(Grebenkemper 1987) describes a directional coupler that has become very popular, especially in commercial implementation.
The 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
The project is to build a test a couple of QRP VSWR detectors by KitsAndParts.com (http://www.kitsandparts.com/bridge.php) 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 KitsAndParts.com QRP SWR bridge
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
Our iiNet / FTTN NBN 12Mb/s service has been connected for just over a month now, and the first full month of download test results (ISP performance graphs) bears examination.
Above is a plot of the measured transfer rate of the test file from iiNet’s site. Continue reading First month of iiNet FTTN broadband Internet access
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
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 “relative 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
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
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.
Whilst 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
I have application for an analogue to digital converter (ADC) in a noisy environment, so a possible solution is to place an ADC module very close to the analogue sensors and use some form of digital connection back to a microcontroller. A possible protocol is I2C, and has the advantage that several ADC modules can be attached to the same bus, along with other peripherals Eg LCD.
The above ADS1115 modules have four input channels, 16 bit conversion, flexible input mux, and were available on eBay for less than A$4, so worth a try.
Continue reading ADS1115 ADC checkout – #1
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