Worcester Radio Systems Issues Continue

[Dispatcher308]

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]

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.

 

[wildbillx]

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...ations_guide_for_the_fire_service.pdf#page=59

 

[emtLarmy15]

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]

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]

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]

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...ations_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]

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.

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Other basic info

Inverse-square law - Wikipedia

en.wikipedia.org

Radio propagation - Wikipedia

en.wikipedia.org

Wireless, RF, inverse fourth power law vs inverse square law

When, in wireless communications, does the inverse fourth power-law become relevant? My understanding is that is that what cause the average signal power to degrade to the forth power is cancellation from self reflections. So by my way of thinking, an LOS point to point system, like a...

www.physicsforums.com

Okumura model - Wikipedia

en.wikipedia.org

 

[DisasterGuy]

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]

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]

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?

 

[boatbod]

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.

 

[DisasterGuy]

Nothing solid or formal at this time. Yes, the 800MHz NPSPAC frequencies you see in CAPRAD are being evaluated by the county (24/7 logging and recording) for cleanliness thus the reason they are currently reserved. Multiple entities with substantial experience in spectrum management are currently involved in assisting with identifying spectrum, analyzing logged and recorded activity and determining the best path forward. Atmospheric induced interference has been significantly increasing over the last 2-3 years and finding interference free spectrum is quite a challenge.

 

[Dispatcher308]

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.

He meant in Worcester County, Talbot County/MFIRST is not experiencing ducting as badly in that region as Worcester County is!

 

[ocguard]

And all of the sudden, 46.18mhz seems like a really happy memory!

 

[ocguard]

There is a thing called simplex for in building fire operations. Why people insist on using TRS or any repeated system for interior work is beyond me. You'll never guarantee interior coverage and there could possibly be system busy outs on a large scale incident.

Stupid, just stupid.

All fine and well if you're talking about fire ground operations, where someone with a radio on the same simplex frequency is at a fixed command post on the exterior to answer you.

But now consider a police officer requesting emergency backup or an EMS provider who's become entrapped in a violent scene. So while I, and the industries, agree with the use of simplex for interior IDLH communications, those aren't the only situations to consider.

 

[dcr_inc]

with law suits and transparency being so rampant. the ablitity to have a recorded account of the event is parimount. Not gonna happen with talk around, in building use.. Build the system to REAL LIFE specs and the rest will take care of itself

 

[dwlipp]

Build the system to REAL LIFE specs and the rest will take care of itself

Truth, with a tweak: "Build the system to REAL LIFE specs and most of the rest will take care of itself". Even with a very well designed system users will sometimes need to fall back to a simplex resource, but if a system is designed and implemented properly simplex should be the exception, not the rule.

 

[dcr_inc]

Agreed dwlipp.. But try to get anyone to A.) Learn how the radio works B.) Learn to work it.. Moving ANYTHING besides the volume control is a MAJOR strain on most public safety folks.. Can't figure that out, they can drive a multi systems vehicle with several sirens and gadgets and can create special functions and sounds but can't figure a channel selector out.. Radio operations drills should be just like Fit testing and Fire arms qualification.. at least once a year, think outside of the box.. After all, you rely on the radio more than anything when your arse is in the corner.. Just my .02

 

[maus92]

with law suits and transparency being so rampant. the ablitity to have a recorded account of the event is parimount. Not gonna happen with talk around, in building use.. Build the system to REAL LIFE specs and the rest will take care of itself

Can to clarify / expand on what you mean by "real life" specs?