Its 2026 and we still don't have flying cars!

 I was wondering what it was like a hundred years ago in Amateur Radio...

So I asked Google - got a lot of guff. But I did come across some magazines from the early twentieth century concerning 'wireless'. The development of 'Radio' during the twenties was of course intent on picking up distant AM transmitters for the entertainment 'content'.

The one article that stood out was - no - is on the Minnesota University web site. This led me to read the article entitled "Is Radio at a Standstill?" Ha! We know it wasn't but it was intriguing to see what the people of the time thought about the new technology.

"that we need not look for any revolutionary improvements in radio at the present time. The chances are against any invention that will entirely upset the radio industry" - quote. We all have 20-20 hindsight. 

"One of the great troubles in the United States at the present time is the heterodyning between different stations nearly on the same wave-lengths.[1] " - This quote is especially for the RAE students. Which links to the footnotes. "In Fessenden’s case, controlled heterodyning could boost a signal. But with the number of broadcast stations crowding the airwaves in the 1920s, natural heterodyning occurred when two stations operating at closely spaced frequencies produced unwanted interference for each of the signals. For more on the heterodyne principle, see The Dynamophone in this book."

This experimentation is still going on a hundred years later. Though I think no patents in the numbers like then:- "During the entire year of 1925 over nine hundred radio patents were issued by the Patent Office; and during the first six months of 1926, almost six hundred radio patents have been issued."

In my early career I was introduced at college and work to all sorts of 'new applications or components'. It was after all the 'golden years' of electronics and communications. New Integrated circuits and improved transistors were released every month. With adverts in the technical magazines of the time. Which we as young people were encouraged to read before getting down to the work of the day.

I went looking a couple of weeks ago for a 'current time' [2020s] Aircraft Transmitter specification. This would have been a simple AM Transceiver with a moderate output power [typically 10 Watts] and sensitivity. I got a surprise  as the 'modulator' was described as a 'Class D' type. Providing more 'punch' in the voice modulation. Achieving 70% amplitude modulation and voice compression without distortion. [No not music quality!] 

In 1926 the valves sets had an 'A' and a 'B' battery. A for the high tension voltage - 90 Volts plus maybe more. The 'B' battery was the filament supply. A few volts later 6 to 12 Volts depending on valve design. These were heavy and non-portable so the invention of a mains power supply was truly an innovation.

Shades of 'Load Shedding'

The later passage tells of the battery suppliers providing connections for the lighting circuit. Remember electric light in the home was a 'new thing' and cost more than a few oil lamps. 

So I had to refresh my understanding of 'Class D'. Then it was slap the forehead time. I remember Clive Sinclair's audio amplifiers which used 'Class D' putting out a few watts. Advertised as 10 Watts per channel. Over the years this type of output stage has developed to a reasonably efficient method for power output. Be grateful it provides the 50Hz from your inverter without suffering from 'issues' as the Sinclair Amp did in the 60s.

At its basis it is a Pulse Width Modulation output stage. Providing the sine wave as the average of a switching waveform. [A QRM generator of note when unfiltered!] But it converts d.c. to a.c. reliably and efficiently.

• Raspberry Pi's core micro (BCM2835) doesn't have any audio outputs per se. Yes, it has the HDMI audio output, but the stereo 3.5mm jack outputs are fed using a pair of PWM generators rather than audio quality DACs. [Raspberry Pi - pages]
• The Sinclair X-10, introduced in 1964 by Sinclair Radionics, was the world’s first commercially available pulse-width modulation (PWM) / Class-D audio amplifier sold to the general public.
  Designed by Clive Sinclair and Gordon Edge, it used high-speed switching instead of linear operation to achieve high efficiency. [Wikipedia] 

The Sinclair amplifier used a set of low frequency transistors to switch the output supply voltage at a super-sonic frequency. Theoretically providing the real power output. Others have tried and failed to provide an efficient audio power amplifier over the years since then. Recently radio amateur designs have with high speed clocked processors managed to achieve AM or SSB modulation of an r.f. signal at high powers. Most new 'linears' use PWM to achieve 1 to 2 kilo Watts output.

Check the RSGB's recent conference videos on YouTube for more information. 

