OBSERVATIONS FROM SCOTLAND 01_May 2008 GM1SXX
The humble Yagi... or should that be 'Uda'
The DL6WU ... for VHF/UHF
Many radio amateurs just go out and buy aerials for the higher bands. After all, if you have lots of money to burn, why not? Here's why not. For a fraction of what you'd pay for a VHF yagi in kit form, that you have to put together yourself anyway... you can design and build a perfectly good (usually better) alternative, using software written by Guenther Hoch DL6WU, and using inexpensive materials from your local DIY store. What's more, it's fun, you'll learn along the way, and you always have something to 'crow' about on the air...... 'my aerials are home brew'. Furthermore, should hill-topping be your thing, you can make an excellent aerial that will outperform all but the best of the commercial offerings.
Mr Yagi and Mr Uda
The yagi, or more correctly the Yagi-Uda aerial (AKA antenna) derives it name from Japanese Professor Yagi and his student Uda. It was not invented as stated recently on HamUniverse by that 'Japaneese expert Mr Yagi Uda' :-)
The design concept was first outlined in a paper presented by Yagi in 1928 although it's probably fair to say that perhaps it should have been the Uda-Yagi instead, since the student, Mr Shintaro Uda, by most accounts, did all the clever stuff.
How it works.
The whole point of a yagi or any of it's derivatives, or any sort of beam aerial is to direct energy in a specific direction. This it does by means of additional 'parasitic' elements. The directors are less than a half-wavelength wide (typically 5% under) and taper slightly downwards in length as you approach the front of the aerial. In some countries the yagi, and it's derivative the Log-Periodic Yagi are called a 'fishbone' and it's easy to understand why.
Elements used to provide directionality are called 'parasitic elements'. The spacing between the elements of most yagi designs is usually of the order of between 0.2 and 0.25 of a wavelength and is not always constant. Making the reflector longer than a half-wave causes it to have inductive reactance while making the directors shorter than a half-wave causes them to to have capacitive reactance.
So, to recap, the reflector element is typically 5% longer than a half-wavelength at the design frequency while the director elements start at 5% shorter than the design 'half-wave' and taper gradually downwards in length towards the front of the array.
Because of it's sheer usefulness, the yagi has been the subject of much study by learned individuals (obviously not GM1SXX & LA2QAA) and various agencies, with the US National Bureau of Standards also being involved. Some of the NBS designs are still used for commercial and amateur yagi designs.
As already mentioned, the effect of making the reflector longer is to cause it to have inductive reactance while for the shorter director elements, they exhibit capacitive reactance.
The parasitic elements of the Yagi re-radiate their signals in a slightly different phase to that of the driven element. In this way the signal is reinforced in some directions and cancelled out in others. It behaves rather like a lens does in optics, focusing energy via the directors.
The phase and amplitude of the induced current in the parasitic elements is depends on their length and the spacing between themselves and that of the other elements in the array. Moving the position of *any* element on the boom changes the behaviour/performance of the entire array.
The impedance at the feed-point of a typical yagi with a driven dipole element is generally of the order of 75 ohms and is largely dependent on it's proximity to the reflector or first driven element. remember also that the dipole feed used in the yagi makes it a balanced aerial and it should always be fed by a BALUN is co-axial cable is used.
Any changes made to the lengths of any element or the spacing between them has an effect on the impedance at the feed-point. In short, everything is interactive. This makes the design of yagis pretty tricky indeed.
It's possible to replace the driven element with a folded dipole element. This is a very popular way to feed a yagi and in this case, the feed-point impedance will be closer to 200R. This is easily matched to standard 50R coaxial cable using a 4:1 BALUN made from an electrical half-wavelength of co-ax. Note that only the inner cores of the co-ax are connected to the feed-point while the braids are bonded together.
Alternatively, the folded dipole element could be fed via ladder-line or open wire feeder.
Half-Wave BALUN
Half-wave co-ax BALUNS made easy. (link)
If you use a folded-dipole to feed the yagi, you should use a BALUN at the feedpoint. This can take the form of an electrical halfwave of cable. Since the Velocity factor (VF) of most amateur coaxial cable is 0.66 you just divide the wavelength you need (in metres) by two and multiply the result by 0.66 for the correct cable length... also in metres. l connections should be as short as is feasible and if used outdoors, the feedpoint should be made waterproof. Just like the aerial itself, the coaxial BALUN is by nature a narrowband device and will only work properly at it's design frequency.
Polar pattern of a typical yagi.
A polar pattern is simply a (logarithmic) graphical representation of the relative power delivered by an aerial. Imagine you are in space, looking down on the aerial. The hatched areas represent unwanted radiation while the large 'major lobe' represents the radiation you are trying to direct. The whole point of using a yagi or any other beam aerial, is to be able to 'squirt' as much RF energy as possible in one direction. Unfortunately, like almost everything in life, there's a downside. Some energy is radiated backwards (rear lobe) while some is radiated sideways (side lobes). In a good design, the side-lobes and rear lobe are minimised, but can never be eliminated.
