Simulcast / Multipath?

[WD0AKZ]

Hi...

I'm in Oakdale and for the past few weeks I've been unsuccessfully trying to use my SDR dongles, UniTrunker, and DSD+ to receive the ARMER Washington county system. I had read just about everything I could find, had very good strong signals (looking on SDR# about 35db over the noise floor), tried all the suggested fix's, and at best could get about 10% decoding which is to say unusable. So a few days ago I bought a new BCD396XT and with it I'm up to about 75% decoding but still have a lot of garbled or missed responses.

So my question is.... Am I located in a Simulcast/Multipath hell hole or am I really doing something wrong here?

System: Washington County P25 Standard Digital XT
Site: RFSS: 1 (1) Site: 005 (5) Washington Co. - Dakota East Simul. 853.7375, 853.1875, 852.7875
GRPs: Corrections, Fire_Tac, Interop, Law_Tac, Public Works

I also added a "Dispatch Only" group to the above that just has TGID's 3750, 3752, 3618, 3620,74. This allowed me to thin out the crowd a bit. I also programed my 2-meter HT to 154.845 so I can hear the VHF analog side of the 3750 WA FIRE talkgroup. The HT of course always hears it all, the 396XT occasionally hears it all but most of the time it only hears pieces or none at all.

Anyhow, I have tried all the tweaks I have found for the 396XT including the hidden menu P25 adjustments and nothing seems to improve the decoding. I live on the 3rd floor and find that the decoding quality seems to change where I have it placed, but it is rare that I get a solid full communication which would tend to point to the Simulcast or Multipath theory.

Any thoughts/suggestions would be appreciated.

tnx
George

 

[Voyager]

Are you between or among two or more sites? Are they about equal distance apart?

 

[WD0AKZ]

Are you between or among two or more sites? Are they about equal distance apart?

I'm not sure about this. The main sight is North of me a few miles (Hwy 5 & Hadley) but I don't know where the other sights and if they are simulcast.

George

 

[NDRADIONUT]

Some old threads with multipath info...

http://forums.radioreference.com/mi...luth-simulcast-recieve-issues-all-sudden.html

http://forums.radioreference.com/mi.../285369-any-strategies-dealing-simulcast.html

 

[johnmoe1]

(I haven't actually used DSD+, but have read some and built my own SDR-based P25 receiver.) Were you using the CQPSK mode?

Have you tried making you antenna somewhat directional? Something like Antenna Magus Evaluation version corner reflector improves home internet reception by 12 dBi. « Antenna Magus blog would be easy to try.


(I know people here are into alternate control channels, they really are very rarely used. Usually for only seconds at a time.)

 

[WD0AKZ]

Some old threads with multipath info...

http://forums.radioreference.com/mi...luth-simulcast-recieve-issues-all-sudden.html

http://forums.radioreference.com/mi.../285369-any-strategies-dealing-simulcast.html

Thanks for these links... Yes, that sounds like what I'm running into and it doesn't really sound like there is a reliable fix.

-----------
Have you tried making you antenna somewhat directional? Something like Antenna Magus Evaluation version corner reflector improves home internet reception by 12 dBi. « Antenna Magus blog would be easy to try.
------------

Actually I have been thinking of something along these lines. I built a small parabolic reflector for the WiFi on one of my phones and that worked out great. I was thinking I might try the same for the scanner and see if I could improve the canceling effect of the multipath (if that is what my problem is).

tnx for the suggestions.

George

 

[WD0AKZ]

(I haven't actually used DSD+, but have read some and built my own SDR-based P25 receiver.) Were you using the CQPSK mode?

Sorry, I should have answered this as well. For the SDR Rx I tried NFM and WFM at a variety of bandwidths, DSD+ was set for Decode P25 Phase 1 altho it was also used in the auto mode with the same results.

George

 

[wesley2099]

Site info

I'm in Maplewood on the border of oakdale and my scanner picks up Hastings/Dakota site which I actually Washington county too I believe try that.

 

[NDRADIONUT]

A guy in duluth built an antenna for this issue by mounting a 3.25" whip inside of a new unused gallon steel paint can and then aiming the open end at the closest site...

 

[WD0AKZ]

A guy in duluth built an antenna for this issue by mounting a 3.25" whip inside of a new unused gallon steel paint can and then aiming the open end at the closest site...

Yup, it just may come to something like that. I loaded up Hennepin E&W, Ramsey, and Dakota counties and get pretty much the same... Good signal, 75% decoding.

