Primer on VHF/UHF voice

As you may be aware, our privileges as amateur radio operators in the US are split up into different bands. Here we are concerned only about two of those bands: 2 meters and 70 centimeters (with a brief mention of 1.25 meters that sits in between them). These are the most easily accessible bands within ham radio in the United States, both in terms of affordable equipment and license access.

There is one thing we want to call out regarding 2m versus 70cm: propagation. 2 meters is capable of spanning a greater distance, but it is more easily blocked by obstacles such as buildings. 70 centimeters penetrates buildings better, but at a cost of going a smaller distance.

Traditional radio is all analog. Analog has some benefits like simplicity and the possibility to have some signal over none with a poor connection, but digital radio brings a number of benefits. The details aren't important here, but you are encouraged to research more on your own!

Some of the benefits of digital radio include:

• Efficiency
• • Spectral efficiency (using smaller chunks of the band for a single call)
• • Multiple access (FDMA, TDMA) further increasing efficiency
• • Audio compression
• High quality audio
• Data being able to be sent in the same transmission
• Trunking (conceptually, this could be done on analog too but isn't)

Some of the downsides of digital radio are the presence of competing standards (FDMA vs TDMA, the various voice protocols, different encodings), the fact that a transmission is is all or nothing, possible licensing fees, and more expensive hardware.

We polled DC ARES NE members and learned that most members who have digital voice capability have System Fusion (Yaesu) and/or DMR (multiple manufacturers). A few had D-STAR. P25, M17, and NXDN are slowly growing in the ham community but are not yet used within our group.

Radios can operate in a few different modes: simplex, half duplex, and full duplex.

Simplex is the simplest, where two or more radios communicate with each other on a specific frequency, with specific parameters (channel width, mode, and so forth). When one device transmits, all others receive that transmission on the same frequency and vice versa. Other than configuring the radio for the desired parameters, which can be as simple as selecting a frequency, there is nothing extra involved, making simplex operation great for a lot of ad hoc and emergency situations.

Duplex simply means transmitting and receiving on different frequencies. This could be done with just radios, or it could involve other infrastructure such as a repeater.

Let's say that Alice and Bob have two HTs. Alice's is configured to transmit on 147.64 MHz and receive on 147.04 MHz. Bob's will be set up the opposite: *receiving* on 147.64 MHz and *transmitting* on 147.04 MHz. By transmitting and receiving on the opposite frequencies, what Alice transmits will be received by Bob, and what Bob transmits will be received by Alice.

Note: There are no non-repeater duplex frequencies in the 2 meter or 70 centimeter bands US following the per the ARRL Band Plan. The example used are repeater inputs and outputs.

Operating on duplex from a couple of personal radios is not a common scenario. What is more likely is using a repeater. A repeater does what it sounds like: it repeats an incoming frequency on an outgoing frequency. It works exactly like the duplex scenario above, except it involves a special radio called a repeater:

• Alice, Bob, Charlie, Diane: transmit 147.64 MHz, receive 147.04 MHz
• Repeater: transmit 147.04 MHz, receive 147.64 MHz

The advantage to a repeater setup is that the repeater is typically set up with an antenna that is mounted on a tower somewhere. Because "height is might" for 2m/70cm, having the repeater working from tens or hundreds of feet in the air means that Alice, Bob, Charlie, and Diane will have better signals to and from the repeater and be able to be *much* further away.

Two HTs communicating via simplex generally have a limit of about 5 to 15 miles. Radio power, antenna, the terrain, and any obstacles (buildings, trees, etc.) in the way can alter this. Putting an antenna hundreds of feet in the air may allow that same HT to reach it 30 miles or further away. Alice could be 30 miles west of the repeater, Bob could be 30 miles east of the repeater, and they are now communicating from 60 miles apart on simple HTs.

There are a few downsides to repeaters:

• Because the transmit and receive frequencies need to be separated a bit from each other and from nearby repeater frequencies, there are not a ton of frequencies available to use. This limits the number of repeaters that can exist within a geographical area.
• To get the most out of the repeater, its antenna needs to be as high as possible. Towers are expensive, tower crews are expensive, long runs of cabling are expensive. There may be other costs involved, such as filters to not interfere or to ignore interference from other things on the tower.
• In addition to the tower, the repeater itself and the antenna have costs associated with them.
• The repeater needs to be up and running to be used. The tower needs to be standing, the repeater needs power, etc.

