Antenna placement on vehicle roof

[Raptor05121]

I tried searching and couldn't come up with anything, so:

I am installing a Laird antenna base in my roof here shortly. I am wondering where would be the best spot to locate it? dead center? slightly rearward? does it matter? I *might* also put a UHF antenna up sometime in the near future if that matters any.

 

[K9WG]

For best omnidirectional pattern, dead center.

 

[Raptor05121]

thank you sir. I just got my FT-2400 programmed tonight (found the manual) and hooked my dual-band RadioShack antenna up to an SWR meter. I got a 1.6 on 154.160 and a 2.3 on 146.270. Put my mag mount Laird on there and both dropped to 1.1 and 1.3, respectively. Static went away instantly. I cannot wait to drop the mag-mount and get that Laird drilled in there.

 

[reedeb]

thank you sir. I just got my FT-2400 programmed tonight (found the manual) and hooked my dual-band RadioShack antenna up to an SWR meter. I got a 1.6 on {154.160 }and a 2.3 on 146.270. Put my mag mount Laird on there and both dropped to 1.1 and 1.3, respectively. Static went away instantly. I cannot wait to drop the mag-mount and get that Laird drilled in there.

Was that a typo[154.160]? If not you do realize it is illegal to transmit on that freq with a HAM rig, RIGHT?

 

[Raptor05121]

Was that a typo[154.160]? If not you do realize it is illegal to transmit on that freq with a HAM rig, RIGHT?

no, not a typo. yes, i do realize the illegality of it.

 

[SCPD]

Probably your fire channel freq.

 

[SCPD]

Why?

Was that a typo[154.160]? If not you do realize it is illegal to transmit on that freq with a HAM rig, RIGHT?

I knew as soon as I read the OP. a comment like this was forthcoming.

 

[reedeb]

I knew as soon as I read the OP. a comment like this was forthcoming.

WHY? Because Aa a ham he [and ALL hams ] should know it's illegal to use such radios on certain frequencies. AND blabbing about it on the internet makes ALL hams look bad.

 

[LtDoc]

Where you mount an antenna will make some differences in it's performance, but it isn't an absolute necessity to put any antenna "dead center" on a roof. It depends on a lot of other factors. Primarily, what else is up there already, or will be up there later. Or, just how difficult would it be to put it some particular place rather than another.
Since you're talking VHF, the typical automotive 'roof' provides plenty of room 'electrically'. there can certainly be some slight 'directionality' to an antenna's placement, but not usually enough to make a lot of difference one way or the other (pun intended).
- 'Doc

 

[990adv]

How does frequency affect the potential for the antenna to be more directional? While in the military it was certainly noticeable that the Hummers had a tough time communicating with anyone to the 6 to 9 oclock of the vehicle. This was with the 8ft tall fiberglass whips mounted on the left rear of the vehicle using a frequency range from ~30 to ~60.

As an example, let's say we put an NMO mount 2/3 of the way back on the roof on a typical extended cab pickup. How would the radiation pattern be skewed on 10m, 6m, 2m, 1.25m, 70cm, 33cm and 23cm?

This is not purely a theoretical question as I plan to change antenna placement on my truck.

I need 2m/70cm for an Icom 2820. 1.25m for a Jetstream JT220M. 23cm for an Icom ID-1. With more to come later.

The truck is a 2004 Chevy Silverado with an extended cab.

Am I getting too worried about things? Should I just space them out inline from front to back? Side to side in some fashion? If I am going to start drilling holes in the roof I want the performance to be worth it.

 

[LtDoc]

Directionality depends on the 'shape' of the antenna, and/or where it's mounted. If you move the elements of a yagi antenna around, it will certainly change the radiation pattern of that yagi. Same sort of thingy with simple vertical antennas, what's around them that might make them directional?
A vehicle's metal body is the 'other half' of the typical vertical antenna. So, if the vertical is sort of 'centered' in that vehicle's body, then the radiation pattern will average out to being omni-directional. Lot's of things can affect that, the same things that affect an antenna in any location fixed or mobile.
At least for me, the 'biggy' is usually where it's most practical for me to stick/mount the antenna, not necessarily the resulting radiation pattern. No two circumstances is ever the same, so give it a shot and see what happens.
- 'Doc

 

[N4KVE]

WHY? Because Aa a ham he [and ALL hams ] should know it's illegal to use such radios on certain frequencies. AND blabbing about it on the internet makes ALL hams look bad.

I'm sure he was testing the TX on several freq's to find out which freq had the best SWR. Now he knows how to adjust the antenna for best SWR on the Ham freq's. GARY N4KVE

 

[Arizona_Scanner]

no, not a typo. yes, i do realize the illegality of it.

I talked to a guy the other day who also does MURS on the ham radio in his car, plenty of people "bend" the rules it seems.

 

[robertmac]

Why?

And the OP first sentence is a troll. One knows there is lots of info. on the internet on the best place. But is there any information on where is the best place for a dummy load?

 

[n5ims]

For some good info on how the placement of an antenna on an auto affects performance see the first two pages of this .pdf (note that values on the diagram are loss relative to a hole mounted center of roof mounting) http://www.repeater-builder.com/antenna/pdf/larsen-mobile-antenna-mounting-and-designations.pdf

 

[kb3elk]

I have 5 antennas on the roof of my chevy blazer, and they are all center mounted and around 2 feet apart, I have no issue with SWR on the bands or any issues.

 

[990adv]

Could you please list more information.

What antennas?
What bands?
Hooked up to what radios?

Thanks