Why do most pocket AM radio receivers have shorter (non resonant) antennas

Question

I would like to know why most pocket radios come with a shorter whip antenna of about 20 cm, which is not a resonant one for FM or SW bands. So, how come they work like that?

Answer 1

Your question is ambiguous, so let me make some assumptions what you really mean:

• By "pocket radio" I assume you mean "small receiver"
• I assume you have not taken apart the radio to see how the antenna is connected, so your assumption the whip is the only antenna or that it is non-resonant is probably wrong
• by AM I assume you mean the MF AM broadcast band (AM otherwise has nothing to do with frequency)
• by FM I assume you mean the VHF FM broadcast band

With that under consideration:

• The whip antenna is probably not used for AM; there is typically a resonant loop stick antenna inside the radio for AM. This is well described in the other answers.
• Antennas do not have to be of a "resonant length" to be resonant if they are connected to additional components like loading coils or tuning circuits.
• For a receiver, it helps sensitivity if the antenna is resonant, but it is not a requirement to receive, especially for receiving high power broadcast stations. Also, a preamplifier in the radio can easily overcome the lack of sensitivity of a non-resonant antenna.
• A frequent design for an FM broadcast receiver is to use a headphones wire as an additional antenna. The typical length of a headphones wire is easily resonant at FM broadcast frequencies.

In short:

• Your assumption that there is one antenna is probably wrong.
• Your assumption that the antennas of the receiver are not resonant because the one you can see is short is probably wrong.
• Your assumption that a receive antenna must be resonant is wrong.

Answer 2

Because it's good enough for people to buy and easier to sell than a radio that comes with an antenna that needs a whole backyard and a front yard.

For MW and SW receiver, the antenna size is usually not the factor that limits the receiver performance. So, it really doesn't matter that much.

EDIT/ADD I see a misconception in other answers and comments. A short rod/whip antenna is quite effective at MW frequencies in generating antenna EMF, compared to decent-sized loopsticks. In most cases, a rod/whip generates a higher EMF than a loopstick. (Just do some calculations with reasonable antenna dimensions!) It is just that a lot of consumer radios do not bother to wire the rod antenna to the MW/AM receiver frontend because that may require a band switch to minimize the interference with the FM frontend. AM only radios may not bother to add a rod antenna because the loopstick alone is totally adequate for the job. But there were plenty of AM-only radio receivers that had both a loopstick and a whip/rod.

I calculated the signal power delivered to the receiver frontend transistor using a small loopstick antenna with or without a 20cm whip. At the optimal condition (the impedance of the first stage matched for maximum power transfer), each antenna alone can deliver about the same signal power or voltage to the transistor. That is, whichever type of antenna is used singly or in combination, the AGC circuit easily absorbs the performance difference in the IF amp; the user is unlikely to notice the difference. This may be one reason for the widespread (at least it seems on this site) misconception that a short rod antenna is ineffective in MW.

However, by far, the largest sensitivity is obtained when a whip is used with a large input impedance amplifier (with or without a tuned circuit attached to it). It is simply incorrect to say a tuned circuit is necessary to use a short whip on MW.

Loopstick antenna is a part of the formulaic AM radio frontend circuit for consumer radio receiver set, but that is by no means the best way to achieve performance receiver. That is, if anyone is to design a high-performance MW receiver radio set, one would not use a loopstick at all. Short whips/rods have weaknesses, but they have nothing to do with their ability to be an effective receive antenna.

Answer 3

Standard circuit for battery-operated AM radio used ferrite-rod antennas. C1 tunes the ferrite rod to resonance. C2, C3 are larger bypass capacitors...

^{simulate this circuit – Schematic created using CircuitLab}

L1, L2 are wound on a ferrite rod. L1, C1 are resonant at the frequency of the transmitting broadcaster. L2 is a link winding on the ferrite rod. Turns ratio between L1, L2 matches transistor base with resonant impedance of the ferrite rod.

The whip antenna on top adds a small capacitive reactance to the ferrite rod. It also adds some signal to the resonant rod. The major part of received signal is from the rod, not the whip.

Ferrite rod antennas are directional, giving a null when the rod axis points to signal source...directionality in a portable radio is unwanted. The whip's added signal tends to reduce directionality of the rod...enough so that radio AGC can act to maintain a relatively constant audio output.