VHF Interference

[Kman]

Maybe there is something to the old saw about keeping VHF antennae away from other electronics.

I made another “doughnut” all-around LED light that slides part way down the Galaxy VHF antenna, but this time the doughnut has a separate circuit with 15 downward-facing LEDs with faceted lenses to light up the cockpit—which it does beautifully. I usually design for 12 volts (3 LEDs in series, resister for 18ma + some goodies for longevity) and feed LEDs via a 12V regulated source, but I got lazy and added two 4002 diodes in series to knock off ~1.5 volts instead. The thing worked great off of a heavily-filtered Tripp Lite 13.8VDC power supply with the diodes bringing the voltage down to a little over 12V.

Here’s the weird part: When I key the mike on the VHF, the LEDs dim considerably. WTF? I traced the problem to the diodes. If I move the diodes to the power supply end of the wires, everything is fine, but as I move the diodes towards the antenna, the LEDs dim, eventually going out altogether when the diodes get about .5” from the radiating antenna.

Learn something every day. Prolly one is that you don’t use diodes to lower voltage, but it was all I had laying around.

Later...

OK did another test, this time with the all-around light and its wires no where near the antenna. Key the mike and no dimming. Now key the mike and move the diodes (still on the power supply side) and it starts to dim at 2" and goes completely out at ~.5'.

I guess that rules out the length of wire along the shaft of the antenna--which was shielded anyhow.

Here's a picture of that bad boy. Man it works good. The all-around LEDs hurt to look at even at 100', and the look-down LEDs light the boat well enough that you can tie a knot anywhere in the boat and lets you identify about any fish before it comes over the rail--important when night fishing as it's no fun to bring a bass into the boat that turns out to be a sculpin :o.

When testing is done, I'll put some silicone grease on the antenna (to act as a bold release), slide the doughnut down the antenna, then pot it by filling the insides with thermally conductive epoxy to waterproof it.

Kurt



PS I keep a "legal" all around light on board just in case.

[Birdman]

huh? Ahhhh.... so what the heck is the purpose of this "thing"?

[diver dave]

Not sure about the lighted space ship issue, but any wiring run along a significant length of the antenna, next to the antenna, can't be good. I suspect that your 1n4001 diodes are rectifying in the presence of high rf fields, causing either too high or too low voltages to the leds. I'm going to say that the space ship is a bad idea on the antenna...

[Kman]

Birdman - 4/19/2005 12:21 PM

huh? Ahhhh.... so what the heck is the purpose of this "thing"?

This is one of those “don’t try this at home” things LOL.

The boat has no top so there is no good place to mount the all around light. We calico bass fish at night a bunch (which means a lot of casting), so the light needs to be out of the way—and running so knuckleheads don’t run over us. It needs to be up high too so as not to ruin our night vision. The only thing high on the boat is the antenna, hence the doughnut idea. I’ve been using one for several months with very good results. The thing is VERY visible and it doesn’t hurt out night vision as it’s several feet above our head.

I’m sure there are 50 electronic reasons not to mount this rascal to the antenna, but the radio’s SWR and output are fine (lights powered or not), and we’ve had several compliments as to the quality of our VHF broadcasts. We carry a back up hand held in case the thing toasts, and this is strictly an inshore boat. BTW: The light is only mounted when needed. It has a waterproof plug that plugs into the helm, and the light itself simply slides off the antenna.

It also is an awesome cockpit light.

Here it is: