How to use a Wilkinson Antenna Splitter the Right Way!
I'm Paul Hollings and one of my jobs at Aareff Systems is to come up with cost effective solutions to customer requests. Frequently I am asked about the Wilkinson splitter from customers that can't get it working right and it gives them a worse SWR than just the antenna connected to the transmitter directly.
Wilkinson Splitter Design
There are many examples on the web of the popular Wilkinson 50-50 ohm power spiltter, but not many of the examples show real antennas with the 50 ohm interconnecting cables.
The 100 ohm load is not required when splitting for antennas, but the lengths of the 50 ohm interconnecting cables have a signicant effect.
Regardless of the antennas type, the 50 ohm cables, if they are foam type, will always be some specific length between 189 and 252 cm for maximum bandwidth and minimum SWR presented to the transmitter.
My example shown below uses two dipoles of 75 ohms. If the LMR400 50 ohm cables between the RG11 splitter cables and the antennas are 3/4 wavelength at 189 cm, this makes the antenna SWR of 1.5 presented to the transmitter worse at 1.7. This will cause some transmitters to shut down or reduce power. If the 50 ohm cables are one wavelength at 252 cm, this improves the antenna SWR presented to the transmitter to 1.3.
Not all antennas are 75 ohms so this cable will not always be 252 cm for best performance. For example, antennas of 33 ohms will also have an SWR of 1.5, in this case the 189 cm cables would work perfectly and present an SWR of 1.3 to the transmitter and the 252 cm cables would be worse at an SWR of 1.7, the exact inverse of the previous example.
If the antennas are perfect at 50 ohms with an SWR of 1.0, then the length of this 50 ohm cable has no effect whatsoever on the final SWR to the transmitter, but in reality this is never the case, antennas are never perfect.
In conclusion, to save you a lot of wasted time, if you use standard dipoles with LMR400 or other foam type cable, then 252 cm will be the optium length. If you use another antenna type, the easiets way is to try diffrent lengths between 189 and 252 cm unitl you hit the sweet spot.
Paul Hollings
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