OpenBCM V1.07b12 (Linux)

Packet Radio Mailbox

SP7YDD

[OBCM_Elk]

 Login: GUEST





  
G8MNY  > TECH     26.04.22 07:33z 188 Lines 7371 Bytes #8 (0) @ WW
BID : 812_GB7CIP
Read: GUEST
Subj: VHF/UHF TVI Filter
Path: SP7YDD<SR1BSZ<IW0QNL<IZ3LSV<I3XTY<GB7COW<GB7CIP
Sent: 220426/0723Z @:GB7CIP.#32.GBR.EURO #:812 [Caterham Surrey GBR] $:812_GB7C
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY (BATC's CQTV 160, & RSGB's Radcom 6/93 p75) (Updated Feb 10)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

In strong signal areas an attenuator may be all that is needed to reduce TVI
but if it is not read on..

I have run up to 400W on 70cm & 2m have found this filter design very
effective. It is basically a suck out filter teed to the aerial socket, & with
effective UHF/VHF braid breaker. UHF losses with this design are much lower
than some commercial filters!

This filter is different than using a 1/4 wave coax stub on 144MHz, that also
works on @ 3x on 432MHz, but an unwanted notch @ 5x on 720MHz!

NEED
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, where 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.

DESIGN
This design suits all UHF & VHF bands, just change the number of turns etc...

 Belling Lee
Aerial socket ||    _FB                        FB_ ===
          ||Ŀ================================ TV
              ||___/-      wave of COAX         - === Plug
           .-.|    |       actual length
          | L |     
           '-' \    
              C   
          
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 for L.
   2-10pF trimmer.
   Paint (to make it more presentable).

COIL
  Band  Turns  Dia
   6M     8    9mm
   4M     7    7mm
   2M     5    6mm
  70cm    3    3mm
  23cm    1    3mm  make C a 0.5-5pF

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 75 coax fed to the common sickout T
point.

1/ Cut tin plate (cleaned old can) into a cross shape (with 3mm tabs to solder
   up) so that it can be folded up into a box 2x2x1cm. WARNING SHARP EDGES!

      <- - - - - - 6.3cm - - - - >
           Ŀ   TV
  |    1.3cm ~/~\~   coax
  |         Ant/   /hole
  |   - -~- -(_Ŀ  
  |   |    |       |    |          |
  |   |    |       |    |  Lid     |
4.6cm |    | Bottom|    |         2cm
  |   |    |       |    | trim    |
  |   Ŀ- - - -  
  |                <1cm><  2cm  >
  |         _ _ _ _1.3cm
           
      <1.3cm>< 2cm  >

   Fold up the box to set the fold lines, do not solder up yet.

2/ Drill/pinch hole to mount the TV Socket on one of the 1cm sides. (Bolt or
   Solder in place)

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 rings are at a voltage maximum, where they
   would have no effect on outer coax current.

      <- -  wave - ->
  =================[]=
    FB   75 coax   FB TV Plug

4/ Cut a hole for coax in a corner of box, & solder the coax outer to the tin
   can without melting the coax, connect the core to the socket centre.

5/ Wind wire to make coil, as per table, scrape & tin the ends.

6/ Locate a position for trimmer leaving room for coil. Make a hole to access
   the trimmer in the tin lid, & mount trimmer with its 2 earth tags shaft
   earthed.

7/ rcoil in place between socket & trimmer.

8/ Fold up box, just tack solder a few tabs. It becomes a very solid box.

9/ Connect an aerial (or sig gen) (50 does not matter too much), & your Ham
   Rx. Check the signal is only being received via the box then, null out the
   Rx signal with the trimmer C. If you find the C is at max or min for only
   slight signal reduction, make new coil longer for max C & shorter for min C.

10/ If all is OK, solder up properly, Glue coax firmly in place (heat clue),
   fix the ferrite rings tight to plug & Box (heat glue).

11/ Paint up the box, & label "xx MHz TRAP" (for thick TV eng!)

12/ Retune for best dip again, & cover trimmer hole.

CONCLUSION
I have measured up to 30dB rejection, with only about 1dB insertion loss @
550MHz.

  Loss
  0dB-------.._       _..-------------
-10dB          `\   /'
-20dB            | |    TV Frequencies
-30dB             V
-40dB>Frequency
                 432MHz

If attenuation is not really a problem, improved interference performance can
be obtained with this filter if a "T" attenuator (3-6dB) is included in the box
& the suck out connected across the middle "T" section.

AERIAL (R1R1=====>TV       dB    R1   R2
             L     coax           3   12  220
           R2 C                    6   27  100
       
This is because a bad SWR of the aerial/TV system will not then de-tune the
filter so much.

IN USE
See where the filter is needed, some times in front of the VCR/DVD/STB.
Remember TVI with neighbours is a delicate problem, as they always think it is
your signal the is "jamming the set", rather than a poor TV Rx or installation.

I still have an old copy of the DTI's "How to improve TV & radio reception",
which I find useful, as it clearly shows the need for filters in the TV aerial
lead & implies this is down to the TV service engineer to sort out!

Aerial/loft boosters/active splitters are bad news, as they are really just
broadband mixers with any large signals. But filters in front of one may cure a
problem if you can get to it! Possible with the LC built inside one!

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

-------------------------------------------------------------------------------

My wife noticed some TVI on an analogue DVD recording from my 2M QSOs, so I
tried a commercial HPF that quoted 2dB UHF loss & -30dB @ 100MHz, but that made
some of the channels too weak to use!

John came around & made one of his filters up for 2M & put it in front of my
amp/splitter. Result no noticeable loss on the weakest channel & no TVI. Peace!

Jude 2E0BXZ


Why don't U send an interesting bul?

73 de John G8MNY @GB7CIP


Read previous mail | Read next mail


 19.05.2022 02:57:32zGo back Go up