Another VHF Issue

According to several "listeners", my VHF broadcasts with a good signal strength, but it often has an accompanying background hum. Also, if I broadcast longer than about 20 seconds, the GPS loses signal?
As background, besides this "fixed" VHF, there is a remote Mike attached about 10 feet away, and the exterior of the wire to the mike is deteriorating.
Reception on the radio is good. There are 3 six to eight foot long "solid" antennas on the roof of my boat. I only have one VHF and one CB radio (almost never turned on), as well as a stereo. (One GPS antenna as well).
Suggestions as to the problem or where to start?
Thanks,
Tom

Do you have an inverter running when the hum happens? How close to the GPS antenna is the VHF antenna?

Dave,
The last time I did a radio check with the Can. Coasties (2 weeks ago) the hum was reported to me, and no inverter running. The charger was on though. It is an inverter/charger in one.

One of the rooftop antennas is only about one foot from the GPS antenna. I am not sure which of the 3 antennas is used by the VHF.
Thanks,
Tom

Sounds like xmit from your antenna is overpowering the GPS receiver. You could separate them horizontally if you have room. Otherwise perhaps an antenna extension that will raise the VHF antenna such that it radiates above the GPS.

You probably need to figure out which antennas are which too.

A 60hz hum is not uncommon when transmitting, and is coming from the AC power system. It could be coupled from AC cable to DC power where the wires run next to each other. The general goal is to keep AC and DC cables separated for this reason. You could test by temporarily running the VHF power directly to a small 12V battery with nothing else connected to it. If that clears it up then you have noise coming in thru the power line. A separately wired power feed for the VHF would then be warranted.

TT answered this as I would have. The only added item that may be of value is using a DC line filter, near the VHF, like the Newmar series of conditioners. I don't have direct experience with those, I tend to make my own EMC type filters. My professional work makes that one easy...

Dave and TT,
Thanks to you both for taking the time to provide me with good information. I will get this checked out and look into the EMC filter idea or the separately wired power supply. I now have a second GPS (puck this time) antenna connected to my new (replacement) laptop using (new to me) Coastal Explorer. Hopefully this new setup will not be affected by the VHF. Currently, most VHF broadcasts are not long enough to affect the GPS, but when I do a Securite (for example when entering a narrow winding passage), that is when it happens.
Thanks again,
Tom

firehoser75 said:

.. but when I do a Securite (for example when entering a narrow winding passage), that is when it happens.
Thanks again,
Tom

Click to expand...

This story jogged my memory a bit. Some years ago, we found ourselves in the middle of a boat/ocean rescue. During my VHF call to LEO, they asked me my Lat/Long. I look over, mic in hand, and see warning flags on the GPS. I was in a marlin tower at the time, the vhf, gps, and associated antennas were in fairly close prox. Basically, I had no good gps readings while I was transmitting. You can imagine the problem.
Eventually, the problem was sorted, and the fire trucks, helicopter, HS ocean rescue boat all, eventually, knew where to go.

diver dave said:

This story jogged my memory a bit. Some years ago, we found ourselves in the middle of a boat/ocean rescue. During my VHF call to LEO, they asked me my Lat/Long. I look over, mic in hand, and see warning flags on the GPS. I was in a marlin tower at the time, the vhf, gps, and associated antennas were in fairly close prox. Basically, I had no good gps readings while I was transmitting. You can imagine the problem.
Eventually, the problem was sorted, and the fire trucks, helicopter, HS ocean rescue boat all, eventually, knew where to go.

Click to expand...

Dave,
Yes, that is one of my fears, but I am hoping that the new GPS puck may provide an option. I also hope I can solve the problem with the other antenna. We don't really know how much we rely on these "modern conveniences" until they don't work when really needed!
Tom

firehoser75 said:

Dave and TT,
Thanks to you both for taking the time to provide me with good information. I will get this checked out and look into the EMC filter idea or the separately wired power supply. I now have a second GPS (puck this time) antenna connected to my new (replacement) laptop using (new to me) Coastal Explorer. Hopefully this new setup will not be affected by the VHF. Currently, most VHF broadcasts are not long enough to affect the GPS, but when I do a Securite (for example when entering a narrow winding passage), that is when it happens.
Thanks again,
Tom

Click to expand...

