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GM7HUD > SIX 22.12.05 01:22l 85 Lines 4530 Bytes #999 (0) @ WW
BID : 083157GM7HUD
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Subj: CB to 6m conversion 1
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<DK0WUE<I0TVL<ED1ZAC<GB7YKS<GB7SYP<
GB7ESX
Sent: 051221/2224z 71790@GB7ESX.#31.GBR.EU $:083157GM7HUD [Witham, Esx]NNA V3.1
*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 .
In 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
73 de Andy GM7HUD
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