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Dec 10, 2008
Phoenix, Arizona
I'm trying to do a sensitivity test on my 446 MHZ repeater. I have a service monitor to inject the signal into the receiver, but what I'm missing is the Iso-tee to allow me to inject the signal while the repeater is actually transmitting. Is there a manufacturer that sells Iso-tees or do I have to build my own to conduct this test?

Any information will be appreciated
Jun 30, 2006
So Cal - Richardson, TX - Tewksbury, MA
The problem with a modified T adapter is its not calibrated and you won't know the actual sensitivity of your receiver without a cal factor.

Another way to accomplish the task is with a calibrated 30dB attenuater that will handle power of your repeater, or usually 100w. You can also use a calibrated directional coupler and in both cases you subtract the amount of attenuation or coupling factor from your signal generator level and you know your receive sensitivity.

A directional coupler or even a modified T adapter that has been mapped out on a test bench for coupling factor can be used for an important but often never used test which is receiver degradation from interference at your repeater site.

First you measure your repeater receiver sensitivity on the bench into a load and also with the transmitter running to make sure there is no desense from duplexer tuning, etc. Then when the repeater is installed at the site into the real antenna, insert the directional coupler or modified T into the receive path and measure the receiver sensitivity again while transmitting and receiving all the crap the antenna will be picking up. You will usually find some degradation and I've seen 10dB or more from the raised noise floor at most busy repeater sites.
Dec 22, 2013
I have used the RFA4059 sampler and similar for years. As PRCGUY points out you will need to calibrate it for each use as they are fiddly in this way. I have best luck putting a 6dB pad at the sample port so that my RF signal generator sees a better than 12dB return loss rather than open circuit.

You can determine the cal factor by comparing the normal measured receiver 12dB SINAD sensitivity with the generator level in dBM required to attain 12 dB SINAD through the sampler when it is terminated into a GOOD QUALITY resistive 50 OHM load. You should do this whether using a T sampler or directional coupler,

Be sure to use quality double shielded cables from your duplexer as well as test cabling. Avoid UHF connectors, BNC connectors (and inter-series adapters) unless that is only termination for the duplexer or sampler.

I keep on hand a pre made RG142 cable with type N male to quality Teflon insulated UHF male for these cases. Likewise a RG142 cable with type N male to quality Teflon insulated BNC male for feeding the sampler.

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