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G8MNY > TECH 29.03.26 17:16l 64 Lines 2117 Bytes #9 (0) @ WW
BID : 55740_GB7CIP
Subj: Scope RF Trick
Path: JH4XSY<JE7YGF<LU4ECL<LU9DCE<GB7YEW<GB7CIP
Sent: 260329/0807Z @:GB7CIP.#32.GBR.EURO #:55740 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Apr 11)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
A few years ago at my local club we had demonstrated an impossible wooden
transformer, that did all its should do!
I recently got a 100MHz scope going and using it for demonstrations, I found
quite a convincing RF trick, that even experienced hams are initially fooled
by.
ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄﾄｿ
ｳRF SIGNALｳ ｳSCOPEｳ Display ｳ/~\ ｳ
ｳGENERATORﾃﾄﾄﾄﾄﾄﾄﾁﾄｴ ｳ ｳ \_/ｳ
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ BNCﾀﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾙ
T
Connecting this arrangement, no one is surprised the scope displays a sine
wave. But adding a short length of coax teed to the scope...
TEED
COAX
ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ ｳ ﾚﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄﾄｿ
ｳRF SIGNALｳ ｳ ｳSCOPEｳ Display ｳ______ｳ
ｳGENERATORﾃﾄﾄﾄﾄﾄﾄﾁﾄｴ ｳ ｳ ｳ
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ BNCﾀﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾙ
T
Making the teed coax quite a short length and the pure OSC frequency set to
make that an electrical 1/4 wave. The scope displays nothing! Many wrongly
think the teed coax must be a DC short!
But shorting the end of the coax out, the sine wave magically reappears!
SHORTED
COAX
ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ ｳ ﾚﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄﾄｿ
ｳRF SIGNALｳ ｳ ｳSCOPEｳ Display ｳ/~\ ｳ
ｳGENERATORﾃﾄﾄﾄﾄﾄﾄﾁﾄｴ ｳ ｳ \_/ｳ
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ BNCﾀﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾙ
T
This is the opposite to intuition and many are confused as to what is going on.
Only when the frequency of around 70MHz (for my short coax) is noted, does the
penny drop!
USES
This of course is a very usefull method of measuring the velocity factor of a
cable (0.6 to 0.8 the speed of light) as well as the losses. Only low a loss
cable will produce a good nul, and of course a good sine wave oscillator!
See my Tech bul on "Tuned Circuit Scope Demo", "Spectrum Harmonic Demo circuit"
"Oscilloscopes".
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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