The "Hidden Node" Problem

In simplex operations (both voice and packet), the ability for every station to hear and be heard by every other station on the frequency is critical for efficient operations. Consider the scenario depicted in the following diagram:

There are three stations ( “A”, “B”, and “C”) operating on a single frequency. Their locations are represented by the colored letters above and their RF coverage areas are represented by their respective colored circles. Stations A and C are field stations with modest antennas which have smaller coverage areas. Station B is located up high, with a greater coverage area. (Station B might be a voice net control, packet BBS, or similar.)

The two field stations A and C are trying to talk to the central station B. Station B can hear both field stations A and C, but they can’t hear each other. This situation is referred to as the “hidden node” problem. Station A’s transmitter is “hidden” from station C, and vice-versa.

Both stations A and C should listen before transmitting in order to avoid interfering with another transmission. In the case of a voice simplex net, each radio operator does the listening; in the case of a data net, the packet TNCs or data radios do the listening. But since stations A and C can’t hear each other, each will likely attempt to transmit while the other is transmitting. Because station B has broader coverage (typically because it is located higher), it hears both transmissions from stations A and C at the same time (a “double”). The result is garbled communications; the transmission will need to be repeated.

In a voice net, the net control will usually announce that there was a double and ask for station A or C or both to try again. In a data network, neither station A nor station C receives an acknowledgment from station B, so they both have to try again after a random delay. In both cases, the total throughput of the channel has been degraded. Communications on a simplex frequency are far less efficient when there are hidden nodes.

To solve this problem, stations A and C need to be able to hear each other. More generally, all stations on a simplex frequency need to be able to hear and be heard by each other. Therefore, we need to improve the RF coverage for stations A and C. There are two basic techniques for this: improve each station’s antenna system, and increase their transmit power. Use both techniques to the extent possible to minimize the possibility that your station is a hidden node. There are also operating protocols to follow that minimize the impact of being a hidden node.

Improving the Antenna System

There are three common ways to improve a station’s antenna system.

Raise the antenna

The most effective way to improve RF coverage is to raise the antenna so that its coverage area includes the locations of all other field stations.

Increase antenna gain

All other things being equal, a higher gain antenna will usually transmit the signal further and allow for better reception of stations that are farther away. As an example, switching from a unity gain J-pole to a 6-9 dB gain collinear ground plain antenna can broaden the coverage area.

Reduce coax loss

Reducing coax loss improves both the transmit and receive situation. More signal is delivered from the transmitter to the antenna, and more received signal is delivered from the antenna to the receiver. Coax loss increases with the length of the cable and the frequency, so if you can reduce either of those, you should. Otherwise, to reduce the coax loss, use higher quality, better shielded coax. See Served Agency Station Feedlines or Field Station Feedlines for more detail.

Increasing Transmit Power

Increasing transmit power has less effect than improving the antenna, but it can help. Increasing transmit power at station A would potentially allow station A’s transmissions to be heard by station C, so station C could avoid transmitting when station A is transmitting. But without an improvement to the antenna, there is no improvement in what station A can hear, so station A may still transmit while station C is transmitting. So both stations A and C need to increase transmit power so that their signal is heard over as much of the coverage area of station B as possible.

The FCC rules require that we use the minimum transmit power necessary. In the case of simplex, this means using enough power so that all other users on the net can hear your station. For city-wide simplex voice nets, this usually means 10-50 Watts. For packet, 25-50 Watts (plus an excellent antenna) is needed to cover the several cities that use a given BBS. Also, users should only use their primary or secondary BBS unless both are unavailable. Remember, it’s not about being heard by the BBS: it’s about being heard by all other users of the BBS.

Operating Protocols

When hidden nodes exist, good net behaviors can limit the inefficiency. For example, when calling in to a voice net, announce your call sign only and wait for the NCO to tell you to go ahead. Your call sign announcement is short, which minimizes the possibility of it doubling with a hidden node. And all stations should hear the NCO tell you to go ahead, reducing the likelihood that a hidden node will double with your actual message. Data radios can use a protocol called “Request to Send / Clear to Send” which is essentially the same approach in automated form. This type of protocol obviously slows the channel down for everyone. If there are many stations, it may do more harm than good. But when there are few stations and severe hidden node problems, it may help.

Some high-bandwidth data networks (such as WiFi) use a different protocol called “time division multiplexing,” where the central hub divides the bandwidth into time slots and assigns them to each field station, effectively telling them when they can transmit.