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G8MNY > TECH 06.01.03 16:07l 53 Lines 2096 Bytes #999 (0) @ WW
BID : 37075_GB7CIP
Read: GUEST OE7FMI
Subj: 70cm TVI Filter
Path: DB0MRW<DB0RGB<OK0PPL<RZ6HXA<SP7MGD<SQ5BAQ<SR9ZAA<HA8FY<LZ0FBB<GB7CRV<
GB7CIP
Sent: 030106/1148Z @:GB7CIP.#32.GBR.EU #:37075 [Caterham] $:37075_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
7 0 C M T V I F I L T E R
I run 400 Watts on 70 cm & have found this filter design very effective. It is
basically a "T"eed suck out, with effective UHF braid breaker.
Parts
Tin Plate (Steel coke can!)
Belling Lee TV Plug
Belling Lee Chassis TV socket (metal type)
12cm 75 ohm TV Coax
2x Coax sixed ferrite cores
5cm 22swg Silver/enamel copper wire
2-10pF trimmer
Paint (to make it presentable)
---------
! !_ FB_ ===
==!-------====================-- TV Plug
SOCKET ==! L & C !-FB COAX - ===
! !
----------
Construction
1/ Cut tin plate into a cross shape (with taps to solder up) so that it
can be folded up into a box 2x2x1 cm. WARNING SHARP EDGES!
2/ Drill & mount the Socket on one of the 1cm sides.
3/ Connect Plug on coax, feed on the 2 ferrite rings, on the coax.
4/ Cut hole for coax in corner of box, & solder in connect the core to
the socket centre.
5/ Wind wire into coil 3 turns approx 5mm dia. & solder to socket centre.
6/ Make tuning hole, & mount trimmer from coil to ground(shaft earthed).
7/ Fold up box, just tack solder a few tabs.
8/ Connect to an aerial (50ê does not matter too much), & 70cm Rx & null
out a signal.
9/ If all OK, solder up properly, Glue coax firmly in place, Fix rings
tight to plug & Box (Heat glue).
10/ Paint up, & label "432MHz TRAP G...." (for thick TV eng!).
11/ Re-tune for best dip, & cover trimmer hole.
Conclusion
I have measured 30dB rejection on the best one, with only about 1.5dB insertion
loss (550MHz).
In strong signal area an attenuator can be all that is needed. Improved
interference performance is obtained if an attenuator (3-6dB) is included in
the box & the suck out connected across the middle "T" section. This is because
the bad SWR of the aerial system will not then de-tune the filter.
73 de John G8MNY on 2001-JAN-06 @ 11:48 local time, from a real IBM XT.
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