BNC bulkhead connector

[Tbone]

What are the thoughts on a standard pass through with grommet and proseal vs BNC connector through the firewall for the gps antenna/MB antenna coax cable? Would there be a noticeable reduction in signal loss? Other options?

[wjb]

Connectors are always a source of problems; two new interfaces to deal with a probably 1-2 dB of loss. If you can swing it, feed the cable through the hole and use a grommet or eyeball feedthrough to close things up. However, from a signal strength perspective, you won't see much if any effect on SNR of the GPS signals.

My personal preference is not to put the antenna under the cowl, but lots of others have done it and are happy.

[snopercod]

In theory, the BNC connectors will only lose about 0.1 dB per connector, assuming you use a good quality connector like Amphenol. That said, I worry about the vibration working things loose. I had the coax partially work it's way out of the BNC connector on my transponder antenna and give me intermittent signal. Having a bulkhead connector might provide for easier maintenance, but it would add several failure points. Not only that, but how will you access the connector on the back of the firewall? I agree with wjb; Just go with a continuous feed line. I would make the hole in the firewall large enough for a BNC connector to pass through. That way, you can purchase pre-fabricated cable assemblies from L-com. They make really good cables.

[RV6_flyer]

According to Amphenol a BNC connector will have 0.2 dB Max loss @ 3 GHz. (GPS is < 1.6 GHz)

I have seen bulkhead connectors that have the coax fastened to one side so the connector count drops to one instead of two connections. That typically gives ease of access / service with an acceptable loss.

I would try to find a UG-909/U connector and go that route if not too expensive.

[RV7A Flyer]

I used a bulkhead connector on the firewall for my MB antenna, because I did the thing with just a stripped length of coax glassed into the lower cowl (so it has to disconnect at the firewall to remove the cowl). No issues at all.

But then again, MBs are going away eventually anyway...

Also, both GPS antennae are under the cowl, and no issues at all, FWIW.

[rv7charlie]

One alternative, especially if you have multiple cables to pass through the firewall, is to pick a convenient spot & use a flanged tube, along with intumescent caulk & firesleeve. Spruce sells a kit for a lot of money, or you can search ebay or the local big box store for equivalents (stainless towel rods or grab bars). You can work around the mess of the caulk by 'running some lines' on a non-stick surface, letting them cure, and stuff the empty space in the tube with the little sticks of caulk.

You can even run lines that at a bit antagonistic to each other (noisy stuff with small signal stuff) by having them diverge on both sides of the firewall, so they are effectively crossing in the tube.

Charlie