The thing you are missing is the duplexer has TWO notches for a reason. Yes the big side is the receiver side notching out the transmitter frequency. BUT you also have a notch for the receive frequency on the transmitter side for a very good reason. The transmitter is going to have some noise on the RX frequency, it’s not enough to be picked up a mile away but it’s enough to dense the receiver with an antenna right next to it. There is a reason this isn’t done.
Both notches are used, 1 blocks the transmit freq and the other blocks the receive from going into the dummy load. The duplexer is tuned the same as a normal application. It works perfectly as described if you transmit high (159Mhz) and receive low (151Mhz). I found that if I reversed the tx and rx frequencies I would get a very large desense that would really limit the coverage. Your timing is amazing on the comment because I did find the cause and a solution to that problem. I learned that most all of the times I blamed desense on a receiver having its frontend overloaded, it was more likely a near-field on-frequency rf product that was desensing it. I just made this video showing how it does it.
That’s not how that works with the dummy lord. You are basically only using one side of the notch. The reason for the transmit side going through the duplexer is so that side notch can be set to notch out any spurious signals coming out of the transmitter that would be on the receiver frequency. The amount of loss you are going to see by running the transmitter thought the duplexer is really not much. Plus by using using the same antenna for both you won’t end up with an alligator or elephant repeater. And I say this from having a mobile repeater in my 4Runner and having a uniform antenna pattern is really the goal and makes you no always wonder about TX or RX issues.
