AM or FM MODULATION FOR OFF-ROAD TRAIL COMMUNICATION

[Nokones]

About a month or two ago, there was a discussion on that the CB Radio FM Mode may be something that should be looked at to consider bringing back the use of the CB radio for uses such as Off-Road Trail Comms. I stated that the CB FM Mode would probably propagate or provide better communications than the CB with AM modulation, and even better than FRS.

Some Forum members disagreed but no one could cite any facts or back up their statements. I related that I think the CB FM modulation would have a better signal-to-noise ratio thus, better communication Farz.

I also stated that as time permits, I would conduct a radio comparison test. Well, that day came, and I conducted the subject test comparing the communications with FRS, CB AM, and CB FM. I kept the test simple, and I did not record any RSSI (Radio Signal Strength Indicator) readings. I just did the ol’ hoot & howler type test to see if you can hear the communication and determine the usable distance and audio quality between the three subject radio transmissions.

The test was conducted essentially in a desert environment on flat terrain, with a slight dip in elevation for a dry creek bed and a test point was conducted in that dip, consisting a few cacti, pucker bushes, and giggly weeds, with no other obstacles.

A 2007 Chevrolet Avalanche was the Stationary Transmitting Point using a Cobra 29 LTD Classic Nightwatch AM/FM CB Radio with a Firestik II 4 Ft. Antenna in the center of the roof. CB Ch. 16 was used for both the AM & FM modes and the antenna was tuned at less than 1.1:1 VSWR. The CB Radio operated in the AM Mode with a Dead Carrier, no modulation, at 3.5 RF watts and was drawing 14.01 DC volts at 1.12 amps; and 3.9 RF watts with modulation, at 14.00 DC Volts and was drawing 1.46 amps. In FM mode, the radio was emitting 3.7 RF watts with essentially no difference in power draw.

The data collection vehicle was a 2023 Jeep Wrangler Rubicon using a Cobra 25 LTD Classic CB AM/FM radio with a Firestik II 5 Ft. Antenna tuned to less than 1.1:1 VSWR and the antenna located on the rear Spare Tire Carrier Bracket near the passenger side of the vehicle.

The data collection consisted of collecting test data every ¼ mile driving away from the stationary vehicle. The test concluded at the 3.25 mile checkpoint. The data collection vehicle operated the radio squelch level at the threshold of squelching out the noise.

The FRS radios were two Midland LXT radios with fully charged batteries. The stationary vehicle transmitted on the FRS radio from inside the vehicle as a user would be doing on an actual off-road trail run and the test collection vehicle was receiving the communication transmission from inside the vehicle as an user would be doing on an off-road trail run.

The RF output power was not checked due to the inability to measure.

The FRS became non-existent passed the ¾ mile mark.

The CB AM signal was good and quiet up to the 1 mile mark. The next ½ mile, the signal was readable but RF noisy. At the 2 & 2 ¼ mile marks the signal was barely readable. At the 2 ½ mile mark the signal was non-existent and covered by possible skip interference.

In the FM mode, the signal was very good essentially full quieting for the first 1 ½ miles. From the 1 ¾ mile mark to the 2 ¾ mile mark, the signal was readable with some RF noise. At the 3-mile mark, the signal was very noisy but readable. At the 3 ¼ mile mark, the signal was very noisy and unreadable.

In conclusion, the FM signal transmissions outperformed the AM signal transmissions.

Based on these results on what would be the best Off-Road Trail Comm., I think the FM CB transmission has a good chance of being a very good trail comm. I think a test on an actual off-road trail with trees/foliage would be warranted to see what would be best under trail conditions.

There is no doubt that any VHF or UHF mobile radio with RF output power higher than an FRS would perform better. I think that I just may conduct that test once I get my XTL VHF radio installed in my Jeep which will occur in the near future so I can conduct a comparison radio test between the VHF & UHF radios on the trail.

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[Mark75H]

Sounds like that Cobra is a VERY good radio. I would be interested in results from other brands and models

 

[Ratbert]

Thanks for doing that testing!

