Canvey Island, Essex, England Justin@g0ksc.co.uk
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Updated 1/01/2010

What is an OWA Yagi?

An OWA or Optimised Wide-band Array Yagi is one that has been designed to present a 50 Ohm impedance at the feed point and therefore requires no matching arrangement. Often, in order to achieve higher levels of 'paper gain' manufacturers throw radiating efficiency, impedance and many other negatives to the wind in order to see a higher gain figure. Once the antenna is completed a matching arrangement is built onto the antenna to present a 50 Ohm load to the coax. However, there are losses in any matching device resulting in large losses before your power gets to that 'high gain' antenna and radiated. Careful consideration needs to be made when choosing a low impedance antenna. Very few antennas are modeled to include the matching arrangement, only the effects on impedance the matching arrangement has. This can result in very mis-leading figures as the end built antenna is far and away from the software model.

The easiest way to achieve gain is to lower the antenna impedance, anyone can do this. However, in so doing smaller bandwidth properties often result, antenna instability ( antenna de-tuned by houses, rain, other antennas etc). With a little skill, lots of time and patience, good levels of gain and performance can be achieved at 50 Ohm impedance with nice wide stable bandwidth. However, I have now been able to achieve excellent results form certain low impedance designs where careful consideration of matching has been made to ensure pattern stability too. Look else where on this site for OWL (Optimised Wideband Low impedance) designs.

What other problems could be seen or covered with matching?

The problem here is two fold. A good matching arrangement can cover a bad Yagi design. This means whilst you see a good SWR at your transceiver, optimal levels of RF may not be being radiated by the antenna. The next is a bad matching arrangement could dissipate your RF energy as heat and a good antenna could be deprived of that precious RF from your rig!

Why risk a matching arrangement issue when you don't need to have a matching arrangement!

What other benefits are there to an OWA Yagi?

OWA Yagis, if designed well show a good SWR across the desired band or at least large portions of it. Generally radiation efficiency levels are very high with absolute minimal losses too. OWA's are not effected by water, close by antennas, structures or tress in the same way as lower impedance antennas can be. In addition, due to the wide-band characteristics of the OWA, small errors in calculations which building the antenna will not make a huge difference to the performance of the completed antenna. This must be a good reason to build and OWA for a start!

Construction and Materials of G0KSC Yagi Antennas

When designing my OWA antennas, I have designed them with performance in mind rather than convenience of being able to pack and post them. Where possible, I design with single piece elements and booms although I am looking to alter some designs in order to provide compromised versions for though amateurs that want to purchase antennas from me.

The element insulators have been crafted by hand, by me out of 15mm thick Plexiglass plastic and all fixtures and fittings (except the antenna to boom mount) are Stainless Steel.

Why one piece elements?

Compromises have to be made when building antennas sometimes. Tapered elements being one such area this takes place. The thicker the elements, the better to bandwidth of your beam so of course this is ideal. However, having a single piece element on a 20 metre beam at 2 inches this just not practical and therefore, tapering is common place. on 70 Mhz however, it is easy to have 1.2 single elements but postage and storage for suppliers is an issue if such large elements are produced and hence, tapering once again occurs.

The most important attribute is this. The more joins you have in your elements, the more points you have for corrosion or water ingress. However small this maybe, it WILL alter impedance and in turn, de-tune your antenna. with one piece elements, any chance of this are eliminated.

Why insulated elements?

Why insulated elements, you have seen lots of antennas built where all elements are bolted to the boom right? The first point is it is much easier to bolt all elements together. Drill the square boom and feed the 1/2 tube through the hole with a nice big self tapper or nut and bolt right?

Remember the point above about corrosion, water and resistance? The Yagi antenna is a tuned antenna array. The designs to isolated element arrays and fixed-to-boom arrays are different. However, with the isolated elements such as in mine, nothing can effect the performance, gain or tuning of the antenna as each element has nothing else (conductor) influencing it's performance.

 

Does the SWR change on your Yagi when it rains? Now you know why.

How was this antenna designed?

