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Author Topic:   Garmin 198C Sounder Affected By Radio Transmission
Andrewdavis34 posted 06-29-2008 12:17 PM ET (US)   Profile for Andrewdavis34   Send Email to Andrewdavis34  
Hi all--I went out on a friend's 170 yesterday. He currently has a Garmin 198C Sounder-Plotter and Standard Horizon Phantom VHF radio with remote mic. While transmitting on VHF in high [power] mode the sounder and transducer would stop working. On the sounder screen is says "Transducer disconnected" and the sounder page would turn blue. If the Sounder-plotter was turned off and turned back on everything would be fine until we tried to use the VHF again. I am not sure why this is happening, but would like to try and help him solve the problem. Thanks,


jimh posted 06-29-2008 01:52 PM ET (US)     Profile for jimh  Send Email to jimh     
Where is the transmitter's antenna located with respect to the other vessel electronic devices?
Andrewdavis34 posted 06-29-2008 02:01 PM ET (US)     Profile for Andrewdavis34  Send Email to Andrewdavis34     

The antenna is mounted on the port side of the console in the upper left corner. It is a 4' Shakespeare Galaxy.

I was thinking that he may be getting interference from the Standard Horizon Phantom box.


jimh posted 06-29-2008 04:29 PM ET (US)     Profile for jimh  Send Email to jimh     
On a VHF Marine Band antenna that is 4-feet in length, the upper 3-feet are the radiating element. If you have the antenna mounted on the console, it is much too close to the SONAR. You need to increase the separation between the radio transmitting antenna and other vessel electronics.

You can test to see if the antenna radiation is the source of the problem. Disconnect the antenna from the radio. Connect a dummy load to the transmitter, or connect another antenna which is located farther away. (Usually the antenna will have a 20-foot length of transmission line, and you can temporarily move it 20-feet away for the test.) Key the transmitter and see if the interference is eliminated.

On the odd chance that the interference is not eliminated, check the wiring of the UHF-Series connector on your transmission line. It may be poorly installed and creating a problem. If the connector is proper, and you still have interference even with the antenna remotely located, then check the battery distribution voltage while the transmitter is keyed. If you have a marginal branch circuit or there are poor connections in the circuit, the current drawn by the transmitter (typically about 7 to 8 amperes) may cause a significant voltage drop in the branch circuit. Such a voltage drop could cause other electronics to malfunction.

If you find the antenna radiation to be the cause, and I strongly suspect you will, vertical separation distance is much more effective than horizontal separation distance. Try to increase the vertical separation between the antenna and other electronics by raising the antenna. Raising the antenna will improve its operation, too.

Some of the fiberglass-encapsulated antennas are rather heavy and extending them vertically with an extension mast may not be practical. This is one of the reasons why I changed to the GAM ELECTRONICS SS-2 antenna. It is very lightweight and can be mounted on a 4-foot extension mast without creating too much weight or wind loading.

Andrewdavis34 posted 06-29-2008 04:46 PM ET (US)     Profile for Andrewdavis34  Send Email to Andrewdavis34     

Thank for the replies. I have the exact same boat that is configured slighly different. My is as follows.

-Shakespeare three foot whip antenna on upper right of console.

-Standard Horizon QuestX mounted inside the console lower left front corner, with remote access mic.

-Standard Horizon 498/Sounder.

My current setup work fine w/o any problem. All my electronics are routed through a Blue Sea System fuel block. I have never had any problems at all.

I think my friend has either his VHF or FF wired through I believe the Stereo Switch on the dash. Could this be causing the problem?


jimh posted 06-29-2008 11:49 PM ET (US)     Profile for jimh  Send Email to jimh     
First test to see if the problem is due to radiation from the antenna. The power wiring could be involved, as the interference could be conducted into the sounder by the power wiring or the transducer wiring.

A sounder is really a broad band receiver tuned to the very low frequency of the SONAR, usually 0.2-MHz to 0.05-MHz. If a very strong radio signal, even one up at 156-MHz, blows into the SONAR, it can cause the amplifier circuits in the SONAR to overload. The amplifiers become driven into saturation, and then they can't detect the real SONAR signals they're supposed to amplify.

But, again, see if radiation from the antenna is the source. If it is, then you can try some other methods of suppressing it, including using some RF Chokes on certain lines and re-routing some lines.

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