How to Modify a Sinclair Q2222E Compact
144-165 or 160-174 MHz Duplexer to 220 MHz

by David Cameron - VE7LTD
IRLP System Designer - http://www.irlp.net
Date: September 10, 2009

Required Supplies

Optional Supplies

Required Time - approximately 1 to 2 hours

Disclaimer

The modification below is provided as is and for information only. This particular modification worked well for me and produced a 220 MHz duplexer that is similar to the specs for a Sinclair made Q2221E. I accept no responsibility for errors or problems you may encounter during your modification.

Background

The Sinclair Q2222E is not a suitable duplexer for the 144-148 MHz amateur band. In order to get suitable isolation, you must increase the inserion loss to about 3 db, which means you lose half of your output power and received signals. Even then, the duplexer offers only 70 db of isolation at 600 kHz frequency split, which in my opinion, is not sufficient for anything solid state over about 20 watts, especially if you use a preamplifier.

The Q2222E has two separate bandsplits available:
Q2222E*1 - 144-165 MHz
Q2222E*2 - 160-174 MHz
Often the *1 and *2 is not shown on the label, however you can usually tell the bandsplit by the factory tuned frequencies. The difference between the two models is in the size of the internal components, and the length of the sections in the harness.

This is the second duplexer type I have modified for use in the 220 Mhz band. The other is the Sinclair Q2220E, and this set of directions is derived from that. The Q2222E is easier to modify, and has a much smaller footprint when complete.

The Q2222E uses a folded cavity design. I am not an RF engineer, so I can not begin to try to explain how it resonates, or how it works. The basic design is to have a fixed resonator (the large internal pipe in the photo), with an adjustable smaller resonator folded inside of it. This modification targets the adjustable resonator which is located inside the fixed pipe. PICTURE PICTURE

To start my modification, I first did some research and calculations. I knew that some parts needed to be scaled down in size to work at 220 Mhz. This included the adjustable tuning rods inside the cavities, and the phasing harness. The inductive loops, tuning capacitors, and duplexer casing do not require modification. PICTURE

As with any modification project, you should verify proper operation on the original band before starting the modification. If the duplexer has internal damage, it will not tune correctly.

Modifying the Phasing Harness

The first thing is measure the phasing harness for length before you start. The length of the RG-142 or RG-400 coax between the cavities should be an electrical quarter wavelength, as should the length between the cavities and the T-connector. To properly determine the length in coax (L), we must multiply the length by the velocity factor to get the actual length of coax. The velocity factor (VF) of RG-142 is 0.695. The existing harness had 12.75 inches length from cavity to cavity and from cavity to T-connector. To figure out the frequency (f, in MHz) associated with this length we use the following formula:

This yields a resonant frequency of approximately 161 MHz, which is about halfway between the designed frequency range of my harness (144-165). Please do your calculations with your harness, as some of the harnesses used over the years have changed both in length and the coax that is used.

To modify to 220 MHz, you have to shorten each cable to center it around 224 MHz. When you flip the equation around to solve for length, I found an optimal length of about 9.15 inches. So you have to carefully remove about 3.6 inches (or more/less depending on your harness) from each length of the phasing harness.

Note: You will only need to desolder and cut the coax feeding into the two connectors that are not the ANT, high-pass (HP), or low-pass (LP) ports. That way you only have to desolder/solder two connectors to shorten all of the coax lengths. If you number the 5 connectors from one end of the harness to the other, you only have to desolder numbers 2 and 4.

(HP-1) ---- (2) ---- (ANT-3) ---- (4) ---- (LP-5)

To accomplish this, carefully work the existing crimp rings round again with a pair of pliers until they are loose. Using a flathead screwdriver, unscrew the top of the T-connector and unsolder the center conductors of the two pieces of coax from the connector. You will need to use a solder braid and/or suction device to remove all of the solder. Once free, carefully pull the coax out of the T-connector. If the coax does not come out easily, keep working the crimp ring. You do not want to break any part of the connector.  PICTURE  PICTURE  PICTURE  PICTURE

If you don't have spare silver crimp rings, use a 3/16 inch drill bit and press the removed crimp ring onto the smooth end of the drill bit, to round the edges of the crimp ring and it can be reused. PICTURE

Use the removed coax pieces as a guide to how you should prepare the shortened pieces for re-solder and re-crimp onto the connectors. Remove 3.6 inches (could vary for your harness, do your own calculations) from each length, and prepare the ends for re-assembly. PICTURE

When re-assembling the phasing harness, solder the center conductor of the coax to the connector. Be sure that no solder bits are loose inside the connectors when you are done. These could flop around and short out the RF inside the connector. Also, use a quality hex crimping tool to reattach the crimp rings to the connector to ensure a good crimp for years of reliable service. You may also want to add shrink wrap onto the outside of the crimp ring for a nice clean look. If you can not get the crimp rings tight, you should replace them or you can also apply solder to the rings and solder them right to the connector, without too much risk of melting the teflon coax. PICTURE  PICTURE

Modifying the Internals of the Duplexer ** See alternate method below

Using a pen, clearly mark the location of the high pass and the low pass side on the top and bottom plates and side of the duplexer. Using an 8mm wrench, loosen the tuning lock screws that hold the four tuning rods in place. Pull each of the four tuning rods straight up and out of the duplexer. You will see that the rods consist of a small diameter silver plated brass resonator and larger diameter aluminum rod, mounted on a rexolite plastic rod. PICTURE PICTURE

Using a drill, remove all of the 50 or so No. 2 Robertson head screws on the bottom side of the duplexer. If you don't have a reversable drill, get one. The top screws can be left alone. You can also leave the large hex nuts on the bottom of the duplexer in place.