There are many very accurate packages out there today which will allow you to design your own Yagi antenna. With time and dedication, a good performing Yagi can be built. I use 'Antenna Model' and 'EZNEC+ 5' which are both commercial packages and 4nec2 which is a free package available on the Internet. This allows me to confirm my designs with different packages using different base engines and algorithms. Once I have an antenna close to where I want it, I use a computer optimization tool which fine tunes the antenna's spacing and lengths (elements) for the criteria that I pre-set.

So can you buy it?

All of my designs are available free for you to build yourself. However, I do build custom antennas for Hams of existing and new designs if they are required.

What is the downside?

The downside is you will need to collect these antenna as all element lengths are too long to post. the Post Office and other services tend not to post items over 1.2 metres in length. Sorry!

Custom builds?

One of the reasons I started to design and build my own was because I could never find anything exactly how I wanted it, boom length, gain, front to back etc was always not quite there. For you, taking the months to learn and master antenna design packages may not be your thing or you may simple not have time. If you have a Yagi you would like to own that is just not commercially available, let me know, perhaps we can build or design it for you.

info justin @ G0KSC.co.uk join the mail address together to mail me.

This is an article I wrote for the UKSMG News Letter. It provides details and producing an effective 5el Antenna for 6 Metres and measurements form producing a 4le 70 Mhz antenna out of the left over components.


Please note: Read the 2 links below for up to date information on building and insulating a Yagi


http://www.g0ksc.co.uk/please-read.html

 

http://www.g0ksc.co.uk/creatingabalun.html

 

Firstly, I would like to thank Pop, YU7EF for creating my extended interest in Yagi antenna design, designing some of my first antennas and helping me along the way with many hints, general help and tips.

The next point I would like to raise is this. This antenna is not meant to be a 'gain beater'. We can all surf the Internet and find antennas with incredible gain and performance boasts. Some maybe correct others may not but the point here is; this antennas has been designed to provide excellent all-round performance from a very compact Yagi and at the same time is simple to build.

 

Directives

 

Earlier this year I installed a nice 3 section tilt over, wind up tower which sits conveniently between my house and the next. The good side to this tower is that it is well hidden and when not wound up, is not an eye soar and well out of general view. The down side is that in order to get antennas passing my house (on the way up) the boom length needs to be relativity short unless I use a longer top section tube which then defeats the object and benefits of my first point.

The above meant I had to produce an antenna with a boom of around 3.5 Metres long and my own target from a gain perspective was 10dBi. As apart of general beam requirements, at least 20dB front to back would be appropriate too. For me, the antenna must be broadband and therefore not effected by bad weather, other close by antennas and not needing any matching arrangement at the feed point. The thing is, I had not seen anything out there producing the figures of that compact size. YU7EF's most compact Yagi, a 5 element, is 4.15 Metres in length and produces 10.1dBi forward gain.

The next steps are to get the antenna software packages to work and see what can be achieved in theory. Perhaps what I am asking is a little too much?

 

Tools for the job

 

When I embarked upon my antenna design, I downloaded and installed (free of charge) 4nec2 which is a great antenna modelling package http://home.ict.nl/~arivoors/ .

It has some sample files which you can alter a play too in order that you may familiarise yourself with the software.

Next I purchased EZNEC+ 5 www.eznec.com  to do my verification with and lastly, Antenna Model www.antennamodel.com . AM is the package I do my early designs with. 4Nec2 and EZNEC use the NEC calculation engine and Antenna Model uses the MININEC engine. It is good practice to test designs with packages using different engines to ensure there have been no miss-calculations. furthermore, AM has a very good optimising package which will get a very basic design very close to an ideal model very quickly. I will not go into detail and list the features and benefits of both but when we are modelling antennas and predicting performance, it is good practice to confirm the designs within other similar packages in case one has made errors within it's calculation engine.

