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

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

Some thinking on ground wave as we enter another solar minimum

During low solar minima, the 40m band becomes very poor for reliable NVIS QSOs, but what of the prospects of ground wave QSOs?

Lets take two ITU-R recommendations for some insight:

  • ITU-R. Jul 2015. Recommendation ITU-R P.372-12 (7/2015) Radio noise.
  • ITU-R. Feb 2007. Recommendation ITU-R P.368-9 (2/2007) Ground-wave propagation curves for frequencies between 10 kHz and 30 MHz.

The first sets out expected median ambient noise in a range of precincts. It is based on measurements made with a short monopole, ie a vertically polarised antenna.

The second sets out the attenuation of ground waves at HF.

screenshot-24_10_16-22_00_39Whilst P.368-9 publishes a set of graphs like the one above for a limited set of grounds, they also publish a program to calculate values for the user’s choice of ground and that is what was used for this article. Continue reading Some thinking on ground wave as we enter another solar minimum

Polarisation of man made noise – an 80m case

Polarisation of man made noise discussed an explanation for the common observation more ambient noise is captured by a vertically polarised antenna than for a horizontally polarised antenna.

This article documents an analysis of a case on 3.6MHz and is to be read in the context of Polarisation of man made noise.


Remembering that P.368-9 publishes a set of graphs like the one above, and that they show that ground wave attenuation is dependent on distance, soil type and frequency.

Though ground wave attenuation is lower on 80m than 40m, the horizontal antenna used in the example is at a fixed height, so it is electrically lower on 80m which increases horizontal attenuation significantly. Continue reading Polarisation of man made noise – an 80m case

Polarisation of man made noise

Ham lore has it that man made noise on lower HF is radiated predominantly vertically polarised, this is offered and accepted by hams without explanation.

It can be shown by simple observation that the ambient noise level on lower HF is quite different in business or commercial areas, residential areas, and rural areas (ITU-R P.372-12). Not only is there a significant difference, the change happens quite rapidly with distance which suggests there is a dominant component (man made noise) and that the propagation path is a very local one (ground wave).

If you look around a typical residential neighborhood where hams might establish stations, the most obvious conductors that might carry and radiate noise currents from noise generators like appliances, leaky insulators etc are aerial power lines… which are usually closer to horizontal orientation (with horizontal E field) than vertical which seems inconsistent with the common observation that vertically polarised receiving antennas tend to capture more man made noise power than horizontal ones.

This article proposes a mechanism that may explain the apparent inconsistency between noise radiators and noise receivers.

Though this explanation is based on experience, the quantitative analysis here depends on interpretation of Recommendation ITU-R P.368-9 (2/2007) Ground-wave propagation curves for frequencies between 10 kHz and 30 MHz.

screenshot-24_10_16-22_00_39Whilst P.368-9 publishes a set of graphs like the one above for a limited set of grounds, ITU-R also publishes the program (GRWAVE.EXE) which can be used to calculate values for the user’s choice of ground and that is what was used for this article. The graph above is for a vertical monopole over ground with 1000W radiated, the antenna has directivity of 3, and the dashed line (inverse distance curve) is the field strength for a lossless ground (PEC). This can be verified with a spot calculation at 1km. Continue reading Polarisation of man made noise

Small signal diode characteristics

We often use diode detectors at microamp currents, and the question arises as to the type of diode best suited to sensitive detectors.

Setting aside zero bias Schottky diodes which are a topic in themselves, the choice is typically between commonly available germanium, silicon and Schottky signal diodes.


Above is a plot of the I,V characteristic of four common signal diodes at currents up to 1mA. It can be seen that at currents below 600µA, the forward voltage drop of the humble 1N34A germanium diode is lower than the others. The 1N270 is an alternative if you really need its higher breakdown voltage. Both of these diodes are reasonably easy to obtain, and cheap at that.