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G8MNY > TECH 17.07.06 18:24l 65 Lines 2498 Bytes #999 (0) @ WW
BID : 38618_GB7CIP
Read: DL1LCA GUEST OE7FMI
Subj: A TV masthead pre-amp
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<GB7CIP
Sent: 060717/0730Z @:GB7CIP.#32.GBR.EU #:38618 [Caterham] $:38618_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Dec 04)
(8 Bit ASCII Graphics use code page 437 or 850)
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!) Bands 1, 2, 3, 4-5, & 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 lead. See my bul on VHF TVI
filters for trap details.
Basic circuit may be using a BFR91...
+12V ÚÄÄÄÄÄÄÂ- - - ¿ ÚÄÄ270Ä<+12-18V
2k7 === u1 )|FB )|
ÚÄÄÄÄ´ n1 ÄÁÄ )|3T )|FB
Hi Pass 1M ÃÄ´ÃÄÄ¿ )| )|3T
n1 n1 ³ ³/ |) | |
AERIAL Ä´ÃÄÂÄ´ÃÄÄÁÄÄ´ |)ÄÄ´ÃÄÄÄÄÄ> COAX >ÄÄ´ÃÄÄ>To TV
³ : ³\e |) n27 n27
L TRAP ³ ³
³ : ³ ³
ÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄ
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. You should be able to get..
@ 2.5mA @ 12mA
Noise F 1dB 4dB
Gain 12dB 20dB
Bandwidth 300-1GHz
So adjust the values to suit your needs.
The L is 2 turns 2mm diameter, the Transformer is small ferrite bead with the 4
turns tapped at 2 turns, & presents a 300ê load to the transistor.
The I/P Cs I said use 2x 100pF (n1) but it may be 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 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 so 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!
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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