Between trunk-mounting and roof-mounting a 2m/70cm antenna, both as through-hole NMO mounts, is there really that big a difference performance-wise? I mean, I know roof mounting is better, but let's face it: looks are a factor for some of us (and especially our significant others), and a trunk mount is much more understated than dead center of the roof. Plus, from a convenience standpoint, if you live in a city, it's nice to be able to park in a garage without removing the antenna.

I've seen the diagram from Larsen's catalog, but numbers don't tell the whole story. Especially not to a new ham like me. -3dB is half the signal strength gone, or put another way, just half an S-unit. I'm more curious about practical experiences (especially from people who have tried both). Plenty of people act like you're asking for trouble if you don't roof-mount, but truthfully, would I really notice those missing 3dB, and if so, would I mind it? Or will there be other effects that I'll mind?

If necessary, to give the question a little more scope, this is my first mobile installation (if it wasn't obvious). The antenna is a Larsen NMO-2/70B. I drive a 2014 Camry and live in the middle of Philly (think multistory buildings), but take that with a grain of salt. If I'm getting in my car at all, I'm pretty likely going out of town.

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    $\begingroup$ I just learned that the roof of my car is not ferromagnetic. Who would have thought! So, I hope the trunk is an acceptable place for the antenna. $\endgroup$ – pupeno - M0ONP ACI1DM LU5ARC Oct 4 '17 at 18:42

Yes there is a difference but it tends to have more to do with the pattern and efficiency of the antenna than anything else.

When an antenna is mounted on the trunk, the roof line and the occupants of the vehicle tend to block RF energy in the forward direction. The rearward direction is minimally affected. So in this case, depending on the direction you are driving this may or may not make a difference.

Placing the antenna on the roof of the car not only gives it an unobstructed view, but it also slightly increases the size of the ground plane under the antenna. This may help to increase the efficiency (and thus gain) of the antenna somewhat.

When communicating on VHF and UHF, the most reliable communications come when the stations are in line of sight to one another. The roof does give a couple of feet of added height to the antenna possibly improving the chance for line of sight communications.

The net of all of this is that it is difficult to come up with a hard and fast rule if one antenna location will allow communications when the other antenna will not but in general, the roof mounted antenna will be superior.

My advice is to do the best installation that satisfies your and your family's criterion for what is acceptable for your circumstances. You will communicate effectively in most practical situations.

  • $\begingroup$ Thanks. I was thinking about the pattern too before I wrote the question but forgot to include that. This is a helpful answer, though. How bad do you think the attenuation in the front would be? $\endgroup$ – Dominick Pastore Sep 29 '17 at 21:18
  • $\begingroup$ Also, the other thing I didn't notice until I was just out looking at my car is that the trunk isn't level at any point. The entire thing is sloped back some. I imagine this would affect performance too, but hopefully not too much. $\endgroup$ – Dominick Pastore Sep 29 '17 at 21:20
  • $\begingroup$ @dominick It will have a negligible effect. $\endgroup$ – Glenn W9IQ Sep 29 '17 at 21:28
  • $\begingroup$ The slanted trunk might give some slight additional gain in the reverse direction. Also consider that a metal car roof blocks a significant amount of power from the occupants -- so if you don't have it on the roof, you may want to operate with reduced power (say, well below 50w). $\endgroup$ – user10489 Aug 5 '19 at 0:53

The movement of the whip,will effect radition pattern more then location where it is mounted while traveling is my though. Grounding vs mag mount, when the coax is on the antenna and you have a grounded antenna thru the radio and negative lead for power is my though on this as well. So why would a seperate ground be necessary?

  • $\begingroup$ Electrical ground is not the same thing as antenna ground. The metal around the antenna becomes part of the antenna, so it has a huge effect, while the motion of the antenna has an insignificant effect unless you are using a wet noodle. $\endgroup$ – user10489 Aug 4 '19 at 13:44

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