Spring 2009 Issue

ANTENNAS

Stacking Broadband Antennas

By Kent Britain, WA5VJB

Photo 1. Wide-bandwidth antennas.

 

One common question from our readers is how to stack log periodic or other broadband antennas. The wide variation of wavelengths used in antennas such as the ones in photo 1 creates a multitude of problems, not the least of which is evenly dividing power between the two antennas. However, there are several ways to divide the RF power between two antennas even over a broad range of frequencies.
Quarter Wave: In photo 2 we have the classic 1/4-wave power divider, two sections of coax 1/4-wave long and 72-ohm impedance. While this is the usual way of stacking two antennas, the power divider has a bandwidth of about ±10 percent. This means a power divider for 2 meters works fairly well as a power divider from 130 MHz to 160 MHz. This is fine for 2 meters, but it is not going to hack it for multiband antennas.

Multi-Stage Quarter Wave: Sometimes known as a “multi-section inline hybrid,” a series of stepped quarter-wave sections with different load resistors can give a pretty good bandwidth of 2 to 1 or more depending on the number of stages. Specifications of 1 to 2 GHz, or even 2 to 8 GHz, are often seen on surplus power dividers. This is good for microwave use but kind of big for VHF antennas. In photo 3 you can see the small chip resistors at the junctions. I sacrificed another power divider on the altar of knowledge so I could cut it up and measured the 100-, 220-, and 450-ohm resistor values.

Ferrite: These simple two-way splitters designed for use with televisions (photo 4) are very handy little splitters or combiners. First, they are not 75 ohms! Rather, they are transformers and are quite happy with all three ports at 30 ohms, 50 ohms, or even 100 ohms. Again, they are transformers and just need all three connections at the same impedance. With adapters on the F-connectors, or taking out the transformer and putting it in your own box will make a handy splitter/combiner with 50-ohm coax systems.

Yes, some electronics stores sell them for $10 or more, but the ones at the dollar stores for $1 work just as well! Furthermore, you can take them apart and put the ferrite transformer directly into your projects. The first time I tried this, I exclaimed, “Boy, look at those long leads. I can made it work much better by using shorter leads!”

That turned out to be another one of those brilliant ideas that didn’t work out very well. The inductance in the long leads is part of the device impedance matching, and you really need to keep the leads the same length as much as possible.

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