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.]

 

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. 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.

 

 

 

 

 

 

RESISTANCE is futile - 'the Borg'

And REACTANCE is imaginary. 

But resistance does 'work'. A resistor 'dissipates' heat when powered. Examples are a 'Bar Heater' - 1 to 2kWatts. A fan heater or a hair dryer. Some of which can be used as a 'heat gun'!

Right now the 'issue' is how to measure power. My long time friend and fellow radio amateur wanted to 'calibrate' a transceiver to S9. What is S9 you ask? Well the convention amongst amateurs is S9 is equivalent to 50 micro volts. Where? At the receiver input. Usually assumed to be 50 Ohms. But it doesn't have to be so low...

Like a lot of signal generators the output level is around zero dBm. Oh sorry! That is 1 milli Watt into 50 Ohms.  (even worse!) What is that in Volts? Time to get the calculator out. 

Power = ( V (r.m.s.)^2) / R  [That is V squared - and r.m.s. because this is an a.c. wave not d.c.]

which when transposed gives  Power * R = ( V r.m.s.  ^2)

Or  V r.m.s. =  SQR( Power * R)   [ SQR mean the square root - see a spreadsheet program/app]

When I worked it out I was gratified to get the same figure as I remember from a long time ago! The SQR of (50 * .001) is 0.2236 volts r.m.s. So the ratio of S9 (50uV) to 0.2236 is 4472...

So how do you get to 50uV from a signal at .2236 Volts? You use an attenuator. 

But his signal generator only had four 20 dB switched attenuators on the output. At this point we MUST discuss the use of dBV. This is because of the use of decibels and a constant impedance/resistance circuit. It is a 'convention' that radio frequency measurements in general use a input and output impedance of 50 Ohms. It didn't used to be... in the early days (last century) measurements at audio frequencies were done at 600 Ohms (in/out).

A voltage ratio of 4472 in dBV is 20 * log10( 4472) which is :- 73.0 dBV Which of course is impossible to achieve with his switched attenuator. He can get to 60 or 80 dB below the output...

So he needs a smaller step switched attenuator... Maybe a 1, 2, 4, 8 dBV attenuator?

Having got to this point I remembered that I had made a switched version many years ago using small toggle switches and resistors. Finding it was no problem. Drawing the circuit - no problem. Calculating the resistor values - time to hit the books. No not the Internet. 

Miniature Toggle Switch Version [Circa 1980]

Well the original was calculated using the Handbook by Scroggie and a new scientific calculator. Using Pi type attenuators at 50 Ohms. I tested it on a Spectrum Analyser and it was certainly 'good enough' for h.f. and above. Might not be accurate at 400 MHz or higher. But if made today with surface mount components might just be accurate. 

The calculated values             Nearest standard value

Loss    Shunt R        Series R 

1 dB    869.549        5.769        820        5.6

2 dB    436.211        11.615      470        12

4 dB    220.971        23.848      220        22

8 dB    116.143        52.844      120        51

Yes I know not very accurate - but close enough for Radio Amateur use. If you really - really want to - you could test a hundred of the standard values to find a close value. The sort of task given to students and apprentices last century. Oh and the 10 Ohm resistor? Well it is actually two in parallel. As I couldn't find a 5.6 Ohm resistor.

 

What about some power?

So someone gave you 20 1K 2 Watt (carbon film) resistors. You put them all in parallel inside a tin with the input connected to a good connector. Yes at h.f. an SO239 is fine. Now you have a QRP 'dummy load'. As an added 'feature' you might want to lift one of the resistors off the ground connection and connect it to another SO239. Put a 56 Ohm resistor across it to make it a known output resistance. This gives you the means to measure the output power into the 'dummy load' reduced by a simple calculated amount.  

What about the carbon film resistors? Yes they are slightly inductive. But as there are 20 of them in parallel, the inductance is 1/20th. So of no significance. 

Power rating? Well it won't be 40 Watts. But 25 should be alright for a short time. Do not pour in oil. Or like a colleague did many years ago only to make the match go from not bad to diabolical! Rather find a 8cm fan and a 12 Volt battery. Add vents and blow the resistors cool.