Multiple repeaters can be connected to one another to extend the range, allowing people from a much wider geographic area to talk together. This communication can be done through any means, including RF, but it is most commonly accomplished by connecting the repeaters over the internet.

A local example of this is the KDØDDU repeater. Using software called AllStarLink, that repeater is connected two repeaters in Thurman, IA; Humboldt, NE; Red Oak, IA; Virginia, NE; Omaha, NE; and others. Listening to one local repeater in Omaha, I have heard people from North Platte to Iowa, Missouri, Kansas, and even Texas. Every individual connects to a repeater that is local to them, but their transmissions go over the internet to the other repeaters and are relayed accordingly.

If you have ventured into the world of digital voice, you may be familiar with hotspots. A hotspot is nothing but a tiny, very localized repeater. Popular options to just purchase and use them include the M1KE (also a radio!) and openSPOT, but more commonly people will purchase a Raspberry Pi and a radio board for it then run either Pi-Star or WPSD.

The details of setting up a hotspot are well beyond the scope here, but once it's up and running they all have some form of configuration (typically a webpage) to configure modes, networks, and either a simplex frequency or a duplex pair of frequencies to operate on, depending on the hardware.

When it's all said a done, a hotspot is nothing more than a very short range linked repeater for digital voice, connecting to dozens or hundreds of other linked repeaters (hotspots and otherwise) via the internet.

As mentioned earlier, FM simplex is incredibly simple these days. Tune the radio to a specific frequency and start transmitting.

In the US, the following frequencies are designated for FM simplex on 2m, 1.25m, and 70cm:

Simplex in the analog world is incredibly easy. Your audio input is modulated and sent out as radio waves. If your radio can do FM on a given frequency, you can talk to anyone else who can do FM on that frequency. But what if your friend's radio can only do AM? You can both send and receive a signal there, but you won't be able to understand each other.

Digital voice has that same problem. There are multiple "flavors" of digital voice. It does not matter if you're doing simplex or using other infrastructure (repeaters, hotspots), two radios that do different flavors won't be able to talk to each other.

*Warning: this is about to get a little complicated. Thankfully, this all happens behind the scenes in your radio, but it's good to understand a little bit about it to understand why you and your friend may not be able to do digital voice together.*

The first step in digital voice is a vocoder (voice encoder). The vocoder takes the raw audio you are speaking into the radio and encodes it as a series of 1s and 0s.

Digital Voice Systems Inc. created a number of standards for encoding voice, with many of the recent ones being a newer iterations of *Advanced Multiband Excitation* or AMBE. With the radios that DC ARES NE members have, either AMBE+ (D-STAR) or AMBE+2 (DMR, System Fusion) are used.

Let's say I have three radios and I press and hold the PTT button on all three of them at the same time. One is an Icom (D-STAR), one is a Kenwood (DMR), and one is a Yaesu (System Fusion).

With DMR and System Fusion both using AMBE+2 for vocoding, the 1s and 0s that get encoded are going to be the same. (Microphone sensitivities and such will make them differ, but for the sake of argument, let's pretend they're identical.) The Icom however is using AMBE+ instead of AMBE+2, so its 1s and 0s are different. We're at the very first step and already have one radio doing something incompatible with the others!

Once the radio has the 1s and 0s, it needs to send it out over that frequency. Similar to how analog has AM and FM modulation options, there are a number of modulation options for digital.

We aren't going to go into the details, but DMR uses 4FSK (4-level frequency shift keying), System Fusion uses C4FM (continuous 4-level frequency modulation), and D-STAR uses GMSK (Gaussian minimum shift keying).

Remember how the DMR and System Fusion radios above had the same data after the vocoder was done with its work? Because of those two flavors use different methods for modulation, that identical input audio is now very different. That signal could no longer be received and understood by the other radio.

One of the benefits of digital voice is that it does multiplexing, allowing for multiple oncurrent conversations to happen in the same frequency. There are two main ways that multiplexing is achieved: FDMA and TDMA.

