Hittade följande när jag sökte efter impedans för bilradioantenn:
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.
I samma tråd hittar jag även:
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.
(darkliferadio.proboards.com - car-radio-coax-impedance)
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.
I samma tråd hittar jag även:
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.
(darkliferadio.proboards.com - car-radio-coax-impedance)
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