CONSTRUCTIONIn this chapter letís discuss some important overall construction points.
Construction ObjectivesWhen I built my first duplexer, I had two primary objectives in mind.
(1) I was determined to use common materials and garage-workshop techniques. Published home-brew designs often discourage would-be builders here. They frequently use hard-to-get materials and often call for costly machining. This design uses neither.
(2) The finished duplexer had to be easy to tune. You'll only need a thru-line wattmeter, a dummy load and an ordinary synthesized transceiver capable of transmitting on both the input and the output frequencies of the repeater. Ease of tuning is also important for the hill top. If you happen to have a spectrum analyzer with a tracking generator, all the better.
Materials and Special ToolsChoosing the type of metal tubing is the first decision. As any home craftsman knows, we really only have three choices, steel, aluminum and copper. I immediately rejected aluminum because it does not solder easily. Commercial manufacturers use aluminum, but they have heli-arc welding and machine shop facilities.
Steel also is unsatisfactory due to its low conductivity and because it rusts. Some commercial manufacturers have successfully used copper-plated steel, but this is difficult for the home builder. The only real choice is copper or brass. Even most commercial manufacturers make this choice.
Starting with copper as the construction material, we again have two choices, tubing made for the metal working industry, or ordinary household copper water pipe. Copper water pipe is best for several reasons. It is easy to obtain and moderately priced. But most important, the excellent ends caps available for copper water pipe make machining unnecessary. That alone makes copper water pipe the only choice.
For the connectors on the cavities I recommend BNC jacks, especially the type with a built-in ground lug. Without the lug it is difficult to install the coupling loops. I have been successful, however, with a little care, using standard jacks and ring-type ground lugs.
For the coaxial cables I recommend crimp-on BNC connectors. A crimp tool is frankly a good investment even for hams. You may use other types, if you wish. Just be sure to prepare the cables carefully. Bad cables are one of the most frequent cause of poor performance in duplexers.
All of the flexible cables used in a repeater MUST have a double shield to make them 100% shielded. On the duplexer, you may use any variety of 50 ohm double-shielded cable with the same velocity factor as the type shown. Do not attempt to scale the length of the cables for a different velocity factor. The cable type you select must, of course, accept BNC connectors.
For good economy, I recommend the variety of RG-58 that has a outer wire-braid shield and an inner foil shield. All major cable manufacturers now make this type. It is an off-shoot of the 75 ohm foil double-shielded cable developed for the TV cable industry. It performs just as well as the expensive varieties.
One the receive side of the duplexer I have provided for an optional seventh cavity. It is a bandpass cavity intended to be inserted after a GASFET preamp. Otherwise, omit it.
For tools, you will need a tubing cutter, an electric hand drill and a propane torch. You should also make up a special tool for tightening the BNC jacks. See figure xx. It is a necessity. Construct it from a short length of 1/2" copper water pipe and a male BNC connector. Select a connector that fits tightly into the end of the tubing. Force it in with a block of wood and an hammer. Apply a little solder if needed.
ConstructionThe follwing are step-by-step construction and assembly instructions for a 440 MHz UHF duplexer. It is a good guide, however, for other bands. Also, these instructions assume you will be cutting the necessary pieces for your duplexer from raw material. If not, merely follow the assembly instructions. No part of the duplexer is highly critical. Ordinary hand tools are sufficient if you duplicate the dimensions carefully.
1. Cut six (or seven) 6 1/2" sections of 1/2" tubing for the inner conductors. Use a tubing cutter, not a hack saw. This length is suitable for 420-450 MHz. Refer to figure xx, chapter x for other frequencies. Clean up the inside and outside edges with a utility knife and a file.
2. Cut six (or seven) 7 1/2" sections of 2" tubing for the inner conductors. This length is suitable for 420-470 MHz. If you use a hack saw, make the ends as square as possible. A tubing cutter is preferable, however. Clean up the inside and outside edges with a utility knife and file.
3. Drill all the pieces to the dimensions indicated in figure xx. Carefully measure and center punch all holes before drilling.
4. Thoroughly clean all parts. Common household copper cleaners or steel wool is satisfactory.
5. Apply soldering flux to the outside of the 1/2" end caps and to the inside of the 2" end caps around the 3/16" hole in the center. Do not use acid flux. I recommend liquid rosin flux.
6. Screw together the two end caps for each cavity with an 10-32 x 1/2" brass screw and nut. Then solder them together (only the end caps, don't include the center conductors). Use a propane torch and standard 60/40 electronics solder. Do not over solder. Excess solder on the inside surfaces of the cavities will increase losses.
7. Insert the center conductor into the small end cap, but do not solder. If the center conductors fits loosely, remove it and gently squeeze the end cap with pliers to obtain a snug fit.
