In ham radio - it's possible to have the transmitter appear on either side of the receiver.? With duplexers that provide symmetrical performance - duplexer ports marked "Transmit" and "Receive" or "TX" and "RX" are meaningless.? Mark the duplexer's radio ports "LPHR" & "HPLR".? This will indicate which radio port provides Low Pass High Reject & High Pass Low Reject frequency responses.? When tuning the duplexer - it will be logical when referencing these marks.? I actually mark each cavity so it's obvious.

Calibrate the VNA with a quality O-S-L.? Use quality cables between the VNA and device under test.? Remove the duplexer harness cables observing where they came from (mark them - so you can put them back).? Connect the VNA's ports to the two connectors of each cavity.? If this is a really old WP-678, the cavities have one connector and a F-M-F Tee.

Tune each cavity individually for best return loss at the pass frequency using the threaded rod.? Maintain the pass to reject spacing (5.0MHz) using the capacitor adjustment.? For the most part the notch frequency (spacing) will track with the pass frequency adjustment - but check it to make sure.? Tune all four cavities that way - two for HPLR and two for LPHR.?

Then, reconnect the harness cables.? Terminate one radio port with a quality (high return loss) load - and reconnect the VNA to the other radio port and antenna port.? Sweep S11 to see the return loss with the two cavities connected together.? Note the result.? You may have a situation where the return loss dips do not fall on top of one another.? You may see one dip, or you may see two dips that no matter what you do, you can't make them align over one another. ?This is not necessarily a bad thing.? For now, do NOT re-peak the cavities in an attempt to make the S11 dips fall on top of one another.? There's a reason for this that I'll elaborate on if you want me to.? If you screwed with the tuning and the S11 dips don't align, go back and re-tune the cavities individually - and don't re-peak after re-cabling.

Now - measure the insertion loss, and note it.? Also, see if the pass response peak (S21) coincides with the return loss dip.? If the return loss dip is not one dip, but two dips, see is the pass frequency is in the center of the return loss response.? Often times the bottom of the return loss response looks like a smoothed W.? On this duplexer - it will likely be one dip.? To put this another way - the frequency you tuned for best return loss should also have the lowest insertion loss - and those should occur at the same frequency.? Concurrent best return loss and lowest insertion loss at the pass frequency is usually the case with UHF duplexers that have a 5 MHz pass to notch spacing.

If the insertion loss and return loss measurements meet or exceed , lock the pass adjustments with the locking nuts (carefully).? Check the opposite side - remembering to swap the load to the other radio port.? The totally cabled insertion loss should be 1.2dB, and the return loss should be -18dB or more on each leg of this WACOM duplexer.? On earlier models - the insertion loss might be a little higher, like 1.4dB or so.

Now - using something else (SA/TG), tune the notches for greatest rejection at the notch frequency - remembering to also terminate the opposite radio port with a quality load.? Do this for both sides.? You can't use your NanoVNA because it doesn't provide enough dynamic range to see the bottoms of the notches.? This duplexer commonly provides 95dB or slightly more notch depth at 5 MHz pass to notch frequency spacing.? If you don't have access to a SA-TG - use a signal generator and receiver to tune the notches.

Clear as mud?? Ask questions....

Kevin W3KKC

On 3/19/2025 7:47 PM, Don Roden wrote:

You're welcome, Don.

One clarification....
The WACOM series of duplexers generally were provided with variable capacitors with enough range to allow the filter to be configured with the notch either above or below the pass frequency.? In other words - each side of the duplexer can be configured for LPHR or HPLR.

One comment....
1.35 dB insertion loss per cavity seems suspect.? I would have expected it to be 0.5 dB, resulting in a total cabled loss of around 1.4 dB or less.? You didn't mention what your total cabled parameters ended up being - so you may want to let us know.

Kevin W3KKC

On 3/20/2025 11:02 PM, Don Roden wrote: