Some potentially useful information that cannot be found with a single Google search.

VK3HN Arduino and si5351 sketch

My sketches for implementing a VFO, BFO, carrier oscillator for CW, rig control, S meter, RF power meter and CW keyer on a Raduino is here. If you want to try this, all the instructions are in the file AND read the repo wiki pages if you want to set up the OLED variant.

Power into a 50 ohm load

7dBm (drive required by SBL-1) is 1.4V peak-peak. The full table is here.


Specifications of some commonly used varactors:

  • Motorola MVAM108 25-500pF; –pF@9v; 30pF@8v; 40pF@7v
  • Motorola MVAM109 30-460pF; 30pF@9v; 35pF@8v; 50pF@7v
  • Motorola MV209 5-40pF; 12pF@9v; 14pF@8v; 16pF@7v
  • Toshiba 1SV149: 25-500pF; 20pF@8.5v; 25pF@8v; 30pF@7v

LiquidCrystal S-meter

Displaying an s-meter (or other meter) on a LCD screen with Arduino code can be done with the following method. Define 5 custom chars with 1, 2, 3, 4 and 5 vertical lines filled using this page:

Custom Character Generator for HD44780 LCD Modules

Copy the byte definitions into the script. Determine how many LCD characters the s-meter will be (a full width meter would be 16 but you might want to make it smaller, say 8). Using a sequence of partial and full block chars you can render a line of 0 to n*5 segments where n is the number of allocated characters. So for an 8 char meter you have 40 segments to play with. Scale the analogue value read from the s-meter pin (0-1024) by n, and experimentally weight it as a crude form of calibration.

The Arduino lcd library allows a cursor to be set under a VFO frequency digit for emphasis, such as to illustrate the current step size. Use

lcd.setCursor(col, line);

SA630D solid state RF switch

The SA630D is a SPDT 50 ohm RF switch suitable for small signal switching in homebrew rigs and projects. It is ideal as a relay replacement, for switching BPFs (not LPFs) in a multiband rig, switching between IF filters, and small signal T/R switching.

Arduino C: convert float/double to string for display

snprintf(destinationBuffer, bufferSize, formatString, data…)
destinationBuffer is a character string to hold the formatted data, bufferSize is the size of the destination buffer (using sizeof() is very useful for this variable), formatString is the normal “printf” style format string, and “data…” is the data to format and place in the buffer.

Can replace sprintf() to avoid risk of overwriting the destination buffer resulting in memory corruption.

Relay drivers

Controlling a relay with a transistor. Variations include: high side (one side of relay earthed), low side (one side of relay on +ve rail), relay closes on low input, relay closes on high input.

For a high side switch use a BC547 and BD140 pair.

For a low side switch use a 2n7000.

Multiplexing switches on Arduino analog input

To save Arduino digital inputs, turn a string of switches into a voltage divider chain on an analog input and map the value read to a logical switch number.

Reading or controlling up to 8 lines with PCF8574

I/O expansion via I2C with a PCF8574.

si5351 clock output levels

Minicircuits DBM LO ports require +7dBm drive, which is 1.4 to 1.5v P-P, 0.5V RMS.

Square wave amplitude measured from an si5351 into a load, as follows:

2ma setting: 190 mv rms. 0.6v P-P. -1dBm
4ma setting: 380 mv rms. 1.0v P-P. 5dBm
6ma setting: 560 mv rms. 1.6v P-P. 8dBm
8ma setting: 735 mv rms. 2.2v P-P. 10dBm

Based on these readings, the 4mA or 6mA output setting should be close enough to drive the L7 DBM LO port at 14MHz (claims G0UPL). A single stage buffer could be inserted between si5351 clock and the DBM for isolation and impedance transformation.

2N5109 replacements

From EMRFD Yahoo group: ‘On another note, I have used for years a BFG591 as 2N5109 replacement. Much cheaper than the 2N5109 and a bit more sturdy that the 2SC5551AF-TD-E. The BFG591 is now obsolete and the BFU590 is a good replacement. 73, Roelof, PA0RDT’.

Here is a table to compare some old and new RF devices.

DeviceTypeVCE (V)fT (GHz)p (W)Ic (mA)hFE
2N3866NPN302N3866 0.5
2N3866A 0.8

Clifton Labs broadband low-IMD RF amplifier

Clifton Labs Z10043A Norton Amplifier kit. The manual for kit has much info on performance and detailed schematic.

Arduino digital pin values

For D2..D13 used as output… The output transistors on Arduino pins have about 40 ohms resistance – however they have an absolute max rating of 40mA current, so any safe use of the output pin will produce 1.6V or less for LOW and 3.4V or more for HIGH.

Capacitors for LPFs

Voltage ratings for capacitors in a high/low pass filter to handle 50 watts. 50 watts into 50 ohms should be 50 volts RMS. However, the P-P voltage is about 141 volts. Voltage is dependent on SWR.

