I’ll start by saying that this article is not about why to bond an HF mobile setup. In my opinion, K0BG.com is perhaps the best, most technical website for building mobile ham radio systems. His bonding page is excellent! He shares more technical information about bonding than I’ll claim to understand here. Instead, my goal is to show you what I’ve done to my car. Alan, K0BG, explains that bonding horizontal surfaces is more critical than the vertical surfaces. This photo shows one of the bonding straps for my hood. I have braided straps to bypass each hinge. I had bonded each of my doors, too, but the braided straps eventually broke from door use (photo below). I did not replace them. I also did not bond my hatch since it is another vertical surface. My most significant bonding efforts happened along the exhaust system.
I’ve long-been one to tinker with my cars, including their exhaust setups. My GTI’s exhaust is custom-made with stainless steel and part of what’s ultimately an upgrade to 400-hp (360 hp to the ground). I chose an unusual compromise between high-flow and sound abatement. So, I have more mufflers/resonators than most sports-oriented cars. As a result, I chose to bond my exhaust in four different places to account for the different pipe unions. I had the builder weld ¼-20 studs at two points in the exhaust. This photo shows an example. The other two bonding points were achieved at a V-band clamp and the turbo down-pipe. Each of the four bonding points are less than 10 inches from a removable factory screw or bolt and then attach to either the unibody or a structural member with direct unibody contact. I use either star washers or serrated nuts to dig into the metal for a solid electrical connection.
Speaking of the unibody, I must mention that most pickup trucks, full-sized vans, and some SUVs (anything with “body-on-frame” construction) require additional bonding between the ladder frame and body parts. RVs present a unique challenge, too! I do not cover any of that here since my car does not have a traditional frame.
This photo shows the turbo downpipe strap which connects the downpipe to the car’s subframe and a screw that goes through to the unibody. The downpipe features a flex-pipe that attaches to the turbo and the rear connection is a pipe clamp that attaches to the catalytic converter. The next exhaust component has its own bonding strap, shown in the previous paragraph’s photo. That pipe connects to the first resonator. The catalytic converter and the first resonator are covered by a skid plate; so, their photos are not clearly depicted on my websites. You’ll see hints of the skid plate(s) in the following paragraphs and in the photo album at the bottom of this page.
There’s nothing exciting about the strap that’s downstream of the second resonator. It’s attached to the second ¼-20 stud. I’ve shared a photo in the album below. For now, I’ll show the V-band connection. I tried three different final muffler setups before settling on the large suitcase muffler. My exhaust builder used a V-band clamp at the last union to make muffler swaps a cinch. There wasn’t much room to fit a stud; and it wasn’t important since I knew that I’d swap the “axle-back” more than once. Therefore, I opted to bond at the V-band clamp. I think it’s electrically sufficient.
Did all of this make a difference? Well, I honestly don’t know! I had read plenty at K0BG.com before installing my first HF rig in a previous car. For me, bonding and RFI mitigation was a foundational process that I accomplished before even buying an HF radio. I wasn’t interested in chasing RFI or a high noise floor later; so, I have no “before and after” testing to share. “Do it right the first time and happy from the start,” I guess! 🙂
Build It and They Will Call!
Scott
