The WB5WPA 6M Duplexer



     General - The WB5WPA Six Meter Duplexer consists of six band-reject coaxial

     "stubs" fabricated from 1 1/4" (or larger) Heliax. The stubs are inter-

     connected with 1/4 wavelengths of RG/58 coaxial cable. The duplexer

     exhibits over 70 dB of attenuation with 1.5 dB of insertion loss - compare

     this with the DB PRODUCTS 30-50 MHz duplexer's insertion loss spec'd at

     2 dB and 800+ bucks! If more attenuation is required, another section can

     be simply added to the transmit leg (to reduce transmitter noise at the rcv

     frequency) or added to the receive leg (to knock down the transmit signal

     more as seen by the receiver) at the cost of slightly more insertion loss.



     Stub construction - Each of the band reject "stubs" is constructed using a

     length of Heliax cable with one end shorted. The length is about 9.8% short

     of the electrical 1/4 wavelength (assuming 88% velocity factor), making the

     'stub' inductively reactively. At the end still open, a series capacitor

     (the fabrication of which is  described below) is connected to the center

     conductor of the Heliax to form a series-tuned band-reject notch circuit

     with a Q of over 800. The high frequency reject stubs are shorter than the

     low frequency reject stubs (naturally). For individual stubs that show 19

     dB at a rcv freq of 52.21 MHz the length is 44 7/8". For an xmit freq of

     52.71 MHz the length is 44 9/16".



     Series Capacitor - is made of a piece of RG-8/RG-213/214 center conductor,

     dielectric and braid, pushed down into the center conductor of the Heliax.

     This yields a high voltage and high Q capacitor with a value of between 5

     and 8 pf. The starting length of coaxial cable required is 5". Remove the

     black jacket, fold the braid back onto itself on both ends about 1". Cut

     one end flush with the braid leaving a dielectric length of 4". Carefully

     remove 1" of dielectric from around the center conductor on the other end.

     This end will stick out of the Heliax when the braid, dielectric and center

     conductor are shoved *into* the Heliax's center conductor. The center

     conductor of the RG/8/213/214 piece is then connected to a piece of .1"

     wide brass or copper stock which is connected between the two center

     conductors of two BNC connectors whose bodies have been soldered to the

     outer conductor of the stub. The BNCs form what we call the 'GOZINTA'

     and 'COMSOTA' (it doesn't matter which is which).



     Shunt "reactance" - A shunt reactance is placed between the BNC to BNC

     connection (called the thru-line RF) and ground. This shunt reactance

     reduces reduces the insertion loss .5 MHZ away from about 1.3 dB to

     .4 dB *without* affecting the notch frequency or the attenuation at the

     notch frequency. To reduce the insertion loss *above* the notch's frequency

     a capacitor of 50 - 70 pF is needed. To reduce the insertion loss *below*

     the notch's frequency a 70 - 120 nH inductor is needed. A 56 pF silvered

     mica can be used for the capacitor.



     Notch Frequency Trim Tab - changes the frequency 60 - 80 KHz is used to

     'fine tune' the notch's frequency. This little square of metal is soldered

     to the outer jacket of the Heliax (ground) and moves the notch freq lower

     when pushed closer to the Heliax center conductor.



     7//8", 1 1/4" and 1 5/8" Heliax were tried as band-reject stubs. 1 1/4" and

     1 5/8" are most suitable. Individual stub attenuation values are measured

     to be 18 to 19 dB; when three 1 1/4" 19 dB stubs are connected with 1/4

     electrical wavelength coax (approx 36") the attentation measured 73 db, an

     "apparent" attenuation of 23 db per stub! Insertion loss with a shunt

     reactances in place (explained above) measured less than .4 dB per stub.

     Equipment used in the R & D of this project includes HP 432 pwr meter, HP

     606A sig gen, Tek 7L12/R7603 spectrum analyzer, Wavetek 2001D, 80386 PC,

     MathCad 2.5, and EESOF's Touchstone (uW linear simulation).