Magnetic mounts and grounding

[MarkEagleUSA]

Everyone (at least the experts here) seems to swear that a scanner antenna needs to be grounded on a vehicle to work properly. I've read where it is crucial for transmitting (i.e. CB, etc), but I wonder about reception.

I have an Antenex mini NMO mag-mount and I just recieved this ultra-mini mag mount. The Antenex has a rubber boot around the magnet and the ultra-mini has a plastic base. Neither one grounds to the vehicle via the magnetic area, and since they're used with handhelds, there's no electrical ground connection through the radio.

Is there some sort of magic happening that I'm not aware of, or is there really no ground, and if not, how is reception affected?

Would a standard NMO body-mount on the Antenex offer improved reception because it would be grounded to the vehicle?

One final question: if they're really not grounded (nor need to be) would any magnetic mount antenna serve as a poor-man's base antenna? I'm not looking to pull signals from 50 miles away... I just want to improve on the standard rubber handheld antenna.

 

[MacombMonitor]

It will be grounded though the outer shield of the coax, which is grounded to the radio, which if not mounted, and grounded to the car, should still be grounded through the negative wire of the 12vdc supply. Unless you're operating from a portable battery, or Gel Cel. For receiving, this is good enough. For transmitting, you would want a better ground.

 

[LarrySC]

What I have found is that portable scanners in a car/trunk simply will not receive as good on a mag mount. The NMO installation is better but a mobile installation with NMO hardware is best. Of course the ant must match the 3 basic freq ranges of the scanner.

 

[MacombMonitor]

What I have found is that portable scanners in a car/trunk simply will not receive as good on a mag mount. The NMO installation is better but a mobile installation with NMO hardware is best. Of course the ant must match the 3 basic freq ranges of the scanner.

Were you using the same antenna on the mag mount? I ask because usually antennas with NMO mounts are of much higher quailty than your typical mag mount scanner antenna. Maybe it was just a better antenna design over all? Possibly the cable on the mag mount was of low quality?

 

[DaveH]

You can transmit just fine from a portable radio inside the vehicle which has no direct ground connection, using a mag-mount. It's done all the time. This assumes the antenna is tuned to the TX frequency of course.

There is no dc ground connection at the base of the antenna, however, there's a capacitive (ac) ground from the base through the paint to the car body. A dc connection is good, but as long as the ac impedence is low at the frequency of interest, it should be OK.

Dave

 

[gg750]

I use a RS mag mount for my pro-96. I often take the antenna in my house and set it on the wooden window sill...works just as well as when it's on the trunk of the car. So I guess I don't understand the importance of it being mounted to metal either. Work just the same sitting on wood!

G

 

[N_Jay]

I use a RS mag mount for my pro-96. I often take the antenna in my house and set it on the wooden window sill...works just as well as when it's on the trunk of the car. So I guess I don't understand the importance of it being mounted to metal either. Work just the same sitting on wood!

G

How do you know "it works JUST as well"?

That would take some very sophisticated equipment.

It obviously works WELL ENOUGH, but you don't know if you are loosing a few dB.

 

[K5MAR]

I use a RS mag mount for my pro-96. I often take the antenna in my house and set it on the wooden window sill...works just as well as when it's on the trunk of the car. So I guess I don't understand the importance of it being mounted to metal either. Work just the same sitting on wood!

G

How do you know "it works JUST as well"?

That would take some very sophisticated equipment.

It obviously works WELL ENOUGH, but you don't know if you are loosing a few dB.

If you ever get a real antenna, you'll probably find that you've been missing out on a lot of signals. Many of us have tried the RS antenna and have been disappointed when compared to a quality antenna. There is a mod to improve the RS antenna on the Strong Signals website:
http://www.strongsignals.net/access/image/antennas/bearwires.gif

Sufice to say, a more accurate statement would be that it works no better mounted on metal than it does on wood, which is pretty sad.

Mark S.

 

[N_Jay]

Sufice to say, a more accurate statement would be that it works no better mounted on metal than it does on wood, which is pretty sad.

Mark S.

You can't even say that without further study.

