Another SDS-200, Antenna and coax question - Impact on my RSSI?

[dynamixone]

hello all,

I have been searching through the countless threads on discones and coax. My question is theoretically how much can I gain upgrading what I have just using the antenna that came with the SDS-200.

My Scanner: SDS-200
My Antenna: Tram - 1411 on 20 ft of top rail outside my home.
My cable: 35 ft of RG6 quad shield and N Connector and BNC on the ends.
What do I scan, a little bit of everything but mostly P25 in the 700 range and conventional freqs in the 150 and 400 range.

My results: I have been connected for a couple of days to the setup that I used to use to play with SDR dongles. I am getting good range for what I need. 30 miles around me that was what I thought was very clear. I decided to run the analyzer and start watching the RSSI on the scanner more closely.
I was getting 95-101 RSSI from local P25 traffic from the scanner read out.

I know the rg6 is not ideal so have been working out running Times LMR-400 but then tested with my supplied antenna that came with the SDS200.

WOW, a lot better. all local stuff is getting low 70s RSSI from the supplied antenna and anything that wasnt "clear" is crystal now. More distant stuff is around the RSSI I was getting from local channels.

My main question is do I go through the cost and effort to run the LMR-400 to my Tram 1411 or will I be chasing that as well and need to replace the antenna? The cheap antenna it came with is doing much better. I'm afraid I will drop all this money and how much reception is expected to be gained? I want a broader antenna to be able to scan more things.

Is my antenna hurting me more than my cable?

I know this is more broad question but up to what I really want to listen to but figured I would through it out there for some opinions.

thank you for your time.

 

[kd8mkg]

Your coax run is pretty short. As such, LMR-400 won't make an enormous difference. At 700MHz, about +1.5dB... with P25 digital, you're either receiving it or not, really. Also, most P25 digital systems, especially if you are listening to like police, ems, fire, etc., are designed to be received well everywhere in the coverage area by vehicles.

Discones are compromised antennas. The benefit is wide receive, but you are ALWAYS better off with an antenna tuned to the band you are interested in. I'd throw up an antenna tuned to the 800MHz band, connect it up and see what happens.

That being said, I personally don't use external antennas for P25 digital on 800MHz. I use the little included stubbies with my Unication G4s and hear everything I want, anywhere I go. I even get good reception in my basement.

 

[Ubbe]

My main question is do I go through the cost and effort to run the LMR-400 to my Tram 1411 or will I be chasing that as well and need to replace the antenna?

Discones usually have 10dB or more loss in the 800MHz range due to the design that points the direction down in the ground and up in the sky at those frequencies and not to the horizon.

But a 30dB difference are much. SDS scanners has a 30dB attenuator so check that it isn't enabled in the config you are using with the discone.

SDS scanners have a feature in the receiver chip that reduce the gain if it receives a strong signal. If the discone receives a too strong signal from a nearby cellular tower it could reduce its sensitivity in the whole 700-900Mhz band. Try and check how it receives in the 400MHz band compared to the telescope antenna.

RG6 attenuate 3dB at 800MHz and 50ft and LMR400 2dB.

/Ubbe

 

[west-pac]

If I understand correctly, the indoor back-of-set antenna receives better than the outdoor discone antenna?.... if that is the case you likely have a bad connection or coax somewhere along your run.

 

[dynamixone]

thank you both. I'm thinking it is the so-239 to F adapter I'm using to connect the tram 1411 to the RG6. Im running the LMR-400 and probably replacing the tram with a diamond so it will be all N connector and 50 ohm. its probably overkill but will see how it goes.

 

[dynamixone]

Discones usually have 10dB or more loss in the 800MHz range due to the design that points the direction down in the ground and up in the sky at those frequencies and not to the horizon.

But a 30dB difference are much. SDS scanners has a 30dB attenuator so check that it isn't enabled in the config you are using with the discone.

SDS scanners have a feature in the receiver chip that reduce the gain if it receives a strong signal. If the discone receives a too strong signal from a nearby cellular tower it could reduce its sensitivity in the whole 700-900Mhz band. Try and check how it receives in the 400MHz band compared to the telescope antenna.

RG6 attenuate 3dB at 800MHz and 50ft and LMR400 2dB.

/Ubbe

the attenuator. is that related to the global filters?

 

[Ubbe]

the attenuator. is that related to the global filters?

No, it's a circuit in the preamplifier before the receiver chip. It can boost the signal and it can attenuate it. Filters and IFX are working at the two intermediate frequencies of something like 250MHz and 350MHz.

/Ubbe