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.
Avoid using adapters to install in line for better
performance and convenience.
The shorter the RF through path, the better. (I’ve 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.
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
The SWR looking into the meter line wobbled around, but
stayed close to 1 to 1 SWR.
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 ¼ watt resistor
piece of 3/16” heat shrink tubing
1” x 2” piece of PC board, solid copper one side
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
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
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
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 ¼” 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
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 ¼” 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
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.
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