Simple steps to building a successful Yagi Antenna
_____
The purpose of this page is to help the professional and amateur builder alike gain some invaluable information to ensure a successful, first time build. Please read All information below and if you need to deviate from anything suggersted be it insulator type, element diameter or something else, let me know via Email and I will do my best to assist.
Antenna insulators
This is probably this most important attribute to follow when building your antenna so follow this section carefully. Deviating from the build criteria I have set may not shift the antenna a long away from the frequency you wish to use it on but your actions may have a big impact on the performance and pattern if directions are not followed fully. The G0KSC insulators on this site have been selected and tested to ensure minimal impact on the antenna and its performance.
Insulator material
Firstly the insulator material is an important consideration. At this point I will introduce you to the concept of IC or Insulator Correction. Many new and old antenna builders are aware of the effect a boom can have upon an antenna when elements are mounted through the boom. correction factors for Boom Correction (BC) are well documented so I will not repeat them here but the concept is, if you place elememts through the boom (or very close and in contact with it) the elements are electrically shortended which inturn means you need to add length to each element. Mounting well above the boom (as we are) means the boom has no or very little effect.
Most black coloured insulators have carbon in them to help with UV resistance. This carbon content makes the insulators 'lossy' and can result in electrically shortening the antenna elements too. If these type of insulators are used, every element on the antenna may need to be lengthend. Only by a few mm or a tenth of an inch or so but this will ensure that the antenna has 'best performance' where you want it and as close to model as can be. Polypropylene or Polyamide (without carbon) are good materials to use. This along with UV resistant Plexiglass (intended for outside signage). Generally if it is not black in colour, the impact on the antenna performance is likely to be small if any infleunce at all. However, check the UV resistant properties of the material you intend to use and before cutting any aluminium, consider the size of the insulator too.
Insulator size - Width across the boom
Even if a very good insulator material is being used, there can still be an electrical shortening of the antenna elements in some cases. One such case is when the insulators are very wide, extending along the element either side of the boom. As the insulators physical effect on an antenna varies with frequency, a good indication as to whether an insulator is likely to have an impact on the antenna or not, is when the insulator extends further than the boom on each side of the antenna. If the width in which it extends on both sides, adds up to more than a boom width, it is likely a correction factor will be need to be added to the elements. Mail me with the size of your insulators in order that I may help make the calculation.
Through boom and insulator mounting
Some insulators provide just one mounting hole for a nut to run through the element (not dipole) and boom. This WILL shift the frequency of the antenna. Tests have shown this will shift a Yagi from as little as 100KHz to 300+ KHz depending on the frequency in use. This is not a trate of the LFA, OWA or OWL Yagi, it is the impact of this type of insulator on any Yagi, don't be told otherwise!
Again, if you are going to use through boom and element mounts, mail me in order that I can guide you to the best changes to make on your antenna.
Antenna Material and Dimensions
If the antenna you wish to build uses one size of aluminium tube (1/4 inch (6.35mm) as an example) and all you can obtain is 6mm, please do not just go ahead and build the antenna, check with me first. While in this example the impact on the model would be small it DOES make a difference. On certain antennas (and with bigger variants in size) it may have a much bigger impact. The last thing anyone wants is to find is the antenna they have spent much time building, does not work well and/or exactly where they wanted it to, frequency wise.
Element materials and variants
All Yagi antennas on this site assume T6 aluminium. If building out of anything other than this, an effect on the performance will result. If another grade of aluminium is used, the differences will be very small. However, if brass or copper, additional modeling maybe required, so let me know.
Over compensation
I have now built many antennas with many different insulators and materials but I still do one thing when building a design for the first time. I add 1-2mm to the end of every element, regardless of following all of the above steps. If any of the above mentioned issues apply or there is something I have missed, the compensation figure will be taken into account. If my antenna was absolutley spot on, all I need to do is trim a little off of each element. This is a good rule to apply for any first time builder especially.
Balun arrangements and fittings
For anyone following my articles within DUBUS magazine, this will have been an area they have read about. For those of you that have not, this is another very important point of interest.
Many unmodeled part of an antenna can have an effect on pattern and performance, one of these being the feedline or coax and any balun than maybe connected to the feedpoint. Firstly, more about baluns can be seen here.
Assuming we are mounting our elements on top of the boom, the 'deal' installation is one where our coax connects to the feedpoint and immediatley passes through the boom (as close to the feedpoint as possible) and then runs along the underside of the boom toward the shack. This will ensure the least amount of interaction between the coax and the antenna. So what about the balun?
My favoured balun is the Pawsey stub but rather than coiling this as I have illustrated in earlier pages, I now keep the balun in a straight line which reduces heating effect, reduces losses (through the balun becoming an essential part of the feedline) and it keeps everything at absolute dead centre of the elements/antenna to ensure the least effect on pattern and performance possible. Below is a photo of the feedpoint of VK7AB's 8 element 6M LFA. Note, this is not simply coax, this is a Pawsey stub balun with a 3-4mm thick wire (if KW's are to be used) running in parallel with the coax (1/4 wave back) until the point it is earthed to the outer braid and antenna boom.
KA0KYZ made a very nice job of his antenna. The thru-boom coax is clearly seen above (correction required for thru-boom/element mounting)
The point at which the Pawsey outer wire connects to the braid of the coax and boom
The photos above show a Pawsey Stub blaun installation at VK7AB. This arrangement will have the least impact possible on the performance and pattern of this antenna.
The G0KSC Antenna Balun
One issue with the pawsey stub is the calculation of velocity factor. All coax cable seems to change drastically from the manufacturers specifications and really needs to be tested and confirmed before making a Pawsey stub. This second issue for some is power handling. With 2KW key-down I have seen these melt, even with the thickest secondary wire. To this end, I designed the 'Antenna Balun'. So called because it is an intergral part of the antenna but does exactly the same job as a pawsey, without the above restrictions.
For more infromation on the Antenna Balun, the new preferred choice of balun, check out the 'creating a balun' page on this site.
Conclusion
Well, you are pretty much ready to go build! Don't be scared off by the above, if you follow all of these steps carefully you will be ensured of a sucessful build first time but as always, if you have any questions, please ask! I always answer every email, if you do not get a response, send it again.
Justin G0KSC