Some methodical experimentation I think will be your best path to solving this. If you can try the GPS puck in a location away from the VHF antenna and show no interference, then move it to the old GPS location and show the problem returns, then you will have a solid understanding of the problem, and a way to figure out how far apart the GPS and VHF antennas need to be.


Same with the 60 hz hum (I assume that's what it is). Any rewiring should be temporary and experimental only to help identify the source of the problem. It's probably coming in over the power wires to the VHF, but it could be getting picked up by the remote mic cable. Disconnect the remote mic cable and see what happens.


My only caution would be to avoid bringing a tech on and asking them to "fix it". I can pretty much guarantee they will take an uninformed and uneducated guess, spend your money, and solve nothing.


Re the Newmar filters, ferrites, etc, I think they generally filter out higher frequency noise, so may not be the best solution. But at least a ferrite is easy to try.

I glossed over the Newmar reference a bit fast. They have a wide variety of items in this space, from HF filters for the DC line, to isolated DC to DC converters, to even 12Volt UPS (like) boxes with a battery inside. The exact unit I was thinking of is this: https://www.psicompany.com/newmar-12-12-6-power-stabilizer-converter/

It costs almost what a cheap VHF costs...But, it should block all common mode and differential mode ripple.

How is your battery health? Liquid level sat? I have come across several boats with vhf hum on transmit that had bad ships batteries.

HW,
My batteries are new this last spring and I keep them topped up checking them every month (my checking has shown that this rate of maintenance is more than adequate). I had this issue prior to the new batteries.

I had a vhf issue as well on our new to us boat in July. I could hear fine but transmission was loud but not audible. My first approach was to hook in another old vhf I had as a spare but I knew it worked, to see if it was really the radio. It turned out the antenna was no good, would receive but not transmit. Changed antenna and problem solved. May be worth trying if you have one available??

Thanks Allan. Certainly worth a good look at the antenna. Unfortunately, I don't own an VHF. But worth considering if I could borrow one.

Hasn't been discussed but if your "fixed" vhf implies a repaired unit, it could still have power amp issues.
See if there is a ground lug/wing nut on the back of it, if so insure it has a good ground connection separate from the DC power connection.

Shakespeare makes a VHF SWR meter to test your antenna. About $120. Wow the price sure has gone up! Checks output power and has a tone generator to check the receiver too. Will not test for hum. You will also need a 3 ft coax jumper with PL-259's on each end.

BTW, is the hum still there on LOW POWER?

WB,
Thanks for that thought. I will definitely check all of the connections. However, I am not sure if the VHF is a repaired unit. I don't think so, I think it was new with the boat when built (2002), but I am the third owner.

I would start by figuring out what antennas you have and what they are used for. I would trace the cables and at the same time look them over for condition. If you think the remote mike is in poor condition then unhook it and do another test. You can eliminate 120 volt hum by unplugging the boat and turning off any inverters. I would look at the PL259 connectors for corrosion and quality of soldering on the antenna that you find going to the VHF radio. Check the voltage at the radio when it is transmitting. Check all connections coming from the battery to the radio and back to ground for tightness and corrosion. You need to methodically check the system from start to finish. Keep in mind that it could be an internal problem with the radio. If you don’t have a backup VHF, then now would be a good time to buy another radio and use it to check the wiring and antenna. Then install it on a different antenna and you will have a backup and also the ability to monitor 2 channels at the same time without relying on the scan function which can be a problem at times.

We have found that LED lights cause interference with boadcasts. Look into trying broadcasts with them on and off and see if that makes a difference.

That said, the issues with antennae that others have raised are possible causes. We have also discovered that feedback occurs when one radio is on 16 and another is on another channel.

Jim