Open Microphone Procedures for Nets

by Stan Pozerski KD1LE and Ralph Swick KD1SM
copyright © 2005, 2006, 2007 KD1LE, KD1SM
Creative Commons License This work is licensed under a Creative Commons Attribution 3.0 License.


We developed the suggestions below after numerous experiences as NCS, observers, and participants in many Public Service events. We welcome feedback from experiences in applying these diagnostic procedures in your situations.

The Problem

Regularly during large events a radio gets keyed inadvertently and jams the frequency or repeater. There are several typical causes for this.

A defective microphone button/switch, or a defective radio itself can cause the problem.

Rainy days are a regular cause of this problem. Wet speaker microphones, headset switches, and radios can cause a locked-on transmitter.

Sitting on a microphone in a vehicle or a microphone that has fallen between seat cushions.

VOX turned on in a loud environment.

Preventing the problem

Proper maintenance of equipment.

Make sure PTT buttons are free and clear. Sticky PTT buttons should be cleaned or repaired prior to arrival at the event. VOX must be disabled.

Keeping equipment dry is a primary deterrent. Plastic baggies for radios and microphones can prevent the problem as will a harness or holster that keeps the equipment under your rain coat or poncho. The plastic bags newspapers come in (long and narrow) will cover a radio and the antenna.

On rigs with such capability, enable the Time Out Timer (TOT) for the shortest practical period. The transmission timeout should be one or two minute’s maximum. The Yaesu VX7R has several settings from one minute up. The Yaesu manual suggests using a one minute timeout. In our opinion any operator talking for more than a minute continuously in a health and safety net probably should be cut off anyway.

Making it easy to know you have a problem

I. Regular identification by the NCS can help. People should come to expect regular transmissions on the net frequencies and expect trouble if they don't hear anything. If the net is active with traffic this will not be an issue. If the net is quiet then NCS should make a regular ID every ten minutes. "This is W4H NCS for the 2005 Walk For Hunger KC1US”. Personally if I don't hear anything for even a few minutes I at least check the display on my rig or kerchunk the repeater.

An operator who does not hear the NCS or other station transmit (no transmissions for more than five minutes) should first check his/her own radio and determine it is working and not jamming the frequency. He may then call NCS on frequency for a radio check. This will benefit everyone who hasn’t heard a transmission for a while. If no response is received after two or three attempts (with appropriate pauses in case NCS is occupied with a priority task), contact NCS on a pre-arranged alternate frequency.

How to troubleshoot a problem

If NCS is confident of having an overriding signal, either to capture the repeater input or to override the interference on a simplex net:

II. NCS and other stations should listen to the audio and try to determine the source. Loud sounds such as a siren may be heard by stations locally and over the radio and help narrow the search. NCS may ask stations to listen on the repeater input (or simplex) frequency with and without an antenna. Keep track of who reports signals to narrow down the possibilities. Not everyone will be able to report due to signal strengths. All stations who hear the NCS request should report even if they do not hear the locked transmitter. NCS will know that none of the stations who report are the source of the trouble. NCS may also be able to rule out other stations based on known proximity to the stations who report not hearing the interference directly. A station that may not be able to communicate back to NCS may still be able to localize the problem if they hear a strong signal with no antenna. This may have to be sorted out locally by verbal contact with local stations since everyone at that location will have trouble communicating back to the NCS.

III. NCS may ask all stations to contact NCS on an alternate frequency. There should be a preplanned alternate frequency for many reasons. By being preplanned everyone can have it preprogrammed. The problem station will not hear the instructions from NCS to move to the alternate frequency and might thereby be identified. When announcing the move only announce it on the problem frequency. If you announce it on the alternate frequency and people are monitoring it you may invite the blocking station to move. The goal of the move is to get the stations to a frequency where they can operate again and to identify the problem station. The repeater output frequency is one appropriate alternate frequency. Stations should have this programmed in case of repeater failure anyway. Make sure any output CTCSS (PL) tone is also programmed in case stations are running tone squelch.

IV. In a closer environment NCS may have a key station at each site physically contact each local unit.

V. After receiving reports, NCS may attempt to contact or may ask other stations to contact non-reporting stations on the simplex or repeater output frequency. This would be effective if there are stations in the net with stronger signals – perhaps due to proximity – than the repeater.

Do not keep saying on the air "Someone has an open mike. Check your equipment". The malfunctioning station will never hear these transmissions and they just tie up the net. On a simplex net it could be possible that some stations may not be aware of the interference; however, in these cases especially NCS should be using techniques II and III.

If the interference lasts for several minutes and then stops, NCS may try asking all stations to feel the backs of their radios. The malfunctioning radio will be the one that is hot and is likely to need new batteries soon. This diagnostic request will fail if the batteries have just died or if the transmitter timeout timer triggered but did not return the radio to receive mode. This is not an attempt to embarrass anyone but an attempt to return the net to normal operation. Stations shouldn’t be afraid to say they have found the problem if that is the case. Otherwise the NCS and other stations may spend time looking for a problem that has already been resolved.

Updated $Date: 25-03-2007 - 10:46$