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The Folded Dipole for 12.5 Ohm OWL Yagis = 50 Ohm Yagi !!
One of the reasons I choose to model most OWL Yagis with a 12.5 Ohm Split dipole feed, is that good quality 50 Ohm Coax can be obtained easily to create the coaxial transformers in order to give a good 50 Ohm connection for your feedline. However, there is another, much more practical and more efficient way to do this; the folded dipole.
A 5 element 50MHz OWL at G8FJG fitted with a folded dipole for a 50 Ohm direct feed antenna
Adding a folded dipole provides several advantages. First of all, you do not need to calculate any coaxial transformers or hope that the manufacturer's velocity factors are correct and true. Additionally, the folded dipole can be arranged above and below the boom in order that your remaining parasitic elements can run through the boom (as in the above example by G8FJG) this saves cost and weight through not having to have additional antenna element mounts/hardware. The antennas are best to be insulated even though passing through the boom although either method may be chosen. One important consideration when feeding elements through the boom is a correction factor will need to be applied. This means that by passing the elements through the boom they are electrically shortern and need then to be longer than the published sizes. The amount by which the lengths should alter depends on both the boom thickness and wether or not the elements are insulated.
A Construction/Layout chart for a 3el 6m OWL with folded dipole
There are many resources on the Internet that help with boom correction. However, if you wish to contact me about your individual needs, i will be more than happy to help. Do also read HERE for more information and steps required to build a successful Yagi project.
Don't be put off by the thought of adding a folded dipole as my methods are very easy (like the LFA) and provides the ability for adjustment too in order that fine tuning can be conducted in order to achieve best SWR.
The Easy-to-build folded dipole using a Stauff clamp either side of a square boom
The easiest way to achieve the folded dipole is as in the above picture. I have used a 1.25inch square boom and mounted a Stauff clamp (available for purchase on this site if you cannot find locally) with in this case, 2 x lengths of 1/2 inch 18 swg tube. One has a gap with a solid nylon rod inserted with a 6mm gap which is hidden by the clamp, the other is a one piece tube of the same length. At the end sections, I have bent 3/8 inch tube as is done with the LFA loop (more information here ) in order to provide a trombone effect; the end sections can slide in and out of the bigger 1/2 inch tubes in order that adjustment can be made for best SWR.
What I did not realise until I started experimenting with the folded dipole is the width and height is not hyper critical. where the above example is spread across a 1.25inch square boom, if a 1.5 inch boom where used, the loop end would simply have to be brought in a few mm's in order to achieve best SWR. Likewise with 1 inch, the end sections would end up being a little longer. Furthermore, The insertion of the folded dipole takes very few changes in order to perform correctly having removed a split dipole from the 12.5 Ohm Yagi. Within a 144MHz example, the ends of the folded dipole will end up being between 15-30mm shorter than those of a split dipole (assuming a 1.25inch square boom). However, whatever the band, adjusting the loop larger and smaller will result in finding the correct and best position for minimum return loss and best SWR. Therefore although I will publish some of the sizes required for certain 12.5 Ohm OWLs to have folded dipoles, it is quite easy to work it out for yourself by trial and error by simply making the folded dipole larger and smaller until you achieve best SWR.
A 9el OWL fitted with an adjustable folded dipole for a 50 Ohm feed point
Other Folded Dipole Advantages
The folded dipole provides a couple more nice-to-have attributes. One of these is the reduction in noise that this arrangement offers, a reduction in rain static and generally, a reduction in most man-made noise. Sky temperature and also the antennas G/T figures are improved too further improve weak signal performance and by feeding the dipole on top of the boom rather than underneath the boom, again these performance figures are bettered.
Finally, in addition to the suggested stability increases a folded dipole should offer, simple baluns can be added to the folded dipole driven OWL too. This includes the simple choke (although not prefered) through to the G0KSC 'Antenna Balun' so called as it becomes an integral part of the antenna.
The antenna balun is simple and does not have losses or power handling restrictions like other baluns. All that is required is a rod, tube or wire of 1/4 wave length to be run form the feedpoint of the antenna (where the coax cable inner core is connected) to a point on the boom where the rod, tube cable finishes.
VE7XDT installed the 'flat bar' version of the Antenna Balun on his 4 x 2m LFA Yagis
The above example by VE7XDT shows one way in which the Antenna Balun can be installed. Provided the balun does not move out of line with the boom, no pattern distortion should occur. More information on the G0KSC Antenna Balun can be found HERE.
If you like the look of a model you see on this site but do not have folded dipole dimensions for it or the tube sizes you wish to use, please contact me and I will help you. the Email me on this website does not always work (sorry) for Email me direct at justin 'at' G0KSC .co .uk
More folded dipole examples below. Enjoy your antenna!
An 8 element 144MHz OWL fitted with a folded dipole for less noise and a 50 Ohm feedpoint
G0KSC built this folded dipole example when first experimenting with the FD/OWL combination
M0WBM built this example using insulated 'above boom' elements rather than insulated through the boom