The long and the short of it.
Yagis come in 'long' and 'short' varieties. Almost all the world's yagis are of the 'short' type. There's a reason for that, it's called 'compromise'. A short yagi gives a reasonable compromise between cost, gain and size. The yagi connected to your TV at home is virtually certain to be a 'short' type unless you live in a fringe area, in which case, it could well be a 'long' one. So, whats the difference? Well, a long yagi that has the same number of elements will outperform a a short one in all but size and cost. It'll cost a little more because the boom is longer... and did I mention, the boom will be a bit longer! The main difference between the two types is in the beamwidth. A long yagi with a comparable number of elements to a short type will have a narrower beam, making it more suitable for use in marginal conditions. The difference in cost is slight, just a bit more for the longer boom length.
As you'd expect, the design of an effective yagi type aerial depends on a great number of variables. Fortunately, we can benefit hugely from the work of others who have made their findings available in the form of some excellent computer software. For VHF and UHF work, DL6WU stands tall. The DL6WU yagi design software has been well proven to produce excellent repeatable results for long yagis. If performance is your aim, the DL6WU long-yagi design is the choice.
When building yagis, you can use one of these three popular methods....
1) Conductive boom with through boom (bonded) elements.
2)Conductive boom with stand-off elements (insulated).
3) Insulated boom.
Of the three systems, the conductive boom with bonded elements is the best mechanical design but unfortunately, the elements require a compensating adjustment. The DL6WU software can do ALL the hard work of designing a yagi for you. All the hard work is taken care of and you only have to put in the numbers, jot down the results, source the material and prepare to do a spot of 'metal-bashing'.
Many DIY stores stock suitable materials for constructing yagis, Sqare aluminium tube and hollow or solid aluminium rod are the staple materials of the Yagi builder.
Yagis for 2M, 70Cms and 23Cms are easily built from cheap raw materials. When cutting yagi elements, you should note that accuracy is important, both with regard to element spacings and element lengths. You should aim to be accurate to the nearest millimetre.
For 2M yagis, the 1M lengths of aluminium rod and tube stocked by many suppliers will be inadequate. Aluminium tube generally comes in sizes that can 'telescope' so they can be joined to form the lengths you need. Drilling and fixing is advised using small stainless steel screws (if you can find them). Alternatively, the elements can be drilled and bolted (model shops stock small nuts and bolts) and the joint smeared with epoxy resin for weather protection. Alternatively, I've had good results using 8mm diameter copper 'microbore' tubing (heating and DIY stores) with the additional benefit that it can be easily soldered to. Apart from the driven and reflector, all the other elements in a 2M yagi are under 1M in length.
For 70 and 23Cms yagis things are easier, Just use 1M long tubes cut down to size for the elements. Use the DL6WU software DL6WU Yagi Design software (Download) to design yourself a yagi to suit the materials available to you. This is a DOS program but can be run under Windoze. Just download the program to your desktop and run it from there.
Various hybrids of the yagi exist but in essence they are all parasitic arrays derived from the yagi.
The Quagi is one example... a hybrid of the quad and the yagi. With loop type reflector and driven element, these tend to have a broader main lobe than a yagi and a broader bandwidth. By comparison, a yagi has a bandwidth of around 1% so can only operate effectively over a relatively narrow band of frequencies. In short, the quagi has a lower Q than a yagi and a broader main lobe as well as a bigger bandwidth.
It's possible to place two yagis on the same boom, one vertical and the other horizontal. This gives a choice of vertical or horizontal polarisation. Although both aerials occupy the same space, they don't really interact much with each other.... effectively they don't 'see' each other.
By means of a phasing harness, it's possible to feed both yagis in a way that produces circular rather than linear polarisation. This has applications in satellite communications (both commercial and amateur) .
It's possible to build a dual band yagi using two sets of yagi elements at 90 degrees to each other (as above) but on different bands. This technique is used on the popular 'Arrow' amateur satellite aerial where 145 and 435Mhz elements co-exist on a single short boom.
It is also possible, but not recommended, to place two sets of yagi elements in the same plane (both vertical or horizontal) but on different bands. This allows the signal polarisation for both bands to be identical. It is however NOT an easy exercise since both aerials interact to a considerable degree. It can be done, but it's not a clever idea. If you have the space, it's far better to have separate aerials for each band.
Some really basic stuff about yagis.
1) Long ones almost always beat short ones for performance (given the same number of elements).
2) Lots of closely-spaced elements doesn't always translate into lots of performance.
3)Boom length matters.
4) Yagis are simple things to build but tough things to design.
5) The DL6WU software allows anyone with decent mechanical skills to build good repeatable long yagis from 144 through to at least 23Cms. With care in measurement, 13Cms is also feasible.
So long as someone else does all the 'hard sums'... someone like Guenter Hoch DL6WU for instance, you can produce a long yagi that's simple to make, repeatable and reliable.
Go on, try it. You won't be disappointed!
Some web links...
73 AL.
GM1SXX
