From: Larry.Mc.Donald@ofa123.fidonet.org
Newsgroups: rec.radio.amateur.misc
Subject: 6m duplexer, ibm chars
X-Sender: newtout 0.05 Dec 30 1992
Date: 31 Dec 92  09:28:02

 LMD> SUCK OUT CAVITY DESIGNED TO BE A DUPLEXER 
 LMD> FOR SIX METER REPEATERS
 LMD> By Will Anderson, AA6DD 
 LMD> (From the Southern California Six Meter Club.)
 LMD> [This  post  uses  the IBM PC extended character set for  the 
 LMD> graphic  renditions  of the figures in the original article.  If
 LMD> your system is  incapable  of these characters,  please see the
 LMD> post under subject "6m  duplexer, 7-bit ascii".] 

 LMD> Here's  a  solution  for a 500KHz repeater  duplexer.   
 LMD> Commercial  cavities for six meters are usually the size of water
 LMD> heaters and  you  need four to five of them. 
 LMD> When we implemented the 500KHz band plan,  Dan N6BKL,  Woody
 LMD> WB6ZCO  and  I  started researching and building prototype suck out
 LMD> cavities.   Woody was successful in utilizing coffee cans soldered
 LMD> together.   Dan  followed  this up with the #10  cans and was able
 LMD> to build  successful  cavities. 
 LMD> I  first started out with a band pass filter described in the 
 LMD> ARRL  VHF Handbook.  This was a band pass filter.  I then asked
 LMD> Woody how to  build  it into a notch filter.   He described for me
 LMD> how their circuit  in  the  coffee  can was.   It consisted of  a 
 LMD> single  link,   series  capacitive tuned.   We removed one link and
 LMD> connected the input to the  output.  I achieved a 12 db notch with
 LMD> about 3 db of loss.  I  soon  discovered  from the RSGB Handbook
 LMD> that maximum Q  in  the  cavity is when it's impedance is 77 ohms. 
 LMD> For a circular cavity with  a  circular tuned resonator the ratio
 LMD> of the diameter of the resonator  (d)   to  the  internal diameter
 LMD> of the cavity (D)   must  be  3.66/1,  according to the following
 LMD> formula:  
 LMD> Z  = 138 * LOG  (D/d) 
 LMD> 0            10 
 LMD> (Cross section is two concentric circles,  D  =  inside
 LMD> diameter of  outer circle, d = outside diameter of inner circle.) 

 LMD> In  order to make my LMB box "band pass filter"  from the 
 LMD> Handbook  optimum Q, I would have to put a one inch (1")  outside
 LMD> diameter (3/4"  nominal)   pipe down the center of the segmented
 LMD> box.  (See figure A).   Here  is the formula for calculating the
 LMD> impedance of the  three  inch  (3") square segments of the box (D):
 LMD> 
 LMD> Z  = 138 * LOG  (1.178*D/d)
 LMD> 0            10
 LMD> (Cross  section is a circle contained within a square,  D   =  
 LMD> the  dimension  of the square's side,  d  =  outside diameter of
 LMD> the  inner  circle.) 

 LMD> The total length of the 3/4"  diameter pipe is 57" long.  A 
 LMD> copper  coupler  can be used to slide back and forth to tune 1/4  
 LMD> wave length  line.    One inch (1")  ceramic standoffs are used to
 LMD> support the pipe  the 3" box.  The most critical part of the tune
 LMD> up is the distance the  link coupling is from the pipe.   It must
 LMD> empirically measure the loss  and  the  depth of the notch to
 LMD> determine positioning.    Because  the  length has such large
 LMD> currents running in it,  it must be at least #12  TW wire.  A brass
 LMD> or copper strap would work even better.  I  was able  to achieve
 LMD> over 25 db of notch with .7 db of loss.  Two  cavities  and  two
 LMD> antennas can be used to  create  a  usable  repeater   system.   
 LMD> Adjust  the  two  cavities  to  suck   out   the  transmitter's
 LMD> frequency,  couple the two cavities with a 37" length of  RG213  
 LMD> or  RG9  coax.   This is an electrical 1/4  wavelength.    The 
 LMD> cavities  are placed between the receiver and it's antenna.   The  two
 LMD> antennas must be vertically spaced at least 40 feet apart. 
 LMD> The  complete duplexer can be built by using three notches
 LMD> tuned to  the  transmitter frequency on the receiver line and two
 LMD> notches  tuned  to the receiver frequency on the transmitter line. 
 LMD> This will achieve  about 80  db of isolation and can be fed with
 LMD> one antenna as in figure  B. 
 LMD> Box specification:  LMB aluminum chassis,  12"W x 17"D x 3"H.  
 LMD> The  lid (cover)  is 12" x 17".  The segment dividers are made from
 LMD> a 15" x  14" cover material. 


