Poor reception/decoding of Philly APCO 25 system

Status
Not open for further replies.

ki1022

Member
Premium Subscriber
Joined
Aug 12, 2011
Messages
38
I am currently trying to monitor a couple talk-groups on the Philly trunked APCO 25 system. I have a BCD436HP and am using the RadioShack 800MHz antenna with SMA/BNC adapter. It seems like the beginning of every transmission is cut-off and transmissions randomly drop-out all the time (seemingly at the worst possible time...). The only way I can reliably follow a radio conversation is by holding the scanner up in the air and pointing it around like an idiot until I find the "sweet-spot" for the current transmission. Obviously this is not very practical and is very annoying. Is this antenna junk or what? I have tried the stock antenna and also another wide-band telescoping and they don't seen any better. When there are no transmissions the control-channel looks like it is reading full signal-strength so I'm not sure what else I can do. Are the police/fire/ems personnel dealing with these same issues daily??
 
Last edited:

ki1022

Member
Premium Subscriber
Joined
Aug 12, 2011
Messages
38
I also wanted to mention this was happening on top of a roof-deck taller than 95% of the surrounding buildings and with 360-degree views for miles around so I doubt it has anything to do with the location I am scanning from. (zip 19125)
 

phillydjdan

Member
Joined
Jan 27, 2011
Messages
1,840
Your scanner and antenna are not the problem. Well, let me rephrase that. There's nothing wrong with either. The issue is that Philly uses simulcast modulation on their system. That means the same signal is being transmitted from more than one tower at the same time. The public safety grade radios are built to compensate for that and thus they have crystal clear reception. The scanners, on the other hand, are not built for it, and even though they say those scanners can decode it, the bottom line is it won't. Well enough to satisfy you, anyway. The only way to remedy that is to use a directional "yagi" antenna, which won't help you if you plan on moving the scanner around. Sorry it's not the answer you were looking for, but that's the truth coming from someone who dealt with (and is still dealing with) the same problem. I've had to abandon scanners altogether for Philly and move to public safety equipment.
 

iMONITOR

Member
Premium Subscriber
Joined
Sep 20, 2006
Messages
8,184
Location
MACOMB, MI.
I'm very familiar with P25 systems using simulcast modulation, and the problems monitoring them. Nothing was worse than Michigan's MPSCS. That's what attracted me to the new BCD436HP. It seems to have overcome those issues, and receptions is excellent!

If you are in close proximity to multiple antenna sites, sometimes less is better, with regards to antennas. I'm using the Comet CH-32 Miracle Baby. Although the stock antenna works well for me, this seems to work even better, in my area. Possibly by getting less signal, from all the antenna sites, it's easier for the radio to ignore some of the more distant antenna sites.
 
Last edited:

ki1022

Member
Premium Subscriber
Joined
Aug 12, 2011
Messages
38
Thanks for the replies. I'm pretty new to radio in general and I never considered that simulcast sites could be the problem, I just figured that would help coverage! I will look into some of the antenna options mentioned above. I'm curious, how exactly are the radios in the field able to deal with the simulcast problem on a technical level?

Also, in the RR database for the Philly system it lists the same lat/long coordinates for both sites, is this a mistake?
 

phillydjdan

Member
Joined
Jan 27, 2011
Messages
1,840
Actually, the system consists of 2 zones (1 for fire/municipal, 1 for police) running side by side at each tower. The db is correct in that regard.

The public safety radios use a far superior receiver and processor that can better determine which site has the better signal and locks onto that one. It has somthing to do with timing of the signals from each tower. If they wanted to use a similar receiver and processor in a scanner, they would have to charge probably double the money for it (which is part of the reason public safety radios are so expensive)

Monoxide, thats up to you. I personally gave up...

And iMonitor does make a valid point. I was able to achieve limited success with a shorty antenna like that. They work great if you are within the city, but if you move too far away from any of the towers (ie travelling outside the city) you lose the system. Ive even had trouble in the outlying areas of the city where a tower isnt very close by.
 

