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Old 23-Aug-2012, 7:10 PM   #10
GroundUrMast's Avatar
Join Date: Oct 2010
Location: Greater Seattle Area
Posts: 4,773
Perhaps the easiest way to describe a practical low loss combiner is to simply have you look at a panel style UHF antenna. The DB4e is just as good an example as any... In the case of the DB4e, they built it from what are essentially two DB2e antennas. The older DB8 is another example of an antenna that is based on four DB2 antennas.

In both cases you can see the primary combining network is made of twin-lead (in the form of side by side heavy wire/rod). The parallel conductors form a transmission line with an impedance of about 1.5 times higher than the antenna terminal impedance.

The goal of these designs is to transform the terminal impedance of each dipole (bow-tie / whisker element) from its nominal 300Ω to 600Ω. This is done by taking advantage of the fact that a transmission line (coax, balance twin-lead, strip-line or even waveguide can be used) can serve as an impedance transformer.

Once we transform the 300Ω impedance of two dipoles to 600Ω each, we can connect them in parallel, which results in a combined impedance of 300Ω. This process can then be repeated to form a set of four or even eight combined dipoles. In theory, one could continue on to sixteen thirty-two...

If there's a downside to this method of combining, it would be that the 1/4 wave long transmission-line / transformer is only 1/4 wave long at one frequency. So, the 1/4 wave transformer becomes less and less efficient as the frequency difference increases. A 1/4 wave transformer is essentially a tuned devise, which makes it less than perfect for application in a broadband system.

In practice, combining antenna elements with a broadband splitter/combiner comes at the expense of loss in the combiner while retaining bandwidth. On the other hand, combiner networks based on 1/4 wave transformer made of transmission line offers lower loss at the expense of bandwidth. If I am combining for the primary purpose of added gain, I would not use a broadband combiner such as a hybrid splitter in reverse. However, if my goal was to combine two antennas for the purpose of nulling out an interfering signal, the loss of a splitter would be of little concern to me.

Last edited by GroundUrMast; 23-Aug-2012 at 7:20 PM. Reason: broadband vs. low loss... when to use
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