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G8MNY > TECH 10.08.04 15:59l 99 Lines 3282 Bytes #999 (0) @ WW
BID : 4981_GB7CIP
Read: GUEST OE7FMI
Subj: VHF/UHF TVI Filter
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<GB7CIP
Sent: 040810/1328Z @:GB7CIP.#32.GBR.EU #:4981 [Caterham] $:4981_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (BATC's CQTV 160, & RSGB's Radcom 6/93 p75, New to Packet Dec 03)
I have run up to 400W on 70cm & have found this filter design very effective.
It is basically a suck out filter "T" to the aerial socket, & with effective
UHF braid breaker.
This filter has been made on the spot once, out of a "cat food tin lid" with
a few components, to successfully solve a 25W ERP 70cms Packet Node TVI case.
The problems were at a not so near neighbour (50M away) of the remote node,
were the TVI had been caused by a so called "satellite expert" who had added an
unfiltered high gain UHF booster amplifier to get TV signals around the house.
This design suits all UHF & VHF bands, just change the number of turns etc...
Band Turns
6M 8
4M 7
2M 5
70cm 3
23cm 1 make C a 0.5-5pF
Parts
Tin Plate (Steel drinks can!).
Belling Lee TV Plug.
Belling Lee Chassis TV socket (solderable metal type).
1/4 wave (12cm) 75 ohm TV Coax.
2 Coax sized ferrite cores.
a few cm of 22swg Silver/enamel copper wire.
2-10pF trimmer.
Paint (to make it more presentable).
ÚÄÄÄÄÄÄÄÄÄ¿
³ ³_ FB_ ===
BELLING LEE ==³ÄÄÄÄÂÄÄÄÄ========================ÄÄÄÄ TV Plug
AERIAL SOCKET ==³ L & ³-FB ¬ wave of COAX - ===
³ C ³ actual length
ÀÄÄÄÄÁÄÄÄÄÙ
Construction
1/ Cut tin plate into a cross shape (with taps to solder up) so that it can be
folded up into a box 2 x 2 x 1 cm. WARNING SHARP EDGES!
ÚÄÄÄÄÄÄ¿
³- - - ³
ÚÄÄÙ- - - ÀÄÄÄÄÄÄÄÄÄÄÄ¿
³ | | | | ³
³ | | | | ³
ÀÄÄ¿- - - ÚÄÄÄÄÄÄÄÄÄÄÄÙ
³- - - ³
ÀÄÄÄÄÄÄÙ
2/ Drill & mount the TV Socket on one of the 1cm sides. (Solder in?)
3/ Connect Plug on coax, feed on the 2 ferrite rings, on the coax.
ÄÄ=±=======±[]=
³ FB FB TV Plug
4/ Cut hole for coax in corner of box, & solder the coax outer to the tin can,
connect the core to the socket centre.
5/ Wind wire to make coil, 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 (or sig gen) (50ê does not matter too much), & your Ham
Rx & null out a signal with the trimmer C.
9/ If all OK, solder up properly, Glue coax firmly in place, fix the ferrite
rings tight to plug & Box (Heat glue).
10/ Paint up, & label "xx MHz TRAP G...." (for thick TV eng!).
11/ Retune for best dip again, & cover trimmer hole.
Conclusion
I have measured 30dB rejection on the best one, with only about 1.5dB insertion
loss @ 550MHz.
In strong signal areas an attenuator can be all that is needed to reduce QRM.
Also improved interference performance is obtained with this filter if a "T"
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.
AERIAL (ÄÄR1ÄÂÄÂÄR1Ä=====Ä>TV dB R1 R2
³ ³ L ³ coax 3 12ê 220ê
³ R2 C ³ 6 27ê 100ê
ÀÄÄÄÄÄÁÄÁÄÄÄÄÙ
Why don't U send an interesting bul?
/QSL
73 de John G8MNY @GB7CIP
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