[a few links:-

https://youtu.be/gXx5sa339i8?si=GyU1xxTh2KEqTPjm

https://www.youtube.com/live/6FheeAv3src?si=VU8WNCsAKpXkpRMU

https://youtu.be/21uE341GtGU?si=ccCg6WwPTbVYZrKR

]

From the 'inspiration' :- 

The Perversity of Things: Hugo Gernsback on Media, Tinkering, and Scientifiction 
 

 

 

 

A 'Starter Project' for 'Newbies/Interested Constructors' - A DC RX

A "Starter Project for after the RAE"

I was thinking about a 'starter project' for the 'newbies' after the RAE. It used to be something like a 'Crystal Set'. Because the AM transmitters are few and far between nowadays, it should be a 'simple DCRX.' And to make it more 'interesting' it should receive SSB, CW and PSK31 and RTTY.

Direct Conversion Receivers have been around for several decades.

Lets try putting an AI on the task...

Based on a search of technical literature and recent hobbyist projects, there are many articles and resources dedicated to direct conversion receivers (DCRs), ranging from historical overviews to modern, high-performance designs.

Academic and Technical Articles:

Numerous technical papers and articles exist on ScienceDirect, MDPI, and ResearchGate covering DCR design, specifically addressing challenges like DC offset, LO leakage, and I/Q imbalance.

Main Themes in Recent Articles:

Simplification:

Many designs focus on minimizing component counts, often using under four transistors.

Performance Improvement:

Articles discuss techniques to overcome traditional DCR limitations (hum, Microphonics) using I/Q demodulators and modern DSP.

Direct Conversion vs. Superhet:

Several articles compare the cost and complexity benefits of DCR, especially for integrated, zero-IF designs.

Overall, the number of articles is substantial (over 20+ prominent technical, academic, and hobbyist resources found in the search results).

So what do 'we' want to build?

Some design features:-

1) The ZS article used a 420mm loop antenna

2) Use an 'active antenna'

3) An IC "Balanced Mixer" to suppress the local oscillator leakage to the antenna.

*** Whoops! That might be a problem...

4) Which 'Band' do we want to use?

5) How sensitive should it be?

Example Picture showing simple construction method 1.

You can't get the wood, you know!

There 'used to be' many integrated circuits for the Direct Conversion Receiver.

SO42 - SL6440 - TIxxx - CA3086 NE602/NE612 and so on. Most are no longer in production or available for purchase. [Also fake ones are available!]

See the note 'Dilbert Cell.' [Not a typo]

A recent Youtube described how a 'Gilbert Cell' worked. Take a look. The simple version would do the job nicely - using 'readily available' - 'general pupose' transistors!

The Gilbert Cell (084d)

https://youtu.be/mQ36yy7mloA?si=2W3HRTWym6j-LcsR

Now we are scavengers

This is where I get on a 'soap box' and tell you about how many transistors I have recovered/scrounged/scavenged from old/wornout electronic circuits. The list includes ATX Power supplies, VHS recorder/players etc.

Just Found this:-

"There are many conflicting technical requirements for a good-quality front-end in an SW receiver. The noise figure and the intermodulation level should be low, the RF insulation between ports LO, RF and IF should be high, and some amplification is desirable. The Type SL6440 high level RF mixer from Plessey ensures a noise figure of around 10 dB, and offers sufficient suppression of the LO signal." - Oh dear! No longer available.

Dilbert Cell ???

In electronics, the Gilbert cell is a type of frequency mixer. It produces output signals proportional to the product of two input signals. Such circuits are widely used for frequency conversion in radio systems.[1] The advantage of this circuit is the output current is an accurate multiplication of the (differential) base currents of both inputs. As a mixer, its balanced operation cancels out many unwanted mixing products, resulting in a "cleaner" output. Gilbert cells can also be used as variable-gain amplifiers (VGA).[2]

It is a generalized case of an early circuit first used by Howard Jones in 1963,[3] invented independently and greatly augmented by Barrie Gilbert in 1967.[4] It is a specific example of "translinear" design, a current-mode approach to analog circuit design. The specific property of this cell is that the differential output current is a precise algebraic product of its two differential analog current inputs.

