Mobile antenna: Antenna length or height above car most important?

[TFin04]

Hi guys,

I'm looking to set up a 2/440 mobile unit in my hatchback. Because of other cargo carrying needs, my antenna options are a bracket off the hood (where a traditional car radio antenna usually is), or a lip mount on the hatch which mounts closer to the bumper.

What I'm curious about is if the length of antenna or the overall height above the vehicle are most important. With the hatch mount I would be able to get more antenna mounted for the same overall height, I'm just wondering if that will make an appreciable difference or not. The hood mount would be a bit easier and cleaner look overall, I think.

Thanks for your time,

Tom

 

[popnokick]

You want to get the antenna as far away from metal... Like the car body... As you can. On a vehicle that usually means higher so the antenna is not obstructed by adjacent metal. A long antenna on the bumper will be nowhere near as effective as a short one on the roof.

 

[TFin04]

You want to get the antenna as far away from metal... Like the car body... As you can. On a vehicle that usually means higher so the antenna is not obstructed by adjacent metal. A long antenna on the bumper will be nowhere near as effective as a short one on the roof.

So given my options, it sounds like the hood mount is the better bet. The A Pillar is a lot less metal than a rear hatch door.

Thanks for your help.

 

[n5ims]

Antenna length is really not an issue, the key to length is the antenna design. Some designs are long (and generally have more gain) while others are short (and don't generally have as much gain). The key is that the length be correct for the antenna's designed frequencies.

Height is generally important on VHF and UHF frequencies, but don't get too carried away about it.
Remember that it isn't just an antenna, but an antenna system. Important factors are antenna design, height, clearance, and counterpoise (aka ground plane).

Dead center of the roof will give you maximum performance (generally highest and with sufficient metal area for a good counterpoise) but they make it twang garage roofs and could easily be pulled out of the mount if you're not careful.

Trunk mount (or hood mount) is often a simpler install and generally does allow for a longer antenna. However the counterpoise area is generally one-sided and smaller. Often there is also blockage from the car's roof area. This blockage and counterpoise imbalance can alter the antenna's pattern.

 

[miserytom]

I prefer NMO mounts , but I don't mind drilling holes in the roof lol

 

[Project25_MASTR]

Larsen has a comparrison chart in their catalog. Though I don't necessarily believe it...it is worth a look.

Height often means more than gain.

 

[KK6RWY]

I just went through this analysis with a coupe that had a small area on the trunk for ground plane, but no practical option for the roof because of the retractable sunroof. (I opted for an NMO mount on the trunk so I can switch between a Comet λ/4 whip and a Larsen 5/8 λ whip — and have found that the shorter and longer antennas are indistinguishable in real-world performance.)

What I would say is that a λ/4 magnetic mount placed in the center of your roof and moved or removed when you need the area for cargo will likely perform better than any other option. Maybe even better, if you feel confident about removing your headliner, would be installing an NMO mount in the dead center of the roof and just unscrewing a λ/4 whip for the times you’re using the roof rack.

The other options you’re considering are suboptimal, but frankly any quarter-wave dual bander will probably get you into local repeaters even without an adequate ground plane. If you can put the lip mount higher up on the hatch, a 19" whip will probably be fine.

KK6RWY