RF drain circuit

[sesmith]

I'm just finishing up the installation of a Raymarine EV-100 wheel pilot on our boat. There is a dedicated RF ground on the controller and on seatalk power line. According to the manual, it looks like they want it run to a dedicated RF ground directly to the battery (my boat doesn't have an RF grounding system already).

Specs call for 10 ga wire for runs less than 3 ft and 8 ga for runs over 3 ft. That seems huge for what I thought was just for noise reduction. Can anyone explain the reason for the large gauge wire. Really necessary on small sailboat with little in the way of electronics other than the wheel pilot, vhf, and a plotter?

 

[Don S/V ILLusion]

The large size cable is not only for purpose of currying high current. So called "skin effect" is the reason for that recommendation - current, including RF, travels on the surface of conductors. The larger the surface, ...

The important consideration is to not tie radio and autopilot grounds together as you are exacerbating the problem. That's why they recommend dedicated grounds.


Unfortunately, having multiple grounds can create an unintended consequence of ground loops and electrolysis. Which compromise can be a tough choice sometimes.

 

[sesmith]

Thanks for the quick reply, Don. I ended up running a 10 ga. dedicated ground directly to the battery bank, even though the run was longer than the recommended 3 ft. 8 vs. 10 ga was determined by the fact that I would have had to buy a 100 ft roll of 8 ga., while I found a more reasonably priced smaller roll of 10 ga. Reality stepped in and the boat will just have to understand

 

[gettinthere]

You could double it up, giving a lot more surface area

 

[Brian D]

Sesmith, I don't think you need an "RF Drain Circuit" since you have no "RF" in your boat. I do not consider VHF to be a strong RF field that will affect you navigation system. Had you had SSB in the boat and operational then I would think the RF Drain would be needed, unless you had a really good RF grounding system.

RF ground IS NOT connected to the battery ground. That makes it an electrical ground. The two shall never meet. I fear by running a "dedicated" ground to the battery, and having an electrical ground in 12vdc circuit, you could create a small ground loop. Just be mindful of this.

 

[sesmith]

Thanks Brian. I'll keep that in mind. I'm probably fussing over nothing, but then I figured their engineers know way more than I do about RFI, hence the recommendation. One thing I have found out since the install, is a little more about the skin effect with ac signals. The higher the frequency, the thinner the skin that the signal follows on the wire surface, so I'm assuming that further explains the need for a larger cable to dissipate any RFI.

Anyhow, the wheel pilot works great, so far. Seems to be a major improvement over the (older and now non-functioning) Simrad / Navico unit it replaced

Here's the wording from the manual, FWIW:

"Grounding — Dedicated drain wire required

This product includes a dedicated drain conductor (screen) for
connection to a vessel's RF ground point.
It is important that an effective RF ground is connected to
the unit. The unit can be grounded by connecting the drain
conductor (screen) to the vessel's RF ground point. On vessels
without an RF ground system the drain conductor (screen)
should be connected directly to the negative battery terminal.

The preferred minimum requirement for the path to ground is via
a flat tinned copper braid, with a 30 A rating (1/4 inch) or greater.
If this is not possible, an equivalent stranded wire conductor
maybe used, rated as follows:
• for runs of <1 m (3 ft), use 6 mm2 (#10 AWG) or greater.
• for runs of >1 m (3 ft), use 8 mm2 (#8 AWG) or greater."

 

[sesmith]

You could double it up, giving a lot more surface area

Thanks. I thought the very same thing. If there's ever an issue, I'd probably try that first.

 

[Brian D]

Sesmith, if connecting your device directly to the battery neg terminal is what they recommend then recommend then I guess that is fine.