• Effective immediately we will be deleting, without notice, any negative threads or posts that deal with the use of encryption and streaming of scanner audio.

    We've noticed a huge increase in rants and negative posts that revolve around agencies going to encryption due to the broadcasting of scanner audio on the internet. It's now worn out and continues to be the same recycled rants. These rants hijack the threads and derail the conversation. They no longer have a place anywhere on this forum other than in the designated threads in the Rants forum in the Tavern.

    If you violate these guidelines your post will be deleted without notice and an infraction will be issued. We are not against discussion of this issue. You just need to do it in the right place. For example:
    https://forums.radioreference.com/rants/224104-official-thread-live-audio-feeds-scanners-wait-encryption.html

Worcester Radio Systems Issues Continue

Dispatcher308

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How is it possible for a signal from 90 miles away to cause problems to a TRS? Seems nearly impossible.
How is it nearly impossible, do you understand Tropospheric ducting? We get Tropospheric interference on our 6Ghz Microwave links from time to time which is a directional path and is a much higher frequency than the 700/800mhz spectrum. Nothing is Impossible!
 

zerg901

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AFAIK ducting does not make signals stronger. So a signal coming from 90 miles away should be minuscule in comparison to any local signals. Then the capture effect takes hold.
 

emtLarmy15

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Moving forward, I hope they can find alternative or additional channels that are less susceptible to ducting interference. Thinking about it, does operating in TDMA mode make you twice as vulnerable since the tech essentially combines two talkpaths into one channel? Are they talking about leaving the 800 band altogether? I wonder if the system could detect interference, and then adjust channel usage "on the fly" to account for current conditions (when ducting is present and your "neighbor" is using the same channel?) Might be a growth area if not...

Anyway, there seems to be a rush to the rebanding / rechannelization because the FCC has some sort of deadline / meeting coming up in the end of September.

Funny you should ask about TDMA. Most primary public safety talk groups (LAW,FIRE,EMS) are all being brought down to phase 1 as it appears to handle the interference better than phase 2...
 

maus92

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AFAIK ducting does not make signals stronger. So a signal coming from 90 miles away should be minuscule in comparison to any local signals. Then the capture effect takes hold.
The county with the assistance of Federal Engineering have been conducting interference testing for several months, and apparently their findings differ from your opinion.
 

maus92

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Funny you should ask about TDMA. Most primary public safety talk groups (LAW,FIRE,EMS) are all being brought down to phase 1 as it appears to handle the interference better than phase 2...
Interesting. I mentioned TDMA in the sense that the channel carries two talkpaths, and channel interference could conceivably kill two separate conversations instead of one. I wasn't suggesting that that TDMA is somehow more vulnerable to interference, but maybe it is. Note that the ORION system in VA Beach is FDMA.
 

ResQguy

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I stand corrected, it was the NFPA 1221 that they quoted in the guide. Thank you for the correction. Page 24 has the info. https://www.usfa.fema.gov/downloads/pdf/publications/voice_radio_communications_guide_for_the_fire_service.pdf#page=59
Still incorrect, this document which refers to 1221 still underscores that is a requirement that an analog simplex channel be provided for use, not that it shall be used.

Many fire agencies that use P25 trunking place an analog channel or two among their home zones to accommodate this.
 

zerg901

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I am searching for a simple explanation of range vs power levels - not finding any simple info - so here is some general info

Understanding Wireless Range Calculations - per chart, signal at approx 10 km goes to minus 110 dB

How to measure signal strength in Decibels on your cell phone? - per diagram, minus 110 dB is "no signal"

Distance per Watt? - ground bounce signals can add gain

Based on those 3 items it looks like signals become very weak after 10 KM.

==============================

Other basic info




 

DisasterGuy

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I am searching for a simple explanation of range vs power levels - not finding any simple info
There is far more to this than what you imply, particularly from your previous post. In simple terms, consider that a typical handheld satellite telephone has a maximum power level of 400mW to communicate with a satellite that is about 900 miles out in space. There is nothing to inhibit it except for free space loss and atmospheric particulate matter (mostly water vapor).

Also be careful of your sources, for example the one lists a level of -110dBm as an absence of signal. In reality in a sterile environment a level of -110dBm can produce DAQ 3.4 results in P25.

To understand tropospheric propagation (ducting and scatter) imagine that you have an air conditioner duct that is built using mirrors. You shine a light in one end and get almost as bright of a light out the other. That is ducting. For scatter imagine that you cover your ceiling in tin foil and then shine a light up on it and it reflects back illuminating the room.

I can tell you that during periods of tropospheric propagation that Worcester Countys EDACS system has been received on scanners in Massachusetts and last year the P25 system was received on a G5 pager in the outer banks. I can also tell you that there isnt a single available frequency that is 100% clear when received in Worceater County with regular reception of mobile and portable radios from hundreds of miles away at times.

When the troposphere is at its peak activity TV and FM broadcast signals are k own to travel 2000 miles or more. Last summer people in NY and PA were able to receive an FM station from St Louis on car stereos.
 

maus92

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Small thought experiment: if the Worcester system is suffering from tropospheric ducting, would the reverse be the same? And would that not incentivize VA Beach to help with re-channelization by moving to (relatively) interference-free channels as well?
 

maus92

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Browsing though CAPRAD data, there are five 800Mhz channels in the 851-854 Public Safety only range are being considered for Worcester County to acquire. Not sure if they will be enough since Worcester currently has eight in their system, but the county did say that 4-5 of their current channels are responsible for 90% of the interference they are experiencing. Worcester's current channels are in the interleaved space in the upper 855-859 range - not sure if that makes a difference. There are also 8 or so 700Mhz channels "reserved" for Worcester County users that are currently unassigned, but not sure how current CAPRAD data actually is. And perhaps there are channels available in the interleaved space, but I guess CAPRAD doesn't show those channels?
 
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boatbod

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Funny you should ask about TDMA. Most primary public safety talk groups (LAW,FIRE,EMS) are all being brought down to phase 1 as it appears to handle the interference better than phase 2...
Well darn, you better tell Talbot Fire, EMS, LEO, MSP, MD NRP they got it all wrong 'cos they're TDMA all the way unless a junky old radio affiliates and pulls the tgid back to fdma.
 
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