What Would You Choose For Car To Car Simplex Communications?

AK9R

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Depending on terrain, you may also consider 50 MHz SSB. Yes, the antenna is bigger, but it can cover a lot of territory. I know this because I used to work for a water utility and we had licensed frequencies on 44 and 48 MHz.
Sideband? The water utility you mention was probably using FM.
 

W8HDU

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Distance wise, (and this was proved last week on a trip), nothing beats low band out on the highway. Caveat: 25 watts or more into a steel whip on the vehicle. Folk should keep in mind that frequency, power, and modulation standard are irrelevant if you can't get that signal cleanly off the antennas radiating on the horizon, and the same applies for receiving.
 

vagrant

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I would not use 70cm as a good (insert laugh here) rubber duck antenna is too long, when vertical, to use in a regular vehicle. For example, the Diamond SRH77CA is a pretty good compromise UHF antenna, but it's just too long. The simple answer is to keep it a wave length that will fit through the window/windshield, like 33 cm. A 900 MHz radio whether amateur, or the Motorola DTR will work well enough for car-to-car communication unless just too far apart.

With the next proposed question of using a rooftop antenna, it depends on the ground plane the vehicle can or cannot provide. Thus, a half wave antenna should be considered if 2 meters, especially for car to car simplex, but one may use 70cm or higher. Much depends on the terrain and or metropolis environment. If I had to choose one it would probably be 70cm when using an external antenna. I have not used 33 cm enough with a rooftop antenna to provide experienced feedback.
 
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prcguy

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Well that certainly narrows things down, thanks. In my opinion that leaves UHF and above like 900MHz or even 2.4GHz. Forget VHF, MURS or 6m or much of the previous recommendations. If the OP and the potential in vehicle users are not amateurs then something like GMRS is back on the table or 900MHz DTR, etc. I have some cool 2.4GHz spread spectrum radios that would get down the road quite a ways but they have butt ugly headsets that I would replace with speaker mics.
 

KA0XR

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Distance wise, (and this was proved last week on a trip), nothing beats low band out on the highway. Caveat: 25 watts or more into a steel whip on the vehicle. Folk should keep in mind that frequency, power, and modulation standard are irrelevant if you can't get that signal cleanly off the antennas radiating on the horizon, and the same applies for receiving.

Interesting observation. Did you compare low band (6m?) with high band (2m?) with all things being equal on this trip and still have better distance? Were you using commercial grade radios or amateur gear? Flat or hilly country? Open or forested?

In my not-so-scientific mobile comparisons over the years, low band (6m) seems to suffer more compared to 2m due to the higher noise floor and physical ground plane limitations, even with a near-quarter wave long (49") antenna placed in the center of the car roof.
 

W8HDU

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Interesting observation. Did you compare low band (6m?) with high band (2m?) with all things being equal on this trip and still have better distance? Were you using commercial grade radios or amateur gear? Flat or hilly country? Open or forested?

In my not-so-scientific mobile comparisons over the years, low band (6m) seems to suffer more compared to 2m due to the higher noise floor and physical ground plane limitations, even with a near-quarter wave long (49") antenna placed in the center of the car roof.

To some extent 6 and 2, but more information is garnered from low band radio to high band radios, of which we (company) have both, as well as UHF itinerant. There is no difference from low band to high band be it commercial or ham. It's not like the radio on 6 meters is better than the low band. As long as the radios are designed to open squelch at relatively the same signal level, and the antennas used are the same type, then same is generally same. The only issue at times is propagation, and to some extent noise which is prevalent in on 6 as is LB at the same physical location. That said, there are some locations where noise can not be overcome on either segment. However, the only places which we have had noise plague us is metro city areas, or in a fully packed truck stop, or factory where welding and processing takes place.

