stripline power divider
One of the ways to improve the over all performance of any radio station is to put up bigger
antennas. Antenna improvements affect both the transmitted and received signal and thus increase your
operating range. Once you reach a certain point, the only practical way to get “bigger” antennas is to put
up multiple antennas phased in such a way so that the total gain of the array increases (hopefully).
For VHF and above this usually requires the ubiquitous power divider.
Over the years I have built power dividers using coaxial cable or round copper tubing to achieve
the necessary matching of the various antennas in an array. Recently, I have been using waveguide to
construct 2 and 4 way power dividers. The waveguide has the advantage of being rectangular which
greatly facilitates the attachment of the coaxial cable connectors. It is also made of copper, unlike
power dividers made of square or rectangular aluminum tubing, which means the whole assembly can be
soldered for long lasting, waterproof, electrical connections.
A few words on how power dividers work is in order. Once you understand the simple theory of
operation of a power divider (combiner also) then you may come up with another way of building 1/4l
transmission line transformers that better suits your needs or available materials list. The basic
objective is to build transmission lines with the proper characteristic impedance to match two different
RF impedances. The shape or size of the transmission line doesn’t matter as long as it is the right
impedance.
The two most common configurations are intended to combine either 2 or 4 antennas, each with a
nominal impedance of 50 ohms. If you were to merely connect 2 antennas with equal lengths of 50 ohm
cable together the resulting impedance at the joined ends would be 25 ohms. This really isn’t too bad,
but we can do better. If we could somehow “transform” the 50 ohms at the end of each cable to 100 ohms,
then when we joined the new ends together we would be back to 50 ohms (The impedances combine like
resistors in parallel). Such a impedance transforming device is an electrical (compensated for the
velocity factor of the transmission line) 1/4l (at the frequency of interest) piece of transmission line
having a characteristic impedance (Z0)
Free download http://www.scribd.com/doc/8616044/RF-Antenna-Power-Dividers
What is described here is just one way of building power dividers. There are other
configurations such as having two or four antennas connected to one end of a power divider and use a
single 1/4l transformer to step up to 50 ohms. The theory is the same for all cases, just different
numbers. I think you will find the construction of the actual device is easier in the configurations
described here.
Below is an end view of a stripline transmission line. In typical stripline applications the sides
are not present. When used as a power divider, the presence of the sides of the waveguide do not effect
the results because of the distance from the center conductor. Eagleware RF design software was used to
calculate the width of the center conductor (w) that is required to achieve the 70 and 50 ohm
characteristic impedances needed for the 2 and 4 way power dividers.