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...

[SteveShannon]

6 minutes ago, LeoG said:

No reference to my-450-sqg-50 on the interwebby

 

And the other number had no reference either.

If you broaden your search to “SQG duplexer” you’ll find several hits. 

[WRDJ205]

Google is sending me to the same link as i found earlier ...

Repeater Duplexer 50W UHF 6 Cavity Duplexer SGQ-450D -

 

[marcspaz]

11 minutes ago, WRDJ205 said:

Google is sending me to the same link as i found earlier ...

Repeater Duplexer 50W UHF 6 Cavity Duplexer SGQ-450D -

 

 

That's the one.

[tweiss3]

2 hours ago, WRDJ205 said:

Google is sending me to the same link as i found earlier ...

Repeater Duplexer 50W UHF 6 Cavity Duplexer SGQ-450D -

 

Be careful, that specification indicates a minimum spacing of 8-10MHz between RX and TX, but the US standard for UHF is 5MHz.

[LeoG]

Conflicting info

 

• 50W RF Power Handling
• 6 Cavity Design
• Minimum Tx and Rx frequency difference: UHF 3MHz
• Maximum Tx and Rx frequency difference: UHF 12MHz
• Fequency Range: UHF 400-470MHz(the full band)
• Bandwidth:±500kHz
• Standard Termination: N Connector
• Frequency Spacing: 8-10MHz
• Insertion Loss:≤1.0dB
• Nominal Impedance: 50Ω
• V.S.W.R:≤1.5
• Suppression:>75
• Isolation:>75dB
• Free Frequency Tuning
• Easy to build your repeater
• Weight:1.8Kg

[LeoG]

Since it's in my repeater and my repeater has 5MHz spacing I would say that it can be tuned for that spacing.

[tweiss3]

If you look at the table, it's performance narrows and insertion loss increases as you approach 5MHz spacing. There is no way in hell that will have acceptable performance at 3MHz spacing. 

[Lscott]

On 6/15/2025 at 10:58 AM, WRDJ205 said:

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?  

The simple explanation is one is used to isolate the transmitter from the receiver. Since the repeater is a full duplex operation, transmit and receives at the same time, any RF energy from the transmitter will get into the receiver. When this happens the receiver will trigger the transmitter. Now you have a feedback loop where the system will stay locked up on transmit until the power is yanked, destruction of the receiver's input at worse, or simply kills the sensitivity of the receiver then it becomes deaf to weak signals. 

The typical notch/bandpass duplexer filter is setup such that the receive half of the filter is tuned to notch out the transmitter's frequency as much as possible. The bandpass filter on the transmitter side is tuned to eliminate as much of a spurious signals that are not on the exact transmitter's frequency, thus preventing them from entering the receiver's circuits.

For the above to work a certain frequency minimum difference between the transmit and receive frequencies are required. This minimum depends on the quality (design) of the duplexer. 

https://horwin.info/en/pro-dupleksery/

https://www.repeater-builder.com/antenna/pdf/w6nbc-duplexer-book.pdf

[WSHH887]

Keep talking guys. I am learning a lot.