 

 

    

 
 

  

 

Time? Spare Time?

It used to be 'procrastination is the thief of time!' But what about time itself. Is it a self?

When you are sick you don't feel like posting - anything. When you are sitting relaxing you don't feel like posting... When you have a cat glued to your lap, you don't feel like posting or moving.

So why today? It is my (step)son's birthday. It is not warm still 20C in the study. In Bristol it is 4C so I can't complain. I need to go to the shops for important stuff like food. I might have to put a jacket on...

I tell myself that I am waiting for the traffic to dissipate. Maybe true but the fact is I don't enjoy the stress of avoiding potholes especially on main roads. At 80 kilometres an hour on Ontdekkers it cost R5500 last year. 

Being retired I should have time to do all the things I was doing in my spare time when I wasn't alone. But projects that I put down to attend to other things are still lying there. Recently I got a tummy-bug that really put me down. Thanks to a friend I have some tablets that work. While down but not out, I gave some thoughts to some recent 'issues'.

The latest is the lack of interest in reworking of some established web sites.  My long-time friend Bob has taken issue with a particular web site which accepts pictures of a certain size in pixels. So I researched the problem. I reached for GIMP [gimp.org] and examined a couple of pictures taken with my phone. It is not a modern phone - 2020ish. But the number of pixels are enormous in comparison to the 90s! 

 

See what I mean?  4000 plus width and nearly 3000 height in pixels.

When reduced / scaled to 780 by 439 pixels.

 

This lead me to put the original pictures on my 'Intranet web-server'.

{Don't have one of those? Ask me how in the comments.}

 

Before skipping away from here ask yourself how the two pictures are the same size on your screen. Because the web site limits the displayed size to the desired number of pixels...

 

Who did that?

The 'webmaster'.

All that is needed is a px adjustment on the web page. The 'server' will do the rest.

 

Or you can the Google AI...

 

From Google AI
Designing a web page for an 800px width typically involves creating a fixed-width container to ensure compatibility with older or smaller monitors () without triggering horizontal scrolling. While modern designs favour responsiveness, limiting content width to around 760–800px (to allow for scrollbars) ensures usability on smaller, older, or non-maximized browser windows. 

Key Aspects of 800px Web Design:

Optimal Container Width: To avoid horizontal scrollbars, set the container width slightly less than, often allowing for browser vertical scrollbars.

CSS Implementation: Use max-width: 800px; or width: 800px; on a main wrapper container to restrict content width, often centred using margin: 0 auto;.
 

Responsive Considerations: While is a common "middle bound" for smaller desktop screens, modern responsive design typically uses media queries to switch to full-width (800px) layouts on mobile devices below
.
Readability: Limiting the width prevents, for example, long text lines on large monitors, improving readability.

You can even use CSS. Example given:-

 

.container {
  max-width: 800px;
  margin: 0 auto;
  padding: 0 10px; /* Optional padding for smaller screens */
}

 

 

 

 

Re-discovering an IBM 80486DX2

 

What an interesting PC.

The IBM label can be orientated according to how you place the box!

A whole 64MB of memory. A date and time remember correctly from last century! A really fast Windows! And programs that load in under a second. Not 11 Seconds like Notepad in Windwoes 11.

More to come...

 

 

Today is the 'Day After End of Support for Win10'

From reading the Register this morning I can only sympathise. As I a 'retired elderly person', I could not afford the subscription rates for the operating system or the office software.  

Frightful Patch Tuesday gives admins a scare with 175+ Microsoft CVEs, 3 under attack
https://www.theregister.com/2025/10/14/microsoft_october_2025_patch_tuesday/

If you wanted to upgrade or install Win11 you might find the installer failed on starting. 

 I have been a committed Linux user for several decades now. 

https://www.theregister.com/2025/09/18/three_decades_in_of_linux/

 Well did your windwoes shrivel up and die? 

Maybe the ever present adverts became more 'in your face'. But it didn't stop dead did it? 

In my cleaning routine the other day I came across a "Y2K" adaptor card. Now nearly thirty years old was supposedly the cure for the PCs of the day. Most won't remember the panic of 1999. Perhaps a good thing.