To summarize: In a flat environment FM CB has better range than AM CB.

FRS pretty well sucks. I wouldn't think that anyone would suggest otherwise. I'm guessing you didn't have GMRS with antennas comparable to those configured for your CBs.

Note that TrailRecon did some sponsored MidlandUSA tests where he was about 11 miles out when using a 6db gain antenna.

 

[Nokones]

Thanks for doing that testing!

To summarize: In a flat environment FM CB has better range than AM CB.

FRS pretty well sucks. I wouldn't think that anyone would suggest otherwise. I'm guessing you didn't have GMRS with antennas comparable to those configured for your CBs.

Note that TrailRecon did some sponsored MidlandUSA tests where he was about 11 miles out when using a 6db gain antenna.

My Jeep GMRS radio is a Motorola XTL5000 Remote radio. We used that radio through my repeater as a communication coordination channel.

I'm about to install a Motorola XTL2500 VHF remote radio in my Jeep in the next couple of days and I intend to do a comparison test between the UHF and VHF radios on an actual off-road trail. Results to follow.

 

[bd100]

I've had GMRS travel down a canyon, bounce off a mountain, and travel up the next canyon over, and we could talk even with a mountain ridge between us. Weird stuff sometimes. It would be so cool if we could visually see the signals flowing through the mountain valleys and spreading out.

I bet you'll get nice results with GMRS and the external antenna. For my vehicle the antenna is a small 1/4 wave mag mount on a stick-on metal disc on the hood where it has a few inches metal on three sides, and I added a copper wire ground radial on the fourth. Also cut back the thin RG176 to only 1 foot to get just under the hood, and ran the rest of the way with RG58, cut to length.

 

[Nokones]

This is the antenna setup I use for my UHF radio. It actually performs very well. I actually got some very good farz with it out in the boonies on the trail.

 

[grimmjeeper]

It's no surprise that FRS was inferior. It has a 0.5W power limit and a requirement for a handheld unit with a fixed antenna.

I would be interested in seeing a test with GMRS. You can even keep it at 5W (most radios will do this) to make it somewhat competitive with CB at its max of 4W.

But then try a higher power GMRS, since you can do that with a license.

I don't doubt that FM CB is far superior to AM CB. But let's see all the options on the table for a more thorough comparison.

 

[dragoneggs]

It's no surprise that FRS was inferior. It has a 0.5W power limit and a requirement for a handheld unit with a fixed antenna.

I would be interested in seeing a test with GMRS. You can even keep it at 5W (most radios will do this) to make it somewhat competitive with CB at its max of 4W.

But then try a higher power GMRS, since you can do that with a license.

I don't doubt that FM CB is far superior to AM CB. But let's see all the options on the table for a more thorough comparison.

I’m quite impressed with my MXT575 GMRS 50watt radio. I have communicated with handhelds at least 10-15mi away... granted a decent line of sight. Through a few trees but no hills.

 

[bugnuker]

Frs and gmrs are essentially the same thing, but frs is capped at 5w and you didn't need a license.

Gmrs is capped at 50w for some eligible frequencies. Gmrs and frs can communicate with escorted without issue on the 5w, low power channels.

Cb is ... Dying or dead. The range sucks. No wide spread support for hardware. Easter Jeep safari will eventually move to gmrs but the hold up is lack of channel space. Instead, privacy codes are needed for gmrs but no one can ever figure them out so people just deal with cross talk.

I have at least one of every radio. Gmrs is most prevalent at Jeep meetups. Ham is sometimes used and I've only seen cb at Easter Jeep safari, but even then, they had gmrs going as well.

I have a Midland 50 watt installed in my Jeep. I'm using a 6db antenna right now and this let's me talk quite far. I have a 7.5 db antenna as well and that goes even farther depending on terrain. My brother has a Midland 20w with a 3db antenna and we can talk about 7 miles apart. The main limitation is his tx power of 20w otherwise it would be farther.

