In a previous Shack Talk article, we looked at the 2 Meter band plan. The 70 cm band (420 to 450 MHz) is another popular ham band for simplex and repeater operating, so we’ll take a look at that band now.
The first thing we need to know is the FCC frequency authorization for our particular license class. Just like the 2 Meter band, Technicians and higher class licensees have privileges across the entire 70 cm band, 420 to 450 MHz. The 70 cm band is BIG…providing 30 MHz of spectrum compared to only 4 MHz on 2 Meters. The FCC rules do not specify any mode restrictions on this band. There is a restriction on operating below 430 MHz if you are close to the US border with Canada. (This is the so-called Line A Restriction. Check the FCC Part 97 regulations if you plan to operate in this part of the band. Most hams will not be operating below 430 MHz unless they are using Amateur Television.)
Knowing the FCC frequency authorization is a good start but we need to check a bit further. Amateur radio operators use a variety of modulation techniques to carry out communications. Often, these modulation techniques are incompatible since a signal of one type can’t be received by a radio set to another modulation type. For example, an SSB signal can’t be received on an FM receiver (and vice versa). One of the interesting modes used on 70 cm is Amateur Television or ATV. These signals are typically 6 MHz wide, so it makes sense to only use a wide amateur band like 70 cm for ATV operating.
We need to use our authorized frequencies wisely by sharing the band with other users and avoiding unnecessary interference. Thus, it makes sense to have a band plan that divides the band up into segments for each type of operation.
70 cm Band Plan
As shown in the table, the ARRL 70 cm amateur band plan supports a wide variety of radio operation. Large portions of the band are dedicated to FM operation, consistent with the popularity of the FM mode. There are portions of the band designated for repeater inputs and outputs. The standard repeater offset used on this band is 5 MHz. Some areas of the country use + 5 MHz offset while others use – 5 MHz. Within any region, the offset will be usually be the same on all repeaters. This means that in some parts of the country, you’ll dial in the repeater output frequency in the range of 442 to 445 MHz and select a +5 MHz offset. In other areas, you’ll dial in a repeater output frequency in the range of 447 to 450 MHz and select a -5 MHz offset.
On the low end of the band, we see segments for some of the more exotic modes, starting with ATV, then Earth-Moon-Earth (EME) operation. EME operators communicate by bouncing their signals off the moon.
|70 Centimeter Band Plan Adapted from the ARRL web site|
|420.00-432.00||Amateur Television (ATV)|
|432.10-432.30||Mixed-mode and weak-signal work CW/SSB Calling Frequency= 432.100 MHz|
|432.40-433.00||Mixed-mode and weak-signal work|
|438.00-444.00||Amateur Television (ATV)|
|442.00-445.00||Repeater inputs and outputs (note overlap with preceeding ATV subband)|
|445.00-447.00||Shared by auxiliary and control links, repeaters and simplex National FM Simplex Frequency= 446.00 MHz|
|447.00-450.00||Repeater inputs and outputs|
Further up the band, we see segments for CW, SSB and beacon operation. SSB is the preferred voice mode for so-called “weak signal” operators. The mode is more efficient than FM when signals are weak, so it is the way to go when you are trying to push the limits of 70 cm DX. Beacons are transmitters that are always on, transmitting a short CW message as a propagation indicator for distant stations. We often think of 70 cm as a local coverage band but when conditions are right, contacts can be made with stations hundreds of miles away. Of course, conditions are not always right so having a beacon on the other end of the desired communication path lets you know if there is propagation in that direction.
Radio amateurs also use 70 cm for OSCAR (Orbital Satellite Carrying Amateur Radio) operation, sending signals to a satellite (uplink) or receiving signals from the satellite (downlink). The OSCAR segments don’t specify a particular modulation type since CW, SSB and FM are all used for OSCAR operation. Because of their elevation above the earth, satellites can hear signals from all over the US simultaneously, so they are very susceptible to interference.
Most of this non-FM operation can be easily interfered with by signals from other users. EME signals, for example, are usually quite small since the signal has to make the round trip from the earth to the moon and back. If a local FM operator fires up in the EME portion of the band, an EME signal that can’t be heard by an FM receiver can be wiped out by the FM signal. Similarly, an operator chatting across town on 70 cm could interfere with a satellite hundreds of miles away and not know it. This is particularly a problem with FM receivers, which won’t even notice low level CW and SSB signals.
The most common VHF/UHF radios are basic FM mobile or handheld transceivers. These radios usually tune the entire 70 cm band from 420 MHz to 450 MHz in 5 kHz steps. The band plan indicates the proper range of frequencies for FM operation but there is more to the story. FM operation is “channelized”, meaning that specific 70 cm FM frequencies are identified by the band plan. The use of channels is especially important for repeaters, since they don’t easily move around in frequency and are coordinated to minimize interference. The idea is to have all stations use frequencies that are spaced just far enough apart to accommodate the signal without interfering with the adjacent channels. You might think that the spacing between channels would be 5 kHz, which is the tuning step of most FM radios. This doesn’t work because a typical FM signal occupies a bandwidth that is about 16 kHz wide.
The channel spacing needs to be at least as wide as the bandwidth of the signal, which allows room for each signal without interfering with the adjacent channel. The vast majority of the country uses 25-KHz spacing for FM repeater and simplex channels. Some areas, most notably parts of California, have chosen to use a 20-kHz channel spacing.
When using a repeater, you just need to dial in the published repeater frequency and set the transmit offset, usually + 5 MHz or – 5 MHz. In some parts of North America, non-standard repeater offsets may be used, which will be indicated in the repeater directory. For repeaters that require a CTCSS tone for repeater access, you will have to set the proper tone frequency on transmit.
Choosing an appropriate simplex frequency can be a little tricky, since local practice varies. Across all of North America, 446.000 MHz is the National Simplex Frequency (also referred to as the calling frequency). In most places, the channels immediately adjacent to 446.000 MHz are used as simplex frequencies, spaced by 25 kHz. This means that 445.925, 445.950, 445.975, 446.025, 446.050, 446.075 MHz are all valid simplex frequencies. But other areas have assigned these frequencies to repeater operation.
While the ARRL band plan sets the guidelines for band use across the US, VHF and UHF band plans are really defined on a statewide or regional basis. This means it is best to find the specific band plan for your region. This may be a challenge to find the right information, but try searching the web for “70 cm band plan” and your state. A good source is your local frequency coordination body, which is listed on the ARRL web site.
For the Colorado VHF/UHF band plans, see the Colorado Council of Amateur Radio Clubs web site.
The fine points of the band plan can be a bit confusing. However, a few simple guidelines can help, especially if you are operating only FM.
- FM voice simplex and repeater operation should only occur in the designated band segments for your area. Stay out of the weak signal, ATV, beacon and satellite subbands.
- When operating through a repeater, make sure you are tuned to the published repeater frequency with the proper transmit offset.
- When operating simplex, use a simplex frequency designated by your local band plan.