VHF Antenna Problem?

[BarryB]

Hi Gang,

I've had a problem for quite a while and have not found a solution.

First: Cobra VHF marine radio attached to a Shakespeare 4400 marine antenna.

Second: Power feed from battery is via 8 guage wire with 40 amp fuse at the battery. This power feed hooks to a power distribution box with individual fuses to each item - VHF, Depth Sounder, GPS, AM/FM/CD.

The depth sounder (Garmin 140) has a volt meter built in. When I transmit at 25 watts, the voltage on the volt meter drops starting at 13 volts and dropping to 11 before alarms go off and GPS goes into "dumb" world.

So, the question is: Could the antenna or antenna wire cause problems with RF energy leaking?

I measure resistence on the antenna cable. From center pin of PL-259 to ground (outside of PL-259) reads open. From center pin to antenna whip reads open. From ground to antenna whip reads zero ohms.

I don't know if this is normal.

According to Shakespeare's web site, this type of antenna reads open from center pin to antenna whip. There is nothing about ground to whip readings.

Any help would be appreciated.

 

[fridaysoff]

Barry,
I can't think of anything that could be wrong with your set up.Did you run your 8 guage wire directly from the battery terminal post to the distribution box? That would bypass the 40 amp fuse. You have fuses on each piece of equipment. That or the PL-259 connection is the only place I can guess there could be a problem.My VHF will not bother my voltage on my GPS. One other solution might be to lower the low voltage alarm on your GPS/depth sounder. My Garmin 545S is a combo unit. I was able to lower my voltage alarm on it because it would show at 11.5 when I would start my engine. Good luck. Let us know what the solution is.
John D.

 

[HighTechRedneck]

I know if you dont cut off the excess wire and just coil it up it can cause all type of problems.

 

[BarryB]

More info...

Hi Friday and HTR,

I cut the antenna cable so there would be no coil. I've had this problem since installing the system, I'm just tired of the entire 12 volt system shutting down (when voltage drops below 11, all the electronics crap out).

I'll borrow a VSWR meter tonight and check it out. I'll report back later.

 

[BarryB]

Okay...

Now I think I have a handle on the problem.

Used two different meters. One is self powered, plugs into the PL-259 and measures SWR and impedence. Antenna checked out okay.

Then took other VSWR meter and installed between the VHF and antenna. Antenna read SWR of 1.4 well within specs and 20 watts output from the radio.

I removed the antenna and moved it to the back of the boat, now, the GPS screen no longer blanks out when transmitting, however, I still read a power drop from the Depth sounder.

Next, I'll power the radio from a seperate battery and do some volt checking.

I think I'm going to find that the RF from the antenna (perhaps because the antenna is so low) is causing interference in the electronics. I just have to see if running the antenna cable under the boat (where all the power and other wires are) will correct or cause more problems.

 

[BarryB]

Repaired with the help of Shakespeare

Hi Gang,

The VHF radio, voltage drop, GPS freezing problem is repaired. Seems, spurious RF emissions from the antenna cable was causing the problem. The RF would confuse the computers in the electronics.

I ran RG-8x from the radio down below the floorboard and aft, then to the port side of the boat. Mounted the antenna on the rear seatback rail and tested the system.

The GPS no longer freezes up and the voltage remains steady on the depth sounder.

Glad that is finally fixed.

So the lesson is: Keep coax cable far away from newer electronics, else the RF may cause problems.