OT: car antenna grounding question

The typical car antenna wire is coax, so two conductors. The center conductor goes to the whip antenna, the outer shield should be grounded to the car body at the base of the antenna.

If you have an ohmmeter or continuity tester, unplug the antenna from the radio and check the continuity for the antenna. The center conductor should be connected to the whip, the shield should be connected to chassis ground. The center conductor and the shield should not be continuous with each other, if they are the coax or antenna are shorted. There are multiple ways for an antenna/coax combination to fail, including shorts, but all easily checked with an ohmmeter. Rust could prevent the antenna from being grounded at the car body, but easily cleaned up with sandpaper.

Richard

Grant Erw> My partner Karen's car radio doesn't work right. We put in a Pioneer

The ground is moderately important and easy to check if you can get to the back of the radio. Unplug the antenna and use an ohmmeter to check for continunity from the outside of the connector to vehicle ground. Also, you can buy/borrow an antenna and plug it in to the radio momentarily to check for improvement.

Vaughn

This isn't pertinent to the question, but relates to my groundplane antenna that I put on the roof to rebroadcast a web stream. Should I expect lousy reception underneath the antenna (i.e., below the groundplane), or is it my radio?

Grant, You didn't elaborate much on "doesn't work right". If the radio works but stations are easily overcome with engine noise and other static and the old radio did the same thing; and the new one offered no improvement. Consider this:

You may have a shielding problem which could be 1) The shield of the antenna lead-in has been "barked" somewhere, that is, the actual metallic braid of the shield is contacting the metal of the body somewhere. Check for this with an ohmmeter while shaking the cable back and forth as much as possible. Even better if you also disconnect the cable at the radio and apply the ohmmeter to both ends.. 2) The shield of the cable does not run physically all the way to the underside of the antenna mounting hole. This is more likely if you have installed a new lead-in cable or have a power lift antenna. Make sure both sides of the antenna mounting hole are cleaned of paint, dirt, etc. The problem could be caused by a length of unshielded lead-in for whatever reason. Make absolutely sure none of the shield is pushed back from the mount - this causes an RF window inside the car. You don't want that unless yours is like mine was; it made a pretty good audible tachometer.

Good luck,

Bob Swinney

I had always considered that the outer shield of the co-ax was grounded through the radio end. All of the antenna fixing gubbins probably do create a second ground at the fender end, though. an antennae does need a ground plane, I recall that installations on plastic body cars need a metal "reflector" under the antenna to work properly.

Brian

If the groundplane is directly between your TX and RX antennas, then you're pretty well guaranteed to have crappy (if any at all) signal

You're correct, I didn't state the problem very well. The car radio reception is fine when the car is stopped, but deteriorates when the vehicle is moving. We never ever hear engine whine, what happens is the "stereo" indicator (which indicates the FM station is strong enough to be reliably received in stereo) starts periodically going off and the station starts getting staticky as though you were a long ways from town. Also, this is a pretty new car, a 2001 Dodge Grand Caravan. Sadly, the radio is buried deeply in the console and there isn't any way I can get to it. Thus I'm looking for background information I can give to the dealer (it's still under warranty) or to the aftermarket deck installer (her new radio/CD player is also under warranty).

I figure the faults could be:

1. Antenna mount not connected to fender
2. Antenna whip shorted to shield of antenna wire
3. Antenna whip not connected to center conductor of antenna wire
4. Fender not connected to engine block

My current understanding is that the braid, or outer conductor of the coaxial antenna cable is *supposed* to be solidly electrically conducted to the car body, so if the outer insulation has barked somewhere and it's connecting briefly somewhere else, that shouldn't hurt anything.

I'm told that "Chryslers and BMWs have problems with antenna grounding because they use very robust auto paint and the antenna masts often don't ground to the body." The aftermarket installers wanted to remove the fender, sand away a spot, and reinstall the antenna digging it in solidly with a star-type lock washer. It seems to me that it makes just as much sense for the fender to not be electrically connected to the rest of the car--the wire that goes from the body to the negative terminal of the battery could also be failing to connect correctly. Some of the ham radio guys recommend putting in a 1" wide copper braided wire from the fender to a bolt on the engine block.

