Grounding an aluminum hood

[bd348]

Midland MXT275.  I get mild alternator noise in the mobile radio.  The radio's power leads are wrapped several times through a ferrite loop.  The antenna is a mag mount on the aluminum hood via an adhesive metal disc.  No ferrite beads on the microphone cable or antenna cable, so far.

I have two foot braided grounding strap which I'd like to use to ground the hood better.  Drilling and bolting into the aluminum hood can lead to corrosion due to dissimilar metals.  The hood hinge bolts are recessed pretty deep so it's hard to get an additional fastener onto them..  I was wondering about the idea of using capacitor coupling for the grounding strap, similar to the mag mount antenna.  The strap could be bolted to a painted metal plate, and the plate could be slid under the hood insulation where it is held by a clip, holding the disk against the underside of the metal of the hood.  Perhaps I would use one of the self adhesive discs which are sold for mag mounts.

Is this a reasonable approach, or should I get the beads for the antenna and microphone cables, or all of the above?

[gortex2]

Id start by moving the antenna. A real antenna and not the little whip would also improve issues. As for grounding the hood. Your on a metal plate stuck to the hook by tape. Grounding the hood will do nothing for you. I'll bet if you checked continuity between the hood and the battery you will see its flows anyway. There are many better options for an antenna that what you are using. 

[Sshannon]

3 hours ago, bd348 said:

Midland MXT275.  I get mild alternator noise in the mobile radio.  The radio's power leads are wrapped several times through a ferrite loop.  The antenna is a mag mount on the aluminum hood via an adhesive metal disc.  No ferrite beads on the microphone cable or antenna cable, so far.

I have two foot braided grounding strap which I'd like to use to ground the hood better.  Drilling and bolting into the aluminum hood can lead to corrosion due to dissimilar metals.  The hood hinge bolts are recessed pretty deep so it's hard to get an additional fastener onto them..  I was wondering about the idea of using capacitor coupling for the grounding strap, similar to the mag mount antenna.  The strap could be bolted to a painted metal plate, and the plate could be slid under the hood insulation where it is held by a clip, holding the disk against the underside of the metal of the hood.  Perhaps I would use one of the self adhesive discs which are sold for mag mounts.

Is this a reasonable approach, or should I get the beads for the antenna and microphone cables, or all of the above?

I think you’re probably thinking about “bonding” your hood to the rest of your car’s electrical system.  Bonding is a way of reliably connecting together parts electrically to reduce RF noise and is commonly done for the doors, hood, and trunk on vehicles by hams.  I suspect that Gortex is right in saying it’s probably already well connected. Capacitors act to block DC so I don’t think that would work.

[marcspaz]

Honestly, if you are hearing alternator hum on a modern FM radio, I would be more concerned about the health of your alternator. The alternator should be replaced if AC ripple voltage of more than 50mV is measured. You usually can't hear it in an aftermarket FM radio unless the radio is trash or the ripple voltage is over about 300mV.

[bd348]

It's a new Jeep with aluminum hood and fiberglass roof, thus the location.  There are many wires in there which could be causing engine RPM dependent noise.  I want to RF ground "bond" the hood to shunt these signals to ground.  While it could be said that the hood hinges and latches ground the hood, I've seen some recommend the braided ground cable for the hood, trunk (where present), exhaust, etc, especially each horizontal surface.

The little stock antenna has an advantage in that you can open the hood and tuck the antenna away under the cowl for storage in case you want to hide radio stuff.  And it doesn't attract much attention when deployed.  I did cut back the 20 feet of thin RG174 down to six feet and added a connector.  According to Coax Calculator this will change my radio from an effective 6 watt unit into an effective 11 watt unit.  If it used RG58 it's be closer to 13W.  Some day I may cut it down to one foot and use connectors to RG58 for the other five feet.

In the end I'll just experiment and see what happens.  The radio is currently at Midland for wide band reprogramming.

 

[marcspaz]

I'm still convinced there is something wrong. I have owned Jeeps most if my life. My last 4 JK's and my current JT all had antennas mounted to the hood and I have never had noise on any of my radios. 

 

Also, I'm assuming you have a JL or a JT, which are already grounded. Adding a bonding strap isn't going to change anything. 

[marcspaz]

For troubleshooting, run the radio on a separate battery and see if the noise is gone. Also, if you unplug the antenna from the radio, is the noise gone?

 

Without getting into a bunch of technical BS about capacitive and inductive coupling and introducing noise, mounting a mag mount on a plate that is sticky taped to the hood is likely contributing to your issue.

I strongly recommend this type of lip mount.

 

 

 

Also, if you absolutely insist on a bonding strap, go from the hinge bolt to the body like the points in the photo.

 

[bd348]

The radio arrived back today, and I reinstalled with the newly shortened antenna cable.  The noise is now gone.  14 feet of RG174 in a coil was really picking up something.

It's a JT, and those hinge bolts are recessed pretty far.  The threads are really back in there deep.  I may be able to find a metric standoff which would work.  But since the noise is gone I guess it isn't necessary.

The mag mount steel plate is not really sticky tape, but rather a thin layer of only adhesive.  No backing.

My concern with lip mounts is that they seem to require the screws pierce the paint to metal.  Unless thoroughly protected these will cause corrosion, especially with the aluminum and steel in contact.

[gortex2]

There are many mounts for a JT that will work fine. Many vehicles have been aluminum over the years. Antennas have been mounted on them for years. First think you should do is toss that antenna and mount out and buy a standard lip mount NMO with UHF whip. There are multiple bolts on the JT that touch aluminum so worrying about 2 small tips of a stainless bolt is not worth it. I run a true NMO mount on a bracket on both my JT and JK. You will find performance much better with a 1/4 wave on a NMO. You can use the lip mount @marcspaz used or a standard one from Laird on the top of the hood. Seen many like that so far. 

[WRQC527]

Good news that your noise is gone. I had this issue with a VHF/UHF radio installed in my Sienna, even after bonding everything. For those of you with chronic alternator whine, one way to tell where the noise is being fed into the radio is by removing the antenna from the radio. If the alternator noise persists, it may be coming in through the power leads, even if they are connected directly to the battery, which mine are. I ended up building a noise filter out of a Quick Link, 20 feet of 12 gauge stranded wire, and a 47k uf capacitor. It knocked out 99+% of the noise. Some will say my alternator is bad, but it's much cheaper to install a $20 filter than it is to drop $300 to replace a working alternator with a working alternator. Here's the link to the build:

http://www.sanantoniohams.org/tips/whine.htm