70cms
TVI Filter
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)
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
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