Building Phasing Harnesses

Building Phasing Harnesses http://www.w4dex.com/kc4fwc/ant.htm

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Phasing harnesses for multiple dipole or folded dipole antennas are simple. They mainly use 50, 75, or35 ohm coax parallelled for impedance matching. Decibel products and Celwave (along with others) useelements that resemble a folded dipole but in reality are not technically a "folded dipole". Folded dipolesare fed balanced and have an impedance of 300 ohms or so, depending on surrounding objects thatcan lower the resonant frequency (and impedance) by adding capacitance. The impedance alsodepends on the diameter of the material used and the spacing.

Decibel Products uses folded dipole elements that are fed unbalanced. In my opinion, they are actually"folded monopoles" with a counterpoise. Imagine a Decibel Products DB-201 antenna without theground radials and just one counterpoise below the feedpoint. Second, the elements are spaced fromthe mast in such a mannar that the pattern is somewhat directional and the impedance of each elementis 100 ohms. This means that there must be some matching done in order to achieve the desired 50ohms that most two way systems need.

The basics of matching are done with odd electrical quarter wavelength multiples using differentimpedances of coaxial cable. By electrical wavelengths, this means you must measure the cablephysically but then calculate the velocity factor of the cable and cut accordingly. By compensating forthe velocity factor, you are taking into effect that RF slows down as it passes though dielectric of thecable. Therefore, the electrical wavelength is somewhat shorter than the physical wavelength.

Most coaxial cables with solid dielectric (RG-213 etc) have a VF of around 66%. Other cables with foamdielectric have a VF of 82% or thereabouts. If in question, consult the technical specs of the cable youare using to create a very accurate measurement when building phasing harnesses.

To calculate an electrical quarter wavelength of RG-213 cable with a VF of 66%, first find the physicallength by dividing the frequency into 2952.

2952 / 445 = 6.63 inches.

Now, take 6.63 inches and multiply by the velocity factor of 66% or 0.66

6.63 x 0.66 = 4.38 inches.

So, an electrical quarter wavelength at 445 MHz using coax with a VF of 66% equals 4.38 inches. Youmay multiply this number by 3, 5, 7, 9 or any odd number and the effect as a matching transformer willbe the same. The same formula above holds true with 75 ohm and 35 ohm coax, or any coax for thatmatter and at any frequency.

When using odd quarter wavelengths to match impedances, you will usually deal with several numbers,mainly 100, 75, 50 and 25 ohms. As listed below, are some typical matches you may deal with whenbuilding antennas.

100 ohm element through odd 75 ohm coax will produce 50 ohms

50 ohm load through odd 75 ohm coax will produce 100 ohms

25 ohm load through odd 35 ohm coax will produce 50 ohms

Another factor to consider is the effect that EVEN ELECTRICAL HALFWAVES can play in matching.Any time a load is connected through this length of coax, the impedance of the load will exactly repeatitself at the other end of the cable. For example, if an antenna element is 100 ohms, and an evenhalfwave or even multiple of a halfwave piece of any impedance coax is placed in line, the impedance atthe other end of the coax will be 100 ohms. See the diagram below which depicts how to match two and


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four elements considering that each element is 100 ohms.

Let's take a look at what's happening to the dual element array on the left. First, the elements operate at100 ohms. Section "A" is an odd quarter wavelength multiple (probably around 5 odd quarterwavelengths) of 75 ohm coax which makes the 100 ohm load turn into 50 ohms at the end of the cable.The bottom antenna element goes through the same thing, making its impedance 50 ohms also. Whenthese are connected together, or parallelled, the impedance at the TEE connector is 25 ohms. You maynow convert this into 50 ohms by using an odd quarter wavelength of 35 ohm coaxial cable, perhaps 7or 9 multiples long shown in section "B". This creates a 50 ohm impedance at the connector which goesinto your transmission line.

If you do not have access to 35 ohm cable, you may wish to try a different method. You may use evenhalf wavelengths of 50 ohm cable in section "A" which makes each element impedance repeat at theTEE connector, 100 ohms. Then parallel the second element of 100 ohms also, and get 50 ohms. Then,section "B" may be any length of 50 ohm coax.

The antenna on the right is the exact same thing as the one on the left, except twice as many elements.Remember in our first example the impedance at the end of section "B" was 50 ohms. Now, just addanother antenna array identical to this with a TEE connector. Parallel two 50 ohms and get 25 ohms.Now, match this 25 ohms to 50 by using yet another section of 35 ohm cable as shown in section "E"and get 50 ohms at the end. This configuration is used by the popular Decibel DB-224 and DB-411antennas.


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The antenna elements are built like this. I can't remember the exact dimentions but can measure themfor a 440-450 MHz antenna, or a 150-160 MHz. You may scale the dimentions and build one for anyfrequency providing you have the tools to bend the aluminum, or copper, or have another way to makethe elements. Remember, the space between the mast and element is critical as it determines theimpedance of the element and the pattern as well.

Sorry the diagrams are so elementary, I drew them up quickly and have hopes of making a better looking model soon. Hope this helps on building the phasing harness. Remember, when buildingcoaxial cables, the length must be measured from the tip of the center pin to the tip of the other end ofthe cable (not where the shield ends!) This means the tip of a type-N center pin or PL-259 center pin aswell.

73, Derek KC4FWC

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