Monoxide16

Member
Joined
Jul 15, 2007
Messages
502
Location
Cinnaminson, NJ
i hear ya about all that and wow and i think i give up too funny thing is i can pick up Montgomery Co loud and in the clear sorry to go off topic here but can get the east system very good but can't hear the west system that great and thank you for the info
 

bailly2

Member
Joined
Jul 29, 2009
Messages
517
Location
suburban hell
buy a yagi or build one from a stick of wood and coat hangers, and use metal screens for a window to block out one or more of the towers, the metal screens should be big, so they are blocking out signals a couple feet higher than the scanner, turn up with squelch to medium or all the way up. where i am in nj there is a simulcast system with a tower 10 miles away and another tower transmitting on the exact same frequencies that gets 3 bars with a 1 inch piece of aluminum foil antenna. both yagi and metal screens is what it takes. Lsm simulcasting is sold as needing fewer towers, but i wouldn't be suprised if its sold as a security measure against mobile scanning

yagi (my first antenna)
7 inches and 6 marks up inch piece coat hanger,
3 1/16 spacing
7 inches and 3 marks up inch coat hanger,
4 7/32 spacing
6 inches and 3 marks up coat hanger
6 31/32 inch spacing
6 inches and 4 marks up coat hanger
10 17/64 inch spacing
6 inches and 1 mark up coat hanger (point to the tower with this side)
 

bailly2

Member
Joined
Jul 29, 2009
Messages
517
Location
suburban hell
The university is in the process of switching to Motorola MOTOTRBO (DMR) equipment throughout their network, which you may be able to monitor with your scanner's discriminator jack connected to the computer running the dsd computer program, if they don't have privacy enabled.
 

jim202

Member
Joined
Mar 7, 2002
Messages
2,654
Location
New Orleans region
I am currently trying to monitor a couple talk-groups on the Philly trunked APCO 25 system. I have a BCD436HP and am using the RadioShack 800MHz antenna with SMA/BNC adapter. It seems like the beginning of every transmission is cut-off and transmissions randomly drop-out all the time (seemingly at the worst possible time...). The only way I can reliably follow a radio conversation is by holding the scanner up in the air and pointing it around like an idiot until I find the "sweet-spot" for the current transmission. Obviously this is not very practical and is very annoying. Is this antenna junk or what? I have tried the stock antenna and also another wide-band telescoping and they don't seen any better. When there are no transmissions the control-channel looks like it is reading full signal-strength so I'm not sure what else I can do. Are the police/fire/ems personnel dealing with these same issues daily??

A big problem with a simulcast trunking system is that it takes some good skill on the part of the radio techs to maintain the system. The audio delay timing between each of the different tower locations is almost the life of the system. If it isn't done correctly, you will never get good coverage in the overlap zones of coverage. The technical term for this is TDI (Time Delay Interference). Have seen a number of large simulcast trunking systems where the audio timing (TDI) was never set up correctly. The users were always having problems.

The best you can do as a passive user of the system is to find out where all the different towers are from you. Figure out which one will provide the best signal. This may not be the closest tower. Then try to lower the received level from the other sites. You can do this by lowering your antenna, using a directional antenna to try and null out the other towers and probably attenuate the signal. In your case, the lower the antenna, is the better your chances are of obtaining a clean signal for the receiver.
 

iMONITOR

Member
Premium Subscriber
Joined
Sep 20, 2006
Messages
8,184
Location
MACOMB, MI.
I tried using both a yagi, and later, a corner reflector antenna. Neither was as effective for nulling out rear signals as I hoped for. I was still picking up Warren, MI PD off the back, like they were right down the road. They were about 13 miles South of me!
 

ki1022

Member
Premium Subscriber
Joined
Aug 12, 2011
Messages
38
Here is my elegant solution. It's not pretty, but it seems to be working a lot better. And it was free since I had the 90-degree adapter laying around.

update: So it was probably my imagination because now that I've been listening for a while it seems pretty much the same...haha, I guess it was worth a try.
 

Attachments

Last edited:

policefreak

Member
Joined
Jan 5, 2007
Messages
1,836
Location
Berlin, NJ
To the OP, are there any cell towers within plain view (up to 1/2 mile of your location)? These can also play havoc with 700/800 receltion, especially so high up.
 

ki1022

Member
Premium Subscriber
Joined
Aug 12, 2011
Messages
38
To the OP, are there any cell towers within plain view (up to 1/2 mile of your location)? These can also play havoc with 700/800 receltion, especially so high up.
There's a T-Mobile tower about 4300 ft away line-of-sight..but I thought they work on ~1800MHz? And I was only up high once or twice to test out. My normal listening location is upstairs in a normal house.



Also when scanning talk-groups the signal bar is is full but keeps disappearing about once per second? Is this normal? Is there a way to tell when I have positioned the antenna in such a way that is not being affected by the multi-cast interference?
 

n1chu

Member
Premium Subscriber
Joined
Oct 18, 2002
Messages
718
Location
Farmington, Connecticut
A bit late, but have you tried antenuating those channels/talkgroups in question? The pic of the scanner with the multiple connectors, allowing the antenna to fold over is antenuation. Each additional connector added to the antenna input antenuates the signal. As previously stated, "Sometimes less is better!"

Bill
 
Status
Not open for further replies.
Top