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What is the OWL Match and How do I Make One?

In this section we will explore the very simple coaxial matching devices used to match both the 12.5Ohm and 28Ohm G0KSC OWL (Optimised Wideband Low impedance) Yagi antennas. The Matching transformer is very simple indeed. It consists of two lengths of coax which are cut to a quarter wavelength for the band they are to be used upon, connected together in parallel and then placed between the feedpoint of the antenna and the coax feeding the antenna from your rig. there is an optional (highly recommended) wire to connect between the inner core of the coaxial transformer at the feedpoint of the antenna and the outer core where the stub meets the feeding coax (earth connection). This will ensure a true balanced input, more on that later.

There is another option for 12.5Ohm OWL's. Read on!

The split dipole maybe exchanged for a folded dipole and in turn, this will provide a direct, 50Ohm feed without any impedance matching required. A 1:1 balun or choke is still needed but power handling and 'fiddly' matches are needed no more! I have not published all Folded dipole sizes as yet but they are easy to make and adjustable, just like the LFA. Mail me for more information if required.

A 9 element OWL sporting the 50 Ohm folded dipole rather than 12.5 Ohm Split dipole

 

 

G8FJG installs a folded dipole on his 12.5 Ohm OWL in order to feed directly with 50Ohm coax via a 1:1 balun

 

The reason the OWL comes in these 2 impedance's is due to the ease of availability of the two coax impedance types needed in order to match the above mentioned antenna types for 12.5/28Ohm to 50Ohm. The 12.5Ohm antennas need 2 lengths of 50Ohm coax connected in parallel (this provides a 25Ohm feedline) and 2 x 75Ohm coax lengths (this provides a 37.5Ohm feedline). Effectively these twin lines in each instance, when cut to a 1/4 wave length for the band they will be used upon, transform the antennas feedpoint impedance to a perfect 50Ohm impedance at the feeding coax end.

When the two lengths of coax are joined together, the inner and outer core (of both parts of the transformer) connect to each side of the dipole at the feedpoint and at the other end, the outer core should be earthed to the boom at the same time as connecting to the 50Ohm coax feedline to your shack. Furthermore, the transformer (twin coax lines) should not be coiled and should run above or below (drill a hole in the boom at the feedpoint and run beneath the boom) the boom and remain at absolute centre of the elements to ensure no pattern influence is made to the antenna. Below is a diagram made by Bob ZL1RS of how he installed his coaxial transformer on this 8 element 50MHz 12.5Ohm OWL. He has kindly adapted this imapge to show either twin 75Ohm or twin 50Ohm lines. Additionally, Bob has supplied a sequence of photos showing the stages of construction of one of these transformers which I have included at the bottom of this page.

Thanks to Bob ZL1RS for creating the above image file

The G0KSC Hybrid OWL impedance match/Balun installed at SP3IYM

 

The additional wire can be clearly seen on this PA0WRS 5el OWL which he installed upon his SteppIR boom

 

Note: the brown 'Pawsey Stub wire' is optional but highly recommended. This wire does not need to be measured it just needs to be cut long enough to connect the inner core at the feedpoint of the antenna with the earth point at the antenna socket. This wire finalizes the 'balun' functionality of the transformer arrangement finishing the coaxial transformer perfectly as now it will not only provide your impedance transformation from 12.5/28ohm to 50ohm, it will provide a balanced input to the antenna while presenting the feedline with an unbalanced connection. This will ensure no common mode currents will exist on your feedline and hence, no de-tuning of your antenna will occur.

The Importance of Coax Selection

First of all we can not simply calculate a free space 1/4 wave length and cut the coax to this size, each coax type has a 'velocity factor' which is introuces a 'slowing' effect to any singal travelling within the coax. Each coax type has different properties and velocity factors and being a very important part of your antenna (the matching transformer), only the best quality coax should be used within your coaxial matching stub.

Currently, I have only been able to test one type of transmission grade 75Ohm coax, RG11A which works well up to 2M (it will most likely be OK at 70cms but I have not tested it) and has an excellent power handling ability. A single length can handle around 1.3KW at 50MHz dropping to around 800Watts at 2M. This figure can be almost doubled when using the two lengths within our coaxial stub match provided the match is kept in a straight line and not coiled.

My favoured OWL is the 12.5Ohm version. This is due to the slightly higher performance which can be seen from the lower impedance antenna in addition to having a much wider range of coax cables freely available to use including some excellent quality types. Using such quality cables will ensure exceptional high-power and long term stable performance. I use one such coax within my coaxial stubs, Westflex 103. This coax is a semi-rigid type which has a thick single core centre section (copper) and a partial air-spaced dialect to help reduce the velocity factor. It also has excellent low-loss properties too so will perform well to (and perhaps above?) 70cms.

Velocity Factor and Coaxial Matching Stub Length

In order to calculate the length of our stub we need to use the following formula in order to present the figures in mm.

