Fellow Radio scientists,
I have asked this question before here and other places and haven’t found a solution yet to a technical problem that has been on my mind for a while and its eatin me up you know how it is when something is eating on you.
I have several late model AM/FM auto radios that I have always wanted to connect to an outside antenna and use in the house.
The question is this: Older car radios used to have an antenna trimmer which you could adjust for perfect antenna tuning for AM reception, the FM band doesn’t seem very fussy for good reception on a long wire antenna.
Using a 30 inch rod antenna made for a car inside the house doesn’t give you a good match like when the radio is in a car, since the car body acts as half the antenna.
I have never been able to make any antenna work real good for AM on a car radio when the car radio is used in the house. I’m talking about radio sensitivity not problems with other man made noises and things like that you may have in the house.
So how do modern car radios tune the antenna for the AM band when there is NO antenna trimmer on the radio?- Is the adjustment automatic or what? There is no antenna trimmer I took the radios apart and looked no trimmer.
I don’t have schematics so it’s hard to tell how the circuits are arranged. These are Chrysler 2006 car stereos.
I tried loading coils/variable capacitors/ all kinds of arrangements of these and had some positive results but never come even close to how the radio performs in a car.
I’m looking for some positive answers on what the deal is here, someone who actually knows how the newer car radios tune the radio rf input circuits for the rod antenna on a vehicular, - It doesn’t make sense that you put the radio in the car and presto- perfect AM reception and then mount on the bench and you can’t get much of anything even with an long wire (30 feet) on the outside of the house, I grounded the radio chassis to a cold water pipe none of those tircks work.
It used to be a short piece of wire in the antenna jack and tuning the antenna trimmer on older car radios worked like charm but not so on the newer radios.
There has to be an answer to this – can anyone give me a technical exploitation as to the answer?
Thank you,
I don’t know why I can’t figure this out for my self
Radio Joe
Searched the web found related post some place man said there is no antenna trimmers on digital radios - sweet- not sure if that is correct but if thats the case then I would think somehow the radio has to adjust to the L and C of the coax and antenna so it would be resonant some place in the AM band. Sombody some place must know the answer to this! I find it hard to beleive that each radio would have been made so the electrical match would be perfect for each model of car the radio was going to be installed in.
Radio Joe this is a great subject. I was already to suggest a few smart ideas, until I read the rest of what you said, and you've sort of covered everything I was going to say.
Just throwing this in, because one of our members invented this idea a while back, probably Ken Norris, and that is to use a mattress spring as a ground plane.
But a water-pipe or electric ground is probably as good or better.
Getting the antenna outdoors, but you tried that...
The auto radio in the house is kind of like the car in the garage, but you covered that.
Ah! Here is my one big idea on the subject!
Start making visits to old-time auto-mechanic-shops and ask them about auto radios and antennas!
Radio Joe,
I know very little about modern receiver implementations, maybe just enough to be dangerous. The new ones use one humungous mostly-digital IC to do almost everything. Here's one example: http://www.st.com/web/catalog/sense_power/FM1934/CL1935/SC201/PF161593
Look at the application example schematic in the datasheet. With the exception of some varactor diodes in the FM side, there are absolutely no variable capacitors or variable inductors among all the discrete components. The AM antenna connects to the pin labeled "LNAin" through some fixed capacitors, a resistor and a fixed inductor. No tuning circuit or antenna trimmer. The IC contains a broadband, untuned LNA front end amplifier circuit. Selection of the input frequency and detection of the AM signal is all done by smoke and mirrors in the IC with Digital Signal Processing (DSP). Basically, the DSP uses digital processing to replace the functions of filters and resonant circuits.
As for your antenna problems, I think a solution lies in grounding the radio effectively. A whip or wire antenna replacing the car antenna should work fine provided you have a good enough ground. I've had some fairly good results using a wire about 4 ft long formed into a circle and connected across the car radio input jack with no separate ground connection. I'm sure this could be optimized, but I never spent the time.
Also, make sure your 12V power supply isn't generating any digital switching noise. The power supply is in series with any possible ground path to the AC line. If it's a switching supply and isn't properly filtered, it's going to flood the LNA with noise. With the almost perfect AGC action of these IC's, the digital noise is just going to sound like normal inter station background noise. Meanwhile the power supply noise is hammering the broadcast stations into oblivion.
You might try a set of rabbit ears. They would be short for the FM band, but would be a balanced antenna (dipole). The other option is to simply buy or make a folded dipole for FM. Radio Shack used to carry these. It is made from twinlead.
Joe
Car radio antennas have a short piece of coax between the antenna and the radio. Maybe that's where the answer is..
Back in the day, I had an old Delco AM radio, using a length of wire wrapped between two thumb tacks on the wall and the other end stuck in the center hole of the Motorola connector on the back. It had a trimmer but it didn't change things much..
Treat it like shortwave through a length of RG-6 or RG-59..
Aparently the answer is in the electrical match between the radio and the antenna ( rod, car body,coax )I will buy a car radio antenna replacement with coax attached, and gound the antenna base at the rod end of the coax, and test. I read some place the Z of the antenna coax in car radios is not the standard 50 or 75 ohms. All I know right now is for top notch reception on AM if you use the car radio out of the car dosent come close to using the antenna and radio in the car. Sure you have local reception but thats it. There has to be a good answer to why this is. I hope to hit pay dirt on this at some point because there to have a simple answer.
It has been many years since I worked on car radios but in the day the coax lead from the antenna was essentially a hollow tube with a shield and the inside conductor was a very small gauge (28 or 30) bare wire which was loose inside the tube. The idea was to minimize the dielectric losses by eliminating the inside insulation which is found in standard coax.
Neil
Like Neil, I too remember the old car radio antenna "coax" as being a shielded hollow tube with a small gauge wire center conductor flopping around inside the tube. Some web research revealed that this is equivalent to RG62 coax. The important parameter is the capacitance per ft. of 13.5 pF, which is much lower than for RG6.
For an AM car antenna, the "coax" does not act as a terminated transmission line, but rather as just a shield around the center wire to block electrical noise fields inside the car along the path from the antenna mount to the radio. In this configuration, it's important to keep the capacitance introduced by the shield to a minimum. A typical car radio antenna has about 5 ft. of shielded cable resulting in 5 x 13.5pF = 67.5pF. This is equivalent to connecting a 68pF capacitor across the antenna jack and using just a straight wire antenna of about 30" (same as typical car antenna length which is probably optimized for FM frequencies).
The standard equivalent circuit for a car radio antenna in the AM broadcast band is a 15pF capacitor in series with a 30 ohm resistor to the input jack along with a 68pF capacitor across the input jack. You can see this in the test setup in the datasheet for the ST TDA7705 car radio IC that I posted in message #4 above. Look at Figure 11 on page 25. The 15pF capacitance and 30 ohm resistor represent the capacitance and ground resistance of the 30" antenna element when mounted on a car. The 68pF capacitance represents the capacitance of the shielded cable.
The 68pF capacitance of the shielded cable will cause a big difference compared to just a 30" wire without the shunt capacitance. This may be the key to getting a short wire antenna to work. Of course, using a real car antenna with cable will produce the same result as shunting a wire antenna with a 68pF capacitor.