Antenna Mounting

[ClemsonSCJ]

Hey guys, I have a question about antenna mounting. I am a deputy in SC and have recently moved to a division where we have “cold cars” (undercover) and we wear plain clothes. I say that, but they’re more like “lukewarm” cars cause they took old road cars and just unmarked them, so the antennas are still on the trunk body making it a dead giveaway. The antennas are those small round barrel looking ones. I moved mine to a metal panel inside the trunk and just plugged the hole with a grommet. With the antenna being in the trunk I get a lot of static now when I get to the outer edges of the county. I don’t want to move it back outside cause it defeats the purpose of having an unmarked car when you have the obvious police antennas on the outside of the trunk.

The solution that I’ve thought about is if I could mount them to that cloth deck behind the back seat inside the back windshield. That way they won’t be inside the metal box like they are in the trunk, but they’ll be hidden by the tinted back windshield. Does this sound like it will work? The only concern I have is grounding. I know nothing about this type of antenna and what type of ground it needs. That should be an easy solution if that is the only issue, cause I could just run a wire to the mounting base from ground and that will solve that. What do you guys think?

 

[mmckenna]

The solution that I’ve thought about is if I could mount them to that cloth deck behind the back seat inside the back windshield. That way they won’t be inside the metal box like they are in the trunk, but they’ll be hidden by the tinted back windshield. Does this sound like it will work?

That's a common approach for undercover cars. Put a tissue box or ball cap over it and no one will know.
Here's the issues, though...
Some tinted glass uses metallic film in the glass. That can reflect RF and create issues. Rear defrosters can create similar issues.
You didn't mention what frequencies/power levels, but for most mobile radios, having the antenna that close to rear passengers heads is way outside the acceptable limits for RF exposure. If you never have passengers in the back seat, you are still probably exposing yourself to a lot of RF.

The only concern I have is grounding. I know nothing about this type of antenna and what type of ground it needs. That should be an easy solution if that is the only issue, cause I could just run a wire to the mounting base from ground and that will solve that. What do you guys think?

I'm kind of surprised (actually quite a bit surprised) that any agency would allow modifications to the antennas like this. Things like mounting them inside the trunk, close to passengers, etc. would be something a reputable radio guy would shy away from.

Inside the trunk it's no surprise it didn't work very well. Reflected power (SWR) was probably pretty high. That can damage the radio, as well as introduce a lot of RF energy into vehicle wiring that likely wasn't designed to handle it.

Rear package shelf -might- work if you know there is no metal film tinting or defroster wires. Checking with an SWR meter would be a really good idea.

Without knowing the frequencies/power levels you are using, it's impossible to give you accurate advice.

There are other options that a radio guy would probably look at first:
"Cellular" look alike antennas. At least they get most of the RF outside the vehicle. Glass tinting/defroster wires are still a concern.
AM/FM replacement antennas that will allow connecting the radio. Our chief uses one of those in his ride.
Satellite radio look alike antennas are an option for the higher frequencies.
There are even some "behind the bumper" style antennas that work for very short ranges.

I gotta believe that somewhere in your agency is a radio guy/shop that can probably help with this. They'll have the knowledge and tools to do it right. I'd recommend talking to them rather than looking for advice on a hobby radio site. I understand the desire to do it yourself, but without the right knowledge and test equipment, there is some potential issues.

 

[kb4mdz]

Do you have any models in mind?

And there is one company, Sti-Co, (http://sti-co.com ) that makes several products ostensibly for your application. I've run across very few of them in my years in the business, so I don't know how well they actually work, but they've been in business since 1967.

There's also Panorama antenna; Panorama Antennas that makes antennas for LMR/Cell/Trunking/ etc. that go in place of the now common shark-fin antennas on a lot of newer cars.

 

[Giddyuptd]

There is a unmarked surveillance antenna kit you can get. I don't have the part number from Motorola so hopefully someone will chime in with it, a dealer should be able to find it easily.

As for mounting under the trunk lid inside, while it would work in a strong signal range for recieving area it isn't ideal and could lead to missed transmissions keying up or receiving to as you noticed on the fringe static to no ability to transmit successfully. It could be a liability if one couldn't transmit and needed backup etc. Which you wouldn't want to have to risk.

The rear mounting in back window deck inside isn't ideal either. The rf energy shouldn't be near someone's head transmitting. Receiving only it may work in a very strong system range for scanning but to transmit wouldn't be ideal.

