Car radio coax impedance?

[Kage]

I am using a car radio as a general FM receiver because it is a really good receiver even if it is made for a car and looks fugly setup in the house
Anyways I notice that even connecting a random wire to this thing for an antenna will pull in stations that most good radios seem to struggle with.
Well what impedance of coax should I use? I know car antennas use a really skinny coax cable and I don't think it's 50 ohm or even 75. I wonder what would work best before I start cutting up a bunch of cable just to see what works best.
I hope 75ohm will do okay.
Pioneer sure makes some good tuners for cars. Too bad half of their house stereo tuners aren't as good as the ones they make for cars.

[Ozone Express Radio]

From years of working in different shops, the car radio is THE choice when looking for a decent receiver to listen to all day and actually receive anything. In my opinion anyway.

I've used everything from coathanger to car antennas, to a random length wire to the center connector, to a lead attached to the metal plumbing pipe. Through all that, I have also tried connecting to a regular rooftop TV antenna via the 75 ohm cable. I can't say I noticed much difference in any of them, except the rooftop antenna obviously pulled in stuff further away. Nothing else made any huge differences.

I have a cheap Wal-Mart Durabrand made in China slide-rule tuner with a tape player in it that I use for the mobile, and it has one of the better AM tuners I've had in a while, and it was around $40. That is with a 20" or so fixed whip antenna on my van.

[sgtpeppers]

All I've ever used for my recievers is measured random wire. if you wanted you could pick up one of those connectors that plugs into the back of the radio, they help just because sometimes they get bumped.

I have used a small chunk of LMR-400 that wasnt really any use so I stripped it, but I didnt notice any real difference in quality.

[Harrrrg !]

The head of engineering answered this question when we worked @ G.E. He told me the Z of automotive coax was around 100 Ohms, but he also said it wasn't hypercritical. I think the type with the spiral center cond. support is/was optimized for the AM band.

[Kage]

Yeah after doing some research a while back I found from a few sources that it is indeed 100ohms. Makes me wonder though how common 100ohm coax even is outside of car antennas?
75ohm coax is so close to impedance that any car radio would work fine with it.

[12vman]

I've always used RG-59 or RG-6 with no issues..

I have Farmer Vision (Outdoor Antenna) and use a splitter to a car stereo. Installed a Motorola plug to the end and plug it in. Works fine..

[Pat B]

Car radios are actually two radios in one (AM-FM).  The whip antenna has a characteristic impedance of about 36 ohms at the 1/4 wave resonant frequency (FM section).  The whip is usually extended to about 31" to 34" to raise its impedance to around 75 ohms. The input impedance of the radio's FM section is usually 75 ohms, so any length of 75 ohm cable may be used to connect the whip antenna.

This whip, however, is far too short to resonate at the AM section's 1/4 wave frequency(hundreds of feet long) so the radio uses special circuitry to match impedance to the short whip. Older radios added a trimmer capacitor with a low capacitance RF cable (any impedance) to fine tune the short whip thus matching the unknown impedance to the AM section's input. Since the length of the RF cable affects the matching network, the trimmer cap had to be retuned to compensate for any changes to the RF cable or its length.

In recent years newer radios now incorporate an automatic tuning network that matches impedance variations of the antenna system to the radio.

Car radios have excellent sensitivity.  If you plan to use the radio to listen to weak AM stations inside a building using a whip or random length of wire as an antenna, you will likely get disappointing results. Whip antenna's are highly vulnerable to localize interference.  A coil loop or ferrite rod antenna will provide far better results inside.  Unfortunately, it will not work as well for the FM section.   

[GumBoocho]

I don't believe there exists 100 ohm coax. The choices are 50, 75 & 93 ohms with 93 ohms (low capacitance) used for square wave digital outputs. I am not sure which is used more with cars: 75 or 93 ohms.

[Kage]

Dec 31, 2016 12:03:19 GMT -6 GumBoocho said:

I don't believe there exists 100 ohm coax. The choices are 50, 75 & 93 ohms with 93 ohms (low capacitance) used for square wave digital outputs. I am not sure which is used more with cars: 75 or 93 ohms.

Wow I wasn't even aware that 93ohm coax existed. Nice to learn.

[Bryan]

The cable used RG62 ,13.5 pF per foot . The equivalent circuit is a voltage generator connected to 15pF capacitor and a 30 ohms resistor in series . An inductance can be used to null out the capacitance at the top end of the BC Band .The received signal is attenuated by the aerial capacitance (15pF) and the cable capacitance approximately 56pF. The attenuation is about 4.7 so an amplifier with a gain of 4.7 should compensate for this loss. I yet to see a compensation circuit so if any one has a copy I would like to see it

[Jefferoonie]

The OEM coax cable in my Chevy Silverado appears to be 125Ω.

www.commscope.com/catalog/cables/product_details.aspx?id=24284

I actually stumbled on this forum trying to figure out what the deal is with the 125Ω coax.

[filter4ever]

He's right - use RG-62. It's expensive but worth it!

Wanted to get my favorite radio station C89 (www.c895.org) in HD Radio. It's only 30kW directional - now I get it all the way up here 75mi away!  It works almost TOO well - when a station is weak, it sometimes swaps in and out with an adjacent station.

I used RG-62 as it was closest to the impedance of the coax used in my 1999 Toyota Solara (93 Ohm). The connector to your deck is called a "Motorola Connector" and can be found on EBay. In order to connect it to the OEM antenna, used a phillips screwdriver of the inside diameter to pound out / "un-crimp" the OEM grounding bracket and slip in the RG-62.  Also, I stripped off 1" of the outer jacket, and soldered it to ground tab under the grounding bolt just like the OEM coax was.

Does anyone have an idea of how to build a diversity circuit that will work with HD radio and could switch between the two antennas to the one with the clearest signal?

I see a separate antenna at the bottom of my rear window.  It has a separate thin coax running parallel to the RG-62, with a resistor rather than a 33nF cap in line. The OEM stereo has 2 antenna inputs on the tuner, and apparently employs a proprietary diversity circuit. I've seen schematics online for a diversity switch - however, they seem outdated, They work by using the subcarrier of FM stereo signal as reference, the circuit taps into outgoing audio to pick it up.  I don't think the subcarrier won't be used in HD / Digital mode and need a better solution

[pbullock]

Dec 30, 2015 9:16:24 GMT -6 Pat B said:

Car radios are actually two radios in one (AM-FM).  The whip antenna has a characteristic impedance of about 36 ohms at the 1/4 wave resonant frequency (FM section) with a ground plane.  The whip is usually extended to about 31" to 34" to raise its impedance to around 75 ohms. The input impedance of the radio's FM section is usually 75 ohms, so any length of 75 ohm cable may be used to connect the whip antenna.

This whip, however, is far too short to resonate at the AM section's 1/4 wave frequency(hundreds of feet long) and highly capacitve, so the radio uses special circuitry to match impedance to the short whip. Older radios added a trimmer capacitor with a low capacitance RF cable (any impedance) to fine tune the short whip thus matching the unknown impedance to the AM section's input. Since the length of the RF cable affects the matching network, the trimmer cap had to be retuned to compensate for any changes to the RF cable or its length.

In recent years newer radios now incorporate an automatic tuning network that matches impedance variations of the antenna system to the radio.

Car radios have excellent sensitivity.  If you plan to use the radio to listen to weak AM stations inside a building using a whip or random length of wire as an antenna, you will likely get disappointing results. Whip antenna's are highly vulnerable to localize interference.  A coil loop or ferrite rod antenna will provide far better results inside.  Unfortunately, it will not work as well for the FM section.