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Home Antennas Matching Sections Power Divider for Stacking 2 Bays or 4 Bays Antenna

Power Divider for Stacking 2 Bays or 4 Bays AntennaWritten by BuSan Sunday, 04 November 2007One 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 andreceived 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 multipleantennas 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 facilitatesthe attachment of the coaxial cable connectors. It is also made of copper, unlike power dividers made of square or rectangular aluminum tubing, which means thewhole assembly can be soldered for long lasting, waterproof, electrical connections.

The Theory

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 comeup with another way of building 1/4 lambda transmission line transformers that better suits your needs or available materials list. The basic objective is to buildtransmission lines with the proper characteristic impedance to match two different RF impedances. The shape or size of the transmission line doesn't matter aslong 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 connect2 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 dobetter. 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 50ohms (The impedances combine like resistors in parallel).

Figure 1. Quarter Wave TransformerSuch an impedance transforming device is an electrical 1/4 lambda (compensated for the velocity factor of the transmission line at the frequency of interest)piece of transmission line having a characteristic impedance (Z0) determined by the following equation:

Z0=Square Root ( Z1 x Z2)With Z1=Input Impedance, Z2=Output Impedance (Figure 1 .)

In the above example we want to take the 50 ohms at the end of the cable and 'step it up' to 100 ohms. The formula predicts that if the 1/4 lambda transmissionline impedance is 70.7 ohms from [Sq. Rt. (50x100)], then we will achieve the desired impedance transformation (See diagram below). This 'transformer' effectalso works with any odd number of quarter wavelengths (i.e. 3/4 lambda, 5/4 lambda, 7/4 lambda, etc.). Aha! So this is why we used RG-11 (75 ohms) to matchtwo antennas in the good old days. The trick was to get the RG-11 the right length (Odd number of electrical quarter wavelengths). In this case the whole pieceof RG-11 was the transformer. If a power divider is being used then the only requirement is that all the 50 ohm 'phasing' lines have to be the same length, anylength, but all the same. Much easier! Figure 2 . is an impedance diagram for matching two antennas.

Figure 2. Two Way Power DividerIn the case of 4 antennas, if we connect two of the antenna feeds together (parallel) with equal lengths of 50-ohm cable then the impedance at this point is 25ohms. The transformer has to then step up this 25-ohm load to 100 ohms. Thus when we connect the transformed 100 ohms from the other two antennas, theresulting impedance at the center will be back to 50 ohms (See Figure 3 .). From the formula above the transformers required must have a characteristic ofimpedance of 50 ohms. Once again this could all be done with coaxial cables of the proper length, but it is much easier with a power divider.

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Figure 3. Four Way Power DividerWhat 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 apower divider and use a single 1/4 lambda transformer to step up to 50 ohms. The theory is the same for all cases, just different numbers. I think you will find theconstruction of the actual device is easier in the configurations described here.

Real Power Divider

I make mine with 1 inch (25.4 mm OD) square section tube ("D"); it has 2 mm wall thickness, and a round centre conductor, using the formula:

Z0 =138log1.08*D/d

With Z0 = impedance λ/4 line (70.7 Ω for two way or 50 Ω for four way)

D = inside dimension of outer conductor (23.4 mm ID) d = outside diameter of inner conductor (7.9 mm OD for two way or 11 mm OD for four way) ID = Inner DiameterOD = Outer Diameter

Now all you need to do is calculating the length of λ/4 transmission line transformers that frequency dependent!!!

Figure 4. Two Way Power Divider

Figure 5. Four Way Power Divider

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RBS FM 92.60 MHz the Best Radio Station in Tulungagung - Power Divid... http://rbsfm.ej.am/joomla/index.php?option=com_content&task=view&id...

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