Comms 'Bleeding' and 'sub' channels

[Cobrajr122]

So at the latest big game, Nightfall 2, there was a lot of comms traffic on channels where it should not have been.

This has been a large issue in the past but we could work around it, but with such a high volume of people and chatter it was hard to work around and it became a largely talked about issue throughout the game.

Many people do not understand why this was happening so I thought I would post this up to try and explain why this happens and how to avoid it.

Comms 'Bleeding'
-What is it
-Why it happens
-How to prevent it

What is it
I am sure by now that just about everybody has heard it. You are listening to your radio and somebody else starts to talk who obviously does not belong on that channel. You can hear them but they cannot hear you.

Why it happens
FRS/GMRS radios operate using a 25KHz bandwidth.
For Ch 1-14 (FRS) this spacing is fine and does not cause any issue between any of them.
When you add Ch 15-22 (GMRS) this causes an issue.

Ch 15-22 operate very closely with Ch 1-7

Bandwidth - A channel uses more then one single frequency to work. To effectively transmit voice(or anything really) a chunk of frequencies are used with the carrier being the main one that it programmed into the radios.
This chunk of frequencies for FRS/GMRS is split in half and the carrier is the middle frequency.

For a 25KHz bandwidth, this means that a frequency of 462.5625 (Ch1) actually uses from 462.550 to 462.5750
462.550 and 462.5750 are the carriers for ch 15 and 16, which have their own 25KHz bandwidths as well.

This means that the channels have 12.5KHz overlaps with each other. Ch 1-7 and 15-22 all overlap with each other.

I have tried to illustrate this with a couple channels.



As seen, 1 overlaps with 15 and 16, and 2 overlaps with 16 and 17(not shown)

HOW TO PREVENT IT

There are many options to avoid this from happening. I will cover 3.

1. -Do not use Ch 15-22 or 1-7.
Instead restrict comms to 1-14 or 8-22

This ensures that there will be no bleeding and blister pack radios can still be used.
Limits the amount of ch that can be used, not good for very large games.

2. -Change your bandwidth settings to 12.5KHz instead of 25KHz on Ch 1-7 and 15-22 (This does not always work)
Not to be confused with the Channel STEPPING setting.
Also known as Narrow band. (Narrow - 12.5KHz, Wide - 25 KHz)
This will eliminate all overlapping, but slightly degrade audio quality.

This ones only works if everybody has non blister pack radios, which, as anybody who has read my game comms briefs knows I advocate.

I know this is possible on the Lintons and Puxings I own, but not sure about any other brand. It seems to be a popular option to add to these radios so I would assume most have it.


I am sure there are a few other things that can be done, but these are just a couple options that will work just fine.

I have not covered the difference between the CDN and US frequencies because we all use the US ones anyways. It also seems that no matter how hard I try, Blister pack radios will still be at massive mil sims like this, and you can ONLY get those with US frequencies.


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'Sub' Channels

Sub channels are bad and they should feel bad.

-What are they
-How do they work
-Why are they bad

What are they

Sub channeling is a method used to block all incoming traffic on a particular channel that does not contain a specific squelch tone.

How do they work

These sub channels operate using a squelch tone.

Squelch - Without squelch, you would constantly hear noise. Squelch turns off your receive circuit until it is triggered either by a specific signal, or a signal with X strength. At this point, it opens up your receive circuit until the tone/signal stops.

While the squelch is open, you can hear EVERYTHING on the frequency you are tuned into.

With FRS/GMRS there are two different technologies used to create 'sub channels' using squelch.
1. CTCSS
2. DCS

They are both quite different in their nitty gritty operation, but achieve the same result.

With CTCSS ch 1 (67 Hz) enabled, your radios squelch will only open up your receive circuit when it receives a 67 Hz tone.


Why are they bad

They are bad because it gives you the false illusion that you are on a private channel and can talk whenever you want without interfering with anybody else.

For this example I will be using CTCSS squelch tones.

---

Scenario:
-You are allotted FRS Ch 2.
-You want to have 3 nets
- Sect 1
- Sect 2
- Sect 3

FRS - 2 - Sub 1 = Sect 1
FRS - 2 - Sub 2 = Sect 2
FRS - 2 - Sub 3 = Sect 3

Radios will be programmed as such.

Sect 1 = 462.5875 MHz CTCSS 1 (67 Hz)
Sect 2 = 462.5875 MHz CTCSS 2 (71.9 Hz)
Sect 3 = 462.5875 MHz CTCSS 3 (74.4 Hz)



A member of sect 1 keys up to talk. All member of sect ones radios are activated and all sect 1 members can hear. Sect 2 and 3 do not hear this traffic.

Because members of Sect 2 and 3 do not hear sect 1 traffic, they do not know that there is traffic being passed. Therefor they do not know that the net is in use and not to key up.

A member of sect 2 keys up. All Sect 2 radios are activated.
A member of sect 3 keys up. All sect 3 radios are activated.
All members of sect 2 and 3 can now hear all traffic - This is a collision - bad.

A member of sect 1, 2 and 3 all end up keying up during the same transmission.
More collisions, more bad.

Because the transmitting stations can't hear the net, they do not know that they are talking at the same time as others. 2 people attempting to talk on the net at the same time is incoherent.

