Updated 12-14-2004
To have a duplexer designed, engineered and custom-built 
to your specifications which is rugged enough to withstand 
shipping via UPS, e-mail me (Jim) here:  "jvpoll at dallas dot net".

Note: a SIMULATION, so no real detail on making one.
Jump to my work Jan 2019

Six Meter Hybrid Ring Heliax Duplexer by Jim WB5WPA

Rcv/Xmit leg plots

Low-band VHF Hybrid Ring Heliax Duplexer Performance on 6 Meters


Return Loss (VSWR)

I prefer using the term return loss to SWR or VSWR since so very few of us actually measure 'VSWR' directly; usually, we measure return loss using a directional coupler (or a directional Watt meter like the famous Bird series) and convert this figure to VSWR using the usual, well-established formulas and via HP's APPCAD utility or the venerable HP VSWR "Reflectometer" slide rule.

To that end, the following close relationships exist between Return Loss and VSWR:

Return Loss
14 dB 1.5:1
18 dB
21 dB 1.2:1
26 dB 1.1:1

The return loss value, directly related to the VSWR (Voltage Standing Wave Ratio), is a measure of the match the duplexer presents to the transmitter, the receiver, and even the antenna. Below is the sweep obtained during modeling of a Heliax duplexer design. Practical experience shows that comparable results can be achieved when building the Heliax duplexer - Return Loss values better than 20 dB translating to a VSWR of around 1.2:1 can be achieved with proper design and tuning.

Sweep frequency range 51.5 MHz to 53.5 MHz of the 4-stub HR Duplexer. For this sweep the following parameters were measured:

  • S11, Red, Antenna leg RL performance.
  • S22, Yellow, Receive leg RL performance.
  • S33, Green, Transmit leg RL performance.
  • Commercial product - comparison

    For comparative purposes, via www.repeater-builder.com/rbtip/duplexerspecs.html we can see what the key performance parameters are for a commercial DB-4032 (an 8-can helical resonator VHF low-band duplexer):

    Type: 8 helical resonators, bandreject
    Minimum freq. spacing      0.5 MHz
    Insertion loss             2.0 dB
    Max. continuous power      150 watts
    Tx noise supp. at Rx freq. 80 dB
    Rx isolation at Tx freq.   80 dB
    VSWR                       1.5:1

    -- History --

    The history of duplexer design for Six Meters using Heliax stubs dates back to the late eighties when I began several years of low-level research involving the mathematical analysis, simulation and 'bench work' on shorted Heliax stubs; work that finally culminated in a proof-of-concept prototype design of a notch duplexer in about '91 which was constructed out of 1 1/4" Heliax.

    Duplexer design, website issues, or to have a duplexer built for use on your VHF low-band commercial (30 - 50 MHz ) repeater or for the 6M Amateur band e-mail Jim (call sign: WB5WPA) at  "jvpoll  at  dallas -dot-  net"  (Be sure to remove the at and -dot- and spaces and replace as required).

    * * * * * * * * * * * * * * * * * * *

    Copyright Notice: The author would like to retain any and all rights to images, text or other creative works (including pictures, sketches, hand or machine drawn art) appearing on this page. Also, should derivative works be created based on this work it is asked that reference or cite be made to same in the 'references' section of said derivative works.

    Jan 2019: Email from Jim WB5WPA This was a SIMULATION.
    He did not build a Hybrid Ring. He did build a notch type.

    An email from Steve N8AR, advice, convert to series loop BP/BR.

    My work ------- My work thus far Jan 2019 ------

    Q/ Can the Hybrid Ring make small cans useful?

    I've constructed a Hybrid Loop for 2m in 1/4" heliax, filed notches in it and soldered on tails of RG223 for the in and out and the stub. No stub yet.

    hybrid, No stub plot
    Cavity is a 6" RFS with about a 2 sq. inch loop in 3/8" copper strip.
    the Hybrid Loop was cut for 147MHz, but was about 153MHz.

    hybrid, with 10p cap
    With 10p cap (trimmer) on the stub port. Now with a decent notch depth.

    Tuning the cavity lower reduces the notch depth.
    hybrid, with 10p, cavity tuned 145MHz

    Tuning the cavity higher also reduces the notch depth.
    hybrid, with 10p, cavity tuned 145MHz

    Now I need to try inductance. This is not easy: 2 turns 1/4"
    hybrid, coil 2t 1/4 in
    Note, it didn't move the notch. I need to swap to notch then pass.

    hybrid, straight wire 1 in
    As small L as I can, straight 1 in. wire 0.3mm
    Scan of the bare hybrid loop, I should have started here.

    hybrid, bare no cavity or stub
    I was expecting an obvious notch where the 180 deg signal would neatly cancel as the Sinclair document intonated.
    Should I have scrapped the heliax and gone for RG-213 for my first test run? Jim's article above I thought used 1/4 in. heliax. I measured the lengths and they are right 920mm for 1/2 wave and 1840mm for a full wave.
    No, email from Jim, ring was 1.25 in. heliax, a simulation.
    Measured again with the Atten AT-5011A and the pieces are on 120MHz, measured VF as 0.74.
    Cut and soldered up again, 1/4 in. heliax, no cavity, notch 153MHz. With resonable attenuation out 25MHz each side.
    I moved the coaxes further apart at the cavity in an attempth to bring the notch down but it went up, 156MHz.
    This is more tricky than I expected, adjusting the ring circumference.
    hybrid, coil 1t in 1/4 in. heliax
    With a trimmer, 10p across the cavity, I have notch then pass, 1MHz split, not yet optimised.
    Again, how to swap the notch & pass ?

    New software for Pocket VNA. NB. Turn mobile phone off !!!! RF leakage into the VNA throws it way out.
    Do not use: pocketVna281018.CentOs6x64.run it gave double the notch depth!

    New software for Pocket VNA. Ver 1.02 Downloaded 23 Jan 2019.
    Still fiddling with the line lengths, small open air link on the stub PCB.

    Bare ring. This 1/4 in. heliax is quite good enough.
    scan, bare ring, 153MHz notch

    With cavity:
    scan, hybrid, coil 1t in 1/4 in. heliax

    Small 1t 1/4 in. coil on stub port:
    scan, coil 1t in 1/4 in. on stub port

    A wider scan
    scan, coil 1t in 1/4 in. on stub port

    Hybrid ring in RG-213
    scan, RG-213 ring for 2m, bare
    It's good enough, hopefully not too much leakage through the braid.
    scan, RG-213 ring for 2m, bare

    Cavity in, 330R resistor on stub port, Rx cavity perhaps!
    scan, RG-213 ring with cavity, 330R on stub port.
    That's more real.

    Back to the heliax, with a small L and 1K on the stub port.
    scan, heliax, 1K and an L on stub port.
    That's impressive but the 1K resistor, could limit high power use.

    Adjusting the notch, ring circumference:
    Adjusting the ring circumference, microstrip.
    The ring is the thick coax.
    Setup with cavity and 1.2K on stub port.
    Ring adjusted, cavity in, 1.2K on stub.