AccuNut

Thanks! That clears it up for me!

(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!

Well, I appreciate the detail, that’s the main reason I posted under the “Technical” forum. For instance, I now understand dBi vs dBd better, and am more confident in going with the Larsen since it has some dBi gain which is what I was looking for to begin with. Just snagged a used Midland MXT575 for a good price, next up the antenna, some research on installing, and put it together!

@SteveShannon thank you that explanation helps a lot! My curiosity is now satisfied. Looks like if I want to go 3dB gain I’ll be getting an antenna built that way. Now to decide between trying a 3dB behind the rear rollbar or putting the puck antenna up there. Then again, that cowl mount is still an option. @nokones was that a gain antenna you had mag mounted to the rollbar? What kind of performance did you get? Also, @Hoppyjr what kind of range are you getting with the cowl setup? I’m curious since it’s a 0dB antenna. Lastly for both of you fine folk: is there a noticeable difference between performance in front vs behind your Jeeps with those respective setups? Still kickin’ around the idea of maybe a single puck mounted high with a custom ground plane mount…but I like options! Thanks to everyone else who chimed in too! I appreciate all the input!

Oops.

Could you explain why please? This is exactly what I am trying to get a better understanding of so any info about why it would/wouldn’t work would be appreciated. Good point. Is this the main reason you wouldn’t recommend trying it? Would the transmit power be so far reduced that any transmission pattern improvement would be lost? For example, would a 30W radio transmitting on a unity gain antenna generally outperform a 15W with a 3dB antenna in terms of range? (Flat terrain, minimal obstructions).

The roof is fiberglass. Kinda a good thing/bad thing since I can’t use it as a ground plane on top, but conversely it shouldn’t affect internal antennas as much. As for the ground plane, I should be able to cook up something for that. The cab above the rear rollbar is basically invisible unless I have the hatch open, so a piece of sheet metal would virtually disappear up there. I was actually thinking the rear rollbar. There is enough clearance for it there if I found/built a mount to account for the backward slope of the bar itself. (Turns out the rear bar is not round). That cowl mount is pretty slick! If I can’t find a good solution inside I might just give that a shot! Similar yes! Except the JKU has a second crossbar in the back due to the longer body. I am hoping to mount it to that. I am still curious if it is possible to co-phase them to achieve a 3dB gain. Even if I do end up with an exterior mount it would be nice to have the extra range from such a low-profile antenna. Is there any reason they couldn’t be co-phased? I’m not entirely familiar with how ghost antennas achieve adequate transmission to start with, so I’m wondering if the design inherently hinders co-phasing.

Hello all! I’m new to radios in general but have poked around for a while gleaning what I can. From what I have gathered, co-phasing antennas increases gain. I am wondering if it is possible to co-phase puck style antennas? Not the 3+inch ghost antennas, more like the Larsen LP450NMO (1.5”). They are unity gain and I am hoping to achieve 3dB. I understand that whip antennas offer better performance, and there are those who detest non-whip antennas. I’m not trying to start a debate about whether I SHOULD use a puck antenna, but I am trying to go “super-ghost” if you will with my antenna install. I have a Jeep JKU (which I know is a difficult antenna platform) with a hard top. If co-phasing would give me 3dB I can mount them to my roll cage under the top. Unfortunately there is not enough room there for a standard ghost antenna, and I intend to avoid adding an antenna outside if it is at all possible. Thank you all in advance for any info you can provide!