The Y2K problem (issue) was known about in the 1980s. Predictably some programmers made allowances for it. When December of 1999 came about there were a lot who predicted the 'sky was falling!' There were some system failures. But nobody remembers them.

In the years following I insisted on the use of ISO8601 date formats. This was especially needed in the payroll industry. There were some who used the SA format and others who ignored the forced/default US format. When information was transferred and the errors lost money, then the brown stuff hit the fan! 

Windows XP was the 'new and improved' OS of choice. I remember having to reset a lot of PCs to a locale of 'South Africa'. So the date and time format along with the currency would not be American. It wasn't easy. I wonder if the Windows 10/11 PCs default to US locale? I don't care - it is your problem.

Relax! It won't fit in your laptop. Or your desktop of this century. It is an ISA adaptor card. Meant for a PC XT or PC AT. Maybe a 386 or 486. But definitely not required in 'modern times'.

At least it wouldn't change the spell-checker to US. Or interfere with your search parameters. The 'knock-on' effect in Windows XP was that the M$ Office settings would now assume all the settings were US. Which made a lot of official documents drive some of us up the wall with laughter. Especially as the hidden properties of a Word document would tell the whole 'story!'

ISA - Industry Standard Architecture

Not this century! And not a 'standard' today. (Look it up if you want: https://en.wikipedia.org/wiki/Industry_Standard_Architecture)

 

 

Components, complaints and complacency

Whilst reading a few issues of Wireless World from the world radio museum web site, I wondered about the issue that happened on my 21st birthday month. Reading the editorial rang a very loud bell! If it reads familiar to you enjoy the feeling. JB ZS6WL

We are constantly receiving letters from private individuals who are finding it impossible to obtain supplies of certain components and whose pleas to manufacturers and distributors are met with stony silence. Even the small company, not in the electronics field, which requires a special component for a one-off job—and which has the advantage of a company letter heading—sometimes receives the same treatment.

One of our correspondents, who was starting a small company, claimed he was asked for two trade references and the name of his bankers, and that was only in order to receive a catalogue!

However, component supply is the result and not the cause of the problem, the whole attitude of the electronics industry towards the private experimenter and the amateur is one of non-co-operation to the point of scorn. Why is this, when many of yesteryear’s major innovations in radio and electronics emanated from the results of work carried out on a kitchen table?

In those days the amateur and the professional (often one and the same person) were working on similar problems and there was a mutual respect. The technology has advanced in leaps and bounds since then and industry is staffed with people who more than likely do not have an amateur background and who have no appreciation of the problems and frustrations that can face anyone trying to work on his own for interest, self-education or amusement.

Because of the great amount of publicity given to electronics, and the aura of mystery surrounding it in the eyes of the layman, more people are taking a practical interest. This has led to manufacturers and distributors being bombarded with letters requesting the solution to private electronic problems, many of which are nonsensical or frivolous, and others could have been answered easily if the writer had shown a little initiative or visited a good library.

To answer all these queries would cost a company a great deal and what would they get in return? Perhaps an order for two or three components, the value of which may be less than the cost of the handling.

By making their components generally available on the retail market, to be bought by people who may not be qualified to use them, a company feels that it is inviting the sort of costly correspondence mentioned. The reason for the reticence in this respect can be understood.

All this has led to the present ultra-low status of the amateur in the eyes of industry and the reluctance of many concerns to accept small orders.

The industry does, however, have a responsibility to the public, even if it is only to maintain its own image, and attempts must be made to give assistance in genuine cases. Refusals because of a couldn’t-care-less attitude can never be justified and small losses should be accepted at times.

Manufacturers could easily set up machinery to ensure that their products can be sold on the retail market through a distributor. Because of the difficulty in assessing the possible quantities required perhaps some sort of sale or return arrangement could be operated with the distributor. At the present time many components are completely unobtainable on the retail market.

In addition, all private individuals seriously interested in electronics should put their own house in order, and as a first step may well think of joining a club. If there is not one in the area—start one. The answer to nearly all the problems likely to trouble the experimenter could be found amongst a group of people with a common aim. Particularly difficult problems could be made club projects. Benefits could be reaped in terms of central facilities, pooled test equipment, tools and literature.

