Radio-Kits SWR meter – build and review

This article describes my build of a Radio-Kits SWR meter (v1.1) and post implementation review.

Advertised features:

  • HF coverage – 1.8-30MHz
  • Displays VSWR, forward power, reverse power and supply voltage
  • Peak reading power meter
  • Bar graph or numerical format
  • Reverse power alarm with adjustable threshold
  • Auto turn on in presence of RF – sensitivity about 1 watt
  • Optional turn off after preset time – 10-240 seconds
  • Backlit LCD display with variable brightness
  • Reverse polarity protection

I purchased the kit some years ago, and on receiving it and reviewing the circuit I formed the view that it was likely to have unacceptable Insertion VSWR on 1.8Mhz, and probably 3.5MHz bands… so I lost interest in assembling the kit. However, I have belatedly constructed the kit, calibrated and tested it.


The kit is supplied as a PCB and parts, no casework is supplied.

The board was difficult to solder, the strain relieved ground plane connections of components have very little donut to contact for heat transfer and are much harder to solder than the other pads. The strain relief is a dubious feature that makes soldering difficult.


Above, the kit assembled in a die-cast aluminium box. An opening for the LCD was milled into the box, and holes drilled for the rest of the fit up. The kit does not lend itself to this boxing as the buttons out the top and display out the front are a problem to fit up. A poor mechanical design.


Above is the interior of the box showing the LCD display and the external BNC connectors fitted (substituted for the ubiquitous UHF connectors supplied with the kit). Continue reading Radio-Kits SWR meter – build and review

Grebenkember’s original Tandem match

(Grebenkemper 1987) describes a directional coupler that has become very popular, especially in commercial implementation.

screenshot-22_11_16-07_23_25The simplified circuit above from Grebenkemper’s article illustrates the key elements of the directional coupler.

An important detail of the design is that the primary of the right hand transformer appears in shunt with the antenna load, and the magnetising impedance of that transformer core compromises Insertion VSWR. It is important that the magnetising impedance is sufficiently high (or the admittance sufficiently low) to not cause significant Insertion VSWR.
Continue reading Grebenkember’s original Tandem match QRP SWR bridge

The project is to build a test a couple of QRP VSWR detectors by ( rated at 10W.


Above are the completed kits.


Above is the schematic. The bridge uses a type of Sontheimer coupler (Sontheimer 1966) and these are commonly poorly designed. The first question is whether the magnetising impedance of T2 which appears in shunt with the load is sufficiently high to not give rise to poor insertion VSWR. Continue reading QRP SWR bridge

DMR distance limit

I have heard ‘traditional wisdom’ that there is a timing limit imposed by the DMR protocol limit that limits radio path lengths to less than 70km.

There is indeed a limit and it is due to the 2ms guard interval between time slots, and half of that time is available for accomodation of round trip radio propagation delay (RTT), so 1ms with c0=3e8m/s gives a one way distance limit of 150km.

To put the ‘traditional wisdom’ to the test, I took a UHF DMR handheld up onto a nearby hill and successfully had a medium length QSO via the VK2RAG repeater netoworked to VK1BGT using the VK1RBM repeater.
Continue reading DMR distance limit

TV upgrade – MER optimisation

At TV upgrade I reported a change in TV antenna pointing to a different and distant transmitter, and gave a Spectrum Analyser plot at the main TV receiver.

At that time, I adjusted the antenna accurately (within 1°) based on compass heading, but antennas are not perfect and two significant path obstructions may have bearing on best signal.

I could have run up and down the ladder making small adjustments and observing amplitude or better, RF S/N on the Spectrum Analyser but that is tedious and suboptimal so I purchased a DVB signal analyser.

Importantly, a good DVB analyser gives measurement of not just signal strength, but carrier to noise (C/N) ratio (which is actually RF S/N), Bit Error Rate (BER) and Modulation Error Ratio (MER), the last two very important statistics in optimisation and validation and not available on an ordinary Spectrum Analyser.

MER is calculated as the sum of the squares of the magnitudes of the ideal symbol vectors is divided by the sum of the squares of the magnitudes of the symbol error vectors. The result, expressed as a power ratio in dB, is defined as the Modulation Error Ratio (MER).

MER is a good overall single statistic for quality, but BER is more sensitive to occasional errors, so they are both important.


Above, the DVB analyser (the red device) enables a view of measurements whilst adjusting the antenna. The analyser here is connected to the masthead amp output and of course powers the masthead amp. Continue reading TV upgrade – MER optimisation

Coil32 inductance calculator

A recent online posting gave unequivocal recommendation for the Coil32 Inductance Calculator for application to a ferrite toroidal HF current balun.

Always interested in these things, I tried to download it to evaluate it but there was rigmorol to create an account and aware that I have never downloaded a calculator that handled that specific problem at all well, I was reluctant.

They do however have some online calculators that are supposed to use the same algorithms and methods as the downloadable software, so lets review the one for a toroidal ferrite inductor.


Above is the data entry form, and warning bells sound. The “releative magnetic permeability” field is a simple scalar quantity, but the permeability of most ferrites at HF needs to be considered as a complex value (ie having real and imaginary components). Continue reading Coil32 inductance calculator

Walter Maxwell’s teachings on system wide conjugate matching

Walt Maxwell (W2DU) made much of conjugate matching in antenna systems, he wrote of his volume in the preface to (Maxwell 2001 24.5):

It explains in great detail how the antenna tuner at the input terminals of the feed line provides a conjugate match at the antenna terminals, and tunes a non-resonant antenna to resonance while also providing an impedance match for the output of the transceiver.

Walt Maxwell made much of conjugate matching, and wrote often of it as though at some optimal adjustment of an ATU there was a system wide state of conjugate match conferred, that at each and every point in an antenna system the impedance looking towards the source was the conjugate of the impedance looking towards the load.

This is popularly held to be some nirvana, a heavenly state where transmitters are “happy” and all is good. Happiness of transmitters is often given in online discussion by hams as the raison d’être for ATUs . Continue reading Walter Maxwell’s teachings on system wide conjugate matching