 

[fourwd1]

I think you're confusing an electrical ground with a groundplane.
An antenna needs to be mounted on a surface area of metal (groundplane) which acts as the 2nd half of the antenna. Usually this surface is part of the vehicle (roof or trunk) which is at the vehicles' electrical ground and thus can make more of the vehicle part of the grounplane. The antenna itself does not need an electrical ground.

Hope that's not too confusing.

 

[MarkEagleUSA]

An antenna needs to be mounted on a surface area of metal (groundplane) which acts as the 2nd half of the antenna.

This is what led me to post my original question... if the 2 mag mounts I'm using make absolutely no contact with the "groundplane" of the vehicle, am I losing reception quality over the same antenna (at least in the case of my Antenex ABSCANC) mounted directly to the vehicle body?

Forgive my ignorance here as I know nothing about how antenna are supposed to work. :?

 

[Al42]

This is what led me to post my original question... if the 2 mag mounts I'm using make absolutely no contact with the "groundplane" of the vehicle, am I losing reception quality over the same antenna (at least in the case of my Antenex ABSCANC) mounted directly to the vehicle body?

Measurably? Yes. Enough for you to notice? Probably not. If you cut the hole for the NMO mount with a hole saw (who comes up with these ideas)? The mag mount will probably work better. (You neet to remove the paint all around the hole for decent grounding - and a hole saw doesn't remove any paint at all.)

 

[N_Jay]

This is what led me to post my original question... if the 2 mag mounts I'm using make absolutely no contact with the "groundplane" of the vehicle, am I losing reception quality over the same antenna (at least in the case of my Antenex ABSCANC) mounted directly to the vehicle body?

Measurably? Yes. Enough for you to notice? Probably not. If you cut the hole for the NMO mount with a hole saw (who comes up with these ideas)? The mag mount will probably work better. (You neet to remove the paint all around the hole for decent grounding - and a hole saw doesn't remove any paint at all.)

Good NMO mounts have teath to fix that problem.

As was said you are getting an RF ground through the paint by capacitive coupling.

 

[MarkEagleUSA]

As was said you are getting an RF ground through the paint by capacitive coupling.

If I only knew what that is! :shock:

How about this... is there any difference between having the antenna on top of my vehicle or holding it in the palm of my hand?

I guess I just don't understand the details of how an antenna works for receiving signals. Can someone point out an easy-to-read reference where I might get my feet wet (a.k.a. antenna's for dummies).

 

[N_Jay]

As was said you are getting an RF ground through the paint by capacitive coupling.

If I only knew what that is! :shock:

How about this... is there any difference between having the antenna on top of my vehicle or holding it in the palm of my hand?

I guess I just don't understand the details of how an antenna works for receiving signals. Can someone point out an easy-to-read reference where I might get my feet wet (a.k.a. antenna's for dummies).

arrl.org has some great books.

 

[Al42]

Good NMO mounts have teath to fix that problem.

If you tighten them enough, which non-professionals usually don't. (Hint to those who will be installing NMO mounts - use 2 open-end wrenches to tighten the mount. Oh, and make sure the O-ring stays in the track, or you'll get a nice leak in your roof.)

Motorola makes a hole cutting tool that fixes the problem, though.

 

[JohnWayne]

If you use a proper NMO hole saw then it will make a proper hole. The Antenex HS34 is such an example. It cuts a depth-limited hole, as well as scraping the paint away around the perimeter. It's the only way to go if you don't want to mangle your roof and/or headliner.

Jeff

 

[gg750]

k5mar,

I don't have a problem with the RS mag mount at all. There are TONS of freqs that the rubber duck won't pick up. Plug into the RS mag mount, and I'm able to listen to activity several counties away crystal clear. That doesn't sound like poor performace to me. And I'm able to pick up the same activity whether the antenna is on the trunk of my car or on my window sill.

A friend of mine has an Austin Spectra on his patrol car. I pluged into that and was able to pick up the same freqs with no difference in signal strength/audio quality, so why should I spend 90 bucks on an antenna when a 30 dollar antenna will do just as good a job for my needs?

 

[Al42]

It's the only way to go if you don't want to mangle your roof and/or headliner.

It's certainly a way to go, and it's probably the cheapest correct way to go, but the Motorola NMO tool does the job just as well. (In fact, the Antenex tool looks like a ripoff of the Motorola tool.)