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Good car FM reception but weak AM after getting another antenna. Need help please.

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thebestofall007

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I got another antenna for a 1996 Toyota Celica to replace its defective power antenna and while I now get good FM reception, I get poor AM (I can put a wire or my hand on the antenna and the AM reception improves, but then the FM degrades). I replaced the Toyota's antenna with a 95 Pontiac Grand Am OEM antenna I got from a junk yard that happened to fit perfectly to the mounts inside the Toyota's fender, as the Pontiac's OEM spring steel whip is much more durable than the (often) cheaply made replacements found at Wally world or elsewhere, fixed or otherwise, plus I hate power antennas (too much parts, plus much more expensive). I have had good results with 31" steel whips reception wise in other vehicles with both bands. I am using the Toyota's OEM cable. I have tried a different RG-6 TV cable (because it was thicker), no go, so I went back to the Toyota's OEM.

Something is telling me they might have used an in-line impedance matching/trimmer capacitor somewhere to compensate for the higher capacitance of a longer cable run or something, as the antenna on my Toyota is also rear mounted and its original antenna might have also had a cap. Would adding a ceramic capacitor to the center conductor of the coaxial cable between the radio and antenna fix this (and what value in Picofarads?), as I have heard that AM radio frequencies are picky about the capacitance of the cable and whatnot due to 31 inches being far too short for the lower frequencies AM uses but not FM, as per this thread in response #7: http://www.electronicspoint.com/threads/car-radio-antenna-has-an-inline-85mfd-capacitor-why.102193/

What can I do to remedy this?
 
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thebestofall007

New Member
I was also wondering if the cable from the Grand Am that was on the antenna would improve the reception? I'll head to the junkyard and get it to find out.
 

JimB

Super Moderator
Most Helpful Member
I was also wondering if the cable from the Grand Am that was on the antenna would improve the reception? I'll head to the junkyard and get it to find out.
That sounds like the best idea.

My knowledge of car radios is a couple of decades out of date, the radios in all my own cars just work with no problems!

Having said that, a short whip type antenna is far from ideal (electrically) at the frequencies used for AM broadcast radio. Which is why the cable from the antenna to the radio has a very thin inner core so that there is as low a capacitance as possible effectively shunting the signal from the antenna to ground.
Using the RG6 type cable is a bad idea, as it is not optimised for low capacitance.

JimB
 

alec_t

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If you have a short whip aerial you could try adding a base loading coil (e.g. a few dozen turns of magnet wire wound helically on a ~1/2" wooden dowel or similar).
 

thebestofall007

New Member
That sounds like the best idea.

My knowledge of car radios is a couple of decades out of date, the radios in all my own cars just work with no problems!

JimB
They usually "just work" for me as well as long as the stock antenna is in good order. Mine just wasn't.
 

KeepItSimpleStupid

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I haven't looked at any recent stuff, but older AM radios did have an AM trimmer capacitor on the radio which generally was adjusted for best reception on a weak station. The trimmer won't affect FM.

Now, I did replace an antenna on an 82 Celica from the window to an active AM antenna + FM booster. Not this http://www.blaupunkt.com/in/nc/produkte/car-multimedia/antennas/products/single/13367/ one, but you get the idea.

http://www.hirschmann-car.com/en/products/antenna-systems/radio-tv/radio/

http://d-nb.info/1018168753/34
 

thebestofall007

New Member
I haven't looked at any recent stuff, but older AM radios did have an AM trimmer capacitor on the radio which generally was adjusted for best reception on a weak station. The trimmer won't affect FM.

Now, I did replace an antenna on an 82 Celica from the window to an active AM antenna + FM booster. Not this http://www.blaupunkt.com/in/nc/produkte/car-multimedia/antennas/products/single/13367/ one, but you get the idea.

http://www.hirschmann-car.com/en/products/antenna-systems/radio-tv/radio/

http://d-nb.info/1018168753/34
I do remember older radios having that trimmer for AM (car CD players when I was growing up were EXPENSIVE and the shaft style decks were all we could afford). Newer radios don't have that and automatically do this, that is, assuming a set cable capacitance, antenna length, physically or electrically yadda yadda as a standard reference.

AWESOME LINKS BTW!
THANK YOU.
 
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thebestofall007

New Member
I replaced the Toyota cable with the original that was with the OEM Pontiac antenna and the AM is now back. I solved my problem. The cable indeed does need the optimum capacitance and whatnot for AM.

The cable was free, too.
 
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