## Baluns – wire size insanity

An online expert recently expounded on detailed design of a balun, this is an excerpt about wire sizing.

The wire gauge used limits the current handling capacity of the wire, run too thin a wire and it will heat up. Run much too thin of a wire for the power in use and it will fuse open. Current carrying capacity of wire is typically rated for either power transmission applications or chassis wiring applications. The latter, and higher, current capacity for a wire is relevant to designing a balun. How much current your 50 watt signal generates depends on the impedance its looking into. If you’re talking about a 50 ohm system, with a perfect match you’ll deliver one amp through your balun wires when driving 50 watts into it. Allowing for say a 4:1 SWR the worst case current(@12.5 ohms) is 2 amps. If you’re using this as a tuner balun, perhaps to drive a multi-band doublet then the impedance can vary widely so over sizing the wires is easy insurance. Here’s a table of wire current carrying capability: https://www.powerstream.com/Wire_Size.htm

For convenience, the relevant part of the table linked above is quoted for discussion.

So, the poster recommends wire with chassis wiring rating of 2A for 50W with reserve capacity for worst case VSWR=4. Continue reading Baluns – wire size insanity

## ARRL guidance on design of ferrite cored inductors

The ARRL handbook for radio communications (Ward 2011) gives guidance on designing with ferrite cored inductors:

Ferrite cores are often unpainted, unlike powdered-iron toroids. Ferrite toroids and rods often have sharp edges, while powdered-iron toroids usually have rounded edges.
Because of their higher permeabilities, the formulas for calculating inductance and turns require slight modification. Manufacturers list ferrite AL values in mH per 1000 turnssquared. Thus, to calculate inductance, the formula is

L=ALxN2/1000000

where:

L = the inductance in mH
AL = the inductance index in mH per 1000 turns-squared, and
N = the number of turns.

Example: What is the inductance of a 60-turn inductor on a core with an AL of 523? (See the chapter Component Data and References for more detailed data on the range of available cores.)

L=ALxN2/1000000=523×602/1000000=1.88e6/1e6=1.88mH

Lets follow the example through. Continue reading ARRL guidance on design of ferrite cored inductors

## Antenna half power bandwidth and Q, concept and experimental validation

Many antennas can be represented near their series resonance as a series RLC circuit, and in many cases R changes very slowly with frequency compared to X. This provides a convenient and good approximation for the behavior of the antenna impedance in terms of a simple linear circuit.

## Series resonant circuit

The response of a simple series resonant RLC circuit is well established, when driven by a constant voltage source the current is maximum where Xl=Xc (known as resonance) and falls away above and below that frequency. In fact the normalised shape of that response was known as the Universal Resonance Curve and used widely before more modern computational tools made it redundant.

Above is a chart of the Universal Resonance Curve from (Terman 1955). The chart refers to “cycles”, the unit for frequency before Hertz was adopted, and yes, these fundamental concepts are very old. Continue reading Antenna half power bandwidth and Q, concept and experimental validation

## IMD associated with colorbond sheet steel cladding

A recent experiment exposed significant IMD of a 7MHz locally radiated signal.

The source used for these tests is a battery powered low power transmitter  driving a 0.6m square loop. Radiated power is very low (of the order of -60dBm EIRP), and received signal on the station receiver is less than -73dBm.

## Loop near steel shed #1

The loop was leaned against the colorbond sheet steel wall of shed #1. Shed #1 is about 40m from the receiving antenna, and is connected to the power mains. The building has colorbond sheet steel screwed to a steel frame. Colorbond is painted Zinc/Aluminium coated steel.

For this test, the submain was turned off at the main switchboard, so there is not equipment in the shed powered up, but the supply neutral is still connected and bonded to the shed steel work and shed PES ground electrode.

Above, the spectral response of receiver output, there are a number of side products at 100Hz intervals, presumably some form of IMD. At higher radiated power, products at odd 50Hz intervals become visible, though at much lower level than the 100Hz products. Continue reading IMD associated with colorbond sheet steel cladding

## Baselining an antenna system with an analyser

I often receive emails from folk trying to validate continued performance of an installed antenna system using their analyser.

With foresight they have swept the antenna system from the tx end and saved the data to serve as a baseline.

The following are example sweeps from one of my own antennas, a Diamond X50N with 10m of LDF4-50A feed line.

Now I have plotted Return Loss rather than VSWR for several reasons:

• Return Loss is more sensitive to the problems that we might want to identify;
• Rigexpert in this case decided that the Antscope user could not be interested in plotting VSWR>5 (Return Loss<3.5dB).

Now a hazard in working with Return Loss is that many authors of articles and software don’t use the industry standard meaning.

## Return Loss

Lets just remind ourselves of the meaning of the term Return Loss. (IEEE 1988) defines Return Loss as:

(1) (data transmission) (A) At a discontinuity in a transmission system the difference between the power incident upon the discontinuity. (B) The ratio in decibels of the power incident upon the discontinuity to the power reflected from the discontinuity. Note: This ratio is also the square of the reciprocal to the magnitude of the reflection coefficient. (C) More broadly, the return loss is a measure of the dissimilarity between two impedances, being equal to the number of decibels that corresponds to the scalar value of the reciprocal of the reflection coefficient, and hence being expressed by the following formula:

20*log10|(Z1+Z2)/(Z1-Z2)| decibel

where Z1 and Z2 = the two impedances.

(2) (or gain) (waveguide). The ratio of incident to reflected power at a reference plane of a network.

Return Loss expressed in dB will ALWAYS be a positive number in passive networks.

