900 MHz mobile antennas

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.

Sorry, I know this thread is old, but I have a question:

So you are saying that you were able to talk 10 blocks away between two DTR550s with 1/2 wavelength antennas? If so, that is pretty good… I really want to get a pair of DTRs!

The DTRs are no better than any other UHF radio which is limited to line-of-sight, but because they are very high end business radios, their range will be about the best of any UHF radio on the market.

Because they have a high audio power output and good speakers and mil-spec components, they are very clear sounding, too.

Two limitations to keep in mind: UHF is good for punching through steel, glass and concrete; VHF is better for punching through trees and vegetation. (In the woods, the MURS radios might be a better choice but I have never had a chance to test them head-to-head.)

The other limitation is the one I mentioned above. They do not work beyond the very limits of their range. Where an analog radio may emit the odd squawk and hiss at the outer limits, digital radios will simply not connect.

All that being said, I love the DTR radios.

Thanks for the reply. I have actually ordered a pair of Motorola i355 (with the longer antenna) from eBay. I am not quite sure what I am going to use them for yet, but I will probably use them for communicating between two cars. I really don’t anticipate using them in the woods much.

Since it seems the i355 gets similar range to the DTR ratios, and are significantly cheaper, I just could not justify the cost of the DTR radios! :slight_smile:

Once I receive them, I’ll do some range testing in my neighborhood and post back here with the results. I am hoping they work a little better than my Midland GXT750 GMRS radios. Even if they don’t, it will be nice not to have to worry about all of the chatter.

I actually almost ordered a pair of the trisquare radios, but didn’t due to complaints about lost sync and lesser audio quality than an FRS radio.

The i355 is actually a cell phone, and while it does have two way radio capability it may not necessarily function solely as a two way radio.

According to the manual, it uses MOTOTalk, which is Motorola’s proprietary system of ten pre-programmed channels and 15 privacy codes. The manual does not specify what these channels are, but based on the general description and the notices such as “MOTOtalk may not be offered by your service provider” and “MOTOtalk is not compatible with older Family Radio Services products”, I surmise that the i355 uses a type of FHSS (Frequency Hopping Spread Spectrum) technology on the 900 MHz band, which is what the Motorola DTR radios use.

The difference is that the i355 is a cell phone with some two way radio capability, a small, consumer grade antenna and consumer grade battery pack. The DTR radios are business class and dedicated digital two way radios, with business grade antennas and longer lasting batteries. While both may use the same spectrum, the DTR radios are more powerful, hence their range will likely be greater than the i355.

In addition, according to the manual, the i355 requires a SIM card and cell service to activate, and since MOTOTalk is part of that service, it is likely that unless you activate the service, those phones may not work as two way radios alone at all.

Thanks again for the reply. After reviewing many forum posts, it seems that people are able to get the Direct Talk function to work without a subscription to cellular service. All that is needed is either a Nextel OR a Boost Mobile SIM. The SIM does not need to be activated.

As far as the frequency goes, it seems to use the 900MHz ISM band, but they are not compatible with the DTR radios. I believe they have an output power of 700mW.

I know that the DTR radios are better and if I actually start using the i355s a lot, then I will probably replace them with DTRs.

A major question I have is what kind of coaxial cable are you using for these tests? Yes, it’s very possible to eat up more power in the cable than the antenna makes up for in gain. Esp. if your connectors are not very good, either.

I’m glad I saw this thread. I have some DTR650s on the way, and was contemplating a mobile magnetic antenna.

After reading the thread I’ll skip that idea.

Thank you for posting the info!

That’s what this forum is for.

Enjoy your DTR650 radios. I would highly suggest you forget any mobile antenna, and invest in the longer rubber duck antenna.

Buytwowayradios got them in for me, quickly and cheaply.

Chickenhawk, I have just purchased six of the Motorola DTR650 radios. Naturally, like you, I thought about adding an external antenna to extend the range. I am glad that I found your comments, which are excellent. However, now I am wondering, if one of the radios could be hooked to a pole mounted base antenna designed for the 900 MHZ range. If this would work, extending it upward around thirty feet, should greatly increase the distance for these neat little radios. My question is, have you already tried this concept? If not, I may do so.

I have never tried a longer antenna, simply because I see any gain just getting lost in the cable run at 900 MHz end of the spectrum.

But if you would like to try, let us know how it works. There are a few of us enthusiasts with a private net of these radios, and I would love to hear your results.

What have you been looking at as far as the antenna is concerned?

Good morning Chickenhawk,

I have a forty foot TV tower and was considering the placement of a base antenna operating on the 900 MHZ frequency at roughly the thirty foot mark on this tower. In my mind doing this would greatly enhance the capabilities of the signal of the DTR radios.

