Batboard

Good day all !

I have a 110Watt UHF A7 Spectra mounted in a CVPI. The antenna is on the middle/center of the trunk. I have been told mixed results that the BEST place to mount the antenna is on the roof of the car, and NOT the trunk. Can anyone provide any info ? Should I move the location to the roof of the car insted of the trunk ? Where do you guys usually put the antenna ?

The roof will have the best omnidirectional pattern - the trunk will have a null due to the roof.

However, the difference isn't huge, and the increased height can lead to the antenna hitting items.

The best place is as high as you can get, and, for an antenna requiring a ground plane, in the middle of the biggest chunk of metal. Obviously that would be the roof.

That said, I suspect the difference will be fairly minimal. On the trunk, you may a slight cardioid pattern with the null facing the passenger area, but I suspect it would be very minor.

If you really want to do this right or are doing this comercially, you should consider RF exposure as well. The occupants will be directly in the beamwidth of a trunk-mounted antenna. You need about 7.2 feet at 450MHz to be below the FCC-mandated limits for an uncontrolled area. The driver is probably OK... the back-seat passenger/suspect, not so much.

As always, vertical separation rules here. Looking at a random datasheet for a typical quarterwave whip from Maxrad, the elevation plot shows a minimum -15dB null below the plane of the antenna. So, your +50 signal becomes +35, and you only need 2' separation from the antenna to be compliant.

In short - the roof is the best place for a new install. If RF exposure isn't your concern, and especially if the antenna is already installed, I wouldn't screw with it.

Wow, thanks for the fast response everyone !

Here is where my antenna is now on my 2010 CVPI (note the Comtelco puck on the trunk) :





Uploaded with ImageShack.us

I'd like to move it to the roof simply do to the reasons above (RF exposure, and most importantly for the BEST possible reception / clarity ). Antenna height will not be a problem, because I will not be going to a full UHF whip antenna. I will either keep the one I have, or move to this : http://www.radio-accessories.com/QWB450 ... tenna.aspx

All of my vehicles, any time the antenna is on the trunk there was always a *slight* bit of differentiation between the trunk, and a car where the antenna is mounted on the roof. So, I think I'll have this one moved to the roof.

I can't believe 110 watts doesn't smoke the matching hardware in that puck antenna. What is it rated for?

Back when I ran 110W on UHF I never used a transit/low-profile antenna.. it was either a 6'' stinger or a center-load gain antenna. For a while I even used one of these monsters (on the trunk of a CVPI, no less) and let me tell you, if I wasn't into the repeater full-quieting it was broken.

The roof is better as it's higher and has a better ground plane.

i always say the roof. but then i drive a minivan.
i have a 110w a3 spectra on uhf with a gain antenna and a 100watt maratrac on vhf hi.

That's a Comtelco A2543B he's running and it's rated at 200 watts. I run two of them on the
roof of my 2011 Outback and they are very good performers.

http://www.comtelcoantennas.com/PDF%20D ... A2543B.pdf

Yup, as Tom has said, that antenna can handle up to 200 watts out.

I don't have the Spectra putting out the full 110 watts though, it's down tuned to around 90 watts out.

I'm having the antenna moved to the roof in the upcoming weeks.

If you were to install the A2543W (white) version it would be a lot less noticeable. Just a thought.

Regards,

Nope, I'll be more than likely ditching (AKA selling here on Batlabs) my current Comtelco, and going with this little booger :
http://www.radio-accessories.com/QWB450 ... tenna.aspx

I doubt you will notice a difference in performance moving to the roof. They are great theoretical discussions, sometimes quite passionate discussions, and some guys love to break out their calculators to prove the rooftop is best. But! Then the real world comes into play. You might get one whole extra mile and a db of extra quieting if you move to the roof depending on where you are in relation to the receiver. Are you down in the valley surrounded by an urban tree canopy, or are you on a bald mountaintop at 2500ft elevation? A few vertical inches and some sheet metal pillars are not going to help you or hurt you.

Which is why I could get away with putting a quarterwave mounted under the trunk on some beater undercover vehicles.

I'm actually in a Suburban area (trees, hills, but no SUPER tall buildings, mountains, etc.). I'm looking to get the best possible reception from my Spectra, so I figured the roof would be the best bet. I should have thought of that before I slapped it on the trunk lol

A few inches higher on the roof will not change the overall terrain aspect, or put the antenna above the tree canopy. Trunk lid is fine. You would never notice the difference.

that black hockey puck will stand out anywhere you put it on that car. Why not just use a simple regular antennae, in this case it will be a visibly lower profile and only stick up about 4 more inches? And cheaper

Welp, I ended up having it moved to the roof a few days ago. I also went with this antenna here:

http://www.radio-accessories.com/QWB450 ... tenna.aspx

Works excellent, and much less gawky then the big round thing I had

Thanks for the assistance everyone !

Oh, and here is a picture of the new antenna, and location:



Uploaded with ImageShack.us

MT2000 man wrote:Welp, I ended up having it moved to the roof a few days ago. I also went with this antenna here:

http://www.radio-accessories.com/QWB450 ... tenna.aspx

Works excellent, and much less gawky then the big round thing I had

Thanks for the assistance everyone !

Oh, and here is a picture of the new antenna, and location:



Uploaded with ImageShack.us

what did you do about the trunk antenna?

I actually gave it to a friend of mine that was installing a Spectra in his car and needed an antenna.

Did you put another antenna on the trunk in its place or plug the hole?

Nope, I plugged the hole with a color-matched hole plug from where the antenna on the roof went . I'd much rather have it sanded down to make it look like nothing at all was on the trunk, but that's too much work