 

 

Antenna Model has a very powerful optimiser too. Intended to fine tune and finish off designs for you, the Antenna Model optimiser can make a reasonable antenna out of a few wires of approximate length for the band you are building it for evenly spaced apart. Obviously there are a number of settings and adjustments within the package that you have to make and when looking to build an antenna which requires no matching arrangement (which we are doing here) things get a little more complicated as the software does not technically support such designs. However, a 'rough diamond' result is achieved within this optimiser much faster than if one were to manually trim lengths then test the antenna. Once a model has been achieved within Antenna Model and tested within EZNEC+.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5 element Yagi

 

Boom Length: 3.411 Metres

Bandwidth: 50 to 50.5Mhz @ 1.1 SWR

Forward Gain: 10.06 dBi @ 50.250 Mhz

Front to Back: 22.89dB @ 50.250 Mhz

If we look at the length versus gain, we can see that we have come to within .04 dBi forward gain of Pop's 5 element beam (albeit we have less front to back ratio) with a full .75 Metre shorter boom length. The SWR has been retained and our target Front of Back ratio of better than 20 dBi has also been achieved.


 

Building the antenna

 

One of the design criteria I had not mentioned above was that of all parts being available in sizes obtainable in the UK and at the best prices. I did a little research before starting the design work and luckily for me, found a local merchant ( http://www.clifdasteels.co.uk/ ) that was able to supply the aluminium tubing I needed but in selected sizes and lengths. 3/8 of an inch was going to cost just £7.00 per 5 metre length whilst ½ inch was just £5.00 per 5 metre bar. This meant I would not use any tapper whatsoever. The ½ is good quality, thick wall so does not show any sag or bend.

Next was the boom thickness. This was a project for the Christmas period and I was not sure how well the antenna would stack up so I purchased an 1,1/4 inch square bar along with an inch square. Again, these lengths (£15.00 and £12.00 per bar respectively) are good quality thick wall and therefore, the 1inch square bar was more than ample to hold up this compact Yagi without any additional support or sag.

The Insulators are being made from clear 15mm thick Perspex which like most things these days was purchased from Ebay. A Table saw and table router would come in handy and make the insulator job a little more palatable. If there were enough people that wanted to build this project and wanted to use my insulators too, perhaps I could build a few extras?

 

 

 

Parts List:

1X 5 metre length 1inch square, square tubing (Clifta Steels) £7.00 per length

5X 5 metre lengths of ½ inch tube (Clifta steels) £5.00 per length

1X A4 sheet of Perspex or Plexiglass 15mm thick (Ebay £13.50 inc delivery)

5x 65mm long M6 stainless steel nut and bolt kits (Ebay) around £6.00

2X 50mm long M6 stainless steel bolts with 6 washers and 4 nuts around £6.00

1X simple boom to mast mount (Truck King, Ebay) £3.50

one 8mm diameter nylon rod or plexiglass rod. I used nylon Ebay £2.00

 

Insulators

 

The first thing for me to do was to make the insulators. If these did not come out right, then another method was going to be needed. Also, this is the most difficult part of the project.

I have a table saw and table router which does make things a little easier. However, the same job can be carried out with hand-held devices too. If you really do get stuck, let me know.

The insulators are cut from the 15mm thick Plexiglass/Perspex and are 100mm long by 30mm wide. First, I cut a 100mm strip of plexiglas and routed a 1 inch grove in the centre from one end to the other which should be between 1 and 2mm deep. When you have done this, you can test your work by placing the plexiglass onto the square boom length and if you have completed the task correctly, the piece should not move side to side.

Now cut the 100mm length into 30mm strips, you will have more than you need, 9 in fact so you have a few spares. As we did with the 100mm strip, we now need to route a grove in these 30mm wide strips from the elements to sit upon. These need not be ½ inch. I made this around 4 to 5 mm wide In order that the elements sit above the insulator just touching it on each side of the grove. However, this is enough to keep it is place. Check out the photos of the insulators to ensure yours look like these!

We will have to do a little more work with the dipole insulator so we will leave this one until last.

 

Fitting the elements and insulators

 

The next job is an easy one, cutting the elements and boom length to size. These sizes are as follows:

Boom Length - 3.411 Metres (cut to at least 3.45)

Reflector - 2.988 Metres

Driven - 2x 1.468 Metre lengths (there is a 10mm gap in the centre of these two at the feed point)

Director 1 - 2.784 Metres

Director 2 - 2.734 Metres

Director2 - 2.654 Metres

 

 

Spacing:

Ref = 0

Driven = .503

D1 = .753

D2 = 2.069

D3 = 3.411

 

