Choosing a Radio
MASTR II InfoSite
Choosing a Radio to Convert
- Bands and "splits"
For the purposes of this article, we will assume that there are only 4 bands
of interest for conversion: 10m, 6m, 2m, and 70cm. A "split" is
a region of a band that a radio built with certain parts will tune. It is
the range of frequencies that the radio is capable of operating on. You
will need to refer to the combination numbers
to determine what radio you want to find to convert.
- VHF Low Band: There are 4 "splits"
available in this band. The only interesting splits are the
"12" split (25-30 MHz, rare!), the "13" split (30-36)
and the "33" split (42-50 MHz). The "23" split should
be avoided at all costs; it is useless for amateur operation. The 12
& 13 split radios can be tuned into the amateur 10-meter band, and
the 33 split radios can be tuned into the amateur 6-meter band.
- VHF High Band: There are 2 splits available
in the VHF high band. The "56" split is from 138-155 MHz.
These are more desirable for amateur work, and much more difficult to find.
(If you find some, get one for me!) The "66" split is from
150.8-174 MHz, and is more commonly available. The high split radios can
be re-tuned into the amateur bands, typically without too much difficulty,
but care must be taken that the transmitter does not have excessive
spurious emissions. The 66 split exciters can be modified to be
56 split exciters by replacing a dozen or so disc capacitors. The 66 split
radios are preferred for 222 MHz conversion.
- UHF: There are five different splits for UHF
MASTR IIs. Only three are interesting for amateur use: the "77"
split (406-420 MHz), the "78" split (420-450 MHz, rare!), and the
"88" split (450-470 MHz). The other two splits (89 & 91)
are in the so-called "T-band" (UHF television spectrum) and are
not useful to amateurs.
The 77 split (406-420) radios can be retuned into the bottom segment of the
420-450 MHz U.S. amateur 70cm band, where they are useful for linking
repeaters, remote receivers, etc. (See
Crossbanding Applications for some ideas...) The 78 split (420-450)
radios are already in the U.S. amateur band, but are extremely rare. Radios
in the 88 split (450-470) split can be re-tuned into the 440-450 band and
used as UHF repeaters.
- Frequency Stability
MASTR IIs are available in 2 PPM (parts per million) or 5 PPM frequency
stability. The difference between the 2 stabilities available is in the
channel elements, which GE likes to call ICOMs, for Integrated
Circuit Oscillator Modules. The ICOMs are available in 3 different styles,
both in receive and transmit varieties, for a total of 6 different ICOMs
for a given band.
- 2C ICOMs: These are the 2 PPM oscillators.
They are what you want if you are going to build a wide-area coverage
repeater that will not be in a climate-controlled environment. The
ICOMs themselves are more expensive, as are the crystals that go inside
- 5C ICOMs: These are the 5 PPM oscillators.
Most mobile radios will use 5 PPM oscillator stability and will have at
least one 5C ICOM. These are reasonably stable, and can be used for
repeater operation if the environmental temperature is relatively
constant. 5C ICOMs provide "temperature compensation" to
- EC ICOMs: These are used in conjunction
with 5C ICOMs. DO NOT USE EC ICOMs IN A RADIO WITHOUT A 5C ICOM
INSTALLED. The oscillator frequency will not be stable. These ICOMs
provide 5 PPM stability when used with a 5C ICOM. They use a
"temperature compensation" signal provided
by a 5C ICOM. Typically, the receiver has a 5C ICOM and the
transmitter a EC ICOM.
You can change the ICOM's frequency by replacing the crystal. However, if
you change the crystal yourself, you risk upsetting the temperature
compensation of the ICOM, which must be matched to the crystal. If you are
going to pay for a 2C ICOM with a 2 PPM crystal, it is well worth the money
to get a reputable crystal house (like International Crystal Manufacturing)
to compensate the ICOM to match the crystal. This typically costs about
The VHF high-band radio has two different exciters available: a
"multiplier" exciter, and a "PLL" exciter. The PLL
exciter is preferred for repeater use, it has a somewhat cleaner output
(less spurs) than the 150.8-174 MHz multiplier exciter does when it is
tuned into the amateur 2-meter band. The PLL exciter still requires a
crystal in a special FM ICOM, it is not "synthesized" in the sense
that you cannot change the operating frequency of the exciter without
changing the ICOM crystal. The PLL exciter is true FM, which may make it
preferable for packet work. (The multiplier exciters are phase-modulated.)
There is a true-FM exciter available for UHF MASTR IIs, but it is somewhat
rare and the prices reflect that. You can recognize this exciter by the
larger ICOM module, the FM ICOMs are about 50% wider and 50% thicker than
the usual ones.
- Power Amplifiers
In most cases, you will want the biggest PA you can find. However, there
is one important exception to this rule: MASTR II PAs don't like to operate
below about 40% of rated power output, especially the UHF ones. If you plan
to drive an external amplifier with the transmitter output, check the input
power level minimum and maximum of the external amplifier and choose your
MASTR II power level accordingly.
The typical accessories for a MASTR II include a control head, microphone,
microphone hanger, speaker, power cable (control head to battery) and
control head cable (radio to control head). If you have no accessories,
you will want to get the all of the above, with the possible exception of
the microphone hanger. Lack of these accessories make tune-up and service
ridiculously difficult. For building repeaters, you can get by with a
control cable alone.
- Handle Color and UHF MASTR IIs
Beware of "silver handle"
UHF MASTR IIs. These typically have a VHF exciter and a "tripler
PA" in them. These radios are notorious for spurs and general noise
on the RF output. The
"painted handle" UHF radios are definitely preferred. The
silver handle radios are an older vintage MASTR II, and are perfectly ok
for low band and VHF.
Many GE MASTR II stations have come on the market recently due to the fact
that all Public Safety, Industrial, and Business users using 25 KHz channels
are required to switch to 12.5 KHz channels by January 1, 2013. These
surplus stations represent a tremendous opportunity for Amateur Radio
repeater builders. The stations have significantly better isolation
between transmit and receive, and many have continuous-duty power
amplifiers with large heat sinks.
- What to Pay
I have seen MASTR II mobiles sold at hamfests for as low as $15, with
accessories, and for as much as $150, without accessories. $75-$100 is
the typical price range. More than $100 is too much. Keep shopping.