Multipath / Simulcast Isuues

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jperkins157

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I just want to share that I have had a significant improvement of reception and clarity of the BRICS P25 system, with my BCD396XT with a Stubby antenna. The shorter the better. It has drastically reduced the Multipath and simulcast issues i was previously experiencing. I have two of the stubbys , both are availible in BNC and SMA connectors and purchased from R and L electronics in hamilton. One is the Diamond SRH805S antenna and the other is a maldol AS-25. My understanding is that the butler county system is a simulcast system with 14 towers. The reciever in the scanner has to pick which of the 14 towers is the strongest signal, switching between towers several times a second, the user hears a Pixelated signal, I dont know the specifics but if you use a smaller , antenna then your scanner will recieve only the strongest singals and not have to work so hard to determine the stongest signal. I know this is a common problem with p25 simulcast systems. I just wanted to share my experience with the group. i have owned RS pro-106 , GRE psr-500 and now the BCD396XT and have had similar preformance from each. A $17.95 solution. 73's KD8GUU

With a simulcast system, your scanner more than likely has to determine the strengths of mulitple towers, if your using an antenna that has gain, then your scanner has to work harder to determine which tower is the strongest. If your using a stubby antenna ( racing antenna) then your scanner may only hear one or two towers and it will not have to work as hard.

End results : Much better Decoding and a Much less pixelated signal.
 

wa8pyr

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With a simulcast system, your scanner more than likely has to determine the strengths of mulitple towers, if your using an antenna that has gain, then your scanner has to work harder to determine which tower is the strongest. If your using a stubby antenna ( racing antenna) then your scanner may only hear one or two towers and it will not have to work as hard.

End results : Much better Decoding and a Much less pixelated signal.
Joe,

Thanks for the first hand report! All too often many people feel that more (antenna) is better, but in this situation less is more. Good to hear some confirmation reports from the field.
 

beischel

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I have the Rat Shack 800 Mhz antenna on all three of my scanners. I really do not notice that much of a difference. However, the scanner that is in the basement seems to perform a lot better because of the natural signal attenuation occurring because the antenna is below ground level and also the scanner attenuator is turned on only - not Global - for the P25 Hamilton County system.

Duffy
 

rdale

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With a simulcast system, your scanner more than likely has to determine the strengths of mulitple towers, if your using an antenna that has gain, then your scanner has to work harder to determine which tower is the strongest.
Your scanner doesn't do any work... It just listens. If you have too much signal the stubby helps, but most systems don't have that problem. So good that it works for you, but the reasoning is a little off ;)
 

GrumpyAeroGuy

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I have the Rat Shack 800 Mhz antenna on all three of my scanners... and also the scanner attenuator is turned on only - not Global - for the P25 Hamilton County system.

Duffy
This is interesting. I, too, have the RS 800antenna. My Hamilton County reception was not very good at all.... UNTIL I enabled attenuation for HC P25 (did it in the TSYS settings of the PSR500 for HC alone). It is now much better, though still suffers from simulcast issues, but not nearly as badas it was before I started experimenting.

I generally have the scanner set with "Multisite OFF" and "Check all CCs on every pass", and ATT to on for this system. It seems that this provides me the best reception.

I also have to add that I am actually a mile or two north of the HC Warren COunty Line in Warren COunty, so, I am sure that doesn't help either.

I would be very interested in experimenting around with one of the stubbys the OP mentioned, just to see if I can detect an improvement with the stubby with and without attenuation enabled, etc. etc.

For the OP: you purchased those at RL ELectronics(?)... i smell a road trip this weekend. Which of the two(?) you mentioned would you recommend if I were to try ONE of them intially??

