[TenTec] PT-340 noise bridge vs. SWR meter

[Dr. Gerald N. Johnson, P.E.]

It only takes an inch or so of extra lead length in the bridge to make a
noise bridge read wrong. What you need is a precision terminator. They
are commercially made by Bird, GR, M/A.

You can make a decent load, but its a bother. You need a 49.9 or 51.1
ohm 1% resistor, a connector an a few inches of coax. With a half watt
resistor, I use type N connector and some material from R-8 coax. To
prepare the resistor, take off the braid intact from the coax and pull
the center conductor from the insulation, preserving that insulation.
Drill the opening left from the center conductor to just fit over the
resistor. Then using a pencil sharpener carve that insulation to a
point. cut a tapered section of insulation the length of the body of the
resistor. put it over the body of the resistor. stretch a couple inches
of braid into a cone to fit over that insulation. Solder the pointed end
of the cone to the resistor lead right at the end of the cone of
insulation. At the fat end of the insulation flare out the braid and
mount it in the connector, trimming the ungrounded end of the resistor
short and soldering it to the coax connector center pin. This could
probably be built in a RG-59 adapter to screw into a UHF connector, but
I don't depend on UHF connectors for measuring anything. This
construction should be good to a Ghz or so, and so not be different from
DC to 30 MHz.

Then you can see which measuring instrument is wrong.

SWR bridges can be in error because of strays too.

An alternative dummy load, is a long roll of coax. Say 100 feet of
RG-174 or 500' of RG-58 terminated with a simple 51 ohm resistor. The
loss of either length of coax is enough that two passes through the coax
is as good as a precision termination.

73, Jerry, K0CQ

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