Regardless, this is *not* the "audible tachometer" problem.

Grant

Robert Sw> Grant,

I might venture that it is a ground problem, but not an antenna ground problem. I had a (new) NUMMI car that would lose radio reception when the rear window defogger was turned on. The dealer replaced the radio twice, didn't fix it, and wasn't interested in finding out how. I got tired of going back to the dealer and did it myself. The radio shared a (poor) power return with the defogger. Activating the defogger pulled a lot of current through the bad banjo lug return point and lifted the voltage at that point by several volts, enough to ruin the RF performance of the radio, leaving enough to make the lights work. The aftermarket fix of hard grounding the coax would work, because the coax braid would now become the power return. Run a shiny clean power return and see what happens.

Kevin Gallimore

I'm not so sure it wouldn't be audible on AM.

Your description of "paint insulated" parts like fenders makes one wonder about receiver desensitization from one of the poorly grounded parts, most likely the hood. Some "desense" fixes call for the hood to be connected to the frame, at the hinges, via heavy braided wire. A poorly grounded hood can act like an antenna and radiate engine noise into a nearby antenna. The general shape and size of the hood is favorable for radiation of engine noise in the FM band.

Engine noise probably won't be heard in the FM receiver. The "noise" is usu. not audible because it is strong enough to capture (shut down) the front end of the receiver. Receiver desensitization from engine noise, as well as from other sources, can be investigated by the following:

With an isolated "Tee" (form of directional coupler) connect both the FM antenna and a standard FM signal generator to the receiver. Meter one of the limiter stages in the receiver as a weak signal is injected into one port of the Tee. Make sure the generator signal is well below the saturation point of the metered stage. (some FM receiver's metering points may be available at the computer service port on some vehicles - otherwise it is necessary to open the receiver). With the engine off, inject a weak signal through the Tee and record the limiter reading and the signal strength in DB from the signal generator. Start the engine. If receiver desense is occurring, the limiter reading will drop off. Increase the signal from the FM signal generator until the limiter reading is at its former level. The before and after levels of the FM signal (in dB) is the amount of receiver desense caused by (RF) engine noise.

Bob Swinney

That doesn't really sound like a grounding problem with the antenna. Unless the antenna is mounted in the center of roof of the vehicle there will be some directional sensitivity. Meaning that you'll get a better signal when traveling toward (front mounted antenna) or away (rear mounted antenna) from the station. However, that directional sensitivity shouldn't be affected be whether the car is moving or not. All that matters is the orientation of the vehicle w/respect to the station.

To me this sounds like it might be an RFI problem related to engine RPM. A simple test should reveal if that's the case. With the vechicle stationary bring the engine rpm up to 2000-2500rpm and see what affect that has on radio reception. Then try driving the car, keeping the engine rpm in that same range and compare that to the signal quality when the vehicle was stationary. Maintaining the same relationship to the station increase the speed, and thus the RPM, up to the point that reception suffers. To be really fair you need to do this test out of a populated area where multi-path reception isn't going to be an issue.

If the first test with the vechicle stationary results is the same symptoms or the last (at high rpm), you're most likely looking at an RFI problem. And for that the first thing I'd try would be to place a large'ish inductor in the power lead to the radio, as close as possible.

Other than an unshielded antenna wire all of the other possible antenna faults should equally affect a stationary or moving vehicle. An unshielded antenna antenna running near an RFI source that's engine RPM related could produce the effect you are seeing.

I'm far from being an expert on the subject but it seems to me that it wouldn't matter whether the antenna is grounded at the mounting point on the fender as long as it's grounded well at the radio. Many CB antennas have a magnetic mount and just sit on top of the paint. They rely on the coax being grounded at the radio instead.

As for normal car radio antennas, the outside of their connector is usually bare metal where it plugs into the radio and the connector on the radio is shorted to the case. With older cars the structure of the dash or instrument panel was metal and the radio was bolted to it so getting a ground was easy. With modern cars (especially Chrysler products) the structure behind the dash is often just molded plastic so it requires a specific ground wire and often has another braided wire connected from the body of the radio to some conductive structure.