NE602/612 ICs

The NE602 and NE612 (including SA602/SA612 variants) are essentially identical, interchangeable Gilbert cell mixer/oscillator ICs often used in RF applications. Originally, the NE612 was introduced as a slightly updated, redesigned version of the NE602 for better high-frequency performance, but they are often identical in modern production and share the same specs, datasheet, and Pinout.

Both are designed by Signetics (now NXP/Philips). The distinction between "NE" (commercial) and "SA" (industrial/automotive) is more relevant than 602 vs. 612.

40m Direct Conversion Receiver

https://community.element14.com/technologies/open-source-hardware/b/blog/posts/simple-dcr-assembling-a-7-mhz-40m-direct-conversion-radio-receiver-part-1

https://qrp-labs.com/images/news/dayton2019/FDIM2019ConfProceedings.pdf

A 40m Direct Conversion Receiver project to upgrade from ZR to ZS

Hannes Coetzee, ZS6BZP, B.Eng Elektronic (Pretoria)

Also in :-

https://zs6wr.co.za/anode/AnodeJuly2010.pdf - Hannes Coetzee ZS6BZP

A 'Work in Progress' JB ZS6WL 2026-05-12

Uses for a 'Tuned Circuit' - work in progress

So I was asked this question last week about how a 'tuned circuit' relates to s.w.r. 

I admit I was a little stumped. When I discussed it with my fellow Radio Amateurs at the club on Wednesday, I was inspired to dig a little deeper.

I first thought about phasing and 'power factor'. Then I considered an ATU (Antenna Tuning Unit). But then as the week progressed I thought of more points. Almost none of these are directly useful to the RAE (Radio Amateur's Exam)! But as a bit of background this might help.

With the advent of Compact Florescent light bulbs and L.E.D. lights as well, 'power factor' has all but disappeared.  Florescent lights have a large inductor (coil) inside as a 'ballast'. To assist in striking the light gas - to basically light!

[If you want a more detailed explanation:- Here ]

When you have one or two of them on the 'mains circuit' the phase difference is not significant. But when you have 20 or more in a large area like a workshop or factory, it is something to consider. 

Similarly when you have a lot of electric motors on the mains again the phasing difference between the current and the applied voltage becomes quite large. This for most electricity consumers never becomes an 'issue'. 

All this was in my college notes from many many years ago. The power metering would read the power consumed incorrectly. And the supplier would check this and apply a 'power factor' correction circuit. Usually a capacitor that compensated for the 'inductive' load.

I am now going to check the new (not necessarily improved) power meters that are fitted to the mains supplied to the homes...

When the current and voltage are 'in phase' the circuit is resistive. Also the 'tuned circuit' is at resonance.

This is important because the electricity supplier wants to measure the power drawn accurately. For billing purposes.

 

Standing Wave Ratio - s.w.r.

In the early days of transistors a lot of radio amateurs used them for power output at radio frequencies. As they were expensive but light weight and didn't need a heater supply. But they were also terrified of 'blowing them' or 'letting the smoke out'. After all replacing them could mean a week's wages!

You will notice - if you look at the CB manuals - all of the CB radios of the 70s onwards had a 'reflectometer' in the output connection. Which would announce in no uncertain terms if you had forgotten to connect the antenna! 

S.w.r became the 'bogey man' of the radio amateur. This has lingered till modern times. While valves would glow a different colour, transistors would silently give up. Most these days just ignore the s.w.r.. Just take a look at the LDMOS device demonstrations on YouTube. 

BUT - s.w.r. is an indication of a non-resistive load (antenna or dummy load). Most 'reflectometers' use a coupled pair of 'transmission lines'.  Some use (QRP) a 'directional coupler'. Sorry QRP is low power which is quite popular this century. This usually means a small ferrite transformer or two. [some more inspiration!]

[Last week RAE I mentioned that an antenna 'looks like electrically' a 'tuned circuit'. Being 'inductive' above resonance. And 'capacitive' below resonance in frequency.]

So this brings me to an A.T.U

An A.T.U. is an Antenna Tuning Unit. It is there to adjust the antenna to resonate at the desired (usually the transmitted) frequency. Which also means the antenna is supposed to be 'resonant' at the same frequency. If it is not it will be 'reactive' - either 'inductive' or 'capacitive'.

How is it like a 'tuned circuit'? Does it have a 'Q' factor? (Bandwidth? -3db frequency power points?)

