Can you use one antenna for AM/FM and CB? Absolutely! But if you have even the lowest level of expectation as to how you want your CB to work, you need to shop wisely.
Back in the late 70ís a couple of manufacturers decided that they would connect a little electronic device to their AM/FM antenna and sell it to people as a 3-way antenna. On the surface, that appeared to be a pretty good idea. You wouldnít need another antenna and thieves wouldnít be able to tell that you had a CB in your vehicle (CBís were being ripped off at a horrific pace). Unfortunately, CB operation with the set up was a huge disappointment. The reason is clear (to us).
The design of a "receive only" AM/FM antenna doesnít really contain that much design at all. The biggest hurdles are the way you want/need it to mount on the vehicle, how tall you want it to be, and what do you want it to look like. If you scanned the antenna for primary and shadow resonant frequencies, you might have a hard time trying to find one that fell into either the AM or FM bands. Why? Because it just isnít that important to "receive only" radios. As a matter of fact, if you broke the antenna off of your vehicle you would probably still receive the major stations in your area. Granted, it may not be as good as when you had the antenna but you could still use the radio. And, if you had a metal coat hanger you could twist up a workable antenna in a few minutes.
Transmitting antennas (such as CB) are required to be resonant (design frequency that matches the radios output frequency) in order to operate. If they miss the mark somewhat you probably wouldnít recognize the difference while in the receive mode. However, CBís are also transmitters. In order for the antenna to absorb the radioís energy it must be, within a fairly small bandwidth, on frequency with the radio. Failure to meet the transmitters needs means poor performance at least and damaged equipment at worst. Accordingly, in the 3-way antenna business, all design functions must first take into consideration the needs of the transmitters. Unless of course you donít care how well the CB performed.
When a CB antenna doesnít resonate in the general frequency ranges of the transmitter, the energy that cannot be absorbed by the antenna is reflected back into the transmitter. High reflection results in highly heated components that will eventually fail. So what did the designers do? In order to protect the radios from going up in smoke they added a circuit in the antenna lead that would bleed away huge amounts of the radios power. What would you think of a tire business that sold you tires that would explode over 40 mph so they removed half of your spark plugs to control your speed? In effect, that is what WAS being done to the antennas in those 3-way set-ups.
If you want a 3-way antenna, start with the CB antenna. Get a good one! Then find a tunable 3-way splitter. First, tune the CB antenna without the splitter in line to establish a set of reference points. Next, put the splitter in-line (connected to the CB, the AM/FM radio and the antenna) and recheck the SWR. Make any fine adjustments for the CB antenna with the appropriate adjusting device on the splitter. And finally, tune in a local AM broadcast station in your area and tweak the AM adjustment on the splitter to get best reception.
Finding an AM/FM look-a-like 3-way antenna today is a difficult task because the marketplace sent them packing after discovering the poor performance they deliver. However, if you end up with a disguise CB antenna, just donít expect it to work very well. With your expectations set low, you wonít be too disappointed.