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G8MNY > TECH 12.04.08 11:33l 47 Lines 1617 Bytes #999 (0) @ WW
BID : 52794_GB7CIP
Read: GUEST
Subj: Coiled Coax Choke Balun for VHF
Path: IZ3LSV<IQ0LT<IK2XDE<I0TVL<HS1LMV<7M3TJZ<ZL2BAU<GB7MAX<GB7YFS<GB7IMK<
GB7CIP
Sent: 080411/2353Z @:GB7CIP.#32.GBR.EU #:52794 [Caterham] $:52794_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Update Dec 04)
(8 Bit ASCII Graphics use code page 437 or 850)
A very practical Balun for aerial work is the choke type. It reduces RFI to &
from the feeder & at next to no cost or loss. :-)
All it is, is a length of coax tightly coiled up, & as close as possible to the
aerial feed point. The length needs to be slightly shorter than a physical 1/4
wave, so that with the added inter turn capacitance it is still high Z.
It is tapped/cable tied to the boom making sure the coil is at right angles
(hanging down) to the horizontal dipole & so it does not "couple" to it!
The rest of the feeder is then tightly tapped to the boom so the coax outer's
capacitace will reduce any outer feeder currents flowing down the coax &
maintain the feeder outer free from RF pickup.
V H F / U H F Y A G I B E A M
@=========@=========@=========@=========@===D==========R
Boom | |.------------/\ÙCoax
Stub | |³Feeder \/ Coil
Mast | |³
| |³
@=directors
D=Dipole
R=reflector
This type of balun can be used in most cases, especially TV & FM designs that
have omitted a balun for cheapness.
Common mode Z is high at & above the resonant frequency, where it eliminates
any outer feeder current on the unbalanced coax.
Z Broad Resoance
Hi ³ L ..'"`-..
³ .-' ``--....
Low³.-'
ÀÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄ F
Fo
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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