Probably much less than an extra inch .... the best article on this
topic was perhaps the one in Ham Radio quite a few years back. The
authors found that toroid lead length was remarkably critical and
showed what they called "inductive rotation" because the error was
frequency dependant.
Their final design provided very simple and adjustable compensation; it
gave a bridge with excellent accuracy over the HF bands. I have never
seen any later reference to this article, which is pretty amazing
considering the work that went into writing it. Maybe its the "not
invented here" syndrome.
I have the mag somewhere in my junk room, but so far I cannot locate
it, or I would be able to give the date/page info.
John G3JAG
On 04-Mar-00 Dr. Gerald N. Johnson, P.E. wrote:
>
> 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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E-Mail: John - G3JAG <patents@dx0man.prestel.co.uk>
To: <tentec@contesting.com>
Date: 04-Mar-00
Time: 10:55:04
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