If anyone is contemplating the purchase of a magnetic-mount mobile antenna for 900 MHz frequency-hopping radios, I have learned some valuable (and costly) lessons.
Don’t do it.
As frequencies go up, the ability to “punch through” steel and glass is enhanced, and this is most obvious with external antennas that screw in to your rubber ducky antenna socket. It appears once you get into the higher end of the UHF spectrum that the loss of gain through the length of antenna cable easily exceeds the gains from a higher external antenna mounted to the roof of your vehicle.
I purchased two well-known magnetic-mount external antennas for my Motorola DTR550 radios and proceeded to test them scientifically. I mounted both to the roof of my truck, keeping them at least three feet apart. One was a ‘mini’ magnetic-mount 3dB gain antenna, about 5" long, designed for applications from cell phones to spread-spectrum two-way radios. The other was an 8" 5dB gain antenna, designed for the same thing.
The Motorola DTR radio uses a female-SMA connector on the antenna, and in one case, I had to order the antenna with a custom female-SMA end on it, and in the case of the other, I ordered the male-SMA end and an adapter female-to-female cable.
It is important to note that cable lengths (8’ to 12’) were sufficient to place the antenna on the roof and run the cable through the back hatch and all the way to the front of the vehicle.
To test the antennas, I connected them to two DTR radios, both programmed in as a private “net” on my system. As a control, I added a third radio sitting on the front seat with the antenna in a vertical placement. The third radio had the optional 1/2 wave Motorola rubber ducky antenna. Not only was it inside the car, but it was three feet lower than the external antennas.
I then walked a few blocks away and kept on calling up each radio on another Motorola DTR. (This is easy to do if you have programmed each of your radios into your Contact List; one merely scrolls to the appropriate radio on your contact list and presses the PTT button. If the radio is within range, you hear two short ‘handshake’ beeps and then the channel is open. If the radio is not within range, you will instead hear a warning tone and the screen lights up with a warning that the user is not available.)
I then saw some very surprising results.
After only three blocks in a heavily-populated residential area, I lost the signal from the 3dB gain external antenna. After four blocks, I lost the signal from the 5dB gain external antenna. I had to walk another six blocks before I lost the signal from the radio with the rubber ducky antenna sitting on the front seat. (Even then, I could sometimes recover a signal by holding my handheld radio higher in the air, at least at the very fringes of the signal.)
What did I learn? External antennas seem to be a waste of money for vehicle use with the frequency-hopping 900 MHz Motorola DTR550/650 radios. You might as well just leave the radios on the seat with the rubber ducky antenna as vertical as you can get.
I also learned the Motorola 1/2 wave rubber ducky antenna does an excellent job in matching the radio and providing a bit more range.
I also learned my appreciation for the Motorola DTR550 went up another notch. (It should be noted that the Motorola DTR410 model has a fixed antenna and you cannot add an external one, but the DTR550 and DTR650 both have replaceable antennas.)
Maybe someone with far more experience on mobile antennas can weigh in here and let me know if my thinking is somewhat on track or not.
By the way, if anyone is reading this and hasn’t purchased a DTR radio, read my review in this forum and remember one very important point: the DTR radios, unlike the TriSquare FHSS radios, are pure digital radios. This means that they either receive a good clean signal with 100% clarity or they don’t work at all; there is no ‘in-between’ fuzzy signal fading in and out at the far end of the radio’s range.
This can be good and it can be bad. P25 digital public-service radios were receiving a lot of criticism when they first came out because you would lose the signal completely when out of range. (There is also a slight 1/2 second delay between pushing the PTT button and when you can start speaking, but that is the nature of digital radios.) Some public safety agencies went back to analog radios because of this, and other problems. Their analog radios would still transmit some essence of static and a few words here and there when used beyond their range.
It appears most of those early problems have been worked out, but one must still understand the nature of digital radios. I personally love the clarity of the sound, and I will NOT miss all that fuzzy, distant static from the analog radios.
Just try a good FRS radio and a DTR radio side-by-side like I did and you will see what I mean.