Matching a quarter wave monopole with two variable caps

Two recent correspondents have discussed matching a quarter wave monopole with two variable caps.

Two capacitor shunt/series match

The matching scheme involves a shunt variable cap at the end of the coax feed line, and a series variable cap to the monopole base. The radials are of course connected to the feed line shield.

This type of matching scheme requires that the monopole feed point has sufficient +ve reactance, ie the monopole is longer than resonant. Lets assume the R component of feed point Z is 35Ω.

This scheme incorporates the simple shunt match, and the value of the shunt capacitor can be found knowing the R value to be matched to 50Ω.

Screenshot - 08_07_2015 , 06_43_07

Above is a Smith chart of a model of the match at 14MHz. The monopole has been lengthened to have 100Ω reactance along with 35Ω resistance. In this case a series cap of 148pF and shunt cap of 150pF are required. Continue reading Matching a quarter wave monopole with two variable caps

Transmit performance of 2m hand held transceivers

This article documents measurements of transmit performance of three hand held 2m radio with several antennas.

Measurements of field strength were done with Lou Destefano’s (VK3AQZ) VK3AQZ RF power meter (RFPM1) and a small loop antenna.


Above, the field strength meter, a RFPM1 with small loop antenna oriented for max gain in the direction of the DUT.  The instrument reads -73.5dBm with no signal, -69.5dBm with the strongest transmitter with the loop removed, and around -30dBm for the various transmitters with the loop in place… so the meter reading is predominantly due to the loop mode pickup.

All three transmitters have different power. The table below reports power into a 50Ω load and does not take account of mismatch with the various antennas.

Screenshot - 06_07_2015 , 12_53_56

Above a comparison of the configurations on a field strength test at 1λ. The relative column factors the different transmitter power and FS to obtain  a comparative figure independent of power. Mismatch is almost certainly a significant part of the explanation of different performance, but it is quite difficult to measure in this sort of application without disrupting the DUT.


It is interesting that there is little difference observed with the Baofeng on two different antennas, when the Boafeng antenna is clearly inefficient, see the thermograph above.


Balanced ATUs and common mode current

This Feb 2012 article has been copied by request from my web site which is no longer online. The article may contain links to articles on that site and which are no longer available.

Many designs have a ‘balanced output’ or an option of a ‘balanced output’, but what does that mean, and are they effective in minimising common mode current in an antenna feed line?

ATUs achieve ‘balanced output’ in one of several ways, the common ones are:

  • a grounded impedance transformation network followed by an internal voltage balun;
  • a grounded impedance transformation network followed by an internal current balun;
  • a current balun followed by a symmetric impedance transformation network that may or may not be directly grounded at its centre;
  • a link coupled ATU where the output circuit is symmetric and may or may not be directly grounded at its centre.

Much has been written about the merits of one approach or another, mostly qualitative and often subjective, but there is little in the way of quantitative analysis of the impedance that the ATU offers to common mode current. Continue reading Balanced ATUs and common mode current

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