Yes it does. Remember that the higher the 'Q' factor the narrower the bandwidth. The more 'selective' it becomes. Don't ask what is the 'best', nobody knows!

So usually you want the antenna for a particular frequency band. This is difficult as at h.f. (1 to 30MHz) as the antenna 'Q' will be high. Why? Because usually it is made of wire... And someone wanted to call this wireless!

[I am going to put a curve here detailing the wire diameter to length ratio. This will show the 'Q' factor.]

If you look at the wartime (WW2) pictures of h.f. stations you will see 'dipoles' of multiple wires looking like sausages. Otherwise known as 'thick dipoles'. These exhibit resistive matches over a greater bandwidth at h.f. This is how I made a dipole to cover the entire 2 metre band with a good match. I used two 20mm aluminium tubes cut to length.

These exhibit resistive matches over a greater bandwidth at h.f. - which means the 'Q' factor is quite low. Larger bandwidth = lower 'Q'. [less selective]

Please don't start putting these antennas in your back yard. Unless you are on a plot or farm. Your neighbours will complain.

 

 

 

 

 

 

A Few thoughts as we start 2025

Hi I was wondering...

I was trying to remember which frequencies are 'new' and which are the 'old' ones. Then I recalled that the h.f. bands  changed 40 years ago!

So I set out to find the up to date charts and listings of our frequencies. Noting that the h.f. bands aren't the only ones to have changed over the years.

You may recall that in the start of Amateur Radio they were given "all those useless bands!" 

I have created a document that details the h.f. and V/UHF Bands for the Radio Amateurs in South Africa. It is a 'work in progress'. So comments are desired please. DO NOT PRINT IT.

Link:-  SA-ZA_Bandplans

Then if you really want to 'get into it' you can go to the new and improved SARL web site on mysarl.org.za to get the information from the 'horses mouth'. 

One thing that caught my eye the other day was the two tone oscillator. This is used to 'test' an SSB Transmitter for purity of output. This 'test' requires a "dummy load" of sufficient power rating for the full output power of the transmitter. Also a connection to the microphone input socket of the transmitter. 

 

So this circuit which originates from the ARRL book has some interesting features. 

It uses a pair of back-to-back diodes to regulate the output. I wonder if it really 'works' at that. So I shall put it on a proto board and check for distortion...

It may be 'good enough' for Amateur use but not for testing a HiFi. Recently I have come across some interesting articles and circuit for 0.001% Total Harmonic Distortion. Which of course are not for Amateur Radio but for 'purists' who believe you can hear the distortion below 0.1%! It used to be the defining factor in defining High Fidelity (HiFi).

 

RAE Backgrounder Article 01

What is the most critical part of a car? The tyres. (tires in us English)

What is the most critical part of a HiFi (High Fidelity) setup ? The loudspeakers.

[We can argue about what constitutes a HiFi later.]

What is the most critical part of an amateur radio station?

It is the antenna installation. Which includes the matching circuitry in the output stage and the cable connection. We usually refer RAE students to the "maximum power transfer theorem" when discussing this. Forgetting the other bits and pieces that make up the whole installation.

When discussing a 'loudspeaker', we usually refer to it as a 'transducer'. It converts electrical signals into sound or air pressure changes. With a radio station it is similar. The radio frequency energy is transferred to an electrostatic wave (also an electromagnetic wave). In the 'very old' days this was called the 'aether'. This field fades away as you get further away from your transmitter. The magnetic field quite quickly. But the electrostatic field not as quickly.

Figure 1: From: Radio & Electronic Laboratory Handbook by Scroggie  

Most times Radio Amateurs get very concerned about 's.w.r.' (Standing Wave Ratio). And the notes also refer to this as a 'bad thing'. Essentially what you need to worry about is getting the maximum amount of power in your transmission 'out there'.

So recently with the supply of 'vector network analysers', it has become possible to measure the antenna with great accuracy. This leads to critical testing of the antenna and a very worried Amateur.

In the early part of the 20th century a current meter in the antenna wire was all that was required. Certainly the s.w.r. was an issue. But the valve power amplifier would ignore this 'reflected power'. Only when the voltage wave became extremely high and broke down insulators, was it 'noticed'!