300/(Frequency) x (Velocity Factor of Coax) x.25

The first section of the above ( 300/(frequency) ) is purely to convert the frequency to metre wave lengths. For example '300 / 144.300MHz = 2.079 Metres'. The next part of the calculation is the coax velocity factor adjustment. The velocity factor effectively shortens the free-space 1/4 wave length in order to account for the 'slowing' effect of the coax. The better quality coax is used, the closer to the freespace 1/4 wave length the coaxial matching stub will be. In the next example, I will use .85, the velocity factor of Westflex 103. Therefore, the next part of our equation comes into play. '2.079 x .85 = 1.767' The velocity factor is a percentage figure. Therefore, in the case of Westflex 103 with a velocity factor of .85, our wavelength of 2.079 metres will be reduced to 1.767 metres, around 85% of the original length. Finally, the last part of the equation '1.767 x .25 = .44175'. This 'x .25' section simply provides us with the 1/4 wave figure, dividing the full wave length number into 4 and presenting us with a final, single 1/4 wave length that our coax lengths will need to be.

Important note

Remember, the coaxial stub length is measured from where the coax is whole each end. This means it does not include the tabs that we will connect to the antenna and to the socket at the other end, it measured from the point where the inner core of the coax is fully surrounded by the outer core of the coax each end. Ensure this is as close as possible to the length the above calculation gives us.

Below I have listed a number of coax types and their velocity factors. There are not many 75Ohm options and even less suitable options for 2M and above so if considering a 28Ohm version, ensure you can obtain RG11A at the very least, if you want to be sure your antenna will provide good, long term performance. Please do not be tempted to use satellite TV coax. These vary greatly in grade and velocity factors are not easy to obtained. Also, their design and construction did not include transmitting purposes.

Coax Cable Velocity Factor List

Coax Type                       Velocity Factor            Impedance                   Note

RG-8                                  .66                            50

RG-8X                                .82                            50              Beldon figures used

RG-213                              .66                            50

LMR-400                            .82                            50              Manufacturer suggests .85, tested to be .82

RG-58                                .66                           50              NOT RECOMMENDED TO USE

RG-11A/U                           .66                           75

UR57                                 .66                           75             Similar to RG11A/U

UR43                                 .66                           50             NOT RECOMMENDED TO USE

UR76                                 .66                           50             NOT RECOMMENDED TO USE

UR79                                 .96                           50             Very low loss 22mm thick

The above is not a definative list. However, it does cover the major/more readily available cable types. If you have information on other good cable types, please let me know.

Waterproofing the match/Balun arrangement

I alway ensure I have plenty waterproofing in place, better this than having to replace after a few months! To this end, I tend to cover the whole assembly with self amalgamating tape paying particular attention to the joints and ensuring plenty of coverage. Next, I cover the match with insulating tape to protect the self amalgamating tape from UV. The exposed joint sections I covered with plenty of silicone (bathroom) sealant, clear is best. Make sure this is a good quality sealant too.

Some hams prefer to box the feedpoint. I am not a fan of this. When you look at some of the self constructed antennas on the Internet, you can see the builder is lead into a false sense of security with regards to these boxes. All cables within are exposed and not waterproofed at all. Regardless of whether or not the box is 100% water-tight, sharp changes in outside temperature can cause condensation inside these boxes. Over a very short period of time, performance will degrade and the assembly will need replacing.

Conclusion

Hopefully, this page has given you all you need to build the perfect matching arrangement for your G0KSC OWL Yagi. Below I have added a photo of the feedpoint on a 9el OWA Yagi built by VK6OX. This shows my preferred feedpoint arrangement (and as illustrated in the above diagram) and also, the completed and installed transformer/balun on my 4M 6el OWL Yagi.

As always, if you are not sure of something or have any questions, please let me know. I will be very happy to help you construct your Yagi and get it right first time.

The nicely finished feedpoint at VK6OX

 

The twin-Westflex 103 Coaxial Transformer/Balun installed and waterproofed on the G0KSC 6el 4M OWL

 

The feedpoint of the transformer waterproofed with a RF Grade sealer. Silicone bathroom sealant would be fine.

 

Tripple sealed feedpoint connection using the RF grade sealer to finish off.

 

The feedpoint from the rear. While I have used one insulator, this was an experiment. The best method is to use one either side of the boom and have the feedpoint connections as short as possible.

 

Two pieces of 50Ohm coax cut ready to make a 12.5Ohm to 50Ohm transformer (144MHz)

 

 

The end sections are now prepared ready for connection to one another

 

 

The 2 pieces of coax are now joined but not yet soldered

 

 

Ring connectors are now joined to each side of the transformer

 

 

Soldered and water proofed (hot glue gun) and ready for installation on the antenna

 

 

Installed and ready to go! Note the point at which the transformer is earthed to the boom

 

 

A nice tidy feedpoint with connections to the antenna as short as possible.

 

 

Until next time,

 

Justin G0KSC