Question about Cobra Radio Communications
My Cobra 29 has developed a very high noise interference (static) level. About 3X higher than it used to be. I can her someone fine if they are within 1/4 mi from me, and they can hear me if they are close to me.I have dual antennas (from factory [Freightliner]). The AM/FM radio also receives through these antennas. When I change the ANL/NB switch there is no difference in reception. The radio is about four years old.
It sounds like something may have loosened up. Given that this is installed in a truck it is probably subject to quite a bit of vibration. This could have broken a connection, etc. If this just started, consider what happened just before you noticed the change. Maybe the truck was serviced just before - consult with the mechanic to learn what he or she may have done to contribute to the problem. Maybe the antenna smacked a low hanging tree limb?
You could disconnect the antenna from the radio, and spin a metal protective cover over it to prevent signals from entering. If the static sounds are reduced, the interference is probably coming from the antenna system. If it remains, it is probably coming in on the power leads. With the ignition OFF, check for noise. If reduced or eliminated, it is related to the engine / electric fuel pump (if equipped), etc.
Check the PL-259 connector at the end of the coax cable. If the braid or center connection has failed, it should be repaired be removing the connector and preparing the cable to receive a new connector again.
Make sure the ground wires and braids are firmly connected at the radio connector end and at the antenna. Rust is a poor conductor and if present will interfere with transmit and receive. Clean it up, make your connections up tightly and then seal to prevent water from entering.
Check antennas for damage. Many antennas on trucks are fiberglass because they're tough. The wire wrapped around it however is soft copper. If the antenna has smacked tree limbs, overpasses, etc. they could have been damaged. The physical shock is transmitted down to the mount point - causing it to loosen up, etc. Repair and replace suspected damaged or loose parts. Make sure the rubberized weatherproof tip on Firestik type antennas are secure and in place.
Check the coax. Make sure the outer jacket is continuous and has not been compromised by being crushed between pinch points, chafed, etc. The jacket must not have splits or cracks that would allow water or oil in. Replace any damaged coax cable with identical type and lengths - this is especially important in a co-phased antenna arrangement as I understand is your current configuration.
A high SWR can also cause problems. Most manuals say 2.0:1 (two point zero to one)is maximum acceptable SWR or "match". I could go along with that provided that value is present of infrequently used channels - like 1 and 40. The channels used most often should have a very low SWR. 1.1:1 (one point one to one) to as much as 1.5:1 (one point five to one) is where I would expect to be on channel 20 (the mid point of the band). There will only be one place that the is the low point in the band. Transmitting further up or down the band from 20 will cause the SWR to increase. Eventually, you will exceed 2.0:1 and too much power will be reflected back into the radio - potentially causing a failure of the RF output final transistors. This is only increased if running a linear amplifier. Transmitting at a lower power make dealing with a higher SWR much easier than sending high power to a mismatched antenna system. Your set up may have a nice, wideband of acceptable match - 1.5:1 on ch1 and ch40 with 1.1:1 on ch20; or be very narrow banded like 3.0 (or more) :1 on ch1 and ch40 and 1.5:1 on ch20. It could even be "way out there" with 4.0:1 on ch1 and ch40 and 2.5:1 on ch20. if you've got a bad match happening, correct it ASAP!
Check you power source. The best place to grab power is directly from the battery. Yours may be there already, and even so, it may also have noise reduction components such as a choke input filter and or capacitor on the power leads. Look for these and other components inline and inspect for damage or contamination by oil, fuel, etc. Replace as needed. Check the power leads for any connected accessory, too.
That's about all I can think of.. good luck!
Posted on Feb 01, 2012
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Posted on Jan 02, 2017
ANL stands for "Automatic Noise Limiter" and NB stands for "Noise Blanker".
The ANL gets rid of a lot of interference noise caused by mechanical noise from motors and other electronic components in a car.
It also gets rid of noise from other sources of interference.
The NB is for extreme cases of noise, but can slightly lessen your receive sensitivity.
Basically both functions help decrease interference noise.
Posted on Nov 24, 2010
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