 LMD> FIGURE A:
 LMD> +--- +-------------------+-------------------+              
 LMD> +-----------+ |    |                   |                   |        
 LMD> |           | |    |   +-----------+   |   +-----------+   |  
 LMD> |           | |    |   | +-------+ |   |   | +-------+ |
 LMD> |               |           | |    |   | |       | |   |   | |    
 LMD> | |<- | -- 3/4" pipe  |           | |    |   | |   |   | |   |   |
 LMD> |   |   | |   |     57" long. |           | |    |   +-+   |   | |  
 LMD> |   | |   |   | |   |               |           | |    |         |  
 LMD> | |   |   | |   |   | |   |               |           | |    |      
 LMD> |   | |   |   | |   |   | |   |               |           | |    |
 LMD> |   | |   |   | |   |   | |   | RF            |           |
 LMD> 17"   |         |   | |   |   | |   |   | |   | Conn.         |       
 LMD> | |    |         |   | |   |   | |   |   | |   +-+             |
 LMD> +-+   +-+ | |    |         |   | |   |   | |   |   | | +---+ --+    
 LMD> | |O+ + +O| | |    |         |   | |   |   | |   |   | | | +-+  
 LMD> |         | +-+   +-+ | |    |         |   | |   |   | |   |   | | |
 LMD> |     |         |     |     | |    |         |   | |   |   | |   |  
 LMD> | | | |     |         |           | |    |         |   | |   |   | |
 LMD> |   | | | |     |  #12 -- | --->|     | |    |         |   | |    
 LMD> | |   |   | | | |     7"  wire. |           | |    |         |   |
 LMD> +-------+ |   |   | | | |     |         |     |     | |    |        
 LMD> |   +-----------+   |   | | | |     |         |           | |    |  
 LMD> |                   |   | | | |     |         |     |     |
 LMD> +--- +---------+-------------------+---+-+-+-+ ----+        
 LMD> +-----------+ |         |         |         |         |        
 LMD> |           | +--- 3" --+--- 3" --+--- 3" --+--- 3" --+  
 LMD> |           | |                                    
 LMD> |               |           | +----------------- 12"
 LMD> -----------------+               +---- 3" ---+ 
 LMD> Adjust total length for resonance notch 29 - 30 db.
 LMD> Adjust link coupling for no more than 1 db of loss.


 LMD> Figure B

 LMD> Antenna
 LMD> /^\
 LMD> |
 LMD> |
 LMD> +-------++-------++-------+ | +-------++-------+ ----------+
 LMD> |       ||       ||       | | |       ||       |           |
 LMD> |       ||       ||       | | |       ||       |           |
 LMD> |       ||       ||       | | |       ||       |           |
 LMD> |       ||       ||       | | |       ||       |           |
 LMD> |       ||       ||       | | |       ||       |           |
 LMD> |       ||       ||       | | |       ||       |           |
 LMD> |       ||       ||       | | |       ||       |           |
 LMD> |       ||       ||       | | |       ||       |         
 LMD> 17" |       ||       ||       | | |       ||       |         
 LMD> | |+-+ +-+||+-+ +-+||+-+ +-+| | |+-+ +-+||+-+ +-+|          
 LMD> | RX ---++O| |O||||O| |O||||O| |O+|-+-|+O| |O||||O| |O++---- TX   
 LMD> | |+-+ +++||+++ +++||+++ +-+|   |+-+ +++||+++ +-+|          
 LMD> | |     | || |   | || |     |   |     | || |     |          
 LMD> | |     | || |   | || |     |   |     | || |     |          
 LMD> | |     | || |   | || |     |   |     | || |     |          
 LMD> | +-----+-++-+---+-++-+-----+   +-----+-++-+-----+
 LMD> ----------+ |     |    |   |    |               |    |     |
 LMD> |     +----+   +----+               +----+     |
 LMD> |       37"      37"                  37"      |
 LMD> |                                              |
 LMD> |                                              |
 LMD> +-------------------- 15" ---------------------+





 LMD> ... Larry.Mc.Donald, N6ZMB, Fullerton, California!  <cough, cough>
 LMD> ___ Blue Wave/QWK v2.10

 LMD> -!- Maximus 2.01wb
 LMD>  ! Origin: Universal Electronics Inc 714 939-1041 (1:103/208)


___ Blue Wave/QWK v2.10

--- Maximus 2.01wb
Newsgroups: rec.radio.amateur.misc