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

ofosot69

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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?
 

spongella

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Excellent question. Most antennas are a compromise between size and performance. A full size, resonant antenna on a portable radio would be very long, much to long to be practical. So, shorter antennas such as telescopic ones of only a foot or two will work for average consumer use. Also, the circuitry in the radio is designed for optimal performance with the supplied antenna for the most part. I say "for the most part" because manufacturers vary in the way their radios are designed and produced so radio performance will vary among manufacturers.

As for pocket AM broadcast band radios, most times the antenna is an internal ferrite bar with wire coils around it instead of a whip antenna. This design, sometimes called a "loopstick" antenna has been around for decades and works very well. These antennas vary in size and length depending on the size of the radio. Below is a picture of the chassis of from an old larger portable AM radio from the '60's. Note the long black rod at the bottom surrounded by a coil. That's the AM "loopstick antenna which works excellent for AM band use. "Pocket" radios will have much smaller ones.

You came to the right place for information and there's a plethora of folks more experienced than me who can answer your question. Hope I've helped to answer your question.
 

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ofosot69

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Excellent question. Most antennas are a compromise between size and performance. A full size, resonant antenna on a portable radio would be very long, much to long to be practical. So, shorter antennas such as telescopic ones of only a foot or two will work for average consumer use. Also, the circuitry in the radio is designed for optimal performance with the supplied antenna for the most part. I say "for the most part" because manufacturers vary in the way their radios are designed and produced so radio performance will vary among manufacturers.

As for pocket AM broadcast band radios, most times the antenna is an internal ferrite bar with wire coils around it instead of a whip antenna. This design, sometimes called a "loopstick" antenna has been around for decades and works very well. These antennas vary in size and length depending on the size of the radio. Below is a picture of the chassis of from an old larger portable AM radio from the '60's. Note the long black rod at the bottom surrounded by a coil. That's the AM "loopstick antenna which works excellent for AM band use. "Pocket" radios will have much smaller ones.

You came to the right place for information and there's a plethora of folks more experienced than me who can answer your question. Hope I've helped to answer your question.
So, this loopstick with a smaller antenna works like a lording coil antenna?
 

kf8yk

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So, this loopstick with a smaller antenna works like a loading coil antenna?

No, in a typical portable radio the whip antenna is for FM and SW only & the internal ferrite loop is only used for the medium wave AM band.
 

prcguy

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So, this loopstick with a smaller antenna works like a lording coil antenna?
Its different than a whip with loading coil. There is a wire coil around a ferrite bar which adds a bunch of inductance requiring much less wire than just wire alone to resonate near the AM broadcast band. Then there is a variable capacitor in parallel with the coil and adjusted to resonance when you select frequencies with the tuning dial. The parallel resonant circuit of the ferrite loaded coil and capacitor make up a very high Q antenna and there is a coupling loop to pull signals off the ferrite stick antenna into the AM receiver.
 

spongella

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So, this loopstick with a smaller antenna works like a lording coil antenna?
The loopstick isn't a loading coil antenna. Prcguy explained it perfectly and as KF8YK stated the whip antenna on a radio is for FM and/or shortwave reception. AM-only radios don't use whip antennas.
 
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ofosot69

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Its different than a whip with loading coil. There is a wire coil around a ferrite bar which adds a bunch of inductance requiring much less wire than just wire alone to resonate near the AM broadcast band. Then there is a variable capacitor in parallel with the coil and adjusted to resonance when you select frequencies with the tuning dial. The parallel resonant circuit of the ferrite loaded coil and capacitor make up a very high Q antenna and there is a coupling loop to pull signals off the ferrite stick antenna into the AM receiver.
But that coil would only work for AM. How come the FM reception from 88 MHz to 108n MHz works without using a longer (75 cm) antenna?
 

prcguy

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But that coil would only work for AM. How come the FM reception from 88 MHz to 108n MHz works without using a longer (75 cm) antenna?
In portable radios a separate antenna is used for FM because the ferrite rod antenna will not work in the 100MHz range. Most have a telescoping whip, some may have wire arranged in a loop inside the radio housing.
 

ofosot69

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In portable radios a separate antenna is used for FM because the ferrite rod antenna will not work in the 100MHz range. Most have a telescoping whip, some may have wire arranged in a loop inside the radio housing.
I know that, but what I don't know is how a smaller non-resonant antenna works for the FM? As per the calculations, at least 75 cm whip is required for 100 MHz reception, but a pocket radio has only 20 cm to 30 cm whip antenna. How that works?
 

mmckenna

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How that works?

Non resonate antennas will still receive. Just not as well as a resonate antenna.

The issue with consumer products is that consumers want form over function. No one would want to use a portable FM radio with an antenna that long.
 

ofosot69

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Non resonate antennas will still receive. Just not as well as a resonate antenna.

The issue with consumer products is that consumers want form over function. No one would want to use a portable FM radio with an antenna that long.
But how come such a portable radio receiver with a smaller antenna is able to receive even distant FM stations? Do they have a tuned active antenna?
 

prcguy

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But how come such a portable radio receiver with a smaller antenna is able to receive even distant FM stations? Do they have a tuned active antenna?
Many FM radio stations are transmitting with 10,000 to 100,000 watts ERP from tall towers or high mountain tops. That's how a small radio and antenna can work well at a distance.
 

mmckenna

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But how come such a portable radio receiver with a smaller antenna is able to receive even distant FM stations? Do they have a tuned active antenna?

I didn't say that a non-resonate antenna would not receive anything. I said that a resonate antenna will receiver better. They are not active antennas.

As prcguy said, these broadcast stations are using a LOT of power that overcomes the poor performing antennas.

Random lengths of wire will still work. I spent a lot of my younger years with random lengths of wire connected to a short wave radio and it worked 'well enough'. But then along comes the ham license and tuned antennas become the norm.

Untuned antennas are not a brick wall that stops RF.
 

ofosot69

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I didn't say that a non-resonate antenna would not receive anything. I said that a resonate antenna will receiver better. They are not active antennas.

As prcguy said, these broadcast stations are using a LOT of power that overcomes the poor performing antennas.

Random lengths of wire will still work. I spent a lot of my younger years with random lengths of wire connected to a short wave radio and it worked 'well enough'. But then along comes the ham license and tuned antennas become the norm.

Untuned antennas are not a brick wall that stops RF.
So, why do the radio manufacturers keep the whip antenna on their receivers, at all? How they decide the shortest length for the antenna on them? Is there any formula for that?
 

exkalibur

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So, why do the radio manufacturers keep the whip antenna on their receivers, at all? How they decide the shortest length for the antenna on them? Is there any formula for that?
No. That is likely a design choice by whoever designed the radio ergonomics, as opposed to who designed the RF section. As others have said, if the transmitter is reasonably close (IE, within 20-30 miles or so from the transmitter), ANY antenna will work just fine, sometimes NO antenna works. If you live in a very fringe area, or are trying to receive distant stations, often the built in antenna won't do a very good job. Good radios have a connector for an external antenna. Car radios have the same issue, especially newer ones without external antennas; Older vehicles often had a full size FM antenna and had very good receive range. HD Radio has somewhat fixed that issue (and that most people probably use Spotify or SXM these days).
 

mmckenna

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