GPS and VHF in conflict

Casey · Posted: Mon Sep 16, 2013 5:59 am Post subject: GPS and VHF in conflict

We have a small Humminbird sounder/plotter installed aboard Katmai that works well.

We also have an iCom VHF that works well.

The respective VHF and GPS antennas are mounted about four feet apart on the "rocket launcher" bar on top of the cabin.

Everything works fine, EXCEPT that whenever we key the VHF it blocks the GPS signal. As soon as the mic key is released the GPS continues to function as normal.

Are there any easy fixes?

Best,
Casey&Mary

jkidd · Posted: Mon Sep 16, 2013 8:52 am Post subject:

Mine are spaced about the same my GPS is on a 2 extension which puts it half way up the VHF antenna. I don't see the same problem. Do you think elevation of the GPS might have something to do with it? I did that because the Radar didn't want the GPS in it's path. It could be above or below the Radar dome.

ghone · Posted: Mon Sep 16, 2013 11:06 am Post subject:

Hi Casey. If you lower the GPS antena height out of the "cone" of vhf transmission the problem should go away. Had a similar issue on a friends sailboat on autopilot. Key the mike do a 360! You may have an internal antenna on the GPS. Try disconnecting the external see if you still get signal. My GPS is internal and thus several feet below the vhf antenna. Best. George

tpbrady · Posted: Mon Sep 16, 2013 1:57 pm Post subject:

Casey,

The difference in operating frequency between the VHF and GPS is so great there has to be something else happening. Check the connectors on the GPS antenna and VHF to be sure they are tight and inspect the transmission lines from the antennas to the radio/chartplotter to be sure the shielding has not been broken or compromised. While the operating frequencies are different, the GPS and VHF may be using some intermediate frequencies that are similar or even the same and a breakdown in the shielding could be the culprit.

Tom

thataway · Posted: Mon Sep 16, 2013 5:47 pm Post subject:

Looking at your picture of the rocket launches I think gives us the clue. There have been far more instances of GPS giving interference on a specific channel of GPS (L1=1575.42 MHz) and the VHF radio (157 +/- MHz)--so there can easily be an issue as a harmonic--although the 10th harmonic is not one which is often an issue--it could be sub harmonics.

Second there is also a major issue with depth finders and VHF radios-the Depth Sounder frequency is 200 KHz--or 80 KHz or 73 KHz or 50 KHz or 400 KHz--you get the picture.

I have had to used ferrite beads to resolve issues with depth sounders and GPS interference. It is not the radiated waves, but the frequency in the wires to the transducers/ or transmitters and receivers.

The way to fix this is--separate the power wires and the radio antenna and GPS antenna wires--you can keep the GPS wires and Radio wires together--but radio and GPS need to be separate. Put ferrite beads on each of the wires by the radio and GPS (4 Ferrite beads)--this works in many instances.

MFJ-700A4 4 PAK FERRITE CHOKES RG8X SIZE about $11.MFJ (662) 323-5869
300 Industrial Park Rd, Starkville, MS 39759

I got my Ferrite beads from a local marine electronics store. Many electronics come with them, and don't need them so they had a drawer full.

There are other rarer issues--one would be that the VHF radio power dropped the voltage to a point that it dropped out--that is easy to check--you can put the volt display on the GPS and see what happens when you key it. Or even use a digital volt meter.

there is also the rare problem with an NMEA back bone and ground loop issue--doubt that is here.

First get the beads, if that does not do it, mover the wires apart--either to opposite sides or to at least 1 foot apart. I know this is a pain!

ghone · Posted: Mon Sep 16, 2013 7:51 pm Post subject:

Holy Cow Bob! I was gonna suggest Casey needed new Dylithium crystals on his hyperdrive unit but them bead thingys wow. George

Casey · Posted: Tue Sep 17, 2013 6:40 am Post subject:

Good ideas ... thanks.

George brings up a good point - I think our Dylithium Crystals are getting pretty exhausted and may need to be replaced.

Hmmm, I wonder what kind of import license we'll need to get the new crystals; last I heard they were only being made in China and off-planet.

Best,
Casey&Mary

hardee · Posted: Sat Sep 21, 2013 2:43 am Post subject:

I have had the GPS signal get lost when I key up for transmission at times. Can't say if it is every time or not, it is only an issue when I am running on the auto pilot and then the track gets lost. Have had to reset it. My GPS puck is mounted flush on the brow, and the backup is inside on the shelf.

Looks like I will be looking for some beads too.

Thanks for the hints.

Harvey
SleepyC Moon

PeterQ · Posted: Sat Sep 21, 2013 8:54 am Post subject:

Gene at Mobile East Marine installed my external Garmin GPS antenna inside the boat, just forward of the helm bulkhead and near the starboard sidewall. Frankly I was a little skeptical but it works perfectly, the wire run is extremely short and in the opposite direction from the radio and radar cables. No interference issues!

Pete

thataway · Posted: Sat Sep 21, 2013 3:22 pm Post subject:

GPS signal goes very well thru the top of the C Dory pilot house. Latest chips will work inside of houses, and cars.