There is various kits out there such as GPS style fin, to glass cellular type mount which some feel aren't too great in specs, then there is the undercarriage type mount kits.

We had a f350 come with a very tiny GPS style fin on roof matching the vehicle paint color from the dealer after it came from ford with the APX the dealer threw in it. Ill see if I can attempt to find a part number.

I've never really needed them but know smaller departments they usually throw it on the most tech savy guy around to do things and learn it all usually on own. Big reality in many parts of rural America where there isn't any radio shops or bean counters want the local alternatives in house on convenational systems.

 

[RFI-EMI-GUY]

The parcel shelf behind the rear seat is a common location for undercover antennas. It is steel and will provide a ground plane at UHF and above. You can place a Kleenex box or hat over the antenna to hide it. If you are using high power radios, you are subjecting the passengers and yourself to EM fields that are above the FCC recommended limits. If you can get by with a 10 or 15 watt radio you would be within the range of a motorcycle radio and those are considered safe.

If the glass is internally tinted, there will be some signal absorbed and thus range and reception will be reduced. That said, it will be preferable to having the antenna in the trunk. P

Be sure the VSWR is tested, especially if reusing antenna cable etc.

There are other antenna solutions like cowl mounted AM/FM/LMR antennas, patch antennas that are mounted inside plastic bumpers, mirrors and any other creative solution can be designed.

It will be a compromise compared to an external whip antenna.


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[ClemsonSCJ]

Thanks for all the quick replies guys! So we are currently running “low band” radios that everyone refers to as “400s”, so I would guess that’s where our current frequencies are. We are switching in a couple of weeks to 800’s with the Motorola APX radios. As for the first guy to respond referring to our radio guy, here’s why I don’t want to try to go through him...our low band radios have been garbage for years now, and we are one of the last hold out agencies in the country (we are the only agency in our state not on 800’s currently) that are not utilizing the latest radio technology. This is only because of a “if it ain’t broke” mentality.

As far as rf exposure, we actually just had a class on our new radio system with a Motorola rep, and he brought up rf exposure. According to him, if you had a radio directly against your body running 100ish watts you’d have an issue, but with the 2-3 watt handhelds and the 40ish watt mobile radios combined with limited key ups, rf exposure would be a non-issue. I’m by no means an expert or even knowledgeable on that topic, that is only a regurgitation or what we were told. I’ll run it by our radio guy but I have a feeling I’m going to be told to kick rocks.

We have one impala like mine and a charger in my division with no exterior antennas so I’ll have to look at what their setup is.

 

[mmckenna]

Low band and 400MHz are different things, but I suspect if you were able to fit the antenna in the trunk, we're talking about UHF 460MHz stuff.

A 460MHz UHF antenna will be about 6" tall for a quarter wave, and require a 6" radius ground plane under the antenna to work ideally. That's something that can be handled by a basic UHF glass mount antenna, or on the package shelf if there is no metal film tinting or defroster wires in the rear window. StiCo makes AM/FM/UHF disguise antennas, but they are expensive.

The Motorola APX line comes in a number of models, both single band and multiband. Knowing which one you are getting will be important as the type of antenna(s) will need to be correct for the radio.

If it's just 800MHz, then there are plenty of solutions that will work. A quarter wave 800MHz antenna is about 3 inches tall and would need a 3 inch radius ground plane under it to work ideally. Same issues as the others if you mount on the package shelf, window tinting/defroster wires. There are also small low profile antennas that can work well, size of a pepper shaker, some small/flat ones, glass mount, etc.

The 800MHz mobiles run 35 watts, and in close proximity to humans, that can still be dangerous. A 3 watt 800MHz hand held with periodic occupational exposure isn't much of an issue. 35 watts on an external antenna, like the Motorola guy said, won't be an issue, but he's talking about a properly installed antenna outside the vehicle, not an antenna installed inside the vehicle. I'd still urge caution if putting it inside the vehicle.

Proper installation and checking SWR is still necessary.

 

[ClemsonSCJ]

Low band and 400MHz are different things, but I suspect if you were able to fit the antenna in the trunk, we're talking about UHF 460MHz stuff.

A 460MHz UHF antenna will be about 6" tall for a quarter wave, and require a 6" radius ground plane under the antenna to work ideally. That's something that can be handled by a basic UHF glass mount antenna, or on the package shelf if there is no metal film tinting or defroster wires in the rear window. StiCo makes AM/FM/UHF disguise antennas, but they are expensive.