The relationship between ReturnLoss in dB and VSWR is given by the equations:

• ReturnLoss=-20*log((VSWR-1)/(VSWR+1))
• VSWR=(1+10^(-ReturnLoss/20))/(1-10^(-ReturnLoss/20))

## Diamond X50N on 2m

So now that we are on the same page about Return Loss, lets look at my 2m plot.

The X50N does not have VSWR or Return Loss specs, but we might expect that at the antenna itself, VSWR<1.5 which implies Return Loss>25dB. Measuring into feed line, you can add twice the matched line loss to the Return Loss target (see why Return Loss is a better measure).
Continue reading Baselining an antenna system with an analyser

## Thompson’s coax common mode explanation

A recent online discussion on common mode feed line current was directed to Thompson’s article with the recommendation that is ALL basically needed to discuss the common mode current.

Above is Thompson’s diagram of currents in a feed coax, and it contains two significant errors that could / would lead to formation of the wrong concepts in a learner’s mind. Continue reading Thompson’s coax common mode explanation

## An RF choke for a 1.8-30MHz coax power injector – LF1260 core

This article describes a prototype RF choke (RFC) for use in a power injector for 50Ω coax over range 1.8-30MHz. Power injector / extractors are often used to connect power and / or signalling on a shared common RF coax feed line to accessories such as remote antenna switches and ATUs.

Design criteria are:

• Insertion VSWR of the RFC in shunt with 50+j0Ω < 1.1;
• Dissipation < 2% of a 100W transmitter.

The core chose is a LF1260 ferrite suppression bead from Jaycar. It is a medium / high µ core readily available in Australia at \$7.50 / 6.

Above is the prototype RFC wound with data cable wire for the purpose of measurement. In application it could be wound with 1mm enamelled copper or PTFE insulated wire (Curie point is lowish at 120°+, but it still benefits from higher temperature insulation). Continue reading An RF choke for a 1.8-30MHz coax power injector – LF1260 core

## Riding the RF Gain control – part 4

This article continues on from Riding the RF Gain control – part 3 and explores the operating advice when applied to the next generation of receivers.

## Direct sampling SDR

Lets jump a generation to the direct sampling SDR configuration.

In this category, I am covering receivers that do not convert the receive signal to an intermediate frequency, the ADC samples the signal at its off-air frequency.

• preamplifiers;
• bandpass filters;
• attenuators.

Because there may be no amplification prior to the ADC in some operating configurations, a voltage controlled attenuator may be used to prevent overflow of the ADC, this is the ‘analogue’ part of the AGC system. Continue reading Riding the RF Gain control – part 4

## Riding the RF Gain control – part 3

This article continues on from Riding the RF Gain control – part 2 and explores the operating advice when applied to the next generation of receivers.

## Conventional superheterodyne communications receiver with DSP demodulation.

The next generation of receivers was a conventional superheterodyne with a DSP based demodulation stage (initially at quite low Intermediate Frequency to suit the power of the available DSP chips).

Communications receivers were enhanced by replacement of the demodulators with a DSP performing demodulation digitally. The DSP sampled the IF signal and digitised it, and channel filtering and demodulation was performed ‘mathematically’ using the digital data stream as input.

There are two significant differences with this change:

• receiver bandwidth can be determined by digitally synthesised passband filters in the DSP; and
• first step in the DSP process is conversion of the IF signal to a digital stream in an analogue to digital converter (ADC).

Critically, the Analogue to Digital Converter (ADC) had an overflow point, and overflow of the ADC creates serious IMD and major degradation of received signal, overflow has to be prevented at all cost. To limit the power delivered to the ADC, a narrow ‘roofing’ filter usually preceded it, and the channel filter was digitally synthesised. Continue reading Riding the RF Gain control – part 3

## Riding the RF Gain control – part 2

This article continues on from Riding the RF Gain control – part 1 and explores the operating advice when applied to the next generation of receivers.

## Conventional superheterodyne communications receiver designed for SSB telephony.

Move on to the generation of superheterodyne communications receivers that incorporated a demodulator designed for SSB, and an AGC system that protected the receiver intermediate and later stages (including the demodulator) from overload for even very strong signals, and you have a receiver that broadly, worked automatically from the weakest to very strong signals with RF Gain control set to maximum and little need to adjust AF gain.

The AGC system’s most important function is to protect the receiver intermediate and later stages (including the demodulator) from overload. Levelling the AF output is a lesser priority.

These receivers were not perfect, the AGC was derived from the signal, and SSB suppressed carrier does not contain a consistent component from which AGC can be derived, so it is derived from the signal using a pseudo peak detector. The dynamics means that the AGC leaks away between syllables, and there is small overload during the attack time of the next syllable as it charges again, but with appropriate time constants, the distortion is small. (Some commercial HF links solved this problem by transmitting a reduced carrier typically at -26dBc from which AGC was derived.)

These receivers were subject to IMD in the front end, and operating them with no more preamplification than necessary improved handling of strong out of band signals, and in extreme interference cases inserting a front end attenuator improved IMD performance. The working configuration on low HF where external noise dominates the receiver is to improve IMD response even at the expense of Noise Figure.

These receivers were not perfect, but by and large, good implementations worked very well hands off most of the time.

### Delayed AGC

AGC reduces gain ahead of the demodulator, typically in both RF and IF stages, and has the effect of increasing the receiver Noise Figure. Increasing Noise Figure degrades S/N ratio at the receiver output.

The diagram above from (Terman 1955) illustrates the behaviour for AVC, the term that was used with AM receivers at the time and superseded by AGC for SSB receivers. Continue reading Riding the RF Gain control – part 2