In the end, it all depends on the channel you are using, if it supports 50w, if your radio supports 50w, the antenna you are using and the terrain you are in.

Gmrs is the new tech, cb is the old tech.

Ham is fun for geeks and nerds like my self.

 

[Johnny B]

Frs and gmrs are essentially the same thing, but frs is capped at 5w and you didn't need a license.

Gmrs is capped at 50w for some eligible frequencies. Gmrs and frs can communicate with escorted without issue on the 5w, low power channels.

Cb is ... Dying or dead. The range sucks. No wide spread support for hardware. Easter Jeep safari will eventually move to gmrs but the hold up is lack of channel space. Instead, privacy codes are needed for gmrs but no one can ever figure them out so people just deal with cross talk.

I have at least one of every radio. Gmrs is most prevalent at Jeep meetups. Ham is sometimes used and I've only seen cb at Easter Jeep safari, but even then, they had gmrs going as well.

I have a Midland 50 watt installed in my Jeep. I'm using a 6db antenna right now and this let's me talk quite far. I have a 7.5 db antenna as well and that goes even farther depending on terrain. My brother has a Midland 20w with a 3db antenna and we can talk about 7 miles apart. The main limitation is his tx power of 20w otherwise it would be farther.

In the end, it all depends on the channel you are using, if it supports 50w, if your radio supports 50w, the antenna you are using and the terrain you are in.

Gmrs is the new tech, cb is the old tech.

Ham is fun for geeks and nerds like my self.

GMRS is the new tech, now THAT is some funny stuff right there.

 

[Nokones]

It's no surprise that FRS was inferior. It has a 0.5W power limit and a requirement for a handheld unit with a fixed antenna.

I would be interested in seeing a test with GMRS. You can even keep it at 5W (most radios will do this) to make it somewhat competitive with CB at its max of 4W.

But then try a higher power GMRS, since you can do that with a license.

I don't doubt that FM CB is far superior to AM CB. But let's see all the options on the table for a more thorough comparison.

The FRS radio was using a 2 watt channel, Ch. 19. The half-watt output limit only applies to Channels 8-14 which are the 467 MHz interstitial channels and by rule can only transmit .5 watt ERP whereas channels 1-7 (462 MHz interstitial channels) and Channels 15-22 (462 MHz main Channels) are allowed 2 watts ERP for the Part 95, subpart B Type-Accepted FRS radios.

 

[roaniecowpony]

Thanks for doing these tests. Gives some perspective on which way to go for some people. For my part, I went with GMRS 5 watt handhelds to fit in with most groups I run with or bump into. Range hasn't been in question ever, since it's more about keeping together and discussions about route changes. I'm mostly a listener, since a lot of chatter on the radio is unnecessary anyway.

 

[Terrymo]

Thanks for doing these tests. Gives some perspective on which way to go for some people. For my part, I went with GMRS 5 watt handhelds to fit in with most groups I run with or bump into. Range hasn't been in question ever, since it's more about keeping together and discussions about route changes. I'm mostly a listener, since a lot of chatter on the radio is unnecessary anyway.

Someone in the group always thinks they need to be the tour guide on Disneys Jungle Cruise

 

[grimmjeeper]

The FRS radio was using a 2 watt channel, Ch. 19. The half-watt output limit only applies to Channels 8-14 which are the 467 MHz interstitial channels and by rule can only transmit .5 watt ERP whereas channels 1-7 (462 MHz interstitial channels) and Channels 15-22 (462 MHz main Channels) are allowed 2 watts ERP for the Part 95, subpart B Type-Accepted FRS radios.

It's still half the power on a handheld with a stubby antenna inside a Jeep. That cripples the FRS radios.

A fair comparison would be CB handhelds with stubby antennas inside the Jeep as well.

Anyone who knows anything about radios knows that antenna type, placement, and tuning can vastly extend the range, or cripple it.

 

[roaniecowpony]

Someone in the group always thinks they need to be the tour guide on Disneys Jungle Cruise

...or Jerry Reed driving a load of Coors.

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