Getting my antenna facts straight

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Kumba

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So I got around to digging up some better info on antennas, and referencing the following sources:

http://www.larsen-antennas.com/docfiles/ASB9/Mobile/MobileSeriesDesignations.pdf
Larsen - Antenna Basic Concepts
http://ndl.ee.ucr.edu/manual.pdf
http://forums.radioreference.com/antennas-coax-forum/175061-gain-antenna.html#post1275243

I believe I get an understanding regarding the 'wave' of an antenna and how it relates to the radiation pattern. But a lot of this seems more tuned with transmission....what about reception?

Considering the image on Page 6 of the ndl.ee.ucr.edu PDF, if that car is transmitting using a 5dbd antenna, thus the signal radiates more towards the horizon, it'll go farther, but not as high, meaning antennas mounted on buildings wouldn't receive signals from the car very well.

But how about the other way around? I.e., what's the usual wave used on the common (if there is such a thing) 800MHz trunked radio system antenna? Are they going to default to some standard, or do they go with different transmitting antenna types depending on the terrain? And thus, what's going to be a good antenna for a car to target that particular setup?

I'm in the DC area, so while driving, I go up and down lots of shallow hilly areas. Not mountainous, but not flat by any means either. I also drive a lot near the Potomac River, so keeping these factors in mind, I'm guessing something in the 3dbd range might be the best. I think this means the Larsen tri-band I currently have is probably all I'll need, but I wanted to get some fact-checking done on my logic to make sure I'm getting this information correctly.


That then leads to my next question. Per Larsen's antenna placement guide, I assume an NMO trunk-lip mount, back-center of the trunk lid against the rear windshield, is going to have some considerable loss? They don't have a mark there for that one, but taking the average of the other listed similar trunk mounts, I would guesstimate I'm still looking at ~2.8-3.4dB of loss? Per that, is a simple mag mount on the roof of the vehicle going to really out-perform an NMO mount? That range of loss implies that's over half of my signal reception. Is an 800MHz-targeted, high-gain antenna going to work better, or do I need a lower-gain? How does this translate into the proper wave-type to get as well?

And I thought understanding all the factors for aperture speed, f/stop, ISO settings, etc.., on DSLR cameras was complicated....
 
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N_Jay

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In almost all cases an antenna acts symmetrically with respect to transmitting and receiving.
 

k9rzz

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Note that a 5db lobe is not like pointing a lazer at the horizon, it's a pretty broad lobe in real life.
 

jackj

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Antennas

So I got around to digging up some better info on antennas, and referencing the following sources:

http://www.larsen-antennas.com/docfiles/ASB9/Mobile/MobileSeriesDesignations.pdf
Larsen - Antenna Basic Concepts
http://ndl.ee.ucr.edu/manual.pdf
http://forums.radioreference.com/antennas-coax-forum/175061-gain-antenna.html#post1275243

I believe I get an understanding regarding the 'wave' of an antenna and how it relates to the radiation pattern. But a lot of this seems more tuned with transmission....what about reception?
The pattern of an antenna is called a transmission pattern. That's because the pattern is easer to measure when transmitting from the antenna. The receive pattern will be identical to the transmit pattern.

Considering the image on Page 6 of the ndl.ee.ucr.edu PDF, if that car is transmitting using a 5dbd antenna, thus the signal radiates more towards the horizon, it'll go farther, but not as high, meaning antennas mounted on buildings wouldn't receive signals from the car very well.
There are examples of high-gain antennas mounted on high towers that have problems with local communications being covered by co-channel traffic from 2 counties away. But remember that the gain measurement is only for the major radiation lobe. There are a lot of minor lobes that would tend to cover close traffic.

But how about the other way around? I.e., what's the usual wave used on the common (if there is such a thing) 800MHz trunked radio system antenna? Are they going to default to some standard, or do they go with different transmitting antenna types depending on the terrain? And thus, what's going to be a good antenna for a car to target that particular setup?
The application will determine the type of antenna used. Higher gain antennas almost always have narrower bandwidths, so if you want to cover a wide bandwidth go with a 1/4 wavelength ground-plane type antenna. If you are only interested in a few adjacent channels in one band then you can go with a higher gain antenna. The type of terrain will have little to no bearing on the type of antenna you choose.

I'm in the DC area, so while driving, I go up and down lots of shallow hilly areas. Not mountainous, but not flat by any means either. I also drive a lot near the Potomac River, so keeping these factors in mind, I'm guessing something in the 3dbd range might be the best. I think this means the Larsen tri-band I currently have is probably all I'll need, but I wanted to get some fact-checking done on my logic to make sure I'm getting this information correctly.
I think the tri-band antenna would work fine for you.

That then leads to my next question. Per Larsen's antenna placement guide, I assume an NMO trunk-lip mount, back-center of the trunk lid against the rear windshield, is going to have some considerable loss? They don't have a mark there for that one, but taking the average of the other listed similar trunk mounts, I would guesstimate I'm still looking at ~2.8-3.4dB of loss? Per that, is a simple mag mount on the roof of the vehicle going to really out-perform an NMO mount? That range of loss implies that's over half of my signal reception. Is an 800MHz-targeted, high-gain antenna going to work better, or do I need a lower-gain? How does this translate into the proper wave-type to get as well?

And I thought understanding all the factors for aperture speed, f/stop, ISO settings, etc.., on DSLR cameras was complicated....
The best place on a vehicle to mount a ground-plane type of antenna for an omni-directional pattern is the center of the roof. Any place else and the antenna will be shadowed by part of the vehicle's body. But that shadowing is not as pronounced as you might think, you probably won't even notice it. So if you were to plot the radiation pattern of an antenna mounted in the center of the trunk lid you would likely see a drop off in signal strength toward the front of the vehicle of somewhere around 2-3 db. But the signal strength will vary more than that as you drive around so I wouldn't worry about it.

As for mag-mount antennas, I'd stay away from them. They have one redeeming quality, you can use one without putting a hole in your vehicle. But almost all mobile antennas require a ground-plane to bring their pattern down toward the horizon. Mag-mounts supply the ground-plane through capacitive coupling to the body of the car. The amount of coupling will depend on the operating frequency and the size of the mag-mount's magnet, higher frequency = more / bigger = more.

A trunk-lip mount makes a mechanical connection with the trunk lid and will give a good ground to the antenna. The NMO type mount is a very robust mount, maybe more than you really need for a trunk-lip mount. But it will work fine.

I hope this helps.
 
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ramal121

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Jackj said a mouthfull and I agree with what is said. Just remember, when you see a diagram of the lobes for a gain antenna, they are low toward the horizon. But it is not an on/off scenario. A high gain antenna radiates (and receives) at all angles, its just a majority of the energy is aimed low. What if an antenna is mounted on top of a building and it's 45 degrees up from your car? This would mean you'd be pretty darn close to the building. Even way off the main lobe, being that close would not make a difference. True you can "drive under a repeater", but other factors are usually involved, such as trees, hills etc...
 
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