8. Solder a 1/4-20 brass nut over the center hole in the inside of the bottom end caps for the tuning screws. Temporarily, use a stainless steel screw and nut to hold the brass nut in place for soldering.
9. Cut and carefully bend the coupling loops as indicated in figure xx. The loops should just barely force through the BNC mounting nuts. Solder them to the BNC jacks. Be careful of the grounding lugs, they break easily. Do not solder the loops for the notch cavities with close-spaced connectors and the curved inductive links.
10. Install the loops and connectors into the filters, slipping the lock washer and nut over the loop from the inside. Tighten the nut by hand until the loop is about 1/4 turn from its final position.
11. Using the special tightening tool, turn the outside of the connector until the nut is tight and the loop is perpendicular to the center tubing. The grounded side of the loop should be inward. Be careful not to use too much force. You may break the small ears on the BNC connectors.
12. Assembly the entire cavity. DO NOT SOLDER THE 2" TUBING AND END CAPS. Skin effect provides adequate connection. The four #6 sheet metal screws adequately hold the cavities together. Tighten them easily.
TuningYou will now tune the cavities. You must do so, one at a time. Don't try to tune the whole duplexer yet. Use the test set up shown in Fig. X. A word of caution. The wattmeter and dummy load must be rated for UHF frequencies. Most VHF and all HF types are unsuitable. I use a Bird Model 43 wattmeter and a small dummy load rated for 500 MHz.
Figure xx Test Setup for Tuning Cavities
1. Key the transmitter on low power and observe the wattmeter as you turn the tuning screw. Bandpass filters should peak, notch filters should dip.
2. With your transceiver on the repeater's transmit frequency, PEAK the transmit bandpass filter. Then, in a similar fashion, but with your transceiver on the the repeater's receive frequency, peak the receive bandpass filter. If you are using a seventh cavity, peak it the same.
3. Now, DIP both transmit notch filters to the receive frequency, and both receive notch filters to the transmit frequency. To obtain better sensitivity for this adjustment, you may wish to substitute a receiver for the wattmeter once you are close to the dip. Be sure it has an attenuator on its input to protect is from too much signal.
4. If the filters do not tune to the desired range, make a slight adjustment to the length of the center conductor. This is why the center conductor was left unsoldered until now. 1/16" is roughly equal to 4 HMz.
5. After checking the tuning range of each filter, remove the loops, solder the center conductor. Use solder flux and minimum solder.
6. Again clean all surfaces.
7. Reassemble and retune each filter.
Loop TuningIf you have access to a spectrum analyzer with a tracking signal generator, you may wish to optimize loop performance at this time. It can also be done with the basic test setup, but it takes considerable patience.
Loop optimization is not mandatory, however. If you have built the loops carefully, they will be close to optimum. The objective of optimization is to adjust for two factors, (1) insertion loss in the bandpass cavities, and (2) separation of notch and bump in the notch cavities.
Let's begin with the insertion loss in bandpass cavities. As we learned in the theory section of this book, a 2" transmit cavity is optimum with an insertion loss of roughly .7 dB. A receive filter is the same unless you decide on higher loss to improve selectivity. In any case, you will know the value.
When measured on a spectrum analyzer, or by patiently plotting power transfer using the simple test setup, you will adjust for the desired insertion loss. If the loss is too low, expand the loops. If the loss is too high, rotate the loops. You can do this by loosing the connectors, or a better way is to twist the loops from the end with a pair of pliers. A little twist will make a big difference. Be gentle. Adjust both loops exactly the same.
In notch cavities, insertion loss at the bandpass bump is more or less constant. Instead you will optimize the loops for the spacing between the notch and the bump. If it is not the same as the spacing between your transmitter and receiver, either insertion loss or isolation will be compromised. Again this is done by expanding or twisting the loops. As coupling increases, the spacing of bump and notch widens.
Final TuningFinally, assembly the cables according to the dimensions given in figure xx, construct a mounting plate or panel and completely assemble the duplexer. Now you will tune it as a unit.
Figure xx Cable Lengths
1. Attach your transceiver to the antenna port and the wattmeter to the either the receiver or the transmitter port. Place a 50 ohm dummy load on the unused port.
2. Set the transceiver to transmit on the frequency of the side under test. If you are using the simple setup, adjust only the bandbass cavities. If you are using a spectrum analyzer you can adjust both bandpass and notch cavities.
Even with a spectrum analyzer, I recommend that you also tune with the simple setup. It is not easy to see the exact peak of the bump in the notch cavities on a spectrum analyzer.
3. After doing one side, change the frequency, reverse the transmit and receiver port connections and tune the cavities as in step 2.
The unit is now ready for installation. It should require little or no fine tuning on the repeater, provided that both receiver and transmitter are 50 ohm devices and the antenna system shows low SWR.