Radio interference

[Akapeterc]

Just installed a new icom commander mic at my helm and discovered that when I turn to channel 73 it shows busy and there is a humming noise. I have discovered that when I switch my anchor light on the interference stops. Also I discovered a month earlier that when anchored in an area with weak TV reception we loose reception when switching on the anchor light. I think there is an LED light in the anchor light as I found a spare on onboard. Any suggestions on why this would be occurring? I did have the mast removed several months ago after purchasing the yacht when the rigging was done. Could the polarity of the light be wrong?

 

[Bill Roosa]

If the anchor light is LED it probably has a pulse with modulator (PWM) to drop the voltage down to what the light can take. These operate in the 100-200 MHz range (channel 73 is around 160 MHz) and are typically RF noisy devices. I have no idea why the interference stops when you turn the light though. No power would mean no PWM. Perhaps a typo and you meant off not on.
One fix is to connect the power and ground wires at the light with a small capacitor to "short the alternating current" and kill the ability of the LED PWM from being a radio. closer to the light is better.

 

[203]

If the anchor light is LED it probably has a pulse with modulator (PWM) to drop the voltage down to what the light can take. These operate in the 100-200 MHz range (channel 73 is around 160 MHz) and are typically RF noisy devices. I have no idea why the interference stops when you turn the light though. No power would mean no PWM. Perhaps a typo and you meant off not on.
One fix is to connect the power and ground wires at the light with a small capacitor to "short the alternating current" and kill the ability of the LED PWM from being a radio. closer to the light is better.

PWM control of a LED is around 200 Hz, not 200 MHz. However, if the switching rise times and fall times are fast enough, the resulting noise can extend into the VHF spectrum.

Possibly the PWM device runs all the time, and creates noise until it's loaded by the LED ? Who knows. That could be tested by removing power from the PWM device while the light is off. I assume it's a separate device down in the cabin area?? if the PWM device and the light are together up on the mast, then the power is already removed when the light is off and this theory goes up in smoke.

Or it's a typo.

Normally if an LED polarity is wrong it just won't light. However, if there is a PWM device in the picture, then again I'm guessing. However, I'd bet the same is true, the LED just wouldn't illuminate.

Bill's solution of a capacitor at the source is a great place to start. Across the power source.. value not too critical, .5 MFD at 50 V is a start. Keep the voltage rating well above the system rating on the boat, capacitance value is not critical. If I got an SASE good for a coupla ounces I'd be glad to send you one (c:

One other source for interference at VHF is handheld pads, computers, phones, etc ad nausea .. but they don't have anything to do with the light being on or off.

It'd be a problem that would have to be solved here, our race committees like 73 (c:

 

[Bill Roosa]

It has been my experience that LED bulbs all have the PWM chip in them so turning the power off to the light would also kill the PWM chip.

203
While I will agree there are PWM units that run in the hundreds of Hz range these are top of the line items because tey want a RF quiet solution. MHz PWM are much more common as that is the freq a lot of the chips operate at and the programing is ultra simplistic if all you are doing is turn on-turn-off-...(repeat 1000 times) ...sense-turn on..........
using Mhz freqs also allows the parts caps inductors to be smaller and actually fit inside the bulb.

 

[Akapeterc]

Hi it definitely stops when you switch the anchor light on.

 

[Akapeterc]

Even though the microphone says busy on it I am still able to talk over it to Marine Rescue and they report back that it just sounds a bit staticky

 

[ben solomon]

I had a similar issue on my 37.5 with the refrigerator providing interference to the helm mic after installation. Got the same hum and the same busy signal. Drove me nuts. Ultimatley, I figured out that it was when the fridge was running when the hum would happen. The vhf and the refrigerator were on the same "row" on the circuit breaker. So...I exchanged the vhf with something else so they weren't on the same row in the circuit breaker panel and the problem was solved.

Plug the ram mic into the back of the vhf and try to reproduce the hum to make sure it is not your extension cable that is the issue.

I must say that when I read these replies, the amount of knowledge the other sailors have is very impressive.

Ben

 

[artboas]

Need to correct some terminology

It has been my experience that LED bulbs all have the PWM chip in them so turning the power off to the light would also kill the PWM chip.

203
While I will agree there are PWM units that run in the hundreds of Hz range these are top of the line items because tey want a RF quiet solution. MHz PWM are much more common as that is the freq a lot of the chips operate at and the programing is ultra simplistic if all you are doing is turn on-turn-off-...(repeat 1000 times) ...sense-turn on..........
using Mhz freqs also allows the parts caps inductors to be smaller and actually fit inside the bulb.

Ok, led lamps are current driven devices. That means that the amount of current going through them determines the brightness of the led.
The simplistic led circuit is a led with a resistor in series. What does the resistor do? Well, a led is a diode, just like any other rectifier.
In one direction it has very low resistance. Therefore, without the resistor it is a dead short across the + 12 and ground.
So, the resistor is chosen to supply a fixed current for maximum brightness.
When trying to dim a led, you cannot do this with a variable resistor, as you can with an incandescent bulb. What you need to do is send a varying pulse width duty cycle to it, which controls it's peak current. This is where you use Pwm; it is for dimming applications .
When a led is driven directly from 12v with no external resistor, it uses a simple current regulator. Part of the regulator accepts a range of voltages from 12-36 v.
It then puts out a fixed current at a constant dc voltage. It will not be a chopped or Pwm signal. It will be constant dc.
Now, getting to your interference on the VHF & tv.
I suspect your mast grounding. Check that the mast has a solid dc ground.
Also, is your tv antenna masthead mounted?

 

[Akapeterc]

Hi, yes the TV arial is fixed to the top of the mast