Mag mount antenna location

[sparks40]

I'm currently using mag mount antennas for my ham rig, and being new to the hobby i have a question regarding the location of the antennas. I'm wondering if they need to be in the center of the roof, or is it okay to have it on the roof, but towards the rear of the truck cab where there isn't as much flex in the roof. Also, will the different locations on the roof affect the SWR? I have a 5/8 and a 1/4 wave, which will not be mounted at the same time. The 5/8 is for the mobile when i use it, and the 1/4 will primarily be for my HT. Thanks


73,
Dave

 

[jim202]

I'm currently using mag mount antennas for my ham rig, and being new to the hobby i have a question regarding the location of the antennas. I'm wondering if they need to be in the center of the roof, or is it okay to have it on the roof, but towards the rear of the truck cab where there isn't as much flex in the roof. Also, will the different locations on the roof affect the SWR? I have a 5/8 and a 1/4 wave, which will not be mounted at the same time. The 5/8 is for the mobile when i use it, and the 1/4 will primarily be for my HT. Thanks


73,
Dave

The use of a mag mount over time will eat the paint. It seems as you drive down the road, the minute sand particles make their way under the mount and abrade away the paint.

With that said, the location doesn't matter that much. Unless you have some large metal object near the antenna, it will not effect the SWR with the position on the roof.

Why don't you just bite the bullet and drill the roof and put an NMO mount and be done with it. My F250 truck has 4 antennas on the roof and 2 glass mounts.

 

[jackj]

Will it matter?

I'm currently using mag mount antennas for my ham rig, and being new to the hobby i have a question regarding the location of the antennas. I'm wondering if they need to be in the center of the roof, or is it okay to have it on the roof, but towards the rear of the truck cab where there isn't as much flex in the roof.

If your antenna is matched and gives you a low reflected power reading then the location will only effect the antenna's pattern. Placing the antenna in the middle of a 1/4 wave of ground plane will give you an omni-directional pattern. Offsetting the mounting location will make the antenna directional toward the longest ground plane.

Also, will the different locations on the roof affect the SWR? I have a 5/8 and a 1/4 wave, which will not be mounted at the same time. The 5/8 is for the mobile when i use it, and the 1/4 will primarily be for my HT. Thanks


73,
Dave

As long as you have a ground plane of at least 1/4 wavelength in one direction, the SWR will not be affected just the radiation pattern.

 

[benbenrf]

I'll concurr with jim202 on this - not-with-standing the role ground-plane does play in localised radiation pattern (both Tx and Rx roles), the reality of "on-the-move" monitoring (and the keyword here is mobile - versus static) is that the constantly changing geo-physical influences & man-made sorruondings a radio wave is subjected to, which lead to multi-path, scatter, polarisation disturbance etc etc..... almost always negate any benefit derived from centralising a mobile antenna with respect to a vehicle groundplane.


Best place to mount your antenna, in my humble opinion - keep safety/mounting integrity and asthetics in mind, and things should be just fine.

 

[sparks40]

The use of a mag mount over time will eat the paint. It seems as you drive down the road, the minute sand particles make their way under the mount and abrade away the paint. Why don't you just bite the bullet and drill the roof and put an NMO mount and be done with it. My F250 truck has 4 antennas on the roof and 2 glass mounts.

I will not have a radio mounted in my truck all the time, same for the antenna. The other issue i have is about three inches or so of clearance between by truck and the garage door.

 

[ramal121]

Sounds like you'll be using VHF and above. A wattmeter will tell you if there are any bad spots to stick it, but I suspect not and any ol' place should be OK up there. Much better than sideways underneath the rear bumper or where-ever people ask about on this forum.

As far as mag mounts, I dislike them as much as the other guy, but they are an evil necessity much like Nextels. If you're gonna use one, keep it clean. Clean it more than you would normally wash the truck.
The whip vibrates in the wind and with dirt under the mount, can you say "orbital sander"? Some people use a layer of sandwich wrap between the mount and roof.

 

[cmdrwill]

The rule for any mobile antenna is one quarter wave from ANY edge. So you need at least 20" of groundplane around your mag mount.

 

[ramal121]

The rule for any mobile antenna is one quarter wave from ANY edge. So you need at least 20" of groundplane around your mag mount.

Whos rule is that? OK I know that centering the antenna on the roof will give a more symmetrical radiation pattern and if you can do it great! But I have installed many antennas towards the rear of pickup cabs (for ease of install, like in those double thick Chevys) without complaint and really no noticeable range degradation.

 

[jackj]

Ramal121 is correct. The reflected power from the antenna will be acceptable as long as you have 1/4 wave length of ground plane in one or more directions. The pattern of the antenna will not be omni-directional if the antenna is mounted off center on the ground plane. But the fact that few people drive in circles means that they are unlikely to notice the antenna's directionality (is that a real word?).

 

[cmdrwill]

Who's rule is that?

Antenna Engineering 101.

Have you ever worked on an antenna test range? In an antenna lab?

The mobile antenna is only ONE part of the antenna system, The 1/4 wave groundplane is the OTHER half. This completes the center-fed dipole, which most mobile antennas are based on.

 

[mrweather]

Go to the Larsen antenna website and look up the "Antenna Placement" document. It maps antenna locations on a standard car and tells you how they compare (dB difference) to a permanent NMO mount dead centre on the roof.

 

[WA1ATA]

Sounds like you'll be using VHF and above. A wattmeter will tell you if there are any bad spots to stick it, but I suspect not and any ol' place should be OK up there. Much better than sideways underneath the rear bumper or where-ever people ask about on this forum.

Your humorous comment about "sideways underneath the rear bumper" comment got me thinking ...... bumpers today are plastic. Maybe "sideways beneath the front bumper" really is the perfect place to put an antenna for picking up the California Highway Patrol mobile extenders on 154.905MHz. That way I'd know what there's a CHP somewhere in the next few miles out in front of me.

....hmmmmmmm

Has anybody really tried an inside the bumper mount? For the CHP extender I might even try just coax with the braid stripped off of the last 19" and shoved through a hole in the back of the bumper.