Mobile antenna grounding: is cable resistance and length important?

[AccuNut]

(Long post, sorry. Synopsis/TLDR at the end)
 

Does impedance or cable length matter on a chassis ground for a mobile setup?

I’ve read several posts and a couple articles about proper chassis grounding of the antenna (specifically the ground plane, and yes I know they are different) but can’t seem to nail this one detail down.

Some sources say use an exposed grounding strap less than 10” long, others refer to grounding NEAR or TO the battery ground on the vehicle body (not the battery itself).
I assume they are not using exposed braid for the multiple feet between their antenna and engine bay, (I could be wrong,) so potentially coax cable? But no mention of what kind.

The only thing consistently stated (which makes sense) is to make sure both ends of the grounding cable actually contact bare metal.

In my mind (not an electrician and new to radios) it would make sense for the ground to have less resistance than the input as it is the “escape route” for unused energy, preventing it from feeding back down the outer braid of the coax.

On the other hand it also makes sense (in my head) for the ground to match the input resistance so it doesn’t allow energy to escape that should be turned into radio waves.

Again, not an electrician, new to radios so I’m probably not thinking of this the right way!

In case it’s useful, here’s what I’m working with conceptually at the moment as far as radio/antenna setup:

I have a MXT575 and am waiting on a Larsen LP450NMO antenna to show up.

I’ll be mounting it on a custom ground plane mount (24”x24” aluminum plate) behind the rear roll bar of my Jeep JKU with a hard top, high enough to clear the roll cage. The radio will be mounted directly under it, maybe 12”-16” of coax between the two.

Before anyone asks, I am trying to avoid external mounting of the antenna as a personal preference. Also, short of drilling holes in my top this gets the antenna above the roll cage which is a plus!

Synopsis/TLDR:

1: does cable length and/or type (shield vs unshielded) and/or resistance matter when chassis grounding a mobile antenna ground plane?

2: does grounding location matter?

Thanks in advance for any info everyone!

[SteveShannon]

7 minutes ago, AccuNut said:

(Long post, sorry. Synopsis/TLDR at the end)
 

Does impedance or cable length matter on a chassis ground for a mobile setup?

I’ve read several posts and a couple articles about proper chassis grounding of the antenna (specifically the ground plane, and yes I know they are different) but can’t seem to nail this one detail down.

Some sources say use an exposed grounding strap less than 10” long, others refer to grounding NEAR or TO the battery ground on the vehicle body (not the battery itself).
I assume they are not using exposed braid for the multiple feet between their antenna and engine bay, (I could be wrong,) so potentially coax cable? But no mention of what kind.

The only thing consistently stated (which makes sense) is to make sure both ends of the grounding cable actually contact bare metal.

In my mind (not an electrician and new to radios) it would make sense for the ground to have less resistance than the input as it is the “escape route” for unused energy, preventing it from feeding back down the outer braid of the coax.

On the other hand it also makes sense (in my head) for the ground to match the input resistance so it doesn’t allow energy to escape that should be turned into radio waves.

Again, not an electrician, new to radios so I’m probably not thinking of this the right way!

In case it’s useful, here’s what I’m working with conceptually at the moment as far as radio/antenna setup:

I have a MXT575 and am waiting on a Larsen LP450NMO antenna to show up.

I’ll be mounting it on a custom ground plane mount (24”x24” aluminum plate) behind the rear roll bar of my Jeep JKU with a hard top, high enough to clear the roll cage. The radio will be mounted directly under it, maybe 12”-16” of coax between the two.

Before anyone asks, I am trying to avoid external mounting of the antenna as a personal preference. Also, short of drilling holes in my top this gets the antenna above the roll cage which is a plus!

Synopsis/TLDR:

1: does cable length and/or type (shield vs unshielded) and/or resistance matter when chassis grounding a mobile antenna ground plane?

2: does grounding location matter?

Thanks in advance for any info everyone!

1. Yes, theoretically, but you’re really asking about how well the various body parts are bonded. If you have enough impedance between where you connect the black wire of your radio and the negative post on your battery, you’ll see a voltage drop to the radio, worse at high current.

2. Yes

Your chassis ground is a DC ground used as the return for the car’s electrical system. Your ground plane does not need to be connected to your chassis ground. You could use a sheet of metal completely isolated from your vehicle as your ground plane, but your radio requires a good low impedance path for its dc power source. 

[AccuNut]

Thanks! That clears it up for me!

[Guest]

I’ve never once chassis grounded any of my 460mhz antennas.   A cb antenna like a Wilson or fire stick yes but a gmrs antenna no never ever.  None of my antennas even have a place to attach a chassis ground.  Now I run my radios directly to the battery both neg and pos with a fuse in both. I have extremely low swr and get 50-60miles easy from my trucks.