Performance of 2M/440 Amateur magnet mount antenna with CB

Question

As I suspect that most people reading know already that a "well behaved" CB antenna tends to be quite large. In the past I compromised on a "proper" antenna to reduce size by using a magnet mounted dual band 2M/440 Amateur antenna when using a CB radio as a temporary measure to communicate between two vehicles on the highway. When spring comes around I might want to go on a road trip and repeat this use of CB radios and magnet mounted 2M/440 antennas but I'm curious on just how much performance/range I'm missing out on by doing this.

I realize the antennas are likely a poor impedance match for the radios but I doubt this does any damage to any hardware. If there's evidence that I'm mistaken in that then I'd like to know about it.

I also realize this is not optimal for getting the best range but I don't have the means to compare to a pair of radios with proper 11 meter band antennas. I have one "ham stick" style 11 meter antenna and trailer hitch mount, but only the one, no antenna for a second vehicle. All I need to get that antenna on my truck is a bit of coax cable to fit the truck and a reason to go through the effort to use that than the magnet mounted antenna.

I don't want to do any permanent alterations to my vehicle, which is why I chose a trailer hitch than something that isn't shielded to the front by the body of my Ford Explorer. Any radio I loan out to another driver would certainly not involve any permanent alterations, for some people I know just getting them to agree to use a CB radio to chat could be too much.

I suspect someone has performed an experiment to test what I described. Or has done some kind of simulation to find out. If so then I'd like to know the results. If the difference is minimal then I'll keep doing as I have going forward. If there is a large enough difference on some kind of temporarily mounted 11 meter band antenna vs. the Amateur antenna then I'll weigh the gains in performance against what this other antenna option costs me to buy.

I doubt it matters but the radios are Uniden PRO 510xl. I'm not sure what the make/models are for the antennas but I suspect they are pretty standard stuff, a steel vertical wire with a little coil bent into the middle and a magnet base with maybe 2 meters of coaxial cable attached. One of the antennas might be from a GMRS/MURS radio but in this case I doubt that matters much.

Answer 1

The performance would be quite bad. A typical 19" dual-bander is 0.043 wavelengths long on the 11 meter band (when you want it to be around 0.25 wavelengths long). It's hard to really accurately model a roof-mounted antenna (where the vehicle itself forms a significant part of the antenna), and harder still when the antenna is capacitively coupled to the vehicle by a mag-mount (which is made for a higher frequency and therefore doesn't have enough capacitance), but a little rule-of-thumb math says you would be getting around 1 ohm of radiation resistance at best (with plenty of reactance).

Even with loading and impedance matching this would be a very low-efficiency antenna. But since you don't have any of that, it becomes even worse. And the possibility of transceiver damage is real: the manual for your radio says that any SWR over 2:1 may cause damage. They're usually conservative about that, but you'll be dealing with an SWR more like 100:1 than 2:1. Not recommended. Not all damage is of the "*pop* and it stops working" kind. It's also possible to get a gradual degradation where output power goes down, distortion goes up, and heating goes up, until sometime later the transmitter becomes useless.

My suggestion would be to get a large mag mount (MFJ makes one with three 5" magnets, but if that's not practical then at least go up to a single 5" one). You can get them with a 3/8" stud, or you can use an NMO to 3/8" adapter. Then install whatever size of "hamstick" type antenna suits your logistics (fitting under underpasses, and not having the magmount tear off if you hit a bump at speed). Or use a "no ground required" (loaded end fed) CB antenna with a lip mount, pipe mount, or whatever. There are lots of means available to attach to a vehicle without drilling holes in it, you just have to make sure that the size of your antenna is in line with the mechanical strength of the mount.

Almost any antenna designed for 27MHz, even a lousy one, will be a great improvement over one meant for 144MHz.

Answer 2

The best tool for the job is a cell phone. Highways and interstates are saturated with coverage; if you have kids, let them handle it. The second choice is getting a pair of FRS radios. Let the kids play with them, it will keep them entertained. They can play I SPY on a cell or FRS.

You do not see any mag-mount HF antennas because it has two strikes against it. It would not be safe due to the size and mass of an HF antenna. The magnets are not strong enough to hold the base securely in the wind while driving. That does not apply to you because you intend to use a VHF-sized whip.

However, you cannot work around the technical issue; mag-mounts use capacitive coupling to vehicle sheet metal for the Ground Plane. The frequency is too low, and the capacitive plates in a mag-mount do not have enough surface area to achieve proper coupling.

Ultimately, you want to shoot yourself in both feet just before a 50-yard dash. It is not going to work out very well. The mismatch might be large enough to harm your radio if you force it into service.