The Motorola APX line comes in a number of models, both single band and multiband. Knowing which one you are getting will be important as the type of antenna(s) will need to be correct for the radio.

If it's just 800MHz, then there are plenty of solutions that will work. A quarter wave 800MHz antenna is about 3 inches tall and would need a 3 inch radius ground plane under it to work ideally. Same issues as the others if you mount on the package shelf, window tinting/defroster wires. There are also small low profile antennas that can work well, size of a pepper shaker, some small/flat ones, glass mount, etc.

The 800MHz mobiles run 35 watts, and in close proximity to humans, that can still be dangerous. A 3 watt 800MHz hand held with periodic occupational exposure isn't much of an issue. 35 watts on an external antenna, like the Motorola guy said, won't be an issue, but he's talking about a properly installed antenna outside the vehicle, not an antenna installed inside the vehicle. I'd still urge caution if putting it inside the vehicle.

Proper installation and checking SWR is still necessary.

Car Antennas » HMS Motorsport » Products

^ That is the style antenna for the UHF radio.

Motorola HAF4013, HAF4013A 762-870 MHz 3 dB Low Profile Antenna

^ That is the style antenna for the 800 radio.

Also I misspoke, the 800 radio in the car is not an APX, it’s an XTL2500. Our handhelds are APX6000’s so I guess that’s why I had APX on the brain.

So if I got a 6” ground plane disc with the 3/4” mounting hole and mounted them on the rear shelf behind the back seat, would they work fairly efficiently?

 

[krokus]

So if I got a 6” ground plane disc with the 3/4” mounting hole and mounted them on the rear shelf behind the back seat, would they work fairly efficiently?

Yes, but keep the radio at a lower transmit power, no more than 15W. Also, there will be a shape to your signal, mostly to the rear & front, with a bit of an RF shadow from the D pillars.

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[mmckenna]

So if I got a 6” ground plane disc with the 3/4” mounting hole and mounted them on the rear shelf behind the back seat, would they work fairly efficiently?

Yes.
IF:
The windows are not tinted.
There are no defroster wires.
You won't transmit with passengers in the back seat.
You get the radio shop to turn the RF power down.
Understand that having that RF energy inside the car will likely produce undesired results, like noise coming through the speakers (even when the AM/FM radio is off), possible issues with the engine/body control modules in the car, etc.
Not going to work as a properly installed antenna outside the vehicle.

 

[RFI-EMI-GUY]

Yes.
IF:
The windows are not tinted.
There are no defroster wires.
You won't transmit with passengers in the back seat.
You get the radio shop to turn the RF power down.
Understand that having that RF energy inside the car will likely produce undesired results, like noise coming through the speakers (even when the AM/FM radio is off), possible issues with the engine/body control modules in the car, etc.
Not going to work as a properly installed antenna outside the vehicle.

Not to mention that noise generated by vehicle electronics is going to be stronger inside the vehicle.

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[RFI-EMI-GUY]

[snip

So if I got a 6” ground plane disc with the 3/4” mounting hole and mounted them on the rear shelf behind the back seat, would they work fairly efficiently?

The package shelf itself might be large enough to provide the groundplane you desire if you drill an NMO mounting hole directly into it.

Whatever you do, make sure the groundplane, and / or the antenna base are electrically bonded to the vehicle body, not through a pigtail wire, but directly screwed to bare sheet metal that is the vehicle body. If not, you will have noise generated that will be apparent in the transmit audio. If you are operating P25, every bump that vehicle takes will mess up the digital audio.

Likewise don't put any other loose metal in the vicinity. To cover, a cardboard Kleenex box would be best, a hat without any metal would be OK as well.



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[iMONITOR]

A 3 watt 800MHz hand held with periodic occupational exposure isn't much of an issue.

I've often wondered about the microphones with the builtin antennas and the officers would wear them on their shoulder with the antenna close to their head!

 

[WB9YBM]

Hey guys, I have a question about antenna mounting. I am a deputy in SC and have recently moved to a division where we have “cold cars” (undercover) and we wear plain clothes. I say that, but they’re more like “lukewarm” cars cause they took old road cars and just unmarked them, so the antennas are still on the trunk body making it a dead giveaway. The antennas are those small round barrel looking ones. I moved mine to a metal panel inside the trunk and just plugged the hole with a grommet. With the antenna being in the trunk I get a lot of static now when I get to the outer edges of the county. I don’t want to move it back outside cause it defeats the purpose of having an unmarked car when you have the obvious police antennas on the outside of the trunk.

