50 Ohm Coax Antenna Power Splitter/Matching Network

   

Stacking beams is a common way to increase the gain in a desired direction.  It is also often useful to split power to different types of antennas. For example, in a domestic contest you might want to work stations both in surrounding states as well as stations across the country. On 40 Meters you might want to use a low dipole to work the close in stations and a beam or vertical for the distant ones. In a DX contest you might want to point one antenna to Europe and another to Africa or South America. Using antennas that favor multiple target areas can improve your QSO rate.

If you are calling CQ on both antennas and a weak station comes back, it is often useful to select one antenna to increase the signal or reduce QRM from other directions. Other times you might tune across a rare multiplier with a big pile up, and you want to put all your power in that direction.

Switching between different antennas or both antennas at once becomes a problem because you want your transmitter or amplifier to see 50 ohms in any combination. It is possible to accomplish this with a matching network consisting of a coax switch, some coax Tee connectors and coax.

The diagram shows the construction of the system.  Note that this is designed for a single band only. The heart is a coax switch. Tee connectors A and C are screwed directly on the coax switch ports. It is important to use a coax switch that does not ground the unused positions.   Quarter wave lengths of coax provide impedance matching.

It is important to realize that the physical length of a ¼ λ piece of coax is shorter than ¼ λ in free space. Typically it will be 60-90% of the free space length. You can look up the specs for the coax you have and calculate the proper length for the band you will use it on, but a better idea would be to start with lengths a bit on the long side and trim them to the proper length with an antenna analyzer. 

The coax to the antennas (L1 & L2) must be the same length if this is to be used with stacked Yagis. They can be any convenient lengths if the antennas are of different types.  Similarly, the coax from the switch to the transmitter can be of any length.

The system works because of the fact that lengths of feed lines act as impedance transformers. The transform function depends on the impedance of the feed line relative to the load, and the length of the feed line.

Two special cases are at work here. A ¼ λ feed line will convert an open circuit on one end to a short circuit at the other. The inverse is true, and a short circuit on one end will appear as an open circuit at the other end. The other special case is that a ¼ λ 70 Ω coax will transform 50 Ω on one end to 100 Ω on the other.

Looking at the diagram, the switch is in the center position.  In this position, both antennas will be connected. Antenna 1 will present a 50 Ω load at Tee connector A.  The 70 Ω coax between connectors A and B will transform the 50 Ω to 100 Ω at connector B.  Similarly, the impedance from Antenna 2 will also appear as 100 Ω at connector B.  The two 100 Ω impedances in parallel form 50 Ω which is preserved through the 50 Ω coax back to the switch and to the transmitter.

If the switch is changed to select Tee connector A, Antenna 1 will be selected.  The coax between connector B and the coax switch will be open at the switch end.  This gets transformed to a short circuit at connector B. If connector B is shorted, everything towards antenna 2 has no effect.

The short at connector B is transformed to an open circuit at connector A. It is as if every thing to the right of connector A is disconnected.  Thus, the only thing the transmitter sees is Antenna 1.  Since the circuit is symmetrical, when the switch selects Antenna 2, the transmitter only sees the 50 Ω impedance of it.

I have used this design for stacking 10 Meter beams and for dissimilar antennas on other bands.  I use one for Field Day on 40 CW. I use a dipole on one antenna and have used other antennas including slopers, half squares and wire Yagis for the second antenna.

40 Meter power splitter used for Field Day.

The 40 Meter version also works on 15 M. It will be left it as an exercise to the reader to figure out why. Hint: a 1/2 λ length of coax of any impedance will reflect the load impedance at the other end.

Power splitting between different antennas helps you get answers to CQs from more than one target area, increasing your rate.  I also can’t count how many times I have been called by a weak station that I can’t copy, only to select the proper antenna to make him Q5.  This slick circuit makes it all possible.

© 2009 - 2014 Gary C. Sutcliffe

  

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