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

A 'Thick' Dipole for 2 metres

The Broadband 'Thick' Dipole for '2 metre's

Once upon a time, John (the first ZS6WL) and I constructed several '2 metre' dipoles and tested them in the lab at Telkor. The premise for the design was to make and manufacture an aerial that needed no tuning (or adjusting), and would cover the whole of the '2 metre' band with excellent s.w.r. / return loss. It should also cost very little and use readily available parts.

Recently I have been looking for my notes about this aerial (antenna in US English). I still can't find them. Not really a surprise but very annoying as I wrote everything in those days into my 'day book'. The original design made use of the “Egatube” or plastic conduit that could be found at any building site or house construction in the form of off-cuts or tossed 'T' pieces that no longer had screws. It used 20 mm aluminium tubing for the element conductors of the dipole. [Reality check! This so called 20 mm aluminium tubing isn't 20 mm! It really is 19 mm outside diameter. So it fits into the conduit tubing fittings.]

Thick Dipole inspection.

I must have given away my '2 metre' dipole as the only sample I have left is the 6 metre antenna. This was identical in construction but with quarter wave elements for 50-54 MHz. As I was struggling to get into the West Rand Repeater from the lounge, I thought I would build another '2 metre' dipole.

Why a 'thick' dipole?

The bandwidth of an aerial is defined by the length to thickness ratio of the conducting elements. So a thin wire aerial has a very high 'Q' factor and will be only usable over a portion of the band. We made several of the monopole aerials for the 70 cm band using 3 mm brazing rod mounted on a SO239 connector. When testing, we found them to be an almost perfect match to 50 Ohms over the entire 70 cm band. The length of the 'pole' was only about 170 mm. So 170/3, 56 approximately, is the ratio. This gave rise to the realisation that to cover the entire V.H.F. aircraft band we would have to build a 'huge tube' aerial. John actually used other words to describe the aerial tube but I can't recall them here.

The elements for the '2 metre' aerial are 20 mm tubing with a length of 490 mm. This gives a ratio of length to outside diameter of 24.5. Adding plastic caps (furniture feet protectors) may add some capacitance. So some adjustment may be needed after testing at 144-146 MHz.

You are operating from where?

When I worked at Telkor with John, I lived in an old mine house in East Chamdor. This was quite far away from any potential lightning strikes and on a moderately high piece of land. Surrounded by trees, I was fairly safe.

When we moved to Roodekrans, the first summer was spectacular to say the least. The church grounds a few tens of metres away had a large re-enforced concrete statue of a pair of hands praying. This got struck fairly regularly and the area around this had a lot of induction which destroyed all sorts of electrical and electronic equipment.

Then 'they' built the gap filler TV transmitter called the 'Roodekrans' transmitter. Irrationally given the name 'Helderkruin' as it was a 'gap filler' for there. This 150 metre tall structure is a marvel of engineering and provides a lightning spike for the top of the hill side. For the most part it drains away the electrostatic charge built up around it. But when it gets struck, that's when the induced voltages in the surrounding area's conductors occur. These induced voltages are extremely destructive. Phones, phone lines, fax machines and modems do not last very long here.

The 'other mode' of lightning which most radio amateurs are familiar with is the electrostatic discharge. This static crack transfers to all aerials locally and over great distances. Whilst the magnetic field dies off rapidly as the distance increases, the electrostatic discharge adds to the QRN heard on H.F. Radios. This is what kills the 'front end' of your receiver. Usually the R.F. stage transistor and any other components associated with the aerial input. Well at least around here (Roodekrans) it does.

So a dipole's no good then?

No dipole is good in South Africa. If it is a piece of wire in the air with no discharge path. So the dipole I am going to make has to have a d.c. short circuit. So a 'stub' must be added to the aerial assembly to provide a d.c. path to ground. It would be nice if it went directly to an earth spike as well. This happens to be one of the main advantages of the J pole or 'slim Jim' aerial. As both have d.c. short circuits for a discharge path.

The other 'thing' to remember is that a dipole is a 'balanced' aerial. That means that the pair of elements are supposedly fed in opposition to each other and neither is at earth potential. When we are talking about an H.F. aerial this becomes tricky and physically difficult with vertical polarised aerials. As the length of elements can be very very long. With horizontally polarised aerials, this still makes life difficult, especially for cluster house dwellers.

Waterproofing and Corrosion protection

Put anything outside the house for a long time and it will rust, corrode or degrade in one way or another. The UV from the sun degrades any plastic and makes it brittle. The rain and condensation will rust any ferrous metal. Aluminium oxidises with the air and forms a 'skin' of high resistance. This does affect VHF and UHF aerials. But the purer forms of aluminium used for television aerials (SABS approved) can withstand a few years of external use.

Remember that dissimilar metals have a potential difference and will react to each other. Ultimately forming a poor contact and causing high resistance or even worse, a diode!

My personal preference for 'protection' is clear Polyurethane lacquer sprayed onto the metal and plastic surfaces. So clean the aerial aluminium with sand/emery paper and wipe clear of dust and aluminium particles. Then spray at close quarters, the clear spray onto the elements. Do this after you have assembled the aerial. If you undo the mounting self tapping screws, remember to spray over the area to cover the exposed metal. Do this after you have replaced and re-tightened the screws.

Testing the Aerial

1 Mount the aerial on a bracket away from any objects such as metal fences or walls. Make the distance at least 5 times the wavelength. If you can place it in the centre of an empty aircraft hanger. But if you can't do that, try and mount it at least 5 * lambda (wavelengths) away. It would be better if it were 10 * lambda away but you probably don't have the real estate for that.

2 Connect any test equipment or hand held transmitter to the aerial using a very short cable and place the equipment in the plane of least signal impedance. This will generally be the centre mounting tube. So make this reasonable in size to support the aerial but not too long so the sag makes the aerial 'slant polarised'! If necessary do the S.W.R.¹ / 'Return Loss'² testing with the aerial mounted above the equipment and horizontally polarised.

3 Place the signal strength measurement equipment at least 10 * lambda away from the aerial. It should also be in the correct “plane” and polarised to match the aerial polarisation under test. So a vertically polarised aerial should be tested with a vertically polarised signal strength meter.

Test Equipment

Having 'put away' most of my equipment and sold nearly all of my radios, I need to test this aerial with some equipment before using it. I may need to rescue a few items from the garage.

1 Sweep Generator

I made a lot of use of swept frequency generators whilst working in electronics. In my first job, I had a 0.1 to 1000MHz sweep generator by Telonic. Later on, I had the use of an HP spectrum analyser and tracking generator that could sweep over an even larger range of frequencies. When I had nothing at home, I made a simple sweep generator that covered 120 to 200 MHz. This I used to check various tuned filters and front-ends for frequency response. It can also check for return loss using another item that I made; a 50 Ohm broadband return loss bridge.

So its time I went in search of these items and set up to test the new 2 metre aerial.

[more in part 2 --- Written in 2008 ]

Notes:

1 S.W.R. means “standing wave ratio”. A ratio of voltage or current sent to the load referred to the reflected voltage or current.

2 “Return Loss” means pretty much the same as S.W.R. but gives it in another way. It means the least amount of reflected voltage or current from the load (aerial). The greater the “return loss”, the better the match and the lowest amount of reflected signal power.

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.

 

 

 

 

 

 

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