Car Antenna Impedance

[Bob Scott]

Does anyone know offhand what the antenna input impedance of a car radio is for AM and FM?
I'm scrapping my old 1990 Lumina minivan but I'm going to rob the Delco car radio and use it in a cabinet. I want to make a loop antenna for the AM, but I need to know the impedance. At first I would assume 50 Ohms, but the car whip is way shorter than 1/4 wave. Darn, you'd think I'd know this stuff having been in the car stereo business in the '70's.

Yes, it's the plastic pachyderm and my continuous reliability liability. It's been parked since the fuel pump failed in 2004. Before that it was the rad, the heater core, the brake lines, the A/C, the oil pressure sender leaking, the rear wiper, the hood latch rusted shut, two water pumps and so many alternators I lost count. Chevy stooped real LOW when they designed this piece-o-crap.

[Externet]

Hello Bob.

Does anyone know offhand what the antenna input impedance of a car radio is for AM and FM?

I would say the Motorola connector that fits in car radios antenna inputs is 50 Ohm.

But in modern receivers means little or nothing as front ends using fets have a much higher impedance and work with whatever piece of clothes hanger as antenna. The length of the whip does not tell the impedance of the antenna. It is made that short to fit in garages. Quarter wave at the 300metres wavelenght AM band would be quite a sight. On the 3 metres FM band, well, measures near to quarter wave...

If the Motorola 50 Ohm connectors are the standard for car stereo antennas, you may suspect the antenna is 50 Ohm. I doubt it. Cannot be. 50 Ohm at which frequency ? 0.5 to 1.6 MHz and 88 to 108MHz is too much of two wide spectrums to present a constant impedance. It is whatever the chinese manufacturer came up with.

So the AM loop may work as well as a random wire.

Hmmm. Here is a random link:
http://www.walcottcb.com/128xmoto29-rg8 ... p-931.html

Miguel

[jwax]

Bob- Here's what I did with a Delco/GM AM/FM cassette radio from my old '87 Fiero. It sits in the shop connected with a 4' length of #36 wire taped vertically to the window behind it. Not suitable for DX-ing, but works fine for the locals.
Didn't need to calculate no stinking impedance.

[frhrwa]

heck, I just ran a piece of antenna extension up and outside, where I mounted a car antenna inconspicuously.. works great.. you can go up and out vent pipes, just seal around where you insert it.. then mount the antenna on the pipe on the roof..

[Robert Reed]

The very short antenna vs wavelength for auto AM antennas has a high feedpoint impedance and presents a high capacitive reactance to the input circuitry. Most of these can just attach to the high end of the front end tuning (tank) circuit If I recall right, the loopstick method utilizes that stick as the "L" in the tank circuit and probably would not work well directly connected to an auto radio's input.
As others have mentioned, the 3-4 ft of wire would simulate the cars antenna. In my shop, I have run a wire up thru the wall and attaches to a 40 ft. antenna wire in the attic and this works quite well-also hidden from sight.
Am planning on checking out some of the passive boosters on the market as they make remarkable claims. Its kinda funny that with all the hi-tech gear on the market now, I still love that AM radio!

[Bob Scott]

Ahhh, but I want to use a loop antena because they are much less susceptible to static interference, like fluorescent lights. I guess I should check out my radio engineers handbook, but the greek letters in the calculus equations give me a sudden urge to close the book.....

JWAX! '87 Fieros made me drool. I was looking for a used V6 model with auto trans a few years ago, but only found one V6AT with sloppily repaired front end damage so I passed it by. They were so much better looking in that last year in '87. I could have worked around the mechanical problems....like reworking the fire prone fuel lines.

[Bob Scott]

Am planning on checking out some of the passive boosters on the market as they make remarkable claims.

Where? Do you have a URL?
I've just heard of the range boosters that use a FET.

Its kinda funny that with all the hi-tech gear on the market now, I still love that AM radio!

Yes. My mind is sometimes way out of sync. Sometimes I still expect to find DJ's and rock'n'roll on AM. In actuality, I mainly only tune AM to the all-traffic-all-the-time information station. But very rarely at night I get an urge to DX, like KGO San Fransisco at night. At least you can DX with old analog AM. You don't get the same feeling of mystery or accomplishment listening to a streaming webcast.

[CeaSaR]

Finally found something for you there, Bob-er-doo! This product page (>here<)
shows an impedance of 75 ohms in the AM/FM broadcast range. You have no
idea how many different searches I did just to find that. There are SO many pages
out there that have general antenna info, but specific to automobile radio input, PAH!

As for "online DX'ing", have you ever tried to find the same video twice on YouTube,
especially if they may have been somehow linked to copyrighted content? How about
going back to a bookmark you made several years ago? Betcha quite a few are gone
and will take some searching to find again, if at all. I know it's not quite the same,
but it can be fun too.