Yagi makes these antennas. However in checking with the dealers, I am told that they are for the GSM 900 MHZ signal. Ironically, none of the dealers I spoke with have any knowledge of the Motorola DTR radio or the 900 MHZ digital signal transmitted by them. I have a hard time understanding why a 900 MHZ operating antenna only functions with GSM, but then again, I am no expert on antennas r radios.

I was almost ready to try one until I discovered that these little antennas are directional only. For now, I have all but given up with this idea, but I still feel placing an antenna thirty foot into the air would be a big help.

I want to thank you for all the information you have shared with people for this radio. It has helped me enormously. As you already now, Motorola has not made a lot of information readily available for these radios. Everything I find seems to be outdated and antiquated.


You should retry your test using low-loss coax designed specifically for 900Mhz bands. You can contact any Ham Radio store and ask them which coax is rated for 900Mhz and higher. You could even use coax rated for 1.2Ghz. If you’re using coax rated for UHF or VHF, the loss will be too great…

Time to dig up an old thread

There is no such thing as coax designed specifically for 900 Mhz. You have to look at your application and decide how much loss is acceptable, then choose a coax that fits.

For example. LMR 400 is a very decent Coax for UHF/VHF use. It’s extremely common in Part 90 communication as well as ham radio. On two meters, you’d have to run about 200 feet of it in order to have half of your power lost, where as on 440 you’d only be able to go about 115 ft before loosing half of your power.

These radios would loose 50% of the transmitted power by the time it got to the antenna with 77ft of coax. I don’t know how much gain the stock antenna on these units have, but to keep things simple for the example, I’m going to say 3dbi. So if you ran 75 ft of coax from one of these radios, up a tower to a 6dbi antenna, you’d loose half of your power in the coax, but your antenna has twice the gain compared to the stock antenna, so when you are at a large distance from the antenna, your received signal quality should theoretically be the same as with the stock antenna.

If you factor in the height advantage, giving you much better line of site between the radios by getting over obstacles and preventing the curve of the earth from getting in your way, you should be looking at a better coverage area. So that begs the question, “Why didn’t it work?”

Most radios designed for interchangeable antennas have settled on the standard impedance of 50 Ohms. For a radio to deliver power to an antenna, the impedance of the radio and coax should match well with the impedance of an antenna, so when you connect everything, it’s best to use 50 Ohm coax, and a 50 Ohm antenna. Most radios will specify this value in the manual or even on the radio. Because the DTR550 radios are part 15 devices, and are not designed to to be used with high gain antennas, it’s possible that Motorola didn’t use 50 Ohm for their radio and antenna. It’s also possible that the 900 MHz antenna that the OP used was for 900 MHz, but possibly not the ISM portion of 900 MHz. Any impedance mismatch or non resonant antenna can cause much greater transmission losses, or if the SWR was exceptionally high, the radio may have dialed back the transmit power to protect itself from being damaged. I’m not saying that is what happened, but I wouldn’t be at all surprised. It would have been interesting to see if the radios in the car were still receiving a signal from you, even though they couldn’t get back to you.

When looking to replace the antenna on something that was not made to have a removable antenna, or does have a removable antenna but the removable antenna is a proprietary design, the ideal procedure would be to measure the impedance of the original antenna, then make your new antenna system match that impedance. If I was in the OP’s shoes, I would have done exactly what he did, buy some antennas and see what happens.

It’s also possible that the coax that was used was much much higher loss… I have a mag mount wifi antenna that uses 12 ft of LMR200, which looses 37% of the energy you put into it at 2.4 GHz. It still performs very well and helped me make a wifi connection back to my house from 1/4 mile down the street. Even if the coax used on the DTR550 was this same small coax, the OP would only loose 24% of his power using 12 ft, which should be compensated for by the antenna. So the coax used would have to be very very small indeed.

I think this could still be pulled off with some tinkering, but whether it helps you out might boil down to your specific location. If you have an obstacle in your way, like a hill, I’d think an antenna system could be made that attenuates the signal much less than the hill does. If you already have line of sight between the radios, there is probably less to be gained from using a different antenna or an elevated antenna.

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Traditoinally, Motorola used a 37 ohm interface on all of their portable radios though I believe much of that stopped with the SMA radios such as the Astro APX, and Jedi series radios. The cellular gain antennas could also be an issue simply based on the fact the gain figures are typically stated for the higher band. This is also evident in the initial post stating the 3 dB antenna being only 5" long and the 5 dB antenna being 8" long. A quick look over at only of my usual suppliers shows 3 dB available in both knob style and whip with the whip being over 12" long.

Thank you for information