Thomas LA3PNA> 100V capacitors is cutting it a bit close in my opinion, I would have used 500V capacitors, there are C0G capacitors around that are cheap and have low lead inductance that would be great at VHF.

See ATC capacitors:

si5351 90 degree phased clocks

Available on CLK#0 and 1 to 110MHz at

ssm2167 limiter/compressor/noise gate

toggle – attach/release (fast/slow)
potentiometer – gain (may be preset)
potentiometer – limit threshold
potentiometer – compression ratio
potentiometer – noise gate.

Discussion on Bitx20,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,16775725


160m inverted L dimensions

There is a lot of inverted-L wisdom on the various forums. KH6AQ, Jul 8, 2016, writes — the formula for this inverted-L is 256/F. I would cut it long – like 270/F and trim it to resonance.

For 1.825Mhz L section length is 140′ 3″ (42.75m), cut wire to 147′ (44.8m) and trim down.

2/10/2020, followed this formula for a 160m portable inverted L, ended up pruning the driven element back to 38.23m to get 1:1 on 1825. A 12m horizontal section just 3m above steel deck roof may be pulling it low.

Semiconductor equivalent catalogue

Use this site to map odd components (eg. power mosfets) to alternatives. Try it out on the STW20NM50 used by VK3XU’s AM class E transmitter.

Winding another secondary onto a 500VA toroidal transformer

Reference: Looking at the specifications of the Altronics M5730C 500VA toroidal transformer ($155) as a basis for determining how to wind on an additional 110V secondary.

M5730C 500 VA Toroid has diameter 127mm.

240V primary is 574 turns, using 0.96mm ECW (AWG#18 shows as 1.024mm dia).
240V primary current is 500/240 = 2.08 amps. recommended fuse 3.5A slo.

30V secondary is 75T of 1.8mm ECW (AWG#12 shows as 2.053mm dia).
75/574 * 240 = 31.4V
1 turn = 30/75 = 0.42V
30V secondary current is 8A.

Therefore 110V secondary = 110/0.42 = 262 turns.

262/574 = 0.45 (cross check: 110/240 = 0.45)

Secondary current rating: 500VA / 110V = 4.5A
Determine secondary wire size (by inference), using 8A <-> AWG12 and 2A <-> AWG18;
Current AWG#   AWGmm   Approx
2A         AWG18    (1.024)     1.0 mm
4A         AWG16    (1.291)     1.2 mm **
6A         AWG14    (1.628)     1.6 mm **
8A         AWG12    (2.053)     1.8 mm.

Length of one turn on intended repurposed toroidal transformer: 21.5cm, therefore 262*21.5 = 5633cm = 56.5m.

150kHz LPF

Some calculations for alternative inductors in the PuWMa PWM module. RM10 refers to winding the inductors on RM10 ferrite cores.  Also T106-26 toroids. 

About RM10 cores: RM10  ferrite core, N87 material, Al 4200nH.  In the case of a ready-made core, the manufacturer gives you directly the AL, in nanohenry/turn². All you have to do is to multiply this figure by the number of turns squared to get your inductance.

 turnsL (nH)L (uH)  
 4.58505085.05 (Target 88uH)
 33780037.8 (Target 36uH)
 turns L (uH)  
 31 88  
 20 36  

Mic preamp

A circuit for a mic preamp, with balanced input and phantom power, using SSM2017. The power supply using 7815/7915 and a multiplier for the 48v phantom supply is also on this page.


Looking for a better PLL/DDS than the si5351, which is fine for simple rigs but places some annoying spurs around the bands. The AD9951 has a 14 bit DAC. The AD9959 has four channels, is readily available from Chinese sources, but its channels are 10-bit DAC. AD9959 prices, now under $AUD40.

See DK7IH’s post on the AD9951, clocked at 120MHz. This appears to be a safe path but involves building up a DDS board and hand coding as I cannot find a reliable library so would need to get my head around Peter’s code.

Switchmode 5v DC -DC regulator for RPis

As used by Pete N6QW: Mean Well 25W Isolated DC-DC Converter Chassis Mount, Voltage in 9.2 → 18 V dc, Voltage out 5V dc.

Suppressing RF noise from HT power supply in an AM Transmitter

Capacitors in parallel with the rectifiers to swamp the diode’s off-state capacitance:

and this:

Additionally, a snubber circuit further suppresses radio frequency noise from the DC rectifier from finding its way back onto mains and radiating:


Shack of VK2KMI

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Ham Historian

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My journey of repairing and recycling anything I put my hands on that I believe is still useful. Not just hardware, but including software with relevant content and issues in the field of Cyber Security, Vulnerability Scanning and Penetration Testing.

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