The solution that I’ve thought about is if I could mount them to that cloth deck behind the back seat inside the back windshield. That way they won’t be inside the metal box like they are in the trunk, but they’ll be hidden by the tinted back windshield. Does this sound like it will work? The only concern I have is grounding. I know nothing about this type of antenna and what type of ground it needs. That should be an easy solution if that is the only issue, cause I could just run a wire to the mounting base from ground and that will solve that. What do you guys think?

When the antennas were initially mounted outside the vehicle, the metal body (which has at least nominal grounding to the vehicles electrical system) acts as a shield so anything inside the car (ignition pops, alternator whine, car computer hash, etc.) would not get to the antennas. So, depending upon your situation, you might be stuck with outdoor antennas.

That being said, there are a few things we used to do at Motorola in our mobile environments that might help. For example, using a metal braid as jumper between the car chassis and hood as well as car chassis to all doors and chassis to trunk lid would guarantee that everything was properly (electrically instead of just mechanically) grounded. Also--and this is going back a few decades--I heard about this stealth device that would connect between the radios' antenna jack and the car body (usually behind the radio under the dash) that would allow the entire car body to be the antenna. I don't know if something like that still exists--unfortunately it's been so long, I can't even remember who made it. Might be worth a Google search, though...

 

[mmckenna]

I've often wondered about the microphones with the builtin antennas and the officers would wear them on their shoulder with the antenna close to their head!

Not any different that holding a hand held up to your face and talking.

Then again, a speaker mic without an antenna would result in your kidneys getting warm from RF.
Truth is, most users don't transmit enough to be an issue. Even if you are doing a full 5/5/90

 

[ClemsonSCJ]

Yes.
IF:
The windows are not tinted.
There are no defroster wires.
You won't transmit with passengers in the back seat.
You get the radio shop to turn the RF power down.
Understand that having that RF energy inside the car will likely produce undesired results, like noise coming through the speakers (even when the AM/FM radio is off), possible issues with the engine/body control modules in the car, etc.
Not going to work as a properly installed antenna outside the vehicle.

Well ****...you can cup your hands around your face and stick your face up against my windows and still can’t see in my car...so yeah, it’s tinted...a lot. And there are defroster wires. And I’m in the warrants division, so yes I transport prisoners...a lot.

 

[mmckenna]

Well ****...you can cup your hands around your face and stick your face up against my windows and still can’t see in my car...so yeah, it’s tinted...a lot. And there are defroster wires. And I’m in the warrants division, so yes I transport prisoners...a lot.

Yeah, I'd recommend going with the low profile antenna outside the car. Or the Stico set up replacing the AM/FM antenna if you need to go fully covert.

 

[brndnstffrd]

Well ****...you can cup your hands around your face and stick your face up against my windows and still can’t see in my car...so yeah, it’s tinted...a lot. And there are defroster wires. And I’m in the warrants division, so yes I transport prisoners...a lot.

As mmckenna said, the Stico replacement antenna would be the best solution in this situation. This is the exact scenario it is designed for, providing decent performance in a covert package. Pursuing and transporting high-risk offenders is not a situation where I would feel comfortable putting my trust in a questionable radio setup.

 

[radioman2001]

Quote"
Stico replacement antenna would be the best solution in this situation.

Most cars today don't have external antenna's even my Expedition SSV has glass AM/FM antenna so you have to have a vehicle that uses an external antenna.

I have been using the rear package shelf of cars since the mid 70's, and there is some front to rear effect on the signal. Did a lot of DEA cars when they would bring in a confiscated car to install a radio on Friday and then we would pull it out on Monday. It was hard to hide a Motrac or Micor in a car, and they wouldn't even let us see the plates when it was brought in and that's how nervous for security they were.
The tissue box is also one of my tricks, the only trouble I ever had was when I used the rear defogger as turning it on caused a farady shield effect (on UHF and VHF). One car we did was a Cadillac and the rear window had a crease in the middle, perfect for VHF whip.
SC is for the most part 800 (Tell us if not for your area) 5db gain antenna on the shelf should work just fine, and with 800 the power level is only around 35 watts max. Other than that you can try the curb feeler antenna by mounting the antenna on the lower portion of the fender like a curb feeler. Cellular antenna's (for 800) work too.
You can do what NYPD does and put a Car Service sign on the windshield of that Crown Vic.
To sum it up anything out of dead middle of the roof is a compromise, you have to decide.

 

[cmdrwill]

Uber or Lyft sticker or sign....