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G8MNY > TECH 18.04.08 11:12l 71 Lines 2934 Bytes #999 (0) @ WW
BID : 53375_GB7CIP
Read: GUEST OM5FA OE7FMI
Subj: Coax Traps
Path: DB0FHN<DB0MRW<OK0PKL<OK0PPL<DB0RES<ON0AR<F4BWT<GB7COW<GB7CIP
Sent: 080417/2346Z @:GB7CIP.#32.GBR.EU #:53375 [Caterham] $:53375_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Sep 07)
To make a coax trap to reject a frequency you basically just T any piece of low
loss coax with electrical length equal to a 1/4 wave for the notch frequency
needed, across the aerial feeder/socket.
Aerial "T"
\³/ feeder notch Rx
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÙ
³
Open coax
Leaving the teed coax "open" a 1/4 wave back from that where the T is you get
the opposite impedance "a short circuit", & it this that gives the attention
across the feeder. So the lower loss in the trap coax the deeper the notch.
To determine the physical length the coax velocity factor is needed (how much
shorter than an actual 1/4 wave the coax has to be). e.g. 0.66 for clear
poly & 0.78 for air spaced & foamed (white) poly.
Cutting the length too long & trimming it shorter at the lower frequency
e.g. 144MHz not 145MHz to start with, so you know the depth of the notch then
trim on frequency until the signals attenuates at the wanted frequency.
Starting point Done Overdone start again!
³---. .---- ³----. .--- ³-----. .---
³ \ /Too ³ \ /Just ³ \ /Too
³ V Long ³ V Right ³ V Short
ÀÄÄÄÄÄÄÂÄÄÄÄÄ ÀÄÄÄÄÄÄÂÄÄÄÄÄÄ ÀÄÄÄÄÄÄÂÄÄÄÄÄÄ
145 145 145
This is more difficult if the trap is for say a 158MHz pager Tx to a 2m Rx,
than say 145MHz to VHF Rx, unless you have signal generator & a Rx for the
frequency (scanner).
Using a 1/4 wave coax stub on 144MHz also works on 3x @ 432MHz, but unwanted
notch @ 5x 720MHz. This may not be what is wanted for a UHF TV Rx !!
Loss
0dB³-----.._ _..--._ _.----._ _.----._ _.--
³ `\ /' \ / \ / \ /
-10dB³ | | | | V '
³ V '
-20dB³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>Frequency
1/4 3/4 5/4 7/4 Waves
See also my bul "VHF/UHF TVI Filter" for an LC trap.
Of course leaving a teed piece of coax across a Tx feeder would upset the SWR!
However using a short circuit 1/4 wave Teed in a Tx/Rx for it frequency, will
attenuate all even harmonics & protect from static. The shorted line is seen as
open circuit a 1/4 wave away at the T point so has no affect on SWR.
Loss
0dB³ .--. _.--._ _.----._ _.----._ _.--
³ .' `\ /' \ / \ / \ /
-10dB³ / | | | | V '
³ / V '
-20dB³/
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>Frequency
DC 1 2 3 4 5 6 7 8 Harmonics
Why don't U send an interesting bul?
73 De John G8MNY @ GB7CIP
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