Antenna tilt

[Jhernan488]

I currently have the tram 1180 dual band antenna. My friend and I ended up mounting the antenna to the car on an area that isn’t completely flat due to stuff being in the way. My friend that helped me told me the outward tilt that the antenna has shouldn’t really affect it.

I have been doing lots of research on antennas and studying for the technicians test that makes me question his theory.

I did find a page that listed the differences drawn out between a 5 degree tilt inward, outward, and vertical. They were pretty close.

Maybe this is a good thing since repeaters have high altitude? What about simplex operation?

What are your thoughts on this?

Thanks in advance.

 

[mmckenna]

Ideally you'd want it perfectly vertical, but with a long whip like that, it's going to bend over a bit at highway speeds, but you'll find it'll still work.

I wouldn't worry about it with an antenna like that. Looks like a 5/8th's wave VHF antenna, and the radiation pattern has a wide enough main lobe that it won't really make much of a difference.

You are way ahead of most by doing a proper NMO install.

 

[Ubbe]

As said that angle doesn't matter much and the roofs groundplane to the right are much larger, which will pull the direction more to ground and the end result are probably equal coverage in all directions.

/Ubbe

 

[W5lz]

That small of a tilt makes no significant difference at all. It will cause the radiation pattern to 'slew' a little bit, but nothing drastic, and everything else in the antenna's environment affects it the same way (a little pattern slewing), so not a biggy.

 

[needairtime]

For best performance you need it parallel to the other person's antenna. You will get a minor signal loss with a few degrees angle depending on the location and orientation of the other antenna.

Keep in mind when using a repeater, its antenna may be in front or behind you, so you can't guarantee where in the lobe your transmission will hit the repeater. It may thus work better or worse depending if you're facing or facing away from the repeater. For omnidirectional use, straight up is best so you don't have to deal with direction, but it won't matter that much until you're at the fringe.

Consider the antennas that are used on HTs... can you hold your HT's antenna straight perpendicular to the ground and keep it like that while using it? Likely not , it's tough to hold it straight perpendicular yet it still works - a few degrees off isn't a big deal. At worst you'd be using something like an HT but with many more watts.

 

[prcguy]

In my experience, you won't notice any degradation in signal strength with the antenna angle shown in the picture.

If your vertical antenna is tilted over at a 45deg angle, signals broadside to the angle will be down about 3dB as you are halfway between vertical and horizontal at that point. As you go back vertical the signal strength will go up slowly and the last 5 or 10 degrees before vertical will be so close to vertical it will be hard to measure any degradation. As you go from 45deg angle further horizontal, the degradation will happen faster and faster and the last 5 or 10 degrees from horizontal will have the most degradation from vertical.

In free space and under perfect conditions you can achieve 40dB or more isolation between vertical and horizontal. For a vehicle antenna driving around town you might get 15 to 20dB vertical to horizontal isolation due to multipath, etc.

 

[Ubbe]

Remember that if you put one or two persons in the front seat the car will tilt down in the front. If you go up or down a street at a 5 degree angle it will change the directional loob over the horizon the same as the antenna tilt on the car. If you have any kind of obstacles besides the road, like houses or hills, it will obsctruct the radiosignals impacting the performance much more than a small angle of the antenna. Only super high gain antennas, not available for car roofs, will need more or less a correct vertical position.

/Ubbe

 

[iMONITOR]

Problem Solved!

 

[mmckenna]

Remember that if you put one or two persons in the front seat the car will tilt down in the front. If you go up or down a street at a 5 degree angle it will change the directional loob over the horizon the same as the antenna tilt on the car. If you have any kind of obstacles besides the road, like houses or hills, it will obsctruct the radiosignals impacting the performance much more than a small angle of the antenna. Only super high gain antennas, not available for car roofs, will need more or less a correct vertical position.

/Ubbe

Good point.
And tall whips going down the road at highway speeds are going to bend back a bit and throw things off. Yet, they still magically work!

 

[Ubbe]

Good point.
And tall whips going down the road at highway speeds are going to bend back a bit and throw things off. Yet, they still magically work!

Well, actually in some cases they don't work. I had a couple of cases where taxi cabs had terrible radio communications while on the highway but worked perfect at low speed or when parked. They used a gain antenna with a flexible phase coil in the middle and when exchanged with a shorter 1/4 wave rod the problem was solved.

/Ubbe

 

[mmckenna]

Well, actually in some cases they don't work. I had a couple of cases where taxi cabs had terrible radio communications while on the highway but worked perfect at low speed or when parked. They used a gain antenna with a flexible phase coil in the middle and when exchanged with a shorter 1/4 wave rod the problem was solved.

/Ubbe

Yep, exactly and one of the reasons a basic 1/4 wave can be a great antenna.

 

[lmrtek]

Actually the tilt is nothing to worry about.

 

[a417]

Good point.
And tall whips going down the road at highway speeds are going to bend back a bit and throw things off. Yet, they still magically work!

that's because they're hollerin "RAAAAAAAAADIOOOOOOOOOOOOOOOO..........." into their *****in' linear... The extra effirt at the mic end makes up for the tilt-loss.

 

[mmckenna]

*****in' linear...

I believe it's pronounced "LYN - E - ARGH"

Passed the scales today while on the road, always a few trucks with the whips canted forward at a 45º angle, you now, 'cuz it looks cool...