SDS100/SDS200: Not Receiving One P25 System With Base Antenna

[sflmonitor]

OK I'm stumped. I have an Omni-X base antenna mounted outside at 28 feet. I have a very short run of RG8x coax connected to a Stridsberg 4 port multicoupler. I have 2 BCD436HPs and 1 SDS100 connected to the multicoupler. The SDS100 seems to be working almost perfectly with the exception of one particular P25 system. I'm trying to monitor the Monroe County FL system https://www.radioreference.com/apps/db/?sid=8095 . I am about 23 miles from the North Key Largo site and can easily hear it with the 436HPs. I can even hear it on my SDS100 with a rubber duck from the second story of my house. For the life of me I cannot figure out why the SDS100 will not receive the system when connected to the outdoor antenna. And the multicoupler is not the culprit. I tried bypassing it directly to the antenna and still suffered the same issues. BTW, the SDS100 has the latest f/w (1.06.00) but it suffered from the same issues with the previous f/w as well. Has anyone experienced this? Thoughts?

 

[RFI-EMI-GUY]

Try a resistive attenuator between the SDS and the antenna. A coil of RG58 cable can serve also as an attenuator. Does the reception improve? If so, the SDS is getting overloaded by some strong off channel or out of band transmitter that your antenna at 28 feet is picking up.

Sent from my SM-T350 using Tapatalk

 

[sflmonitor]

Try a resistive attenuator between the SDS and the antenna. A coil of RG58 cable can serve also as an attenuator. Does the reception improve? If so, the SDS is getting overloaded by some strong off channel or out of band transmitter that your antenna at 28 feet is picking up.

Sent from my SM-T350 using Tapatalk

I think you’re on to something. A coiled cable didn’t improve it but I tried a ferrite bead and I began to hear sporadic transmissions. Not enough to be called a success but a little bit of an improvement. I have no doubt that it is some kind of overload. This thing is terrible with overload. I’ll have to play with it but I think I’m going in the right direction. Thanks for the input.

 

[RFI-EMI-GUY]

I think you’re on to something. A coiled cable didn’t improve it but I tried a ferrite bead and I began to hear sporadic transmissions. Not enough to be called a success but a little bit of an improvement. I have no doubt that it is some kind of overload. This thing is terrible with overload. I’ll have to play with it but I think I’m going in the right direction. Thanks for the input.

A ferrite bead on the coax will only knock down common mode current and I don't think that's the problem. My suggestion for the coax was to get 6 or 10 dB of attenuation before the front end of the SDS. The SDS might have an attenuator function, but I don't trust them unless it is an resistive attenuator, not some biasing of silicon..

Do any of your receivers have a panoramic spectrum display? You might look around to see if there is 900 MHz paging or a nearby cell site.

 

[sflmonitor]

A ferrite bead on the coax will only knock down common mode current and I don't think that's the problem. My suggestion for the coax was to get 6 or 10 dB of attenuation before the front end of the SDS. The SDS might have an attenuator function, but I don't trust them unless it is an resistive attenuator, not some biasing of silicon..

Do any of your receivers have a panoramic spectrum display? You might look around to see if there is 900 MHz paging or a nearby cell site.

Unfortunately the coax coil did not fix the problem. It did however allow for very intermittent reception, which did not occur without it. Like you, I agree there is something affecting the reception, in this case the control channel. I’ll have to work on this some more but I definitely think I’m on the right track. Thanks for the information and ideas.

 

[RFI-EMI-GUY]

Try 25 to 50 feet of RG58 coax.

Sent from my SM-T350 using Tapatalk

 

[Ubbe]

You should always have a variable attenuator in your toolbox as soon as you use an external antenna. You then easily check if you have interference issues or overload by adjusting the attenuation for best reception.
https://www.ebay.com/itm/Goobay-Adjustable-attenuator-0-20dB-F-male-to-F-male-67009/282941627117

/Ubbe

 

[sflmonitor]

Thanks for the advice guys. I’ll have to give those a try. Man, talk about more is not better!