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G8MNY  > TECH     14.12.17 11:12l 65 Lines 2965 Bytes #999 (0) @ WW
BID : 2189_GB7CIP
Read: I0OJJ GUEST
Subj: High Gain 70cm Preamp
Path: IW8PGT<IZ3LSV<IR2UBX<SR1BSZ<GB7CIP
Sent: 171214/1010Z @:GB7CIP.#32.GBR.EURO #:2189 [Caterham Surrey GBR] $:2189_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                              (New Dec 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This is a design I used for ATV Rx as well as a linear 400W SSB PA wrap around
RX path.

I used it in a 1 or 2 stage configuration giving 14 or 33dB gain, one of the
main advantages is it is a narrow highly tuned design keeping out strong out of
band signals.

CIRCUIT
                  ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄ<+12V
                 2k2                    680      === u1
           1n ÚÄÄÄÅÄÄÄÄ¿           1nÚÄÄÄÅÄÄÄÄ¿  _³_
      ___    ===  ³  | )     ___    ===  ³  | )     ___
      Û  ³   _³_ 220k| )L    Û  ³   _³_ 68k | )L    Û  ³
      Û  ³        ³  | ) n68 Û  ³        ³  | ) n68 Û  ³
      Û  ³        ³  |_ÃÄÄ´ÃÄÛ  ³        ³  |_ÃÄÄ´ÃÄÛ  ³
BNC   Û  ³        ³ ³/       Û  ³        ³ ³/       Û  ³     BNC
 o)ÄÄÄÛÄÄ)ÄÄÄ´ÃÄÄÄÁÄ´BFR34A  ÛÄÄ)ÄÄÄ´ÃÄÄÄÁÄ´BFR34A  ÛÄÄ)ÄÄÄÄÄ(o Output
  ³   Û  ³_  n68    ³\e      Û  ³_   n68   ³\e      Û  ³_    ³
  ³   Û  ³/|         |³      Û  ³/|         |³      Û  ³/|   ³
  ³   Û ===          |³      Û ===          |³      Û ===    ³
  ³   Û /³2-10p      |³      Û /³2-10p      |³      Û  ³2-10p³
  ÀÄÄÄßÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÁÄÄÄÄÄÄßÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÁÄÄÄÄÄÄßÄÄÁÄÄÄÄÄÁÄ

HOW IT WORKS
The 1st transistor is lightly biased for 2-3mA to give the lowest noise figure,
the base Z is low, so it is fed at the 50ohm air spaced strip line point
opposite the input.
The collectors are all both tapped into 2-3x the length of the 50ohm point for
best match (200-500ê).

The collector chokes Ls are 3cm of thin enamelled wire coiled up on a 2mm drill
former. The base C (n68) has next to no leads on it.

The 2nd stage is biased for 10mA for max gain & higher power handling. If only
a single stage is used just put in the BNC socket as on the 2nd stage.

LAYOUT
          cÄÂÄÄÄÄÄÂÄÄÄÄ+12V        The PCB box is built around the BNC
     ÉÍ _ÍÏËR _ÍÍËR _ÍÍ»           sockets that fill in the lower 2 corners.
     º / \ÚÄ´/ \ÚÄ´/ \ º           Start with bottom side & input corner,
     º \ø/³Çc\ø/³Çc\ø/ º           then solder in the strip.
Inputº  Þ ³º³ Þ ³º³ Þ  ºOutput     Middle partitions have 2 holes, 1 for the
 BNC º  Þ ³ºLÚÞ ³ºLÚÞ  º BNC       transistor & 2nd for the bias wire.
 ÚÄÄÄÝ  Þ Rº³cÞ Rº³cÞ  ÞÄÄÄ¿       With all the outer walls in place & all the
 ³   ÝÄÄÞcÁTÁÙÞcÁTÁÙÞÄÄÞ   ³       components fitted it can be powered up.
 ÀÄÄÄÝ  Þ  º  Þ  º  Þ  ÞÄÄÄÙ       Check the currents are about right if not
     ÈÍÍÊÍÍÊÍÍÊÍÍÊÍÍÊÍͼ           change the bias Rs to suite your Transistor.

 ÍÍÍ =Double sided PCB box & dividers      Ý = brass/copper strips 3cm long
/ \ = a 10pf Trimmer.  c = capacitor. L = Choke.     T = Transistor.
\ø/

When all is OK & the preamp all tunes & peaks the band, tack on a lid (you may
need to unsolder it!) & the gain & selectivity will improve.
   

Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP


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