Mounting big antennas

[bngl3rt]

I enjoy having a CB radio and a public safety scanner in my car to stay informed during long trips. I have a small VHF antenna mounted to my engine bay garnish/vent thing and a handheld CB radio at the moment. I am not getting the range I want out of these, and want to put some much bigger antennas (fully removably) on the car.

I have seen license plate mounts for CB antennas. However, I'm not sure how putting the antenna on the back of the car influences my ability to pick up signals from ahead of the car - since there is a whole big chunk of metal in between the antenna and the people in front of me.

My grand master plan was to buy a second targa top (any color), paint it Midnight Pearl, and destructively mount my antennas directly to it. That way for cruising around town I could pop on my OEM top, and for long trips I could switch to the antenna top. However, it turns out t-tops ain't cheap (even ones off wrecked cars) so this plan might turn out to be very expensive.

Another thought was to buy the lexan engine hatch - still expensive, but less so than a t-top since it's aftermarket and not OEM. Some kind of bracket could be fabricated and drilled into the lexan to get the antennas up nice and high. Switching to my lexan antenna hatch in advance of a long trip is slightly more effort than swapping out the targa roof which was designed to be removable, though.

To recap, my three options at the moment (plus a guesstimate of cost parts and labor) are:
* second t-top repainted and wired up ($2000+)
* lexan hatch w/ custom bracket for antennas ($1000ish)
* license plate mount w/ dubious forward reception ($200 to get my stereo shop to wire it up)

Still weighting the pros and cons of each of these - for example, in the t-top scenario, all the requisite radios and other equipment could be mounted to the inside of the targa top so that when I am cruising around town with my OEM top on I don't have a bunch of valuable electronics in the car - but I was wondering if the Prime collective has any other ideas or insights on this.

 

[d_prada1]

You could do another oem spoiler, bend it forward and have it hook up somewhere in the front of town while driving through the streets and un hook it so its all the way up when your on you trips. And then when you want that oem look its only 4 bolts. I've seen oem spoilers go for under 100

 

[bngl3rt]

You could do another oem spoiler, bend it forward and have it hook up somewhere in the front of town while driving through the streets and un hook it so its all the way up when your on you trips. And then when you want that oem look its only 4 bolts. I've seen oem spoilers go for under 100

That's a great idea! All I really have to do then is build a breakaway connector for the 3rd taillight so I can easily attach and detach both spoilers.

 

[d_prada1]

That's a great idea! All I really have to do then is build a breakaway connector for the 3rd taillight so I can easily attach and detach both spoilers.

Glad I could throw in some ideas !

 

[Kinan]

I have seen license plate mounts for CB antennas. However, I'm not sure how putting the antenna on the back of the car influences my ability to pick up signals from ahead of the car - since there is a whole big chunk of metal in between the antenna and the people in front of me.

Typically, any car surface acts as a ground plane and improves the gain of the antenna in the direction of the metal. So, if you add an antenna at the rear of the car, you'd get better gain looking towards the front of the car. Every situation is a little different but that is the general rule as it applies to RF propagation and CB antennas. Use a top load coil antenna to get the radiating element above most of the car.
http://www.rightchannelradios.com/choosing-the-best-cb-antenna

http://www.wilsonantenna.com/images/antenna_location.gif

 

[Rope]

The take of point on antennas is at the tip of them. The length of antennas is for wave propagation and the ability to receive transmissions.

 

[bngl3rt]

Typically, any car surface acts as a ground plane and improves the gain of the antenna in the direction of the metal. So, if you add an antenna at the rear of the car, you'd get better gain looking towards the front of the car. Every situation is a little different but that is the general rule as it applies to RF propagation and CB antennas. Use a top load coil antenna to get the radiating element above most of the car.

In that case, putting them on the license plate or the spoiler is ideal for hearing stuff up ahead, no?

 

[Rope]

*off

Also based on the fact that it is a single pole it would make the antenna Omni directional putting out equal amounts of power in all directions. the height difference from putting it up on top of the car versus on a foot lower for the base on the license plate will have minimal effect on range. Terrain is a much bigger concern.

- - - Updated - - -

I have seen license plate mounts for CB antennas. However, I'm not sure how putting the antenna on the back of the car influences my ability to pick up signals from ahead of the car - since there is a whole big chunk of metal in between the antenna and the people in front of me.

Typically, any car surface acts as a ground plane and improves the gain of the antenna in the direction of the metal. So, if you add an antenna at the rear of the car, you'd get better gain looking towards the front of the car. Every situation is a little different but that is the general rule as it applies to RF propagation and CB antennas. Use a top load coil antenna to get the radiating element above most of the car.
http://www.rightchannelradios.com/choosing-the-best-cb-antenna

http://www.wilsonantenna.com/images/antenna_location.gif

What you speak of is a counter poise and that is not true. The only thing all the metal is doing is helping to push the signal straight up. you place a larger piece of metal behind the antenna to amply the power in a certain direction as it acts like a mirror.

 

[Kinan]

In that case, putting them on the license plate or the spoiler is ideal for hearing stuff up ahead, no?

That is correct, you also want to use an antenna with a top loaded coil (since that is the primary radiating element) to get it above the plane of your car - this insures the car can be fully used as a ground plane. If the coil is below the plane of the car, the metal of the car could detune your antenna and reduce its efficiency as well as alter the radiation pattern by blocking some of the radiated energy. These are general rules, the biggest thing is get the right antenna (top loaded) and mount it correctly, having a good ground to the car so the car can become part of the RF circuit.
But I don't see how you could mount it to the spoiler securely since it isn't metal and wouldn't be rugged enough to withstand wind loading or anything inadvertently hitting it.

- - - Updated - - -

*off

Also based on the fact that it is a single pole it would make the antenna Omni directional putting out equal amounts of power in all directions. the height difference from putting it up on top of the car versus on a foot lower for the base on the license plate will have minimal effect on range. Terrain is a much bigger concern.

- - - Updated - - -

What you speak of is a counter poise and that is not true. The only thing all the metal is doing is helping to push the signal straight up. you place a larger piece of metal behind the antenna to amply the power in a certain direction as it acts like a mirror.

You are confusing the addition of director/reflector antenna elements with antenna ground plane. He is using an omni antenna. Additional elements will give it gain but mounting position in an all metal car affect this as well. Try clicking on the gif link in my initial post for a graphic illustrating how this works..

 

[bngl3rt]

But I don't see how you could mount it to the spoiler securely since it isn't metal and wouldn't be rugged enough to withstand wind loading or anything inadvertently hitting it.

Oh? What is the spoiler made of, plastic?