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.....

[WRYZ926]

Outstanding! 

We look forward to your test results. As mentioned by many of us, a duplexer tuned for all of the GMRS repeater channels will not work as well as one tuned for just one channel.

[LeoG]

Never expected it to.  It's absolutely a compromise.  But what it does is allow you to figure out what channel might work best for you in your area.  And when you figure that out you can commit to a tuned duplexer.

Of course when I first started out I didn't know squat about any of this stuff.  And that was a little bit more than a year ago when I got a pair of HTs.  But I'm a quick learner and generally don't need to ask questions because I can find answers myself.  But duplexers are simple and complex at the same time and it took many vids before I understood how they worked.  I always knew what they did.

I'm excited to see if it'll improve my reception.  We have 2 repeaters nearby.  Commercial repeaters all gussied up with the best of the best.  And you can tell by how they receive, very sensitive.  While mine being much closer wasn't that sensitive.  Kinda frustrating.  Hoping for a similar sensitivity with the better tuned duplexer.  Of course the commercial ones have the advantage of being 700ft+ in the air while mine currently sits at 40ft.

[marcspaz]

9 hours ago, LeoG said:

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.....

 

Thank you for the kind words.  It was my pleasure!  I'm glad I was able to help.

[LeoG]

Any chance you can enlighten us on the equipment you used and the techniques to tune it in this particular instance?  Details like brand names and models of the equipment so I can looke them up and see them type of details.  Unless you don't want to give up your secrets LOL.

[marcspaz]

Sure! On the way out right now, but I'll share what im using for this job. Its easy to do and the tools are very affordable.