G8MNY TECH 05.09.19 03:11l 52 Lines 1952 Bytes #365 (0) @ WW
BID : 20249_GB7CIP
Subj: Pawsey Stub Coax Balun
Path: OK0NBR IW0QNL JH4XSY JM1YTR JE7YGF N9PMO VE3UIL I0OJJ I0OJJ GB7CIP
Sent: 190905/0604Z @:GB7CIP.#32.GBR.EURO #:20249 [Caterham Surrey GBR]
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From: g8mny@gb7cip.#32.gbr.euro
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Dec 04)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This type of balun is to be found in most textbooks & many aerial designs. It is a 1:1 balun & does not alter the impedance.
Balun is ¼ wave of scrap coax alongside the feeder (a pipe will do).
┌────────────┐
B─┬─@ dead coax │ @=Solder
│ ┌════════════╧@─────────────────
└─) FEEDER
B───┴───────────────────────────────
The feeder inner is connected to the dead coax braid B & B are the 50Ω balanced feed points. The dead coax is what it says, a shorted out at both ends bit that could even be copper pipe of the same diameter. It is connected to the feeder ¼ wave back from the aerial where the braids join together. At the aerial end the dead coax is connected to the feeder's inner.
This works by making the aerial connections look "balanced", as the ± voltage phases appear to radiate equally from both halves of the balanced ¼ wave lines & hence cancel, the 2 balanced folded ¼ waves look like a ½ wave where the centre is at RF ground & therefore can be connected to the feed coax outer without any RF left on the outer of the coax skin.
Common mode Z is only high at the resonant frequency, where it eliminates any outer feeder current on the unbalanced coax.
Z
Hi │ .".
│ / \ /"\
Low│...-' `-....-' `-.
└───────┬───────────┬──── F
Fo 3 Fo
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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