A great deal of useful work can be done by a well-run organisation of this nature and the local community can benefit. For instance, club projects could aid local handicapped people, small electronic systems for local firms could be designed and constructed (power supplies, control systems, photo-electric switches etc.). Often these firms can advantageously use electronic equipment, but, because only a one-off is required, it is uneconomic to employ professionals to do the job.

The companies who supply components would, we feel sure, be more than willing to assist such organisations so long as things were done on a business-like basis. A good example of the sort of co-operation that can be achieved is to be seen in the components list for the Logic Display Aid in this issue.

From: Wireless-World 1969-06 pg69 

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).

 

Ham-Comp - before the Pi

Yes before the Raspberry Pi we had computers of all sorts. As a radio amateur I had some experience of using a microprocessor for an application. It was the 80s and RTTY was in great favour. This usually meant acquiring a teletype machine. Which the post office of the day was dumping in large quantities. These were hernia-inducing electro-mechanical machines weighing the best part of 50kg.

They also only ran at 50 Baud. Most radio amateurs were already running at faster speeds. And ASCII was just 'allowed' so long as it wasn't faster than the Posts and Telecommunications could read! So 300 bits per second of 7 bit ASCII. Packet Radio was just around the corner.

On my return from the UK with a ZX81 I unwisely mentioned this on the Johannesburg repeater. Being told that this was just a toy incapable of doing much. I should buy an Apple which could do much more. At this time CP/M personal computers were flooding the market with reasonably priced models. Much lower cost than the Apple II. With inexpensive software and adequate performance they were taking the word processing and spreadsheet market by storm. The Z80 versions were also quicker than the 8080 versions. Hard disks were starting to become available. I know you will laugh when I tell you the size of 5 Megabytes was considered enormous by us then.

So onto this scene came the low cost ZX81 later the Z80 based 'Spectrum'. Both had reliability 'issues' with the back connector. Which always seemed to fail at the worst possible moment. But onto this connector could be plugged all sorts of 'peripheral' cards/adapters.

As I was working at a company that had just invested in an 8080 development machine, I started taking an interest in the microprocessor. This unit had an EPROM programmer as part of the front keyboard and screen. This allowed you to write an assembler program [in 8080 code] and program the EPROM with the code. Hang on! How was I going to write for the Z80 based ZX81? ...

Well it wasn't easy but the assembler would allow 'define byte' inserted into the code. This is where I was exposed to 'code compatibility' in no uncertain terms. The Z80 had literally grown out of the INTEL 8080. Because the founders of Zilog had left Intel and designed the Z80 as 'replacement' for the 8080.

So the majority of the instructions were applicable to both processors. The assembler would produce a program that would run on the Z80 without problems.

With the help of a friend and colleague I programmed the ZX81 by hand. Wrote the program into an EPROM and took it home to try it...every night!

This 'project' taught me everything about the hardware interfacing with the software writing that forced me down a path I had not considered. I learnt how to set up the 8251 UART for 5 bit operation [Baudot]. I also learnt how to read and write to the registers and memory. It took a month but I did demonstrate it working to the Rand Amateur Radio Club. This used the TV screen and the ZX81 'listening' to a cassette tape of a RTTY transmission. 

I still have the sterilisation lamp I bought to erase the EPROMs. It is in the garage. I used to test the program I had written and erase the ones from the previous night. Then sit down with the Z80 Assembly Language Programming book by Lance A. Leventhal and pencil in the next bit of the program. I still have the book. 

The wires went to the PCB connector which was cut to size and a key added to ensure I didn't plug it round the wrong way. The 8251 is above the 5 Volt regulator heat sink. The crystal is the baud rate generator crystal with a DIP switch to set the required baud rate. As time went by I added another 2k of RAM as well as some 8 bit parallel ports. 

I don't think I could wire it up these days!

 A more complete picture with the connector/extender. Oh yes there was all sorts of possible expansions.

Later there were Z80 Assemblers and EPROM programmers. But this was 'hand-crafted'. The EPROM is missing probably somewhere amongst the EPROMS I still have somewhere! The program listing? I know I wrote it all into an exercise book. But again it is somewhere - to be found later...

73 ZS6WL John Brock

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