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.
