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G8MNY  > TECH     27.04.22 09:11z 72 Lines 2938 Bytes #12 (0) @ WW
BID : 851_GB7CIP
Subj: A TV masthead pre-amp
Sent: 220427/0906Z @:GB7CIP.#32.GBR.EURO #:851 [Caterham Surrey GBR] $:851_GB7C

By G8MNY                                     (Update Oct 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Following a reply to a question on TV Preamps:

As a HAM I would not consider a broadband mixer @ the aerial a good idea!
Better 'gain' is to use low loss coax.

However it is possible to put low & high pass (UHF Band) filters into preamps,
but most commercial ones are a poor attempts at this. Many makers insist on
broadband amps that cover 40-900MHz so that can be used to amplify (& also
MIX!) Band 1 TV, 6M, 4M, Band 2 Radio, 2M, Band 3 DAB Radio, 70cM Band 4-5 TV,
& 900MHz cellphones etc.

Traps for specific ham bands are most effective if placed right on the 1st RF
transistor's base, rather than in the feeding aerial lead.
(See my bul on VHF TVI filters for trap details.)

Basic circuit...

                +12V  - - -             270<+12-18V
                     2k7      === u1   )|FB         )|
                 Ĵ n1          )|3T         )|FB
     Hi Pass    1M    ĴĿ         )|           )|3T
        n1    n1   /      |) T      |            |
AERIAL ĴĴĴ   4:1 |)Ĵ> COAX  >Ĵ>To TV
                 : \e     |)  n27                 n27
 75->   L1 )   Trap  BFR91 

You can power this one from local 12V, the 2k7 & 1M are supposed to bias the
transistor to 2.5mA for minimum noise maximum sensitivity. But higher gains &
more linearity can be achieved if you change the bias as needed..

          @ 2.5mA       @ 12mA
Noise F     1dB           4dB
Gain        12dB          20dB
Bandwidth      300MHz-1GHz

So adjust the values to suit your needs.

The L is 2 turns 2mm diameter, the Transformer T is small ferrite bead with the
4 turns tapped at 2 turns, & presents a 300 (4:1 Z) load to the transistor.

The input Cs I said use 2x 100pF (n1) but 10pF may be better at removing VHF
with not too much degradation @ 470MHz.

DC powering can be line fed, just put the 2k7 to the TV socket & decouple @ TV
end to feed 12V in.

Tight UHF layout (next to no component leads!) is essential on a ground plane,
so try an old tobacco tin (if U can find one!) & UGLY construction for best
results. Etched PCB is so easy @ UHF with a sharp knife!

As there is no NFB in this design, the cross modulation is poor! Some
commercial designs use 2 transistors with heavy NFB to throttle back the gain &
attempt NFB @ UHF, Linearity may just be a few dBs better!

Poor linearity may affect the ability to Rx Digital TV, especially in the
presence of strong analogue signals. Indeed I have seen a commercial UHF
distribution amplifier fail on digital signals (made them worse), while the
analogue was much improved & had NO evidence of any picture cross modulation!

Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP

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