Duplexers for dual band radio are usually high pass filter and low pass filter combined in the same box. Following is a design within a cost of US$10.
The two filters are built on the same piece of printed circuit board with copper strips etched out as per the diagram shown below. The copper foil pattern formed the two groups of coil L1, L2, L3, L4 and L5. These coils with capacitors C1,C2,C3,C3,C4,C5 and C6 completed the circuit. All sections must be screened by vertical metal plate. These plate can be made from pieces of un-etched double sided printed cirucit board. The most important issue is that all intersecting edges must be covered by melted soldering tin smoothly. A semi-circular mouse hole can be filled on the bottom of these walls where the foil (or capacitors) passes between sections.
The next diagram shows where the capacitors should be located on the foil board.
In order to tune the two filters, the best way is to get a sweep/tracking generator while the responce of the filter can be plotted. If such equipment is not available, another one way is to use R.F.though put power to assist judgment. A S.W.R meter inserted between the transmitter and the VHF or UHF input is also helpful.
To tune up the UHF section, change the value of the 5pf capacitors ( C4,C5,C6) to get maximum power output at antenna port while minimum reflect power should be experienced back to the UHF transmittor. The larger the capacitor's value, the lower the lost but the higher the chance to pick up interference from VHF side, best lost figure should be around 0.5db.
To tune up the VHF section, change the value of C1 to obtain minimum reflect power to VHF transmittor while re-positioning the paired capacitors which formed C2 and C3 (two 22pf makes 44pf) until the maximum power output is observed at the antenna port. Generally speaking, a smaller value of C2 and C3 will push the roll off point of the low pass section closer to UHF frequencies. Before this roll off point is higher then 250MHz, the decrease will favour a lower VHF power lost. But as soon as the roll off point approaches over 300MHz, it starts to cause serious impact on the UHF section. Like the tuning of UHF section, try to settle at a lost of above 0.3db should be pretty safe.
Just a reminder for those who are not familiar with db calculation.
Insertion lost (db) = 10 log (power output / power input)
Know the output power from the transmitter before start tuning. Use the above formula to calculate the corresponding power level which stands for 0.5db and 0.2db. Use the calculated result as a guideline during future tuning.
For 0.2db lost, power output = 0.9545 x power from transmiter.
For 0.5db lost, power output = 0.8913 x power from transmiter.
Always pay attention to the coupling factor of both band but do not push for minimum lost. Always remember that this hardware is to protect the front end of both VHF/UHF receivers from the transmission of the opposite section. Minimum insertion lost is not the major concern.
The foil pattern image is on 2:1. It can be transfered to a mica film or a transperent paper with a 50% reduction via print function. Such ture sized film can then be used to prepare a printed circuit board with photo sensitive paint. All capacitors used in this application must be of high voltage nature (recommend at least 0.5k volt). Silver mica base product will be the best choice if lower lost is required on the UHF portion.
Click here to download the 2:1 scale printed circuit board image.
by VE3RGW ...Aug-1997
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