Enquiring minds want to know....:D
 

rdale

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You would never want multisite or check all CC's ON, since a simulcast only has one CC and one "site"
 

GrumpyAeroGuy

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well, I'll switch that over and see what happens. Not saying that I understand all of the details, but am not avert to trying and correcting what I've done....
 

wa8pyr

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You would never want multisite or check all CC's ON, since a simulcast only has one CC and one "site"
Not true, in this case. The Hamilton County/Cincinnati system is a networked P25 system with two separate simulcast cells, Cincinnati and Hamilton County. Talkgroups do roam between the two; with multisite and check all CCs enabled and all control channels entered, the scanner can monitor both sites, since they're part of the same system.
 

rdale

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I don't think that's considered a full simulcast then... Too many topics mixing in one unlabeled thread ;)
 

jackj

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My understanding of the simulcast problem is:

1. A simulcast system consists of multiple sites or towers but only one control channel.
2. Each traffic channel broadcasts from all sites at the same time.
3. A traffic channel broadcasts the same traffic from all sites at the same time.
4. A digital signal consists only of ones and zeros.
5. A one is the presence of a signal while a zero is the absence of a signal.
7. Since you can't detect something that isn't there, a zero is detected by measuring the time between ones.

The problem with simulcast systems is that the propagation delay (tower to scanner) will vary depending on the distance from each tower to your scanner. If the signal from two towers gets too far out of step then your scanner could detect a one from tower #1 where a zero from tower #2 is in the data stream. Multipath distortion will add to this problem.

One solution is to reduce the sensitivity of the scanner enough so that you only pick up one tower. This will work well with stationary scanners but might make the distortion worse on a mobile scanner. The only truly effective solution is for the scanner to use the parity info to reconstruct the data stream. But this requires much more computing power in the scanner and computing power equals money.

If I'm wrong in any of this then I'm sure I will be corrected.
 

wa8pyr

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I don't think that's considered a full simulcast then... Too many topics mixing in one unlabeled thread ;)
It's actually the equivalent of two full simulcast systems. Each is a true simulcast site, and they're networked together for better coverage, interoperability, etc. It's also very appropriate to this thread, since reception of a simulcast site is the topic, and instead of one simulcast site there are two. This just complicates the reception issue.

The problem insofar as reception is that sometimes a talkgroup which might normally be on one site all of a sudden moves to the other site as the users wander between the coverage areas; to leave one site out of your scanner programming, or to set Multisite and Check All CC off, means there is a good chance you'll miss communications when talkgroups move between sites.
 
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Since Lucas County switched from a two tower analog sytem to a 12 site simulcast system, my scanner has been worthless with nearly contstant crash & burns of the audio and deocding values going from 90% to 30% to 0% to 70% in 5 seconds time. I've tried an 800MHz yagi with decent cable, trying to point to only one tower, and had no sucess. Maybe I'll try the stubby and see if I have a snowball's chance in heck of it getting bacl to at least hearing half the traffic consistantly - otherwise this has been the most frustrating lack of reception I've ever, ever had since my radio listening /DXing/ ham / PS band days started 36 years ago from BC/HF/VHF/UHF to 800 P25.. This is the only system I've never been able to deocde worth while, and I doubt that they sell a Motorola that could listen to these talkgroups at a semi-reasonable price.
 

GrumpyAeroGuy

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It's actually the equivalent of two full simulcast systems. Each is a true simulcast site, and they're networked together for better coverage, interoperability, etc. It's also very appropriate to this thread, since reception of a simulcast site is the topic, and instead of one simulcast site there are two. This just complicates the reception issue.

The problem insofar as reception is that sometimes a talkgroup which might normally be on one site all of a sudden moves to the other site as the users wander between the coverage areas; to leave one site out of your scanner programming, or to set Multisite and Check All CC off, means there is a good chance you'll miss communications when talkgroups move between sites.
My obervation has been, and I do not have sufficient knowedge to understand why, that when I program BOTH sites, my reception is in the basement. If I limit my programming to the HC site, AND set up my scanner for stationary, AND have attenuation for this site set to "On", I seem to get the best results.

Again, not sure this makes sense, but my powers of hearing tell me that this works best (for whatever reason).
 
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