A lot of our comms fall into two lists. On site coordination, and long distance deployment and logistics. Think of the coordination as being where the the truck with supplies is within several miles of the site, and you call the site to ask where to offload. Logistics is somewhat the same way, but at times it's hooking up with others on the road who are coming from other directions and want to all come in together. Deployment is where one is in a group on the highway were you may, or may not be traveling close together. This is the one area where any thought of a UHF Baofeng in the cab can be ruled out. A high power Retrivis VHF helps, but still is not helpful, not to mention a PIA to operate when driving.

The locations are normally rural, and noise free. In the city I may get S8 of noise on 51 MHz at a steel plant. Out at a job site, S1. Same applies to LB commercial. The locations can be a Saskatchewan or North Dakota prairie, or we can be deep inside a Tennessee woods, or up in northern Michigan among dense pine trees.

So to be fair, the bands can not be compared without the assumption that the antenna for the radio is outside, roof or fender mounted, and is unity gain or better on both 6 and LB, or 2 and HB. I've gotten a pretty good read on LB simply because my truck runs both a stainless whip, and Hustler resonator, for example for ops on 29.750 MHz and 29.600, both in FM mode. Same with 6m and 2m. After a decade of use, you get used to the performance of each. And no, I don't mind pulling into a rest stop to make an antenna change.

But back to the LB point, LB has never failed in the woods. UHF is worthless if you can't see (visually) more than 50 yds due to trees. HB is better, but LB makes the trek. LB is also fantastic on the prairie as flat terrain can give us the most distance, and thus sometimes as much as an hour's heads up to arrivals.

The key to comparison is same antenna architecture, gains, mounting, and power. And if the radios are similar in their sensitivity and selectivity, then it's fair. A lot of times in discussions, those factors get clouded as the transmission path is not equal.

In our mounting, heavy trucks have mirror mounts. Light trucks/pickup have fender mount LB, and roof VHF/UHF. Cars, only one car has LB but it's the same as the light truck in performance. Cars usually have trunk/fender mounting with nothing on the roof.
 

Xiphos007

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Last month I had to pleasure to essentially be the convoy commander of a contracted team to replace some microwave gear on a mountaintop covered in light snow and ice. The contractors were from a local radio shop, had gear on the next mountain over, yet lacked any mobile comms for vehicle-to-vehicle use in the four vehicles they showed up with.

Halfway up the mountain (in the lead vehicle) I lose sight of the rear vehicle, then the one in front of that. Luckily, me and my own crew had some MURS radios (VHF) which were distributed to one person in each of five vehicles as I walked back to check on what was happening. The last vehicle got stuck in snow and ice, as the driver had not engaged four-wheel drive yet. The vehicle in front of them stopped to help out. That rear vehicle had to be left behind as even in four wheel drive the tires were street rated only. So now four vehicles have comms. The driver on my crew was now in a hurry, and we managed to get about a mile within a pine forest ahead of the rear (fourth) vehicle. The MURS five-watt radios worked very well within that distance. We could not test further as I did not want to get too much further away from the other vehicles. I know the distance between vehicles as a certain government agency thought it odd that five white vehicles were heading up a mountain so late into the snow season. They provided me with the distances and where we were transmitting. Seems they monitor MURS/FRS/GMRS and 2-meter amateur for lost hikers often, and even have some direction-finding gear on this particular mountain.

My note to self, always keep a few spare radios to give to other vehicle users in your convoy. Whether amateur, FRS or GMRS or something else, anything is better than nothing.

For my own personal use, when with family we tend to use the 1-watt DTR series 900 MHz ISM band Motorola radios, which get you about one mile range. I frequent ski resorts where everyone seems to have FRS/GMRS and getting a clear channel can be rough. DTR keeps only my crew on the "net" and gives enough range for our needs.

Good luck on whatever you decide to use.
I also use the DTR 900 mhz FHSS radios for our family. They are fantastic. Only once did I go somewhere where we ran into interference, and it was because they place we're running the same radios. And totally agree about keeping spare radios with you. I keep mine in a center console vault.
 
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