On the Washington Co. Site:005 (853.7375) that I'm interested in I have a System set up for the 3618, 3620, 3750, and 74 (MSP) Call Groups. Assuming that it is a Simulcast or Multipath overload condition I tried the internal 20db attenuator the signal strength dropped from the full 5 bars to a 1-2 range and got a decrease in decoding most likely due to a noisier signal. I tried using a 3.5" wire antenna (1/4 wavelength @ 850mhz) to lower the incoming bandwidth and signal was back at full 5 bars, decoding is now at about 80%, Then I put in a SMA-BNC adapter with a BNC 50 ohm resistive load, signal strength still full 5 bars and decoding is about the same as the 1/4 wave antenna, maybe a bit better.

The interesting thing is that I can be listening to for example 3750 (Fire Dispatch) and have Error rate of 0 with a steady threshold of 6 (100% decoding). Have a break in traffic and when they return have a error rate of 50 and the threshold is bouncing between 6-11 or at times it will stop and stay on the 3750 with no audio (still 5 bars) or not even stop at all, yet my 2-meter HT is listening to the VHF side fine so I know their transmitting. Switching to different transmitter sight that may have more multipath?

So.... It appears that switching from OEM antenna to a 50ohm load actually improves things a bit but still not the reliability of the 2-meter HT.

"Just a Analog Man stuck in a Digital World"

George

 

[fredva]

Was it cost that steered you to the BCD396XT instead of a BCD436HP? The x36HP scanners do better on P25 simulcast.

 

[johnmoe1]

I'm in Maplewood on the border of oakdale and my scanner picks up Hastings/Dakota site which I actually Washington county too I believe try that.

Except for the law dispatch channels, almost all other Washington County talkgroups are found only on the Washington County site.

 

[fredva]

Here is something you might want to consider trying:
http://forums.radioreference.com/in...07265-reception-improved-idps-system-1-a.html

 

[WD0AKZ]

Was it cost that steered you to the BCD396XT instead of a BCD436HP? The x36HP scanners do better on P25 simulcast.

Yup it was the cost. I picked up the 396XT on ebay NIB (with current firmware) for $260 shipped. The seller may have been someone who raided the RS stores when they were shutting down, he had more than one for sale.

George

 

[WD0AKZ]

Here is something you might want to consider trying:
http://forums.radioreference.com/in...07265-reception-improved-idps-system-1-a.html

On first glance one would say it's a dipole but using the BNC Tee connects the two antennas in parallel basically ending up with a center tapped wire. There might be a possibility that two wires (antennas) in parallel could have a phase canceling effect under certain conditions? Usually you see this with some sort of phasing harness to act as a delay line.

Maybe as fredva suggested, I have heard that the 436HP has better decoding.... But I do like having the colored back lighting on this 396

George

 

[fredva]

On first glance one would say it's a dipole but using the BNC Tee connects the two antennas in parallel basically ending up with a center tapped wire. There might be a possibility that two wires (antennas) in parallel could have a phase canceling effect under certain conditions? Usually you see this with some sort of phasing harness to act as a delay line.

George

I would struggle giving a technical explanation for why it helps. But if I'm not mistaken, using two antennas to receive one signal could potentially result in the signal being out of sync. So maybe receiving two signals that are already out of sync with two antennas puts them back in sync when they are merged? Basically what you suggested - a phase cancelling effect.

 

[WD0AKZ]

I would struggle giving a technical explanation for why it helps. But if I'm not mistaken, using two antennas to receive one signal could potentially result in the signal being out of sync. So maybe receiving two signals that are already out of sync with two antennas puts them back in sync when they are merged? Basically what you suggested - a phase cancelling effect.

Sounds reasonable to me. I did try simply putting two 3.5" wires into a BNC Tee with a BNC/SMA adapter on my 396XT and could not detect any change in signal levels or decode quality. I really wasn't expecting to see a change, but I've been playing with this kind of stuff long enough to know that there are a lot of things I don't know so I thought I'd give it a shot.

The more I try things I'm beginning to think that while Simulcast still might be my problem, multipath probably is not. Using the 50ohm load in place of the antenna reduces all signals levels so low that at that point FM should be capturing the strongest signal (direct path). Too bad that the NFM modulation is fixed and the wider bandwidth FM cannot be selected. I'm not saying the bandwidth is the problem, but at times it sure sounds like the decoded audio is clipping do to over deviation.

George

 

[NDRADIONUT]

Simulcast is the multi-multipath problem... Try the paint can antenna,,,

 

[WD0AKZ]

Simulcast is the multi-multipath problem...

Your right of course, but I was looking at it as Simulcast being multiple sourced signals transmitted with a time delay that the software should be designed to sort out (as it does in commercial units) whereas Multipath is reflections of a single source signal. I guess I'm saying that Simulcast is Multipath but Multipath is not Simulcast?

I think I should take the 396XT out of the area a ways and see if the decoding improves. I'm assuming that if Simulcast/Multipath were removed that the decoding should be 100%?

tnx

George

 

[NDRADIONUT]

Take it right up close to one of the sites and see how it does...