FDMA, or frequency-division multiple access, splits the channel into multiple smaller chunks that can easy be doing their own thing at once. In radio terms, this is similar to LSB or USB compared to AM. In more accessible terms, this is 4 people in a room where 2 go to one side, 2 to the other, and speak to each other. The whole room is being used for a conversation, but by moving to a dedicated area and speaking at a lower volume, multiple conversations can happen at once.

D-STAR and System Fusion both use FDMA, though the technical details differ between them.

TDMA, time-division multiple access, takes a different route. Instead of splitting up the frequency into smaller pieces, it uses the entire frequency but the different conversations take a small sliver of time one after another. In that same analogy, this would have 2 people go into the room and speak for a set amount of time, then they leave and the other 2 come in and speak for the same amount of time, then this process repeats. Whatever 2 people are in there have access to the entire room, but they only have it for a little bit of time then they have to wait while it's shared with the other.

DMR uses TDMA. This is why DMR has you set a time slot: you are joining the first group or the second, and ignoring everything in the other. Of course, the time slots that DMR uses are tiny tiny fractions of a second so the people using it never even realize it's happening.

Oof, that was a lot!

This is why the terms "digital voice" or "digital simplex" on their own doesn't tell us much. Whether simplex or using a repeater, digital voice uses some vocoder, some form of modulation, and some method for multiplexing, and the combination of them differs between those common flavors.

A Yaesu FT-70D (System Fusion) and an Icom 52A (D-STAR) can both do digital voice, but they cannot do digital voice over simplex together. (Reflectors, another topic related to hotspots and repeaters, can allow cross-band communication in digital voice, but that is beyond our scope here.)

Below are the common configurations for digital simplex within the US for the systems used by DC ARES NE members. Consult your radio's manual for information on configuring it.

System Fusion is arguably the easiest option for digital simplex. Simply tune to a frequency, tell the radio to operate in digital mode, and start talking.

The VHF and UHF calling frequencies used by Yaesu employees are 145.5625 MHz and 445.5625 MHz.

If you are using a DG-ID (needed to select a specific module on Fusion), set it to 00.

Unfortunately, DMR is a little more complicated to configure, requiring a talkgroup, timeslot, and color code. Fortunately, the simplex calling frequencies all utilize color code 1, timeslot 2, and talkgroup 99, so once one is configured, configuring the rest is fairly simple.

The DMR calling frequencies are 145.51 MHz and 145.79 MHz in VHF, and 441.0 MHz, 443.45 MHz, 446.075 MHZ, and 446.5 MHz in UHF.

D-STAR only has a VHF calling frequency: 145.670 MHz. Specific areas or events may specify a UHF frequency to use for simplex, but there is no standard UHF frequency.

Repeaters typically transmit and receive within a single band, e.g. 2 meters. These days, that is generally not an issue with most radios that support 2m also supporting 70cm. However, a lot of older equipment is either VHF or UHF. In an emergency situation, you may only have access to an older radio that can only work on one band or the other.

That's where cross-band repeat comes in. A single radio supporting cross-band repeat can function like a repeater, listening on a single frequency in VHF or UHF and transmitting it to a single frequency in the other. For example, it could be listening on 441.450 MHz in UHF and transmitting on 147.25 MHz in VHF.

In addition to older or limited equipment, you may want a radio set up for cross-band repeat to handle the limitations of each. For example, a radio could be connected to a fairly high antenna in a forest in a valley that can reach a distant repeater outside of the valley on VHF. Meanwhile, multiple radios communicate with that cross-band radio within the forest, allowing for better penetration of the foliage at the expense of range.

FM is just the tip of the radio iceberg. There's AM, SSB, CW, and a myriad different non-voice digital modes like FT8 as well as other digital voice modes (for example, P15, NXDN, and M17).

Based on a survey during a net, the vast majority of our members have radios capable of System Fusion (Yaesu) or DMR (many manufacturers), and we want quick and easy so we're unlikely to ever want to expand beyond FM. Of course, you're encouraged to play around with any option available to you at your license class!

• A Comparison of Amateur Radio Digital Voice Systems (PDF)
• A Digital Communications Guide for Amateur Radio Operators (PDF)
• ARRL Band Plan
• D-STAR: First Steps
• DMR Simplex Frequencies
• Ham Radio Range: A Guide to How Far You Can Talk
• KD0DDU Ham Radio
• Multi-Band Excitation
• Simplex, Duplex, Offset and Split
• US C4FM national simplex frequencies?