Radio Guy
Well-Known Member
I swapped my antenna out for a 13" rubber coated one with copper coil, yada yada inside. I reported the results in another thread but the bottom line is a shortened antenna will be worse than stock for several reasons.
The stock antenna is roughly 1/4 wavelength in the middle of the FM broadcast band and its a microscopic fraction of a wavelength in the AM broadcast band operating in a completely different mode. In the FM broadcast band the whip will be fairly low impedance and on the AM band its extremely high impedance and probably several thousand ohms. The coaxial cable from antenna to radio is a special high impedance coax and all AM/FM radios are designed to match up with the equivalent of a roughly 30-32" whip antenna with the special coax.
When you shorten the stock antenna you can coil wire inside to make it electrically the equivalent of a 30-31" whip but the efficiency goes down. It doesn't matter if the coil is copper or made of solid gold, if you swap a 31" long antenna with a 13" you will loose a lot of reception and you can't get it back by any means. Its gone and aint commin back.
When I swapped my antenna I took note of various strong and weak AM and FM stations then visited them again after installing the 13" replacement and many weak stations were gone, could not receive anything. Some stations that were fine got noisy and some strong stations did not seem to change. This happened across both AM and FM bands.
If a station is really strong and you degrade it a bunch with a tiny antenna it could still be really strong and you just won't perceive a change on the radio, but if you hook a test receiver or spectrum analyzer to the antenna lead you will see exactly how much degradation there is. I may do this some day when I have time but its more for the benefit of others as I know and accept the degradation from my swapping antennas.
Years ago most car radios had a small variable capacitor to match the antenna on the AM band since the combination of antenna and length of coaxial cable can affect the circuit. The variable capacitor was accessible either on the back of the radio or through the cassette door opening (what's that??). I have not serviced car radios in many years and don't know if they still have a variable capacitor for antenna matching, but if they do you will get back some of the lost reception on AM by adjusting the capacitor if it has one.
The stock antenna is roughly 1/4 wavelength in the middle of the FM broadcast band and its a microscopic fraction of a wavelength in the AM broadcast band operating in a completely different mode. In the FM broadcast band the whip will be fairly low impedance and on the AM band its extremely high impedance and probably several thousand ohms. The coaxial cable from antenna to radio is a special high impedance coax and all AM/FM radios are designed to match up with the equivalent of a roughly 30-32" whip antenna with the special coax.
When you shorten the stock antenna you can coil wire inside to make it electrically the equivalent of a 30-31" whip but the efficiency goes down. It doesn't matter if the coil is copper or made of solid gold, if you swap a 31" long antenna with a 13" you will loose a lot of reception and you can't get it back by any means. Its gone and aint commin back.
When I swapped my antenna I took note of various strong and weak AM and FM stations then visited them again after installing the 13" replacement and many weak stations were gone, could not receive anything. Some stations that were fine got noisy and some strong stations did not seem to change. This happened across both AM and FM bands.
If a station is really strong and you degrade it a bunch with a tiny antenna it could still be really strong and you just won't perceive a change on the radio, but if you hook a test receiver or spectrum analyzer to the antenna lead you will see exactly how much degradation there is. I may do this some day when I have time but its more for the benefit of others as I know and accept the degradation from my swapping antennas.
Years ago most car radios had a small variable capacitor to match the antenna on the AM band since the combination of antenna and length of coaxial cable can affect the circuit. The variable capacitor was accessible either on the back of the radio or through the cassette door opening (what's that??). I have not serviced car radios in many years and don't know if they still have a variable capacitor for antenna matching, but if they do you will get back some of the lost reception on AM by adjusting the capacitor if it has one.
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