {dipole} )  of highest current segments and height may affect this too!

135 foot Length
The ARRL Antenna book lists a 135′ doublet recommended length … this length may work better than 120 with much lower 80 and 40m SWR before the balun (the 1:1 , i.e. no transform, swr)

135′ dipole SWR at rig end from the 4:1 balun 
 λ     f 
80  3.5       5          4
80  3.88     4          5
60  5.35   14          12
40  7.04   25        24
40  7.18   25        24
30  10.1    2           1.6
20  14.0    9           8
20  14. 2   9           9
17  18.1    5           5
15  21.2    3           3.4
12  24.9    4           5.4
10  28.2    3           2


Yaesu MH-31 Microphone Failure/Fix

•February 17, 2019 • Leave a Comment

Problem: No transmit audio on the FT-857D tranceiver.  FM/AM/CW produced a carrier.

mh31a.PNG       mh31cart.PNG

RFO: Dynamic microphone cartridge failure.

Detecting failure.  Besides no transmit audio, measurement of the resistance between the two cartridge leads enabled diagnosis.  The cartridge should have with a normal resistance of between 200 to 600 ohms.  Instead the faulty cartridge presented an open (infinite) resistance as measured by a DVM



This was a bad dynamic mic cartridge that checked out with infinite R instead of 200 – 600 R. Given the mic was dead, seemed like a no-brainer to do the electret mic conversion since I had the parts.  Used the electret mic cartidge that came with the uBITX and followed m0ukd’s schematic. The utube is excellent with a very similar solution. QSO on 40m confirmed decent sound on the FT-857D with the new refurbished microphone.

Performed this mod with parts on hand , an electret mic cartridge from the uBITX project and a couple of caps on hand (47nf and 1 nf).

It’s important that there be capacitors in the signal path to block the +5v dc voltage.  I took some pains to insure there were no shorts since 5v would be bad news for a mic pre-amp or other circuitry downstream that is not expecting it.


The microphone now works and reportedly sounds good.



TS-590SG Reduced transmit power, Reboots. Known Problem. Easy Fix.

•February 17, 2019 • Leave a Comment

20190216  Incident – TS590SG rebooted once while transmitting.   Later under testing into a dummy load, power was limited around 50 watts in CW mode, sometimes more, sometimes less.  The reboot occurred once or twice.  The lack of power affected all bands.

Reason for failure – the 25amp fuse holders and other connectors on the power cable for the TS590 oxidize over time and can cause significant power loss over time.  When the total current carrying capacity of the power cord assembly is reduced due to oxidation and gradual loss of the necessary gas tight metal to metal contact, the rig reduces power output or may reboot.

Recovery Actions Needed –  remove power cable, inspect, wipe all connectors clean, pull out, clean fuse contacts, or simply reseat the fuses.  Inspect Johnson power pole connectors and insure all connectors are seated fully.  When I inspected the fuse blades, they looked shiny and clean, but the reseat worked.  The power pole connectors are known to be subject to oxidation and losses.

Result of maintenance actions:  Restoration of normal operation.

Other – the power limiting and reboot behavior made me wonder if  a serious electronic failure, e.g. of the final mosfet transistor amplifier, occurred.  Other accounts on the web may likewise be misleading, if only because fragments of information can be  misleading.

The  problems caused by power loss in the power supply cable are not widely advertised but known to TS590 owners and amply documented online if you know where to look – but misleading information is also present.  See: https://groups.io/g/TS-590/s  or on QRZ.  Before too long I was in contact with two hams in particular with direct experience with the problem. CE3DNP and W2JDB helped me zero in on the problem. This has been characterized as a “classic” TS590SG fuse problem.  Thanks guys – saved me a lot of time and effort!



Low noise event – 2Jan2018 …

•January 3, 2019 • Leave a Comment

1/2/2018 – on or about 2 jan 2019, I noticed that  the 40 meter noise floor  dropped from S9+5 to S3 (no preamp).

Noise canceling is no longer necessary.

The source of the noise is a mystery but seemed to be related to power, maybe a noisy always on device.  Hoping for the best.


FLDIGI CW Split Operation

•October 25, 2018 • Leave a Comment

oct 2018
Use FLDIGI for CW and how to use SPLIT in contests and for DX stations.