---


Feel free to add to this or ask questions.

[Drake]

Should be noted: when programming Chinese radios there's a Freq Step setting which can be set (among other settings) to 12.5 and 25kHz: these are NOT the bandwidth settings, only the "step size" between frequencies when turning the knob. The setting you want to adjust is Wide/Narrow (Wide = 25kHz, Narrow = 12.5kHz). For all programmable radios and most new blister pack radios Narrow band will work. Some older FRS-only radios may have trouble with this: buy a better radio. Srsly.

Another thing to consider is transmitting on Low power. Most people are always set to transmit on full 4W but it isn't always required (in fact it often isn't required). Most programmable radios have 100+ programmable channels, its easy to program two sets of 22 (FRS+GMRS) channels, one in Low Tx Power and another in Full power. I usually keep the high power channels on 101+ for easy acces, i.e., if I want to go on channel 12 in high power I can just enter 112.

Excess power will cause bleeding.

Blister pack radios will be 2W Tx max so its not an issue.

[Six By Ten]

Excellent post, and this explains a lot thanks for the info!

[Kokanee]

Excellent post Matt, this deserves a sticky.

[Cobrajr122]

Originally Posted by Drake

...
Excess power will cause bleeding.
...
Blister pack radios will be 2W Tx max so its not an issue.

Unless the radio is poorly designed or poorly/not filtered, "higher" powers will not cause this, especially at such a low power (4W)

However, even with a bndwidth setting of 12.5KHz there is still no buffer between ch, so when I get off course I'll hook up my puxing to a frequency analyzer to see what it looks like vs my Motorola XTS2500.

It was also my understanding that blister pack radios were limited to 0.5W, I could be wrong though.

Originally Posted by Kokanee

Excellent post Matt, this deserves a sticky.

Thanks!

[Reignman]

Excellent man. Makes sense. I will be conferring with you and FlyGuy in regards to channels next year for NF3. Hopefully we can lick the issue.

[lurkingknight]

here's a question though, if game admin acquires the use of the frequencies outside the frs/grms range, does that allow anyone at the event to use those frequencies, or only those with hamm radio licenses/certificates to broadcast on those frequencies?

[Cobrajr122]

Originally Posted by lurkingknight

here's a question though, if game admin acquires the use of the frequencies outside the frs/grms range, does that allow anyone at the event to use those frequencies, or only those with hamm radio licenses/certificates to broadcast on those frequencies?

To be 100% certain I would have to defer the question to flyguy (I'll draw his attention to the thread) but from my understanding, anybody the licence holder has given permission to can use the frequency.

[lurkingknight]

ultimately though, getting a hamm radio license isn't all that difficult from what I've been told, one of my local car clubs offers a course for certification to use at rally events, all volunteers at those events (canadian rally circuit) need to have completed the course.

[Drake]

Originally Posted by Cobrajr122

Unless the radio is poorly designed or poorly/not filtered, "higher" powers will not cause this, especially at such a low power (4W)

Most people here are using cheap UHFs and I've witnessed it first hand (couldn't tell you which brand radio it was though). My Puxing bleeds a bit at full power / close range.

Curious to read the results of your experiments with real measuring equipment.

It was also my understanding that blister pack radios were limited to 0.5W, I could be wrong though.

As I understand it they're 0.5W on FRS channels but upto 2W on the 8 GMRS-only channels (this was the Cobra GMRS radios IIRC).

[FOX_111]

FRS channels are 0.5W and GMRS channels are 1W and sometime, rarely 2W.
When you select a FRS channel on a "GMRS" radio, it will transmit to 0.5W automaticly.

That need to be taken into account when distributing channels to teams.
If you use a programable radio, it won't mather unless you try to reach a "noob" with a FRS/GMRS. You won't hear him.

[Cobrajr122]

Added why sub channels should feel bad.

Originally Posted by FOX_111

FRS channels are 0.5W and GMRS channels are 1W and sometime, rarely 2W.

GMRS can actually get up to 5W, in the US anyways, its so hard to find the CRTCs regulations on this stuff.

[Hectic]

Here they mention 2 wats alot not sure if they use that just as an example or if they use that because its what they have restricted frs/gmrs radios to.
Seems like crtc isnt the governing body but industry canada is.
http://www.ic.gc.ca/eic/site/smt-gst...g/sf08450.html
Ill see if i can find more info about power resrtictions as this is just what turned up in a search about frs/gmrs reguoations in canada

Edit.
Here we go. http://www.ic.gc.ca/eic/site/smt-gst...10211.html#sA6
A6.2.4 Output Power

A*GMRS*transmitter may transmit with a maximum power of*2.0*We.r.p.

The same document says 0.5w for frs

So does this mean running my puxing on high power is a no no or im good provided im outside of the city?

[FOX_111]

It's a legal no no. You need a license to operate a 4watt emitter.
But like you mentionned, outside of the city, out of reach of the crowd using FRS to chit chat at the mall.... I'd be suprised if it bothered anyone.

[Hectic]

Thats what i figured. So long as im not near a radio station or the cop shop or near a large population i shouldnt cut into/block anyones transmisions.
Like they say "it aint a crime if ya dont get cought" lol