Duplexer Tuning Question

[WRDJ205]

I've been looking into building a briefcase repeater using a couple of HT's.  It's a summer project I'm working on with my sons.  It'll be used mainly for personal communication at some rural property where we don't quite get good coverage with HT's.  I will probably order a tuned duplexer from BuyTwoWayRadios.  The 462.725/467.725 pair is fairly open in the areas where I might be traveling and there are no repeaters near the property. 

I can't find a good explanation as to why the duplexers need to be tuned for a specific frequency pair.  Why wouldn't a duplexer tuned for GMRS UHF bands be able to cover all pairs as long as the TX and RX radios had the proper offsets on them?  

As usual, thanks for your help and Happy Father's Day...

[WRYZ926]

Tuning a duplexer for a single pair gives the best performance with minimal insertion loss and better isolation from other frequencies/channels.

Yes you can tune a duplexer to cover all eight GMRS repeater pairs but that is a compromise. Doing this increases insertion loss and reduces isolation. Both will lead to poor performance. And the whole point of a duplexer is to isolate the repeater pair from other frequencies/channels.

[SteveShannon]

13 minutes ago, WRDJ205 said:

I've been looking into building a briefcase repeater using a couple of HT's.  It's a summer project I'm working on with my sons.  It'll be used mainly for personal communication at some rural property where we don't quite get good coverage with HT's.  I will probably order a tuned duplexer from BuyTwoWayRadios.  The 462.725/467.725 pair is fairly open in the areas where I might be traveling and there are no repeaters near the property. 

I can't find a good explanation as to why the duplexers need to be tuned for a specific frequency pair.  Why wouldn't a duplexer tuned for GMRS UHF bands be able to cover all pairs as long as the TX and RX radios had the proper offsets on them?  

As usual, thanks for your help and Happy Father's Day...

Happy Father’s Day to you!
This might help:

 

[WRDJ205]

@SteveShannon

Thanks a bunch for the link.  

I did try to search just like that poster and read several of the results but did not see the one you sent. 

I have some reading to do. 

[nokones]

The duplexer and cables in itself will cause you to lose approx. 30% of inserted power. If the duplexer is not tuned properly, you will lose more of your inserted power and you may be very well left with just only a handful of milliwatts of output power and that will affect your Farz.

[WRYZ926]

You will always lose some power output through a duplexer. The power loss can be anywhere from 30% to 50% depending on the duplexer and the repeater used. The Retevis and Midland 10 watt portable repeaters lose half. They only putout 5 watts after the duplexer.

Why compromise your setup when you don't have to. Tune the duplexer for the exact repeater pair you plan on using for the best performance.

[WRDJ205]

51 minutes ago, WRYZ926 said:

Tune the duplexer for the exact repeater pair you plan on using for the best performance.

That’s my plan.  I was just curious about why that was necessary and if other non-turned for frequencies might work.   It appears like they might but at a costs in performance.   Once I get it set up I’ll do some test to see what the difference is.  

[marcspaz]

Due to there being a 5MHz spread between the uplink and downlink, you can make a custom duplexer that would be low loss and provide great separation, but you would have to have a pretty good understanding of electronics and how LC networks work.

 

I am unaware of any commercially available, but if there is one out there and configured to tolerate any reasonable power, it would likely be well over $3,000. I wouldn't be shocked if it was more like $5,000.

 

Actually, the repeaters available from companies like Midland are as expensive as they are, and have such low power, due to having a broad frequency duplexer that covers all channels. They tried to make them as efficient as possible without pricing themselves out of the market. 

[WRXL702]

6 hours ago, WRDJ205 said:

I've been looking into building a briefcase repeater using a couple of HT's.

I think you should revisit your original 1st sentence, "building a briefcase repeater using a couple of HT's" - And further address your future repeater site location, including elevations, antenna gain & total loss, as compared to your current HT user elevations & issues.

If this suit case repeater will not be placed at an elevated location, in relation to your current HT user areas, such as a tower, hill or mountain top, with the loss in cabling & a duplexer - you will be simply wasting funds.

Duplexer tuning, is irrelevant, unless all other parameters have been adressed.

[WRDJ205]

51 minutes ago, WRXL702 said:

If this suit case repeater will not be placed at an elevated location, in relation to your current HT user areas, such as a tower, hill or mountain top

Thank you for that info.  That was already taken into account in my plan.  

[WRDJ205]

1 hour ago, marcspaz said:

the repeaters available from companies like Midland are as expensive as they are, and have such low power, due to having a broad frequency duplexer that covers all channels.