Starting with the reflector end, place the insulator over the boom in its mounting position and align one edge with the end of the boom. Having measured the width again (100mm) and marking the centre, drill (M6) a hole through the centre of the insulator (in the recess for the element) and through the boom too. Now measure the reflector element and establish the centre point. Drill an M6 hole here too. Using one of the M6 65mm length stainless steel nut/bolt/washer sets, secure the reflector to the boom. Hopefully, everything fits fine, now remove the element and bolt the insulator to the boom on its on. Now, turn the boom on its side so as the insulator is pointing straight up and down. Hook a tape measure end on the back of the insulator where its end meets the boom end and measure along the boom to the point where the driven element would be in this case, .503 metres. Mark the boom with a pencil of knife. This is the point where the back of the driven element insulator will align with.

 

Now conduct the process we did with the reflector by drilling through both insulator and boom and placing a stainless steel bolt through it. Once all elements are completed, we need to go back to the driven element insulator and remove its insulator.

The dipole section will have our nylon 8mm rod inserted into each piece of the dipole and just 10mm of nylon showing between the 2 aluminium lengths. This is the only element where the stainless steel bolt will NOT go through the element itself. Instead we need to countersink the drilled M6 hole with a hole big enough to fit the head of our bolt into. This should be 5mm deep into the insulator in order that the bolt sits well below the dipole centre. Next we need to align the dipole upon the insulator and measure along each side 5mm from the nylon joint and drill through both the element and the insulator. This time, we need to countersink the holes as we did above but this time, the countersink will be from the bottom. Insert the stainless bolts (50mm M6) from the bottom up into the elements, place a washer upon these with a nut and tighten them. The protruding portions of the bolt will be used to bolt our coax to with the left over nuts and washers we have. All elements can now be bolted to the boom.

 

Balun and coax connections

 

Whilst there are more complex baluns that can be installed, the simplest is a coaxial balun at the feed point. This is where a few turns of the feed coax help reduce any standing waves on the outer shield of the coax. I have created one (see photos) with a diameter of between 3 to 4 inches across and 4 to 5 turns. I have added some clip on ferrite chokes too. The builder may choose to install a lambda type balun consisting of a ¼ wave length and a ¾ wave length piece of 50Ohm coax connected at the feed point. Details of construction of this type of balun can be found on the Internet.

In order to seal and connect the coax, I have soldered two large ring connectors to the coax. It is important to note that as soon as the inner and outer cores are separated, they become a radiating part of the antenna and add to the dipoles length. Therefore, these should be keep as short as possible. We also need to consider the weather gaining access to the coax and therefore, need to ensure (with lots of rubber adhesive tape) that this is secure and well sealed.

At this point, you should be ready to go! My antenna is now on the tower and shows a flat SWR of 1.0 from 49.9 through 50.275 at around 50.5 Mhz if starts to rise quickly and by 50.7 it is out of acceptable ranges.

 

Important notes

 

If this is a first time project or you are not confident with your measurement or cutting abilities, cut each element length a few mm too long. If the antenna does not sit where you want it to be, you can file the elements down. If it is too short when you measure afterwards, you can not add length to it!

Whilst these elements are electrically connected to the boom, they perform as isolated elements due to the fact the connection is at 'centre zero' position of the element but will still require correction. This amount is around half as muhc as would be needed if there were no insulator at all. Email me for correction figures on your antenna as it varies with boom diameter.


Take your time and don't rush. If you do not have everything you need, wait until you do have before starting the job.

 

Added bonus

 

Remember the extra few insulators we made and all those off cuts? I have designed a 4 element Yagi for 4 metres for all those interested which can be built out of the waste. Construction is the same as the 6 metre antenna and the sizes are as follows:

 

Element lengths in Metres:

Reflector = 1.054

Driven = 1.018

D1 = .985

D2 = .946

Spacing

Ref = 0

Driven = .356

D1 = .556

D2 = 1.457

 

I have sacrificed a little front to back ratio (as 4 is such a quite band) in order to give a little more gain. The antenna achieves 8.96dBi forward gain at 70.200 Mhz and a F/B of just under 16dB. All with just a 1.457 Metre boom too. In addition, this antenna provides a very good pattern when stacking.

 

Enjoy!

 

Justin G0KSC