I can’t hear call signs at 30 wpm.  I could hear my call sign at 30wpm,  I could do limited contest and dxPedition cw without fldigi.  Until that day …



Computers in the shack 2018

•August 14, 2018 • Leave a Comment

2018-Computers: I use an Intel I3-4150 Dell mini-tower for its disk space and as my Win10 outpost until microsoft licenses me out into the cold.   CPU: i3-4150 @ 3.50 GHz / 8 GB

We’ll probably go one more round of hardware upgrades before we ditch Microsoft completely.  Ham radio apps, SDRs in particular, have not migrated fully to openSource, i.e. Linux, at least for the hardware I have.  And Windows is convenient for now (2018).  I use hand-me-down macbooks from my daughter and my sister for portable and some digital ops and for wireless ssh/x11 for the raspberry pi’s, and hard wire USB for arduino projects.  Often I’ll use a rapspberri pi to front end an arduino, and do arduino development from the pi, from a ssh to the pi from windows or osx.  I use raspberry pi’s for programming and development – mainly python, C, and perl languages.  The pi’s are more or less a dream come true for an old unix dog like myself.  As the need arises I use arduino’s, mainly the nano’s for projects needing microprocessors – they are to me suprisingly powerful C/C++ language developments platforms too which seems outlandish, but they can do some  significant number crunching on a thumbnail size chip.

MFJ-4125P Power Supply Fan Noise and Fix

•April 7, 2018 • Leave a Comment

The MFJ-4125 switching power supply works really well but is notorious for fan noise. MFJ uses a 60x60x15 mm fan from Yate Loon.  The high noise level is a result of running the fan at full blast.  Any noise abatement comes from dialing back the voltage going to the fan at the cost of reduced airflow, which brings us to the black art of form vrs function.  The problem is to reduce fan noise without adversely affecting the power supply circuitry of the power supply.

How much cooling is enough? Chose a fan, watched the failure rate of the power supply and if acceptable from a business standpoint, good enough.

Curiously, the fan noise level is described by MFJ as “whisper quiet”.  On the sound scale, “whisper quiet” is 30dB.   My cell phone sound level app does seem to confirm a 30 dB level of noise coming from the supply fan. It is whisper quiet.

Noise increases with airflow. I plotted the data (see the Yate Loon product data sheet (2) for it’s fan product line further down ).  I plotted out their four data points below which shows the interesting relationship between Airflow and Noise.


To reduce noise, reduce airflow by reducing RPM.  The human ear detects sound intensity in increments of a decibel or two.  For every increase in one  cubic foot a minute  airflow, there is a one to two dB increase in noise.

Reduce RPM by reducing voltage – 4 diodes in series

A number of hams reported using 60Ω to 100Ω power resistors to drop the voltage and reduce fan rpm.  I am trying diodes instead of resistors since I don’t have the needed power resistors.  4 diodes in series produce a 25% voltage drop from 12V to 9V.  Noise decreased noticeably; every diode drops .7 v drop and each results in a small drop in noise, so this might have been 4 to 8 dB.  At 9v, the fan still makes noise. The question is what’s the impact on airflow and necessary cooling.  E.g  a 25% drop in airflow from  22 CFM  would be 16 CFM …  is that enough to make a difference in the power supply temperature?   {9/1/18 after a few months, still seems to be fine with no apparent ill effects.  Using a noise app on my phone, the fan now elevates background noise in the shack from  30 dB to 35 dB with the mic a few feet away from my operating position.}

There are many comments out there of hams who have installed resistors. Nobody has reported any issues with dropping the fan speed using resistors.  We’ll just sort of see how it goes with diodes.

While I was at it, I put a thermistor on the heat sink to enable monitoring the temp on it and set up my arduino temperature IR monitoring probe to also use a thermistor as a second sensor channel.

From April to September, the power supply ran with diodes in the fan circuit with no ill effects.

9/1/19.  Did some power supply temperature test with TS590sg  transmitting into a dummy load.  PS heatsink temp rose from 84º F to 103º F  100 watt cw.  That’s not too bad I think.  (the ts-590sg rig remains cold to touch).   Under digital condition the temperature of the heat sink however goes to 123º F.   That’s a limitation of a permanent install of the diode pack.  For a lot of transmitting,  we’d want to switch on full power to the fan.

Temperature controlled fan driver (4)
(Taming the MFJ-4125 PSU Fan)
Below is a schematic from G0KLA who developed this nice little circuit for regulating span feed from temperature for his linear amp power supply.   He used the same circuit with an MFJ-4125 and posted it ( see 4 ).   When the temperature is low, the fans runs at a low level.  When the temperature goes  high enough however, the voltage to the gate of the MOSFET Q2 rises high enough to switch Q2 on fully, and the fan runs full blast.

