Transmitter & Receiver Combining Info

[freddaniel]

For guys interested in designing, building or just understanding how combining systems come together, I came across a technical article on cavity loops & probes, otherwise “How Cavity Filter Coupling Works.” This area is often perceived as black art, but can be examined and learned without difficulty. To become proficient, you need at least a quality Spectrum Analyzer [SA] with tracking generator [TG], which many Service Monitors include. A better choice would be a Vector Network Analyzer [VNA]. Many of these have appeared on Ebay lately for under $400, including shipping. For tuning duplexers, you will need something that will provide at least 100 dB dynamic range, with 120 dB being ideal.

Keep in mind the VNA also displays phase and return loss, while not having the full functionality of a Spectrum Analyzer. Therefore, there are tradeoffs with everything. Many people ask about the inexpensive TinySA and LiteVNA products, but they lack dynamic range for duplexer tuning. They will generally provide up to 80 dB, so they will work to build and test transmitter and receiver combining.

 

[prcguy]

I've been tuning duplexers of all types for 40+ years and the 120dB jobs can be a problem without good test equipment or adding an amplifier to the generate port of the analyzer to increase measurement dynamic range. However, after reading over some old duplexer alignment instructions from Celwave and Motorola, they used to recommend disconnecting all cables, tune each cavity individually for perfect pass and reject, then reinstall all cables and put into service.

I tried this method several years ago testing the completed duplexer after individual cavity tuning and it worked quite well, even on 120dB rated PD/Celwave/EMR 526 models, which are considered about the best UHF duplexer ever made. More recently including yesterday, I used my two port Chinese antenna analyzer to align a four cavity BpBr UHF duplexer by Telewave that I got cheap at Dayton this year. The antenna analyzer has plenty of dynamic range and accuracy for tuning just one cavity and since the cavities on the Telewave were single connector with a Tee adapter I used a 6dB attenuator on one side of the Tee adapter back to the analyzer as recommended in some old instructions. Each cavity tuned up perfect and I checked the final tuning of the complete duplexer with my HP/Agilent 8920 service monitor and it was also perfect. Total pass and notches were centered and I could not improve on the tuning as a fully assembled unit.

So while its recommended to have a spectrum analyzer or VNA/Scaler Network Analyzer with more dynamic range than the duplexer offers, using the single cavity tuning procedure will allow much cheaper equipment to perform the task and the results here have been excellent.