Carefully remove the bottom plate of the duplexer. It will yield 4 fixed resonators with "finger stock" attached to them. Note in my picture that one of the resonators has already been shortened. The stock length of the fixed resonators with finger stock is about 5.5 inches for the duplexer I modified. The tuning is achieved by the tuning rod adjusting the electrical length of the fixed resonators. PICTURE PICTURE

The only parts that require modification are the fixed resonator and the small diameter brass resonator on the tuning rod. The stock length of the fixed resonator (without the finger stock) is about 5 inches (see picture). The finger stock is soldered onto the resonator using a silver plated brass ring. For this modification, you will cut the fixed resonators to be 1.5 inches long WITHOUT the finger stock (when measured from the bottom plate of the duplexer). So first, you must remove the finger stock and brass rings.

Using a torch, carefully heat the ends of the tubes and remove the finger stock and the brass ring. Some important warnings here:

• Do not overheat the finger stock or the tube.
• Heat the tube, not the ring or the finger stock.
• Using solder flux or a small bit of fluxed solder can help the old solder flow easier.
• Do not allow solder to coat the finger stock.

I used a pair of pliers to slowly work the finger stock and brass ring off in one piece while I heated the resonator. I also tapped the ring and finger stock on the workbench when it was still hot to remove the excess solder.

** START ALTERNATE METHOD ** - Added July 11th, 2012

I was happy to see this modification quoted on an ebay auction recently, and I decided to contact the seller to see if there were any changes he would suggest to the modification. He suggested that instead of removing the finger stock, you instead removed the tube and finger stock (intact) from the bottom plate, unsolder the tube, cut it to length, re-solder the tube to the nut it came from, and re-attach the assembly to the bottom plate. It must be removed from the bottom plate, as the plate acts as a heat sink, and the solder will not flow if attached to the plate. Caution must be used to ensure the tube is centered in the nut. This removes the need to remove the finger stock, and provides a cleaner final product.

I have not tried this myself, but plan to do this on the next Q2222E I modify.

** END ALTERNATE METHOD **

The small diameter brass resonator also must be reduced from about 6 inches to 2 inches. The resonator can be removed from the rexolite rod by unscrewing the Phillips screw on the bottom of the rod. PICTURE PICTURE

The best tool to use to cut the resonators is a pipe cutter, but a hacksaw can be used as well. Once the fixed and smaller diameter resonators are cut to length, remove any burrs from the inside and outside of the resonators with a file. I also found that the edges of the cut end of the fixed resonator had to be rounded a bit to place the finger stock and brass ring back onto the end of the resonator. I removed a bit of the brass around the edge of the resonator with the "teeth" of my pliers. Position the finger stock square onto the end of the tube. Using a torch, heat the tube and solder the finger stock and ring back into place using solder. I recommend silver solder here if you have it. PICTURE  PICTURE  PICTURE

Using a pencil's eraser, carefully clean the contact points of the finger stock. Using a soft cotton cloth, rub any oxidation, finger prints, or solder flux off of the tuning components. Do not use anything too abrasive, as the silver plating is very fine. Do not use silver polish as the finish it leaves behind is not conductive and can affect tuning.

Re-assemble the duplexer. If the screw holes do not line up, don't force them. Instead, lift the bottom plate out, re-align the holes, and re-insert. Insert and attach the loops into the duplexer using the same orientation as when they were removed. Attach the harness. Tune the duplexer using whatever means you use. There are tuning instructions for the Q series duplexers that can be found on the internet. Once tuned, determine the length of tuning rod you want to keep exposed and cut the remainder. Like the case, you do not have to cut the rods, but it makes the whole installation much cleaner.

When tuning the duplexer, you will want to adjust each loop individually to set the insertion loss. I found that approximately 0.6 db pass loss yielded about a 40 db notch at 1.6 MHz separation.The following scans were obatined from my HP8920A service monitor.
Single Cavity Pass - 224.300 Mhz
Single Cavity Notch - 222.700 Mhz
High Pass Pass - 224.300 Mhz
High Pass Notch - 222.700 Mhz
Low Pass Pass - 222.700 Mhz
Low Pass Notch - 224.300 Mhz

That's it! You should now have a fully functional duplexer for the 220 MHz band similar to the specifications of a factory Q2221E. PICTURE