Ah, AM radio. I miss the days when I used to listen to... some petroleum company's
(ARCO?, Texaco?, Shell?, ??) "Mystery Theater" (live actors) on sunday nights on the
old 6 transistor AC/DC Panasonic. The programming back then was so much better
than most of what is out there today (or is this just an age thing?).

CeaSaR

[jwax]

Bob, I too loved that Fiero. I had 3 over the years, an '84, 4-cylinder I bought new, a steal on a '85 but was full of problems, and finally the '87, 6 cylinder, red of course.
It was in the body shop for a week, came out looking showroom new (this was in 1998), and was stolen a few days later. Out of my driveway in a very nice middle-class neighborhood. My guess is a body shop kid couldn't resist copying the key. Just finished putting in a 6 CD changer too.
It was found, stripped, a month later. Totalled by insurance company standards, and mine.
Body panels and seats were removed!
Lesson is that if you have a car in better than BlueBook condition- get the insurance company to take pictures and adjust its value, otherwise like me, you get book value.
On the bright side, I could still be in jail for shooting a car thief.

Sorry for the off-topic diversion.

Didn't we all AM DX at one point? Loved it!

John

[Robert Reed]

Bob
Google up PASSIVE ANTENNA-AM RADIO and you will find a lot of sites. One in particular is www.selectantenna.com. There are also a lot of radio and antique radio forums on the web. I haven't started a thorough research on all of this yet but plan to in the near future. A word of warning though is that these are fairly large and somewhat expensive.

John
Ours was an '84 four cylinder and red of course! Bought it for my wife and she loved it. On the occasions that I drove it, the first 15 miles were agony and once settled in it was a dream to handle, almost seemed like part of me.
Only exceptions were when my car was in the shop and I had to drive it to work. I was suffering from chronic back pain then and literally had to crawl out of it on all fours upon reaching the company parking lot.

PS - how do you guys do that "here" thing that contains a URL?

[CeaSaR]

My fave Fiero was in Car Craft (I think) that had a 400 small block transplanted
in the back. They said it would pull a Porsche at 200!

Robert,

Click the URL button and place the web address within the first set of brackets as such:

Code: Select all

Click URL button
[url][/url]
then make it look something like this:
[url=http://www.somewebaddress.com]Place text here, such as (>here<)[/url]

This will give you a text link something like this below:

Ceasar Software's SSSDCalc and Justaclock

instead of: http://www.geocities.com/ceasarsoftware/index.html

Both are links, but the first one is much more polished and descriptive.

You can do this at any point in your post. Just be very deliberate about how you
make your link stand out, as the board doesn't underline them and the color of the
link is very close to the standard text color. That is why I have taken to placing
them between the symbols (> and <) when in line with other text. I don't bother
when it is labeled as a standalone link as shown above.

CeaSaR

[jwax]

Like this:
FIERO

[Bob Scott]

Google up PASSIVE ANTENNA-AM RADIO and you will find a lot of sites. One in particular is http://www.selectantenna.com. There are also a lot of radio and antique radio forums on the web. I haven't started a thorough research on all of this yet but plan to in the near future. A word of warning though is that these are fairly large and somewhat expensive.

That URL doesn't work.
I was thinking of making my own loop antenna with about 5 to 10 turns of wire inside a hoola hoop with a ~300p variable capacitor wired in parallel. Regular Romex should work. The guage is not that important. I just need thick insulation to minimize interwinding capacitance. I might have to make my own variable capacitor out of vanes cut from SS copper PCB, 'cause variable caps are obsolete.

Ours was an '84 four cylinder and red of course! .......... I was suffering from chronic back pain then and literally had to crawl out of it on all fours upon reaching the company parking lot.

I had the same thing with back pain and getting out of a Corsica. Now I have the little S-10 truck, egression is better but the ride over speed bumps will be tough on a sensitive spine.
A colleague of mine had a 1984 Fiero. I liked the speakers built into the headrests.
It gave me the idea that wing-backed chairs would be ideal for building in speakers for TV watching.

PS - how do you guys do that "here" thing that contains a URL?

If you use the QUOTE box to reply, you whould be able to see what he entered in order to enter the URL and change the color too.
In the color scheme you have chosen for this forum, do you see the QUOTE boxes? I think some too basic schemes of this BBS software don't have quote boxes and also limit other capabilities.

[jwax]

Come on now Bob. You know you have a pile of old AM radios in the junk pile with a 365 pfd variable cap just waiting for a new application!
BTW, a hula-hoop? Are you planning an AM DX-pedition?
John

[Robert Reed]

Bob
Tried to relocate that URL on the same websearch from yesterday and guess what- it wasn't there! But no never mind as the google search should provide you with enough info to keep you busy for the weekend. The commercial units shown appear to be 18-24 inches in diameter. there were several DIY sites there also. Air vari caps are still available in some corners of the country. For one google up DANS SMALL PARTS (I forgot the URL but Google will get you there just as fast).