
                *CB TO SIX METRES* Part1 - The Introduction

By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa

  *Anyone  is  free to use the following information for private use on the
provision  that  it  is  not  used  for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the  following on the condition that recognition is given to the author .In
the  case  of  it  being  used  in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress.*

 Having allways had an interest in "band 1" propagation , and being spurred
by  reports  of  TV  signals  from the old Crystal Palace transmitter on or
around  45Mc  being  recieved  regularly  down here in Cape Town during the
Seventies  ,  It  was particularly frustrating not to be able to operate on
the  50Mc  band . This band was for exclusive use by ZS-call holders and as
you can see by my call , I hold a "VHF-only , no-code" ZR-call . 
  Things  changed  in  1981 , when it was announced that the six metre band
will be opened up for holders of the ZR-call on the 1st of January 1982 . I
was extatic ! But elation was soon followed by disapointment when it became
apparent  that there were very few rigs to be had , and the prices of those
available were way above what I could afford . 
 I was given an ex-military rig by OM Dave , ZS1SG , but soon realised that
if  I was to really exploit the amazing fare of different propagation modes
possible  on  six  metres  , I would have to use SSB/CW and have more power
at  hand  ,  when  it  was  needed . A transverter seemed to be the obvious
choice  at the time , and I proceeded to aquire an SSB/AM CB rig , with the
idea  to convert it for operation on the 10 metre band , which I would then
use to drive a transverter I was still going to construct . 
  After the mod to the rig , (which worked extremely well) , I proceeded to
experiment  to  find  the  best circuit for a transmit converter that would
eventually  form part of my prototype . At this point , I realised that the
whole  idea is not going to be all that simple . The local oscillator would
have to have a frequency of 22 Mc . (22 + 28 = 50) . Unwanted products from
the  mixer would be (28 x 2 = 56) and (22 x 2 = 44) These unwanted products
from the mixer i.e. 56- and 44 Mc are only 6 Mc away from the wanted output
and  were  going  to  be  very difficult to attenuate to the required -50dB
(min) level required and to have a consistant clean output across 50-52 Mc
would  be  asking  quite  a lot from the mixer and filter stages . To cover
from 50-54 would paint an even bleaker picture and would involve switching
bandpass filters to enable the unit to stay within minimum spec .
 At this point , something else dawned on me . For the rig to operate on 28
Mc , the local oscillator would have to be on (28.000 + 10.695 = 38.695) .
The  10.695  is  the  IF  frequency  used  on  the  rig I had and the local
oscillator  is  injected  on  the  "high  side"  of  the  desired operating
frequency  . It now becomes clear that if the Local oscillator signal could
be  treated  as  if  it would be injecting on the "low side" of the desired
operating  frequency , the new frequency (image of 28.000 , with 10.695 IF)
would be 38.695 (LO) + 10.695(IF) = 49.390! 
  If  the Local oscillator signal could now be made to go up to 39.305 (rig
operating on 28.610) we now have (39.305 + 10.695 = 50.000!) . This forms
the  basis  of  the  modifications to follow . I part2 , I will discuss the
frequency  synthesizer  , and how to actually go about moving the rig up to
round  28.600  , which will be the first step in the mod . Also , bypassing
the inherent problems when using a 40 CB channel switch in any mod , albeit
to  10M  or  to  6M  .  (5 "gaps" where the frequency "jumps" 20kHz and the
sequencing  problems  round  ch23-25 , and the nuisance of having to have a
typed  piece  of  paper  stuck  to  the  rig  to tell you what channel=what
frequency)  The conversion is well worth the effort and I have already done
dozens of these mods . The bugs in the early prototypes have been fixed and
due to  presure  from the local guys , I have decided to "put pen to paper"
and share what I found from years of experimenting and perfecting .
  Hope  you will be able to enjoy it as much as I have . Please let me have
your comments and input .

CU in part2

vy 73 de Shawn ZR1EV  JF96ic
SMIRK#5673