4/8/18  I built out the circuit for testing.


C17  – electrolytic,  filter/time constant ?   R31 – the gate resistor 10k seems to be typical to protect the gate from transient voltage at the expense of switching speed.  Switching speed for turning on a fan is hardly critical. (5)

Test Session #1 – Standby noise of about 35 dB and 109º up to 50 dB+  and temps above 118º on 9/13.



9/13/18 Testing temperature driven fan controller-  finally.   Removed the  diode breakout and inserted the test unit.    Insertion notes:  for testing I tapped the 12V on the rear panel, and attached fan leads directly to fan + and  minus – .   There is a better way.  Make a power cable to get the 12V from the fan power socket on the board, and provide two header pins for the fans power plug.  This plugs into the fan power jack.   The fan plug now plugs into the pins on the board.  This refinement allows the fan circuit insertion without any cutting / soldering  on the MFJ4125.

9/10/18 Results.   The circuit works well.  Maximum temperatures are about 118º F with full power digital operation.   I set the trigger point to occur at something close to 105º F by trial and error .  At this setting with transceivers powered up, the fan idles with heat sink temps around 105º and 110º and fan noise is barely audible with total ambient noise of about 35 dB.    On transmit, the fan speed revs up, and with a lot of transmitting activity like with a digital mode, the fan goes to full speed as the MOSFET switches on to deliver the full input voltage to the fan. The fan at full power adds another 10 to 15 dB of background noise.


The spec we’re most interested in is the junction temp of the power transistors attached to the heat sink .   Then the heat sink temperature might tell us if the cooling is enough.  We’re leaving this to another time.

9/10/18 Production Insertion Notes.   Came up with the better idea for how to make the insertion. Soldered two header pins to plug the fan end connector in.   Used a two pin socket to plug into the fan power supply socket on the rear of the 4125 main board.

(1) http://w9xc.net/w9xc-html-site/projects/mfj4125mods/mfj4125p-mods.html
(2) http://www.yateloon.com/en/product-38825/DC-FAN-SERIES-60x60x15.html
3) http://www.nmbtc.com/fans/white-papers/dc-brushless-cooling-behavior/
4) http://www.g0kla.com/workbench/2016-05-06.php  – heat sensitive ran speed reduction circuit.
(5) http://www.electronic-products-design.com/geek-area/electronics/mosfets/using-mosfets-as-general-switches
(6) https://www.google.com/search?q=using+mosfet+to+control+dc+motor&safe=off&tbm=isch&source=iu&ictx=1&fir=8VQ36z01EUPryM%253A%252CKpP3Zyk_V6miYM%252C_&usg=__qMZ6oTM2b7Ruq27wP5WeLTlBarU%3D&sa=X&ved=0ahUKEwiKrZ6a6a3aAhUQTawKHbWGAU4Q9QEIUzAE#imgrc=8VQ36z01EUPryM:

Fan Specifications 

The fan used by MFJ  is a 60x60x15 mm 2 wire YateLoon S60SM-12 run at 12V with the fan load, 13.8v no load.   So the fan OEM is YateLoon.  If I was manufacturing something and needed a fan, I’d probably use YateLoon too.

The specs and replacement information are included here because the originals bearings are on borrowed time and multiple re-lube operations.  The PS is about 4 years old.  Apparently the fan is the first thing to go.

Below: product sheet from Yate Loon (2).  YL has data on four different models of 60x60x25 fans with different motors.  Airfow and noise increase with RPM

fan specs

OEM Source (3) 60x60x20   

D60SM-12 Technical data:
Dimensions: 60x60x20mm
Nominal voltage: 12V
Nominal speed: 2700rpm
Airflow: 18CFM
Noise level: 30 dB(A)
Power consumption: 1,2W
With 3Pin Molex plug, 60cm cable and rpm signal

Naturally, the original fan finally gave out.  Instead of a genuine YL, here’s what I bought below.  It’s now plugged into the power supply and seems to be running fine.


12/13/2018  So far so good.  With only receivers on, the power supply fan is off.  At some point the fan my idle as the load increases.  On transmit, the heat rises in the PS, the fan rpm rises rapidly and continues until the heat drops.  If we stop transmitting the fan idles and eventually turns off.

Aug 7 2020 – still holding up very well and plan to duplicate it for another MFJ-4125P.  I wonder how long these supplies last?  As we found out, the fan bearings are the first thing to go.