SGQ-450D Duplexer - Tune-up and Review

[marcspaz]

Hey folks!  For people looking for an affordable duplexer that works well, I may have a solution for you.

 

For those of you who have been following the thread, you may be aware that our friend @LeoG had a problem with a duplexer in his B-Tech repeater.  They exchanged it for him, but I offered to take a look at the duplexer, hoping that we could get a spare available, or a second for another machine.  I have to say, I am impressed.

 

The duplexer is a SGQ-450D.  It is a traditional six cavity mobile duplexer, made in China, and the cost is about $115-$120.  When I first got the duplexer, I connected it to a VNA.  The unit looked like it was tuned perfectly, the notch and SWR were great... it was just on the wrong frequency, preventing it from working on GMRS.  I connected it to a radio and a dummy load, checking both sets of filters, and it worked perfectly.

 

The downfall with duplexers setup like the ones installed in the B-Tech repeaters isn't the duplexer itself, but rather the tuning.  What companies do to make these wideband (and what I found with this duplexer), they tune the 3 receive cavities to slightly overlap instead of perfectly align. Also, the 3 transmit cavities to slightly overlap instead of perfectly align.  What you end up with is about 500KHz-700KHz of filtering at about 45dB-50dB on each side, for a max of about 100dB of isolation.  While not terrible, it leaves a lot of room for improvement, which is easily accomplished if you align all 3 cavities to work on the same frequency.

 

@LeoG asked to have the duplexer aligned to 462.600/467.600.  On the high side, I was able to get a minimum of -85dB notch centered at 462.600MHz.  On the low side, I was able to get a minimum of -87.4dB notch centered at 467.600MHz.  That is better than -172dB of isolation... almost double what the previous tune was.

 

Here are the final tune-up results.

Isolation = > -172dB

Transmit SWR = 1.08:1

Receive SWR = 1.18:1

Loss due to SWR = 0.0064dB

Total Insertion Loss is <1dB

 

For a test, I hooked it up to my portable repeater while at my house.  I got the same range as with my personal duplexer.  For comparison, these specs are on par with my SinTech/Sinclair duplexer, which is priced 4 times higher than the SGQ-450D.  The duplexer will be shipped back to Leo next week.  I am looking forward to his real-world feedback after the tune-up.

 

Below are a few images of the duplexer and the tuning results.  

 

 

 

 

 

 

 

 

 

[WSIK532]

@marcspaz Great Job! i know @LeoG will be excited!

[WSIK532]

@marcspaz How long did it take to tune it?

 

[marcspaz]

@WSIK532 the entire process from setup and initial testing to the finalized range test and cleanup was about 2 hours. The actual tuning itself only takes a few minutes. 

[LeoG]

Yep, I can't wait to do a real world test when I put it back in the RPT50 Btech.  Right now I have a wide band tuned duplexer in it that they sent as a replacement.  And this duplexer worked much better than the one I gave Marc to tune.  As soon as I swapped out I noticed a marked improvement in reception.  Not sure how it's tuned, if it's the same tuning as the one I gave to Marc or something else.  It's probably tuned better than the original just from the fact I could get better reception out of it.

I've done a lot of real world testing with my current setup.  I know where I can hit it and where I can't and where it's iffy.  It's the iffy spots I'm hoping to see better results.

And Marc I just want to say it so the people here see it too.  Thanks for doing this.  I appreciate your effort and I'm looking forward to putting it to good use.  I know you just wanted to play with your toys, but it's hopefully going to be a real world difference for my repeater.  This way we can both have our fun.  And just think, now I'll have that other duplexer.....