Updated: Repeater on the Cheap: The Chinese SQ-450N Duplexer Test Results

Background:
This was purchased from jaco3378 on 1/12 with tuning service. Shipped the 18th and received the 29th to Boston, MA.

I ordered a Motorola “duo-repeater” at the end of 2012. It consisted of two SM120 radios and came with two antennas, something I later learned was not a fantastic way to make a repeater. The high power of the transmitting antenna would “de-sense” the receiver of the repeater (I assume some early gain stage is overloaded by the high level of the nearby signal and some gain control effectively reduces the level of your intended signal into the receiver).

On eBay, a new slew of duplexers exist (some more of the direct from China electronics). After a little research, I decided to take the plunge and purchased a SQ-450N model, which is spec’ed for UHF from 380 to 485 MHZ with a minimum TX separation of 5 MHz and a maximum of 20. It included a free initial frequency tuning. People had warned me not to trust that the tuning would stick during it’s circumnavigation of the globe, so I was wary. I had to get UHF mini->N connector cables to connect my SM120 radios, as no UHF-mini version of the duplexer is available in higher than 20 watts.

I finally got enough adapters to plug it into my (also new) service monitor, the HP 8924C. The results were pretty good. Both notches were spot on with tuning (to the best of my measuring ability). I haven’t yet tried to alter them.

Update:
As a result of my not knowing how to properly use the service monitor, I was in some strange mode that caused it to display a notch and not actually measure (the brilliance of people who put that in there…). A re-measure indicates that the tuning was slightly off (almost as if it had been tuned perfectly before the locking nuts were tightened, but they moved the tuning over a hair or seven).

I got the necessary wrench and re-tuned the sucker. The result was a greater than advertised attenuation on the transmit frequency (nearly 88 dB of attenuation at 462.600 mHz) between the low and high port. At the same time, I measured ~1.5-2 dB of attenuation between the low side and the antenna, still quite good. I hold my opinion that it is a very good duplexer for just $91.00.

According to the Repeater Builder page on duplexer tuning, that level of attenuation is sufficient for greater than 350 watts of transmit power. Since this is only rated for 35 watts, I assume that’s sufficient.

Below you can view a picture the tracking generator pass through between the Low port and the Antenna port. It’s off by 1 mHz. That’s proof that everyone needs a service monitor 🙂