So, your VHF/UHF FM station is not performing as well as it should during simplex (station to station) communications. The discussion on this page is designed to provide a better understanding of how to troubleshoot a VHF/UHF station with poor receive and/or transmit performance. I’ll describe typical troubleshooting techniques as well as providing a “punch list” to guide you through the process. It is assumed that you have access to and know how to use test equipment such as an RF wattmeter and antenna analyzer.
We’ll assume that the equipment line up includes the following:
- VHF/UHF FM transceiver – typically 25 > 50 watts output power
- Coaxial cable – Rig > Antenna
- Vertically polarized antenna – ground plane or Yagi type
Other assumptions include; the antenna is mounted above a roof and in the clear and the “other station” is performing properly and can hear and be heard by other stations.
Defining the problem
Before calling for additional resources, you should take a moment to define the problem. Most EOC stations participate in training exercises where the station’s communicating ability is tested and compared against other stations to communicate over a defined distance. There is a detailed log of how well the participating stations perform – and, this is a good place to start gathering information. Answers to the questions below will serve to help everyone involved understand the problem and your station.
Is the problem related to receiving only – in other words, can your station be heard by other stations and you cannot hear other stations ?
Or, is the problem that you can hear other stations and they cannot hear you ?
Does this condition exist all the time or some of the time ? When ?
Is there a noise problem that opens the radio’s squelch ? All the time or some of the time ? (assuming the receive PLL/Tone is set to OFF)
Has a computer model of the path between the two stations been done ? This would offer some insight into if communications is actually possible. The “Radio Mobile” program would be a good choice of software.
Transmit problems are defined as “the other station cannot hear you”.
The first test would be to connect a wattmeter and dummy load to the transceiver (in place of the antenna/coax). Test for the output power value – what is the power level ? Does it meet the manufacturer’s written specifications ?
The next area to interrogate would be the antenna and coaxial cable. First, insert a bi-directional wattmeter in series with the transceiver and the antenna/cable – measure both the Forward and Reflected power. The second test would be performed using an antenna analyzer to look at the antenna/cable to determine its impedance.
If the computer model says that communication is possible along the desired path, and the receive performance at the other end is acceptable, then you need to start looking at the use of a directional antenna (Yagi) pointed in the direction of the other station.
Receive problems are defined as “you cannot hear the other station”.
First, the antenna and coax tests described above should be performed and the results verifier to be OK before proceeding.
The next test would be to test the receiver’s sensitivity using a signal generator or communications test set. A “field” test would be to swap out the transceiver with one that is know to be well working. If you can hear the other station (the problem goes away), then the first transceiver has an RX problem and should be repaired.
FM mode receive problems can also be caused or influenced by local RF interference and noise. It may manifest itself as an increase in the noise floor (higher than normal minimal S-meter reading). A spectrum analyzer connected to the antenna/coax will show whether the interference is located in the amateur band (between 144 > 148 MHz) or outside the band. Wideband, continuous noise (ranging across the amateur and adjacent frequencies) may be caused by an electrically powered device; i.e., an inverter, lighting, etc. Sporadic, repeating, narrow band and, short duration noises are typically caused by local RF emitters (communications/paging transmitters or repeaters) which may result in the station’s receiver to experience overloading and de-sensing. The use of a defined filter (wide band-pass or narrow band pass/reject) will help clean up what the transceiver “sees”.
If it is determined that the station is not plagued by RF or electrical interference, it is possible that the transceiver’s receiver section may be deaf (common in a lot of radios) and requires a bit of receive amplification. The use of a low noise preamplifier (LNA) will help “pull out the weak ones”. The use of a pre-amp may also subject the station to hear some long range interference, so you may need to add a band-pass filter too.
Directional antennas – Yagis
If you station is located in a valley or very remote area, then the use of a directional antenna will make it possible to aim the RF energy (both transmitted & received) in a single direction. This is effective if the signal path between desired stations is over a hill, through a pass or just at the edge of the calculated range. The vertically polarized Yagi antenna can replace the ground plane vertical antenna or be installed with its own coax cable and a switch (located in the shack) to enable the operator to select the “best” antenna for the communication path. A short Yagi antenna (3 to 5 elements) may be mounted on a 10 to 12 foot tall mast/tripod and located on the ground or rooftop to test its effectiveness – if it works, then you should think about a permanent installation.
So, with the various options described above, what should you plan on doing ? The “punch list” below provides a few preliminary tests designed to funnel the problem. Then, call in additional resources.
- When does the problem exist – all or part of the time ?
- Transmit problem – cannot be heard by other stations.
- Receive problem – cannot hear other stations.
- Swap out the rig and note whether the problem stays with the radio or remains.
- RX noise – S-meter reading – with and without the antenna/coax connected. A spectrum analyzer may pinpoint the problem.
- Antenna – tests OK using analyzer ?
- Coax cable – test with antenna using analyzer.
- Directional antenna – test a portable version first.
- Pre-amplifiers and RF power amplifiers may be tested to increase the effective range.
For additional assistance and discussion, contact W1SMS via email: “w1sms @ arrl.net”