Differential and common mode components of current in a two wire transmission line

A pair of conductors in proximity of some other conductors or conducting surface (such as the natural ground) can operate in two modes simultaneously, differential mode and common mode.

Differential mode is where energy is transferred due to fields between the two conductors forming the pair, and common mode is where energy is transferred due to fields between the two conductors forming the pair together and another conductor or conducting surface.

The currents flowing in the two conductors at any point can be decomposed into the differential and common mode currents.

Differential current Id is the component that is equal but opposite in direction, it is half the difference in the two complex line currents I1 and I2.

Common mode current Ic is the component of the line currents common to both conductors, it is half the sum of I1 and I2.

  • Id=(I1-I2)/2
  • Ic=(I1+I2)/2
  • I1=Ic+Id
  • I2=Ic-Id

So, for example, if I1=2A and I2=-1A, Id=(2–1)/2=1.5A, Ic=(2-1)/2=0.5A.

A line that is operating with perfect current balance has only differential current, ie common mode current is zero. It is unlikely that a feed line in a practical antenna system is perfectly balanced, but with due care, it can have very low common mode current, 20dB or more less than the differential component.

Review of inexpensive Chinese thermostat – 8010F #2

This is a review of an inexpensive 8010F Chinese bang-bang  thermostat that was purchased on eBay for around A$13 complete with thermistor sensor and postage.


Above is the front view of the thermostat. There are many thermostats on the market with similar front panels, but they differ in internals and most importantly, performance and quality.


Above, the rating label is clear and informational.  Continue reading Review of inexpensive Chinese thermostat – 8010F #2

Programming jig for STC15Fx DIP-8, DIP-28 chips

The STC15Fx chips use a simple TTL/CMOS async programming interface that is suited to the common USB-RS232(TTL) adapters, some of which are less than A$2 on eBay (CH341 chip).

STCMicroPgmAdapterAbove, the completed adapter. Both DIP-8 and DIP-28 are located furthese from the operating lever, and pin 1 towards the operating lever, the same jumper connections are used for both chip sizes for STC15F104E and STC15F204E.

There are two spare Gnd pins next to the black jumper above but hidden from view. They are for grounding jumpers that may be required to enable programming of some ‘bootloader protected’ chips.

The 6 pin male and female headers at lower left accept a USB-RS232 adapter (break out board style or cable) with the common Arduino pinout. The only thing that commits the pinout is the 1µF bypass capacitor between Vcc and Gnd pins and the spare Gnd pins. The USB-RS232 adapter powers the chip being programmed, and it needs to be a 5V adapter.

Alternatively one of the little MAX232 adapter boards could be used with a physical RS232 port, but power will be required.


Fox flasher MkII

Flashing LED driver using an ESC described a LED driver for an animal deterrent using a repurposed brushless DC motor electronic speed controller.

This article describes a simpler implementation based on a Chinese 8051 architecture microcontroller, the STC15F104E.


Above, the schematic. A very simple circuit with just a handful of electronic components (one capacitor, two resistors, one LDR, one Polyswitch, 4 x LEDs and the MCU). Continue reading Fox flasher MkII

Using the AIM to measure matched line loss

A correspondent wrote seeking explanation of difficulty he was having measuring line loss using the advice given in the AIM manual using a scan with either O/C or S/C termination:

Note the one-way cable loss is numerically equal to one-half of the return loss. The return loss is the loss that the signal experiences in two passes, down and back along the open cable.

Because my correspondent was using one of the versions of AIM that I know to be unreliable, I have repeated the measurements on a cable at hand using AIM_900B to demonstrate the situation.

The test cable I have used is 10m of RG58C/U which I expect should have matched line loss (MLL) of 0.26dB, but I expect this to be a little worse as it is a budget grade cable that I have measured worse in the past. Continue reading Using the AIM to measure matched line loss

Thermistor for CBAIV

I wanted to embed some thermistors in battery packs to use them with CBAIV and sought specifications from Westmountain Radio who declined to supply the information.

It is a straight forward matter to measure the resistance of a thermistor immersed in a stable bath of water, and similarly to observe the software response to standard resistors. Continue reading Thermistor for CBAIV

Baofeng GT-3TP MkIII review

This article is a review of the Baofeng GT-3TP MkIII, a hand held 2m/70cm FM transceiver.

The radio is supplied with a bunch of useful accessories (even if the power supply lacks the necessary Australian approval) and a brief and inadequate manual.


Above, the Baofeng GT-3TP purchased for about A$85 delivered overnight from Sydney. Continue reading Baofeng GT-3TP MkIII review

Antenna span tensioner using a counterweighted halyard

Correspondents have asked about application of the technique used in Antenna span spring tensioner using Antenna wire catenary calculator to a span tensioned with a counterweighted halyard.

The scenarios bear a lot of similarity, the main difference being that the tension from the counterweight is constant up to the point that the counterweight travel reaches its limit.

The tension applied is the weight force of the counterweight with a little increase to allow for friction in the sheave block.

So, lets say the scenario is a 42m wire plus halyard that adds 1m to the span under no wind conditions and can pay out a further 1m at which it reaches its limit. Lets say the counterweight is 5kg weight so 49.0N tension. Continue reading Antenna span tensioner using a counterweighted halyard