W0IVJ's RF Sampler, 2nd version, by N9MW
The two versions of the RF sampler. The copper tube is the latest iteration.
Core assembly ready to place in the T fitting.
The PC board configuration.
Coupler Construction Notes
Considerations:
Avoid using adapters to install in line for better performance and convenience.
The shorter the RF through path, the better. (Ive been told that an impedance hump in a transmission line less than 5 degrees (a little over 1% of the wavelength in use) is acceptable.
Use readily available products.
Results:
The RF through path measured 1.03 to 1 SWR up to 170Mhz. (A recent test showed the output to be -39.6dB +/- 1dB from .5Mhz through 300Mhz.)
The SWR looking into the meter line wobbled around, but stayed close to 1 to 1 SWR.
Materials:
1 each ½ copper water pipe T fitting
1 each ½ copper end cap
short piece of ½ copper pipe
1 each FT-50-61 core
2 #28 enameled wire
4 RG303 teflon coax
1 each SO-239 UHF female connector (chassis)
1 each PL-239 ? UHF male connector (chassis)*
1 each 15 ohm 2 watt resistor
1 each 36 ohm Ό watt resistor
piece of 3/16 heat shrink tubing
Epoxy glue.
1 x 2 piece of PC board, solid copper one side
Construction:
The typical copper T fitting will not pass the 5/8 connector through the center section so it must be carefully drilled out. It can be done with a drill press vice, 5/8 bit and a lot of cutting oil if you are very careful and patient.
The flange of the SO-239 needs to be removed. Common tin snips will remove most of it, but it all must be removed. I was able to do so by using the edge of an electric grinder. Be careful to leave as much as possible of the plating on the connector adjacent to the removed flange as you need it to solder to.
At this point the SO-239 should slide through the T fitting.
Prepare the coax by removing about 2 of the outer insulation. Then comb out the braid and fold it back over the insulation. Leaving a small amount (1/16) of the center insulation, strip the insulation off the center conductor and trim it to fit into the UHF male connector. Solder it.
Cut a couple of pieces of heat shrink tubing to the length between the braid and connector housing so it will cover the connector center pin. One at a time slide the tubing over the coax with the shield pulled back and shrink it onto the connector pin. Now comb the shield over the center conductor (covered with two layers of heat shrink tubing) and onto the flange if the connector.
Wind 15 turns of #28 enameled wire onto the FT-50-61 core. Leave a couple of inches of wire for the leads. Slide it onto the coax.
Trim the outer insulation and shield back to the point that the torroid core sits nicely on the remaining outer insulation. Again leave about 1/16 of the center insulation from then end of the shielding and remove. Trim the center conductor to length and solder to the center pin of the SO-239. It is desirable to keep these components aligned.
The core will be somewhat free at his point. Roll as loosely as possible the core leads around the center conductor so it can be slid into the T fitting.
Measure from the beginning of the threads of the SO-239 to the base of the male connector flange. Subtract a small amount from that measurement, perhaps 1/32 or 1/16 and cut equal amounts off the ends of the T fitting to leave the measured length. Cut several Ό slots in the T fitting ends. That helps reduce heat transfer when soldering the connectors in place.
Slide the unit into the T fitting after carefully combing out the braid shield over the remaining male connector flange. The torroid core should be accessible through the 90 degree opening. Unwind the core leads and bring them through the opening. Check to see that the end of the fitting falls on the plated part of the SO-239. If you are happy that it is matching properly, solder the T fitting to the connectors. A 250 watt soldering gun will do the job.
Using a small screwdriver or dental pics, position the core at the end of the shielding and epoxy into place.
Using a vice to hold the unit, cut several slits in the remaining T opening. Be careful to not cut the wires. Two of the slits should be wide enough to accommodate the PC board in a position offset from center so there is room for the components.
Remove the copper creating solder pads at the bottom of the board and at a distance equal to the 15 ohm resistor at the top of the board (however, the resistor does not connect to it). I used a Dremel tool to rather crudely grind of the copper. Cut off the PC board just above the top solder pad.
Remove the enamel from the core leads. Feed the 15 ohm, 36 ohm and one core lead through the lower solder pad hole. Trim off some of the excess wire. Slide the PC board into the T fitting slots. If the board is perpendicular to the T fitting, solder it in place with a big iron. With needle nose pliers or the likes, pull one of the core wires through the solder pad, solder with a pen style iron and remove any excess. Solder the remaining core lead to the grounded part of the board. Solder the top end of the 15 ohm two watt resistor to the grounded part of the top of the board. Put the 36 ohm resistor lead through the top solder pad hole.
Trim the PC board so the piece of ½ tubing will slide over it. Cut matching slots in the tube end so it will slide into the T fitting over the tabs of the PC board. Cut the tubing at a length that it will slide into the T fitting and leave about Ό above the PC board.
Solder a desired length of RG174 or coax of your choice with the center conductor to the top solder pad of the board and the shield to the grounded portion of the board. Slide the tube back into place and solder with a big iron. Drill a hole of the appropriate size through the center cap to fit the coax and slide it over the coax and onto the tube end. I drilled the end cab and tube at an open location and secured it with a very short self tapping screw.
Place the desired connector on the end of the coax cable to match your meter device.
The flange should be removed from the male UHF connector. With some grinding, filing and polishing it can be made to look very nice.
You should have continuity on the RF through ports and see 36 ohms across the meter leads. Checking for shorts and continuity at various times during construction is a good idea.
Additional Considerations:
Allow time for the glue to set and the unit to cool off after soldering. During construction there will be times when the unit is very hot. Be careful!
While this can be done with every day tools and materials, obviously if one had copper or brass stock and a milling machine it could be far more professional
EST 06-23-MMX