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G8MNY > TECH 05.12.09 13:16l 53 Lines 1756 Bytes #999 (0) @ WW
BID : 44157_GB7CIP
Read: GUEST
Subj: Scope RF Trick
Path: IZ3LSV<IK2XDE<DB0RES<ON0AR<UA6ADV<IK0MIL<GB7CIP
Sent: 091204/2209Z @:GB7CIP.#32.GBR.EU #:44157 [Caterham] $:44157_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
(8 Bit ASCII Graphics use code page 437 or 850)
By G8MNY (New Oct 09)
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 & using it for demostraghtions, I found
quite a convincing RF trick, that even experienced hams are initially fooled
by.
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³RF SIGNAL³ ³SCOPE³ Display ³/~\ ³
³GENERATORÃÄÄÄÄÄÄÁÄ´ ³ ³ \_/³
ÀÄÄÄÄÄÄÄÄÄÙ BNCÀÄÄÄÄÄÙ ÀÄÄÄÄÄÄÙ
T
Connecting this arrangement, no one is surprised the scope displays a sinewave.
But adding a short length of coax teed to the scope...
TEED
COAX
ÚÄÄÄÄÄÄÄÄÄ¿ ³ ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄÄ¿
³RF SIGNAL³ ³ ³SCOPE³ Display ³______³
³GENERATORÃÄÄÄÄÄÄÁÄ´ ³ ³ ³
ÀÄÄÄÄÄÄÄÄÄÙ BNCÀÄÄÄÄÄÙ ÀÄÄÄÄÄÄÙ
T
Making the teed coax quite a short length & 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
ÚÄÄÄÄÄÄÄÄÄ¿ ³ ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄÄ¿
³RF SIGNAL³ ³ ³SCOPE³ Display ³/~\ ³
³GENERATORÃÄÄÄÄÄÄÁÄ´ ³ ³ \_/³
ÀÄÄÄÄÄÄÄÄÄÙ BNCÀÄÄÄÄÄÙ ÀÄÄÄÄÄÄÙ
T
This is the opposite to intuition & 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!
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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