What is 900 MHz all About
BEGINNERS GUIDE TO THE
900 MHz? Maybe you did not know we had an allocation at 900 MHz. Surprise, surprise! Now you know. So how does someone make use of this band? What equipment is available for 900 MHz? Radios? Antennas? What modes can be operated? How does 900 MHz propagate? How much power can I, or should I use? In this article, I will try to answer some questions to get you started on this interesting band. Also, to keep things short, I will concentrate on the predominant mode of operation, FM.
We hams have had an allocation at 900 MHZ for a long time. It is a shared allocation. Just like we share 440MHz with the government, we share our 900 MHz allocation with several other services, some on a ‘not-to-interfere’ basis. The exact allocation is 902.005 MHZ through 927.995 MHZ, inclusive.
We hams share many of our bands with other users, mainly, the government. They take band segments that, because of the intended uses, would be worthless to most users (usually because of the potential for interference), and allow hams to use them. This arrangement gives us the opportunity to use and experiment with these frequencies so long as certain rules are followed. The biggest is that we do not interfere with other licensed users of the band, or our government benefactors.
WHO DO WE SHARE THE BAND WITH?
This is really a ‘trash’ band. You throw in hams, a few other licensed users, some industrial and medical RF radiators, the government (who occasionally uses it for radar), and a mishmash of Part 15 users (cordless phones, wireless headphones, and other such consumer electronics), and the result is a ‘trash’ radio band. We, as hams, only have to worry about interfering with the government and other licensed users (i.e., TeleTrak). If you mess up you neighbor’s cordless phone or open his garage door, too bad for him as Part 15 radio devices are subordinate to ham radio, and are required to accept any interference received, including that which might cause undesired operation. Most of the time, though, this kind of thing will not be a problem.
WHAT RADIOS ARE AVAILABLE FOR 900 MHZ?
Unfortunately, you are not going to be able to pop over to
your favorite ham radio store and get a rig for this band. This is because
it is not a band that is available for ham use in most other countries.
Therefore, you won’t find 900 MHz equipment on the ham radio market.
The only place to get radios for this band is the commercial market.
In fact, the most popular radios for general FM use are mobiles
and handhelds made
by Motorola. If you know where to look, or who to talk to, you can,
for example, obtain a Motorola
Spectra for around 200 bucks or so. The Spectra is actually a 12
or 30 watt (depending on model), 128-channel mobile radio for the trunked
service. They come in 800 MHz and 900 MHz models. Only the 900 MHz models
will modify for ham use. Kenwood also makes commercial 900 MHz radios,
as do other manufacturers, but the Spectra is the prevalent rig because
of its ease of modification and programming. The 10-channel GTX
model is a popular mobile rig for use as a base station. It is compact,
and has a built-in, front-firing speaker. Fancier Motorola rigs and
Converta-Coms (for mobile use of handhelds) are available, but they
WHAT ABOUT ANTENNAS?
Antennas for 900 MHz are available from many dealers that make ham antennas. You can get vertical antennas, panel antennas, and beams for fixed station and repeater use, and mobile antennas are available, from simple quarter-wave spikes to colinears that give several Db of gain. A fifteen-element beam for this band is only about three feet long, is lightweight, and, when used with a cheap TV antenna rotator, gives pinpoint directivity for repeater or simplex work. Obviously what you would end up using would depend upon your particular needs and situation.
IS COAX CABLE IMPORTANT?
Absolutely! The most important thing to understand is that,
more than the antenna you use, your selection of coax is paramount!
Transmission line losses rise with frequency, and coax power losses
are a deadly killer at 900 MHZ, double that found at 440 MHz. You can
easily cancel out the gain of a good base antenna by using the wrong
coax is a must if your transmission line will be longer than 50 feet.
Anything of lesser quality and you will be operating at a net loss in
many cases (antenna gain minus coax loss). For example, you might get
away with 9913 or LMR-400 if your line length was 25 feet or less. Hardline
coax is not easy to work with, but it is a necessary evil in most cases.
Type ‘8X’ or better should be used, at a minimum, in a mobile
installation. One should avoid the use of magnet mounts as they do not
couple well to the vehicle body at these frequencies, and are lossy.
You should consult a transmission line attenuation chart or similar
information when planning a 900 MHz station.
In this band, you will be operating at near microwave frequencies. RF exposure limits can easily be exceeded if you are not careful, particularly in a mobile installation. Antennas for 900 MHz and higher should NEVER be installed on a vehicle where the main lobe of radiation would beam directly into the passenger compartment of a car (such as the trunk lid), or the cab of a truck (a bed-mounted tool box). Antennas for these frequencies should always be installed on the roof of a car or truck to use the metal as a shield, and to enhance antenna performance. When using a hand held radio, an external speaker microphone is highly recommended, and the radio should be kept away from the body when transmitting, both to avoid RF exposure, and to enable the radio to radiate efficiently.
SO HOW DOES 900 MHZ PROPAGATE?
900 MHz is a tough band to operate on. It is almost completely
‘line of sight’. Almost anything can block or hinder communications
on these frequencies. Even trees! This is why almost all commercial
800 and 900 MHz systems for two-way communication use multiple linked
repeaters to cover an area. Using one central
repeater to provide mobile coverage over a huge area of real estate
rarely works well unless the site is very high (such as a very tall
building or mountain) and the surrounding area is relatively flat and
clear of obstructions or multiple receivers and a signal-to-noise voter
is used. Many Ham 900 MHz repeater systems are used as a fixed-station
entry into another repeater system (such as six or two meters) thus
keeping the regular repeater inputs open for mobiles and H/T’s,
with the audio between the repeaters allowed to mix together (think
full-duplex). Some 900 machines are finding handy use as a fixed station
entry point into EchoLink and
other VoIP systems.
WHAT MODES CAN BE USED?
Any communications mode that is legal on other bands can be used on 900 MHz. This includes CW, SSB, data, fast-scan TV, as well as FM. Several band plans exist for this band, and they vary by region. Most ‘gentlemen’s agreement’-type band plans have FM repeater outputs between 927 and 928 with a 25 MHz negative offset (902 thru 903 input). Simplex is usually done on the repeater output frequencies. Other modes are scattered around the rest of the band while avoiding frequency segments where other commercially licensed users are known to operate.
ANY OTHER GOOD ATTRIBUTES?
Yes. 900 MHz can offer a certain level of communication confidentiality or privacy. While there are receivers available that offer frequency coverage of this band, most of the cheaper radios have poor sensitivity, receive intermod, images, or are numb at these frequencies. Also, because of the fact that transceivers for this band are not widely available, if you need to get away from someone, this band can be a refuge, or ‘hideout’. It is much cheaper to hide out on this band than to hide on, say, 2-meter SSB, or 1296 MHz FM.
Well, there you have it! A quick run down on 900 MHz. This was not intended at a technical tutorial, just a quick intro to the band, who’s out there, what gear will get you there, and a few other things. Ten years ago, you could hardly find a repeater on this band. Now, thanks to the availability of equipment, repeaters for this band are popping up all of the time. This site has a repeater list for the band. Check it. If there are no repeaters in your area, consider checking with your local club ‘techie’ about maybe putting one up. It is important that we use what we have, or else someone else will!
73 de Stacy (KC6STT)
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