The availability of those repeaters covering multiple frequency pairs is what brought this question up.  
 

I was curious why the packaged repeaters were able to cover multiple pairs when the one on BuyTwoWay had to be tuned for a specific frequency pair.  
 

It seems like it’s somewhat of a compromise but you get the convenience of a pre-packaged unit.  Something to think about.  

[LeoG]

I have the Btech RT50 and it has a full spectrum GMRS duplexer.  It says it's rated at .25µV at 12dB SINAD.  Can't say I can prove it but it seems to receive well for the antenna and it's height.

 

This is the replacement duplexer they sent me.  The originals sensitivity was much worse than the replacement.  Tuned full band.

[marcspaz]

For those new to radio, and aren't a complete radio dork.... yet....  12dB SINAD is a standard measure to describe receiver sensitivity.  SINAD stands for 'Signal to Noise and Distortion'.  This type of measurement is particularly useful for testing analog FM receivers.  It represents the point where the desired signal is 12dB stronger than the combined noise and distortion. A lower input voltage at 12dB SINAD indicates a more sensitive receiver.

 

Receiver sensitivity is the ability of a receiver to detect weak signals. A lower input voltage (measured in microvolts or dBm) at 12dB SINAD means the receiver can detect weaker signals and still produce a usable audio output.  A 12dB SINAD measurement of 0.25µV (about -118dBm, -119dBm) is pretty good.  Most expensive radios are about 0.200µV (about -121dBm).

 

I looked at that SGQ-450D duplexer specs a few minutes ago.  If someone is interested in buying one, while it's only rated for 50w, it actually looks pretty good on paper.  1dB insertion loss is great and both the suppression and isolation are on par with other mobile duplexers that are 3 times the price.  Again, zero personal experience with this particular device, but it looks good on paper and @LeoG hasn't thrown it in the trash yet... so those are both good signs.  LOL

[WRDJ205]

7 hours ago, LeoG said:

This is the replacement duplexer they sent me.  The originals sensitivity was much worse than the replacement.  Tuned full band

@LeoG  Where did you order the SGQ-450D from and did they do the tuning before sending it? 

[LeoG]

7 hours ago, marcspaz said:

For those new to radio, and aren't a complete radio dork.... yet....  12dB SINAD is a standard measure to describe receiver sensitivity.  SINAD stands for 'Signal to Noise and Distortion'.  This type of measurement is particularly useful for testing analog FM receivers.  It represents the point where the desired signal is 12dB stronger than the combined noise and distortion. A lower input voltage at 12dB SINAD indicates a more sensitive receiver.

 

Receiver sensitivity is the ability of a receiver to detect weak signals. A lower input voltage (measured in microvolts or dBm) at 12dB SINAD means the receiver can detect weaker signals and still produce a usable audio output.  A 12dB SINAD measurement of 0.25µV (about -118dBm, -119dBm) is pretty good.  Most expensive radios are about 0.200µV (about -121dBm).

 

I looked at that SGQ-450D duplexer specs a few minutes ago.  If someone is interested in buying one, while it's only rated for 50w, it actually looks pretty good on paper.  1dB insertion loss is great and both the suppression and isolation are on par with other mobile duplexers that are 3 times the price.  Again, zero personal experience with this particular device, but it looks good on paper and @LeoG hasn't thrown it in the trash yet... so those are both good signs.  LOL

I would actually like to get one tuned to the frequency I use to see if it changes anything.  Not planning on moving the frequency anytime soon so a hard tuned duplexer would be just fine.  I would just remove the broad tuned one out of the machine and replace it with the single band tuned.  Pretty simple operation.  Probably less than 1/2 hour.  That's about what it took me to do when I didn't know what I was doing, although not knowing what I was doing was just based on never had done it before.  It's just removing some screws, swapping out the coax one for one and returning the screw back in place.

[LeoG]

1 hour ago, WRDJ205 said:

@LeoG  Where did you order the SGQ-450D from and did they do the tuning before sending it? 

The repeater I ordered was new and apparently they got a bunch of defective duplexers and needed to replace them.  OffroaderX can tell you all about that.  So the company just sent me a new one already tuned to receive the GMRS frequencies.

[WRDJ205]

10 minutes ago, LeoG said:

I ordered was new

From BuyTwoWayRadio?

[LeoG]

No, directly from Btech.  I ordered the repeater from them.  They decided the duplexer was troublesome and sent me a new one already tuned to the GMRS spectrum.  I removed the old and installed the new.  I still have the old one, they didn't want it back.