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I have an old Sears Dashmate 8 track AM/FM radio from the late 70's, and it kicks a**! Really nice radio. I would love to build a proper cabinet for it.

Anyway, it only has one antenna hookup. Now, I'm no RF engineer, but last time I checked AM and FM antennas are wildly different due to the radio waves being different sizes.

Sure enough, if I hook up my CB antenna, it gets EXCELLENT FM reception, but AM reception is essentially nonexistent. The opposite happens if I clip the antenna input onto a large grounded object (like the HVAC vents in the house,) AM reception is pretty decent, but FM reception sucks.

How is this all supposed to work? How do I build a proper antenna for this thing? Do I just tie a loop and monopole antenna together?

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  • \$\begingroup\$ AM broadcasting is narrowband and usually strong, or used to be, so just adding an AM loop antenna was usually good enough and didn't mess up the FM antenna reception. The loop wasn't ideal. It has to be a very long tuned dipole for that. But OK. Some loops are much better than others so there was a market for better ones. For now, use the FM antenna and as you say add the loop to it. The loop is directional. It won't hurt. See if it works for you. \$\endgroup\$ Commented Mar 27 at 15:42
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    \$\begingroup\$ You could just buy a replacement aerial for an older vehicle, just what the radio was expecting in the first place. \$\endgroup\$
    – Jon Custer
    Commented Mar 27 at 15:46
  • \$\begingroup\$ But since this is a car radio it was likely designed for a single long antenna often seen on old cars. \$\endgroup\$ Commented Mar 27 at 15:54

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AM/FM car radios are designed to use the same antenna for AM and FM. For FM, a 1/4 wavelength whip is optimal, but it's not terribly critical, so ~1m is good. For AM, an electrostatic probe works well: it need not be scaled to the wavelength if the receiver input is designed for it (high impedance). So the same antenna will work for both.

A self-resonant antenna at AM frequencies is very large. It has a huge collecting area and thus delivers a very strong signal, along with a lot of natural and man-made noise. You don't need such a powerful input for a well designed receiver.

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What you do is use an antenna made to be used with car radios. As an example, here's one on Amazon.

Car radios were made to work with car radio antennas. The antennas are designed to give useable performance for the two different bands. It is not as simple as "just tie a loop and monopole antenna together." I don't know how they make them work, I just know that they do work.

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Simple: You cannot have cake and eat it. Take your pick: AM or FM. Antenna length and frequency are directly related. Designing an antenna starts by finding 1/4 wavelength. The formula to find the length in feet = 234/ Frequency in MHz. For the FM band, we use 98 MHz because it is in the middle of the band. 234 / 98 = 2.38 feet or 28.7 inches. This is very doable on a vehicle or your home.

Let's try AM. The middle of the band is 1.1 MHz. 234/1.1 = 212 feet. Good luck putting a 212-foot whip on your car or a tower at your house. The other option is to accept that you can install a decent FM antenna and take whatever you get for the AM band, which no one listens to anymore.

FWIW do not use a CB antenna for FM; they are cut for 27 MHz. Sure, it works, but it works poorly.

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