| |
G8MNY > TECH 19.06.08 00:42l 152 Lines 5738 Bytes #999 (0) @ WW
BID : 59500_GB7CIP
Read: GUEST OE7FMI
Subj: VHF/UHF TVI Filter
Path: DB0FHN<DB0MRW<DK0WUE<I0TVL<ED1ZAC<GB7CIP
Sent: 080618/2211Z @:GB7CIP.#32.GBR.EU #:59500 [Caterham] $:59500_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (BATC's CQTV 160, & RSGB's Radcom 6/93 p75.) (Updated Packet Jan 05)
(8 Bit ASCII Graphics use code page 437 or 850)
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. It is differnet than using a 1/4 wave coax stub on 144MHz
also works on 3x @ 432MHz, but unwanted notch @ 5x 720MHz.
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 distant neighbour (50M away) to a 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).
ÚÄÄÄÄÄÄÄÄÄÄÄ¿
³ ³
===³ ³_ FB_ ===
BELLING LEE >³ÄÄÄÄÄÂÄÄÄÄÄ========================ÄÄÄÄ TV Plug
AERIAL SOCKET ===³ L ³-FB ¬ wave of COAX - ===
³ C ³ actual length
ÀÄÄÄÄÄÁÄÄÄÄÄÙ
Construction
It is made in the smallest box possible to keep the UHF TV impedance mismatch
losses down. If you use a bigger box (e.g. for several band filters) then the
input & output will have to be properly 75ê coax fed to the common sickout T
point.
1/ Cut tin plate (cleaned old can) into a cross shape (with tabs to solder up)
so that it can be folded up into a box 2 x 2 x 1 cm. WARNING SHARP EDGES!
<- - - - - - 6.5cm - - - - - >
/|\ ÚÄÄÄÄÄÄÄÄ¿
| ³- - - - ³1.5cm
| ³ ³
| ÚÄÄÄÄÙ- - - - ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ /\
| ³ | | | | ³
5cm ³ | | | | ³ 2cm
| ³ | | | | ³
| ÀÄÄÄÄ¿- - - - ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ \/
| ³ ³
| ³- - - - ³
\|/ ÀÄÄÄÄÄÄÄÄÙ
<1cm>< 2cm >< 3.5cm >
2/ Drill/pinch hole to 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 1/4 wave
apart at the frequency of the filter (or lowest) (no velocity factor
correction!). This ensures neither are at a voltage maximum, where they
would have no effect on outer current.
<- ¬ ,\ ->
ÄÄ=±========±[]=
³ FB FB TV Plug
4/ Cut a 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 a tuning hole in the tin, & mount trimmer from coil to ground (shaft
earthed).
7/ Fold up box, just tack solder a few tabs. It becomes a very solid box.
8/ Connect to an aerial (or sig gen) (50ê does not matter too much), & your Ham
Rx. Null out a Rx signal with the trimmer C.
9/ If all is OK, solder it up properly, Glue coax firmly in place (heat clue),
fix the ferrite rings tight to plug & Box (heat glue).
10/ Paint up the box, & label "xx MHz TRAP" (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.
Loss
0dB³-------.._ _..-------------
-10dB³ `\ /'
-20dB³ | | TV Frequencies
-30dB³ V
-40dBÀÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>Frequency
432MHz
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
IN USE
See where the filter is needed, some times in front of the VCR.
Aerial/loft boosters are bad news, as they are really just broadband mixers 1st
& linear RF amplifiers 2nd. But filters in front of one may cure a problem if
you can get to it!
Try to educate your neighbours so that if they move away, they leave the trap
for the next tenant & do not take it with them where it will do no good.
------------------------------------------------------------------------------
Date/Time : 31-Dec 14:54 From: G6OHM@GB7HOL.#22.GBR.EU
I set about building a 4m, 2m, 70cm compound suck out filter in a tobacco tin.
I added as suggested 75ohm coax inside the box, & soldered it to the box &
where the coax comes into the box. I wound each coil on a pencil & soldered
each coil to the coax socket. Next I soldered the variable caps to each coil &
soldered the other end of cap to tin box. I fitted it to my tv set this morning
& played around until I found the best spot between the TV antenna coax & the
digital box. Each band has tuned up very well.
I now have no EMC from 4m, 2m or 70cm. For the first time in years I went onto
2m SSB & had contacts without the xyl yelling at me you are patterning the TV.
Thanks Andy G6OHM @ GB7HOL
Why don't U send an interesting bul?
73 de John G8MNY @GB7CIP
Read previous mail | Read next mail
| |
