nanovna-H – measure 144MHz Yagi gain – planning / feasibility

This article documents a feasibility study of using the modified nanovna-H to measure the gain of a 4 element 144MHz Yagi, the DUT.

The intended configuration is the DUT will be connected to the tx port (Port 1 or CH0 in nanovna speak), and a known ‘sense’ antenna connected to its rx port (Port 2 or CH1 in nanovna speak).

nanovna |s21| noise floor

To make useful measurements of the received signal, the rx signal level must be a reasonable amount higher than the noise floor, 10dB should be sufficient.

Above is a plot of the |s21| noise floor around 146MHz. Continue reading nanovna-H – measure 144MHz Yagi gain – planning / feasibility

nanovna-H – measure 40m low pass filter for WSPRlite flex

This article demonstrates the use of a nanovna-H to measure the response of a low pass filter designed to pass 7MHz frequencies but attenuate harmonics. The inductors and capacitors make a seven element Chebyshev filter as designed by G3CWI for use in a 50Ω system.

Implementation

Above, the filter is assembled on a piece of matrix board with two BNC connectors. The inductors are fixed with hot melt adhesive, and the whole thing served over with heatshrink tube. It is not waterproof. Continue reading nanovna-H – measure 40m low pass filter for WSPRlite flex

Transmitter / antenna systems and the maximum power transfer theorem

Jacobi’s Maximum Power Transfer Theorem

Jacobi’s law (also known as Jacobi’s Maximum Power Transfer Theorem) of nearly 200 years ago stated

Maximum power is transferred when the internal resistance of the source equals the resistance of the load.

Implied is that the internal resistance of the source is held constant, it does not work otherwise. The source must be one that can validly be represented by a Thevenin equivalent circuit. This was in the very early days of harnessing electric current, direct current initially.

Later adaptation dealt with alternating current and it became

Maximum power is transferred when the load impedance is equal to the complex conjugate of the internal impedance of the source.

Again a necessary condition is that the source must be one that can validly be represented by a Thevenin equivalent circuit. Continue reading Transmitter / antenna systems and the maximum power transfer theorem

Walter Maxwell’s teachings on system wide conjugate matching – a SimSmith example

I have written on Walt Maxell’s proposition about simultaneous system wide conjugate matching in antenna systems. I will repeat a little to set the context…

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, anthropomorphism over science. Continue reading Walter Maxwell’s teachings on system wide conjugate matching – a SimSmith example

Average power of SSB telephony – experimental verification

Average power of SSB telephony used 80 year old research by (Holbrook and Dixon 1939) to come up with a ratio of peak voltage to RMS voltage of a voice waveform, and from that derive the ratio PEP/Pav..

(Holbrook and Dixon 1939) explored the subject measuring the voice characteristics of many talkers (as there is variation amongst talkers) to come up with an average characteristic.

Whilst in its day, obtaining instantaneous samples of voice was a challenge, it is trivial today and if you can’t believe the numbers given, try your own experiment (but realise it is for your own voice rather than the general population).

Many modern PC sound applications are capable of the measurement, I will demonstrate it with the feed Windows application Audacity with the stats.ny addin.

Above is a screenshot of a 6s recording of my voice made without stopping for breath. The statistics window shows a peak of -8.9dBFS and RMS of -27.4dBFS, giving a peak voltage to RMS voltage ratio of 18.5dB. Continue reading Average power of SSB telephony – experimental verification

Average power of SSB telephony

Some components used for SSB telephony need not be capable of handling the Peak Envelope Power (PEP) continuously, many components for instance respond to the average power (Pav) which is quite less. Essentially, components that are subject to voltage breakdown (usually as good as instantaneous) must withstand the PEP, those that heat relatively slowly must withstand Pav.

In estimating the power dissipated in components due to an SSB telephony waveform, a good estimate of the ratio of Average Power (Pav) to Peak Envelope Power (PEP) is very useful.

Long before hams had used SSB, the figure has been of interest to designers of FDM or carrier telephone systems to size amplifiers that must handle n channels of FDM multiplex without overload which would degrade S/N in other channels of the multiplex. The methods are applicable to SSB telephony, it uses the same modulation type and the overload challenges are the same.

(Holbrook and Dixon 1939) gave the graph above which characterises the ratio of instantaneous peak to RMS voltage of voice telephony for different numbers of channels in a multiplex and different expectation of overload or clipping. They recommend a very low probability of clipping at 0.1% to avoid significant intermodulation noise in adjacent channels. Continue reading Average power of SSB telephony

Exploiting your antenna analyser #30

Quality of termination used for calibration

Some of us use a resistor as a load for testing a transmitter or other RF source. In this application they are often rated for quite high power and commonly called a dummy load. In that role, they usually do not need to be of highly accurate impedance, and commercial dummy loads will often be specified to have maximum VSWR in the range 1.1 to 1.5 (Return Loss (RL) from 26 to 14dB) over a specified frequency range.

We also use a known value resistor for measurement purposes, and often relatively low power rating but higher impedance accuracy. They are commonly caused terminations, and will often be specified to have maximum VSWR in the range 1.01 to 1.1 (RL from 46 to 26dB) over a specified frequency range.

Return Loss

It is more logical to discuss this subject in terms of Return Loss rather than VSWR.

Return Loss is defined as the ratio of incident to reflected power at a reference plane of a network. It is expressed in dB as 20*log(Vfwd/Vref). Continue reading Exploiting your antenna analyser #30

Elecraft CP-1 directional coupler – magnetics review

Elecraft produces a directional coupler that may interest QRP aficionados. It comes with instructions for 20dB and 30dB coupling factors rated at 25 and 250W respectively from 1 to 30MHz.

This article reviews the magnetics design of the -20dB / 25W coupler.

The coupler 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 Elecraft CP-1 directional coupler – magnetics review

Should you trust your VSWR meter – linearisation

Should you trust your VSWR meter? asked an interesting question, and Should you trust your VSWR meter – detector linearity discussed a problem apparent in may VSWR meters.

This article illustrates one method of linearisation of the detector response of a practical VSWR meter.

Radio-kits SWR meter

This article contains an analysis of the analogue circuitry of the Radio-kits SWR meter.

The directional coupler at top left contains half wave peak detectors for forward and reflected waves. They are wired to the two compensated op amps at lower right (the connections are not shown on the circuit as the coupler may be remote, follow the terminal designations). Continue reading Should you trust your VSWR meter – linearisation

Should you trust your VSWR meter – detector linearity

Should you trust your VSWR meter? asked an interesting question, and based on experience, including a relevant example, concluded:

The answer is no, like any measurement instrument, prove that it is trustworthy in the intended application.

It went on to ask:

If the VSWR meter is designed to fail, why does it fail?

This article contains an analysis of the analogue circuitry of the IC-7300 directional coupler to explain the likely cause of its poor behaviour.

IC-7300 directional coupler schematic


Above is an extract of the IC-7300 circuit in the area of the directional power coupler used for VSWR measurement. The circuit is a quite conventional Bruene coupler, and its response is similar to several types of directional couplers that produce a DC output voltage from a half wave detector. Continue reading Should you trust your VSWR meter – detector linearity