I would try running a jumper from the case of the radio to a known good ground to see if it makes a difference. Even if the radio is hard to get to you can probably get to the case well enough to clip a wire on it temporarily to see if it helps. If that doesn't help I doubt you have a ground problem but you may have a bad coax to the antenna.

Best Regards, Keith Marshall snipped-for-privacy@progressivelogic.com

"Even if you are on the right track, you'll get run over if you just sit there." - Will Rogers (1879-1935).

I suspect that your radio is probably removable from the front. Look at the sides of the radio for a couple of small holes. Stiff wire in these holes release the radio and it will slide out.

If a single wire was used, points and plugs would create noise like all business!

Ground lug on radio and coax to the antenna. You might need to get an antenna adapter that matches foreign radio coax to the U.S. large wire size.

Martin

Martin, would you mind rewriting your post to be somewhat less terse? Like using verbs and stuff? Are you saying "check the ground lug on the radio, and check the coax to the antenna" or what?

This radio was installed by a high-end car stereo place's installer. Not that they can't make mistakes. But it's vanishingly unlikely that they didn't hook up the antenna, or that they somehow missed that the coax rattled around in the hole meaning they needed an adapter.

Grant

As Richard stated - coaxial cable -center conductor carries the rf from the air to the radio - the outside braid shields the center conductor. The outside braid is grounded (or should be) where the antenna is mounted to the fender, and also at the point of entry into the radio equipment itself. Generally speaking, the poor connection will be right at the base of the antenna where it mounts to the fender. HTH Ken.

Reception right below the antenna is not normally very good but if you move out away from the antenna , like down the street, it should be ok if your antenna is working.

And if the fender is non-metallic(quite possible these days), then no ground. Braid would have to be run back to a metallic body part. Check with a magnet.

Stan

I believe this is the one you want. Having installed way too many stereos to count in the last 25 years, this was by far the most common poor reception problem I ran into. Inside the antenna coax is a VERY thin single strand of wire, usually not even bonded in plastic. I've torn apart many antennas and found this to be the case.

Try springing the antenna by hand and letting it whip while the car is stationary.

I was responding to another who was talking about a single wire. Sometimes I get distracted or have much on my mind to be fluent. Sleep has something to do with that also. Beg Pardon.

Here goes another shot:

I'm glad to hear that a pro type place put it in. Was the radio ever working ? It sounded like it was a home job. I did those for my Son and Friends when pressed into work to save them money. :-)

I hope the demo CD wasn't in it and Here you go young lady ... type of problem.

On the backs of these radios are plugs that a coax is plugged in. Perhaps if there isn't any radio but the CD or Tape functions (checks out amp and speakers) it seems to verify the cable connection to the radio and to the antenna is the issue. On the antenna there is often a short cable with socket for yet another connector that can come off. The cable from the radio plugs into the radio and the antenna. Perhaps this is off - as it might have been 'untied' to be retrofitted and not 'tied' back when the job was done. Is the Antenna extended ? or extends when the radio is turned on ? Maybe that isn't being noticed - antenna always down. Not there to know.

Sometimes there is a different size of coax to adapt to. My Sony XPLOD unit in my truck required a small coax to large size coax. I know that because when I had mine installed, I paid extra because my truck had a big cable (like all of my other cars for years).

Any of these could giggle off or loose. If the cable is attached to the radio and the signal is weak in the radio sound system - it would indicate that the antenna isn't attached. e.g. Low gain on the antenna.

The cable to the antenna might have a bad connection - but the mounting ground is likely not part of the issue. clipping on a wire to the metal case and grounding it to the frame would be an indicator if needed.

Guess there isn't a problem of not picking the correct sub scale for a station. Most like mine have to be programmed for a station - I have to scan and accept or enter a number and save it. Hope that isn't the issue - might have to check out the booklet.

When I had my truck worked on, the saved channels were wiped out. No flash memory - shucks.

I think the real issue is not knowing what happened and what the status prior to replacement as well as the post mortum issue.

Martin