RF Sampler - W0IVJ.pdf

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MORE ON A HIGH POWER RF SAMPLER (MAY 2011)
QST
Technical Correspondence, July 2011
ARRL Lab Engineer Zach Lau, W1VT, pointed out an error in the May 2011
QST
Technical Correspondence letter from Tom Thompson, WØIVJ. On page 53, in the text
describing Figure 4, we incorrectly listed the wall thickness of the hobby brass tubing that Tom
used to build his sampler as 0.14 inch. Of course that specification should have been 0.014 inch!
I apologize for any confusion and inconvenience this may have caused our readers.
Zack and others also raised some questions about the resistor type Tom used, and also
wondered how he formed the concave half-round end on the tubing to be soldered to the “top” of
the T. Tom answered those questions and provided some additional construction information for
those interested in duplicating his sampler. —
73, Larry Wolfgang, WR1B, ARRL HQ;
wr1b@arrl.org.
Here are some further comments about my RF Sampler Technical Correspondence:
1) The tubing is actually 9/16 inch OD with a 0.014 inch wall thickness.
2) The 15
,
2 W resistor I used is a metal oxide resistor that I obtained locally. It
measures 15
with 20 nH of series inductance. Digikey carries a resistor that should work: part
no. P15W-2BK-ND. I don't have this resistor but I have some 100
resistors from the same
manufacturer that measure 100
with 4 nH of series inductance. The lower inductance should
improve the VHF performance of the sampler.
3) The 34.8
,
¼ W resistor is a 1% metal film component, Digikey part no.
CMF34.8QFCT-ND.
Construction Steps
Construct the BNC/toroid assembly as shown at the top of Figure 4, on page 53 of May
2011
QST.
Cut the tube to length so that the flanges on the BNC connectors just fit inside the
through tube, and drill a ¼ inch hole in the center of the tube as shown in the Figure 4 photo.
Insert the toroid assembly into the through tube and snake the toroid wires through the ¼ inch
hole.
Solder the BNC flanges to the tube, filling the slight gap between the tube and the flanges
with solder. Solder a ¼ inch wide brass strip to the through tube just below the ¼ inch hole. Cut
the strip so it is just slightly longer than the 15
,
2 W resistor. Cut the lead on one end of the
15
resistor to about 1/8 inch and fold the other end 180° so that it is parallel with the resistor
body and solder it to the far end of the brass strip.
Next, solder the 34.8
resistor, along with one of the toroid wires, to the 1/8 inch stub of
the 15
resistor. Solder the other toroid wire to the near end of the brass strip. Connect the other
end of the 34.8
resistor to the BNC connector center pin.
File a notch into the T tube with a small round file, so that the curvature matches the
curvature of the through tube, slip it over the resistor assembly and cut it to length so the BNC
flange just reaches the end of the tubing.
Solder the tube to the BNC in the same manner as was done with the cross piece, and
solder the curved part of the T tube to the through tube, filling in the gaps with solder. — 73,
Tom Thompson, WØIVJ, 990 Toedtli Drive, Boulder, CO 80305;
tlthompson@qwest.net.

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