Mobile Antenna - Angle vs. TX

[KR7CQ]

If you were able to gradually adjust the angle of an otherwise properly mounted mobile dual band antenna (e.g. - Diamond NR770HNMO) away from vertical, while maintaining proper connection to the sheetmetal (let's assume that the sheetmetal of the car could bend as needed)...

... how much of an angle from vertical would you be able to adjust to before a noticeable drop in TX range would occur? Would an antenna ten degrees or less off from true vertical still perform basically the same?

And a related question...

I have noticed that with a HT, a slight tilt of the antenna / radio has no apparent effect on effective TX range, but why? Shouldn't a verticaly polarized antenna rapidly lose effectiveness as it is tilted away from vertical?

 

[LtDoc]

Actually, I don't think you'd even notice a difference with something like a 10 degree tilt, just about like that HT being tilted.
- 'Doc

 

[prcguy]

In free space if you rotate a dipole from vertical to horizontal by 45deg you drop 3db. Its hard to estimate a roof mount vertical due to the ground plane interaction but it will degrade some as you get away from vertical. It also depends on the receiving station and what kind of reflections your signal takes getting there, etc.
prcguy

If you were able to gradually adjust the angle of an otherwise properly mounted mobile dual band antenna (e.g. - Diamond NR770HNMO) away from vertical, while maintaining proper connection to the sheetmetal (let's assume that the sheetmetal of the car could bend as needed)...

... how much of an angle from vertical would you be able to adjust to before a noticeable drop in TX range would occur? Would an antenna ten degrees or less off from true vertical still perform basically the same?

And a related question...

I have noticed that with a HT, a slight tilt of the antenna / radio has no apparent effect on effective TX range, but why? Shouldn't a verticaly polarized antenna rapidly lose effectiveness as it is tilted away from vertical?

 

[jhooten]

One problem with real world antennas is they seldom act the way theoretical antennas in "free space" say they will.

Here is another one for you, see if you can figure it out without looking it up. FM Broadcast receive antennas on vehicles are vertical yet for fixed use, home for example, they are horizontal. The signal is sent from one antenna and is received equally well at either receiver. Why is that?

 

[AK9R]

I was under the impression that many FM Broadcast transmitter antennas use circular polarization or some other means to provide a good signal for both horizontal and vertical receiving antennas.

 

[W2PMX]

As far as tilting the antenna on an HT, it's not vertically polarized to begin with. (It's "guess what" polarized.) With some HTs, in some situations, vertical may give you the least loss. With others, holding the antenna horizontal may work best.

On a mobile, though, if you have at least a 1/4 wave ground plane under the antenna (or it's a half-wave antenna), tilting from the vertical will increase cross-polarization loss. How much? In the real world, the only way to tell is to measure it. The theoretical loss doesn't include any signal reflections or other real-world effects.

 

[jhooten]

I was under the impression that many FM Broadcast transmitter antennas use circular polarization or some other means to provide a good signal for both horizontal and vertical receiving antennas.

Some do some don't. There is also a dual polarity antenna with both vertical and horizontal elements. The elements have to be phased properly or you end up with circular anyway.

 

[zz0468]

Some do some don't.

Not being in broadcasting, I can't speak with much authority, but after 30+ years roaming transmitter sites across several states, I can't say as I have ever seen an FM broadcast station that didn't have a CP antenna, with the exception of 1 or 2 low power translators. Even most of them run CP.

There is also a dual polarity antenna with both vertical and horizontal elements. The elements have to be phased properly or you end up with circular anyway.

Are you sure those aren't just CP antennas?

 

[zz0468]

If you were able to gradually adjust the angle of an otherwise properly mounted mobile dual band antenna...

... how much of an angle from vertical would you be able to adjust to before a noticeable drop in TX range would occur? Would an antenna ten degrees or less off from true vertical still perform basically the same?

One effect you might see is an increase in capacitive reactance in the antenna, which would have the effect of slightly detuning it. How much would depend on unknown variables, and it's not likely to be bad if you're just talking 10 degrees. If you were to sweep test the antenna, I guarantee you would see the effect, even with just a few degrees of tilt. But your radio wouldn't care a bit.

 

[jhooten]

Not being in broadcasting, I can't speak with much authority, but after 30+ years roaming transmitter sites across several states, I can't say as I have ever seen an FM broadcast station that didn't have a CP antenna, with the exception of 1 or 2 low power translators. Even most of them run CP.



Are you sure those aren't just CP antennas?

Not according to the station engineer I was talking to as the tower monkeys went up to service the beacon.

 

[zz0468]

Not according to the station engineer I was talking to as the tower monkeys went up to service the beacon.

Interesting. I wonder what the advantage would be for dual polarity vs circular.

 

[prcguy]

There is a multipath advantage to circular pol plus some comparability with both horizontal and vertical pol. I don't see any advantage to using dual linear pol over circular and have never encountered that in FM broadcast. Only vertical linear pol on some low cost, low power stations and of course pirates.
prcguy

Interesting. I wonder what the advantage would be for dual polarity vs circular.

 

[zz0468]

There is a multipath advantage to circular pol plus some comparability with both horizontal and vertical pol. I don't see any advantage to using dual linear pol over circular and have never encountered that in FM broadcast.

Yep. That's the question I have... why dual linear polarization instead of circular. I'm more inclined to think that particular broadcast engineer isn't really clear on the difference and the antenna in question was actually CP.

Only vertical linear pol on some low cost, low power stations and of course pirates.

Those are the only applications of linear polarization on FM broadcast stations I've ever heard of.