With the advent of power transistors it became of vital interest to make sure the 'reflected power' did not damage the expensive transistors. Most CB radios in the 60's and 70's had a simple 'reflectometer' in the output connection. These would usually announce a bad s.w.r. in no uncertain terms.

s.w.r - How do you measure it?

Standing Wave Ratio is a ratio of transmitted radio frequency voltage or current, to the reflected voltage or current from the 'load'. The load is usually the antenna system. A 'dummy load' should provide a perfect match for the transmitter. That is no reflection of voltage or current. All of this is complicated by the fact that we are talking about radio frequency voltages. And the phasing of voltages and current will definitely not be 'in-phase'.

What is required is some 'directional' coupling to the circuit. To allow measurement of 'forward' r.f. at the same time as the 'reverse' r.f. . Most radio amateurs reach for a transmission line 'reflectometer'. This works fine over the h.f., 1 to 30 MHz range. As the coupling is usually quite small with regard to the wavelength. The well known example of this is the "Bird Thru-line Wattmeter."

Antenna Measurement

An alternative method is to 'measure' the antenna electrically. With h.f. it is relatively easy to build a 'bridge' which will indicate the impedance of the antenna. Note that I said 'impedance' not resistance. This is simply because most wire antennas 'look like electrically', a resonant LC tuned circuit. This is why an ATU proves very useful in matching an antenna to the transmitter. The bridge type circuit is the most common with transformer types the next most common. Recently though it seems that a lot of amateurs have lashed out and bought a VNA. The VNA has the advantage that it 'sweeps' the frequencies around the antenna resonance. Showing the resultant matching on a display. What about the 'monetary challenged' Radio Amateur? He/She will have to build a test unit to check the antenna.

Bridge Test Unit

The simplest form is a resistive bridge. With some provisos this can be fabricated in a tin box using readily available resistors. The downside to this is the fact that when the 'bridge' balances, nothing 'comes out of it'! So a simple diode detector stops working at the load matching frequency...

In the 'old days' this would use 2 to 5 Watt carbon composition resistors. Which in the 'old days' were readily available from component shops. Usually 'downtown'. Those shops have long ago closed never to be seen again. Component suppliers today will insist on 'minimum order quantities' and prices that bring a tear to your eye!

So what is the alternative? Radio Amateurs and Electronic Enthusiasts have become 'Electronic Scavengers.'

Let us see if we can't make a simple 'bridge'. Some of us and some clubs have a stock of resistors. Either bought over the years or left to the club as part of a deceased estate. So what can we use?

[I actually bought some decades ago 51 Ohm resistors.] It is highly unlikely that you will find them near you! But two 100 Ohm resistors in parallel make 50 Ohms. With twice the power handling of a single resistor. Or four 220 Ohm resistors in parallel make [erk! I had to check using a calculator!] 55 Ohms. Just remember that resistors in parallel have also 'self-capacitance'. Which when you use four in parallel makes four times the self-capacitance...

Figure 2: A 'simple' bridge

So this is the circuit of a 'simple antenna bridge'. It is really quite simple. BUT turn the transceiver power output down to a Watt. Otherwise the resistors will get hot! Maybe even burn out. So before checking or testing, connect your dummy load and turn the output power (CW) DOWN!!! [Don't have a dummy load? Another article link!]

The 'balanced' condition when the antenna is approximately equal to the 51 Ohm resistance produces the lowest signal level out of the 'bridge'. Which is why the bridge needs a Watt or so to drive the diode to provide a d.c. reading. A germanium diode is used as it has a low forward voltage for conduction. To give a reading at 'balance'.

There are of course other methods of measuring the antenna impedance. And it doesn't have to be at a set frequency. The source could be a swept oscillator covering the adjacent frequencies. This would show up any 'out of band' resonance. That could be corrected quickly. Also the antenna impedance can be measured accurately. So that the compensating reactance could be connected to get it resistive at the desired frequency.

...

NOTE

Even a Watt will go a long way. So other methods of lower power signal sources have been used over the years to reduce the chances of interference. The lower level of d.c. from the diode is usually amplified by an operational amplifier.

...

I intend adding to this article. Please let me know in the comments if you would like more information on this subject.

73 John Brock ZS6WL