Category: Transmitters

WIA 4:1 current balun – further measurements

4-101a

I mentioned in my article WIA 4:1 current balun that the use of a single toroidal core in the above graphic compromises the balun. This article presents some simple measurements and analysis that question whether the balun works as so many users think.

The popularity of the balun derives from the work of VK2DQ and is often known as the VK2DQ 4:1 current balun (though probably not his invention).

Analysis at the limits

Often, analysis of a network as frequency approaches zero or infinity can simplify the analysis whilst allowing a reasonable test of the sanity of the design.

Above is a conventional transformer schematic of the WIA 4:1 current balun on a perfectly symmetric (balanced) load. At frequencies where the electrical length of each winding is very short, we can assume negligible phase delay along or between windings, simplifying analysis greatly. Continue reading WIA 4:1 current balun – further measurements

Last update: 5th April, 2020, 6:23 PM

nanoVNA-H – Port 2 attenuator for improved Return Loss

nanoVNA-H – measure 40m low pass filter for WSPRlite flex describes measurement of the response of a filter.

The filter is of a type that depends on its source and termination impedance for as designed performance.

The article mentioned the use of a 10dB attenuator on the nanovna-h Port 2 for the purpose of improving the accuracy of the load impedance for the filter.

Like most low end vnas, the nanoVNA Port 2 VSWR or Return Loss is not wonderful, not as good as needed for some types of measurement. Return Loss can be improved by placing an attenuator ahead of the port. The nanoVNA-H v3.3 already includes an attenuator on the PCB, and the nanovha-H v3.4 increased that attenuation by about 5dB to improve Return Loss by about 10dB.

In my own case, I am using a nanoVNA-H and upon measurement of |s11| (-ReturnLoss) I determined that it needed to be improved by 20dB for my use so I purchased and installed a 10dB attenuator semi permanently on the Port 2 connector.

Above, the 10dB attenuator is semi permanently attached to Port 2 and also serves the purpose of a connector saver. There is a connector saver semi permanently attached to Port 1. Continue reading nanoVNA-H – Port 2 attenuator for improved Return Loss

Last update: 30th March, 2020, 6:32 PM

A common mode choke for a VDSL pair – LF1260 core

This article describes a common mode choke intended to reduce RF interference with a VDSL service.

The MDF is located where the underground cable enters the building. From here it rises vertically and travels some 25m across the ceiling to the VDSL modem. Continue reading A common mode choke for a VDSL pair – LF1260 core

Last update: 19th March, 2020, 11:08 AM

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

Last update: 2nd February, 2020, 7:11 AM

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

Last update: 2nd February, 2020, 7:11 AM

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

Last update: 21st October, 2019, 4:34 AM

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

Last update: 19th October, 2019, 5:42 AM

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

Last update: 23rd June, 2019, 7:40 AM

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

Last update: 28th November, 2021, 12:48 PM

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

Last update: 12th July, 2019, 5:16 PM