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...