Cons to installing antenna under unmarked vehicle?

scterran

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Dec 10, 2010
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For our unmarked cars, our vendor has mounted our 1/4 wave 7/800 antenna to the undercarriage of the vehicle for an APX radio on a trunked P25 system. I'm now hearing some concerns from other sources. So, should I be concerned or not? I'm interested in hearing arguments for and against.
 

WB9YBM

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I'm not sure what you're concerned about...but I've heard these types of antennas being used several decades ago (although at that time they hadn't pushed to such high frequencies as what's being used today, which changes the picture a bit). I'm guessing that given today's technology if someone wants to communicate discretely they'd probably use cell 'phones instead of worrying about "Secret Squirrel" radio equipment.
 

ladn

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Cons would include poor performance on both TX and RX because of shielding from the vehicle. I hope the installer checked the SWR of the antenna!

Pros include being really hard to detect the antenna. Hopefully the radio was also hidden from sight.

Depending on vehicle, a disguised antenna that replaces the "shark fin" roof antenna used for in-car navigation systems or mounting the 1/4 wave spike inside the vehicle on the rear deck and covering it with a hat or tissue box may have offered better performance.
 

maus92

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Installing a hidden or disguised antenna on an Explorer, Taurus or Malibu would defeat its purpose.... Many agencies around here use cell phones and personal vehicles, and some use an smartphone app that links with their radio systems - no need for special antenna installs.
 

RRR

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Remember the "UnTenna"?
 

kb5udf

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Admittedly, while I've never mounted an antenna under a vehicle, if I did, I would be very concerned about
greatly accelerated wear and tear.
 

jonwienke

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SWR is going to be crappy because of the signal having to navigate the narrow space between the vehicle and the ground, and the antenna is going to break a lot, especially if you have to drive in snow or mud, on dirt roads, etc.
 

prcguy

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There are also covert antennas that mount behind licence plates. When installed on both the front and back license plate they work, although degraded from a roof mounted antenna. They come in VHF/UHF/800 and tri band.
 

R8000

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Putting the SWR concerns aside, if you are in a major metro area with a well engineered radio system, there's probably more than enough signal and overlap that a crappy antenna will still work fine.

You can tell if you radio system is well designed when the rubber ducky falls off a officers portable and he comes into the shop 3 weeks later saying "it still works ok, but I thought I would stop in...". If you are in a rural area or on a system that doesn't have "melting" coverage, then it may be a concern.

Sometimes the word comes down from a higher rank that doesn't know anything about radio to put the antenna where it is.

Besides the Sti-Co disguse antennas and bumper antennas, there's the good ole standby of the NMO phanton in the back window with a tissue box or trucker hat sitting on top of it.
 

Joe_Blough

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Speaking of tissue boxes in the back window, did anyone hear the story of the guy who was driving on the freeway with a box of tissues on the rear window deck and got in a crash? When the car came to an abrupt stop from the crah the tissue box continued to move forward from the momentum and struck the driver in the head killing him. Not an issue if the box is held in place by the antenna.
 
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jonwienke

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Speaking of tissue boxes in the back window, did anyone hear the story of the guy who was driving on the freeway with a box of tissues on the rear window deck and got in a crash? When the car came to an abrupt stop from the crah the tissue box continued to move forward from the momentum and struck the driver in the head killing him. Not an issue if the box is held in place by the antenna.
That's an old urban legend. Mythbusters did an episode testing whether that was possible, and concluded the tissue box is incapable of inflicting any kind of significant injury at highway speeds. To do any real damage, it has to be going bullet speed, or else be filled with rocks or metal objects instead of tissues.
 
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Speaking of tissue boxes in the back window,
Mythbusters did an episode testing whether that was possible
"In KILLER TISSUE BOX, the myth states that, in a car smash, a box of tissues on the rear shelf will be propelled forward with enough force to crush your skull. What they discover-regardless of what the tissue box is doing, you're more likely to be distracted by the engine suddenly coming through the dash onto your lap. "
 

techman210

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Putting full power (20+ watts at 150 MHz) antennas in passenger compartments would exceed MPE limits in most cases.

Dialing down the power to 5 watts - HT power - should prevent this - but many technicians don’t know different.
 
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