Antique Radios | Wave Trap & Antenna Coil

What is a wave trap? How does a wave trap work? What is an antenna coil? How does an antenna coil work? As the signal enters the radio from the antenna, it first encounters capacitor C3. This is to protect the circuit in case the antenna wire touches an external DC current.
After the capacitor, the signal goes through the wave trap circuit. Taking a closer look, it consists of an inductor, and variable capacitor. You may recall from the section on capacitors that I briefly showed that an inductor and capacitor can be used to tune to a specific frequency. This is called the resonant frequency.
The wave trap takes the resonant frequency and sends it to ground, where it is no longer part of the signal that goes on to be amplified. This is done to block that specific frequency.
The frequency that the radio wants to block is 456 kilohertz. As you’ll learn, this is the radio’s important intermediate frequency.
Unfortunately, some marine transmitters of the day broadcasted Morse code at the same 456 kilohertz. A nearby AM radio would pick up the Morse code, which would be heard over the radio broadcast. A properly adjusted wave trap would filter the Morse code before it could cause interference.
Nowadays, radio frequencies are strictly regulated, so the wave trap in an antique radio is no longer necessary. Some people disconnect the wave trap, but I’ve never found that necessary if it’s tests ok. In our radio, it was working correctly, so I left it in place and adjusted it during the alignment process, which I’ll explain later.
Antenna Coil
After being filtered by the wave trap, the antenna signal is fed to the antenna coil. Here’s the antenna coil from our radio. It’s actually one coil, adjacent to a second, with a third coil wrapped around it.
All the coils are very close, but they are electrically isolated from one another. When coils are arranged this way, the circuit is called a transformer.
The coil which receives the input signal in a transformer is called the primary. The adjacent coils are called secondaries.
When current flows through the primary, it creates a magnetic field. Secondary coils pick up the magnetic field, and the signal from the primary, is induced in the secondary. The signal transfer is magnetic only, and there is no wired connection between the coils.
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Transformers got their name, because they’re able to transform voltage. If the number of windings on a secondary are the same as the primary, voltage from coil to coil stays the same. If the number of windings on the secondary are fewer, voltage is reduced. If the secondary windings are more numerous, voltage is increased.
On the schematic, our antenna primary is indicated here, and the two secondaries here.
There are two secondary windings, because our radio receives two different broadcast bands. A higher frequency band for shortwave. And a medium band for standard AM.
The schematic shows accurately that the shortwave secondary has fewer windings than the AM’s. But unlike shown here, both secondaries have more windings than the primary. This is to transform the tiny microvolt signal coming from the antenna, and boost it a little before it passes to the next stage.
The shortwave circuit uses capacitor C4 to shift the frequencies to the higher band. A switch selects either the AM coil, or the shortwave coil. The switch is accessible from the back of the radio.
I hope you join me for the next video where we’ll discuss the fascinating process used in almost every radio: superheterodyning. To stay updated, please subscribe and click the bell. And if you like this video, give it a thumbs up. See you soon.

[@jw228w]

just started in radio electronics and found your videos, clear concise and to the point.i like the images and diagrams that explain what you are talking about. thanks John

[@VitorFThome]

Your videos are very clean! Tys for the content.

[@FluxCondenser]

Vítor, thanks so much for taking the time to leave a nice comment. really appreciate the feedback. Take care.

[@bingojamas4786]

Hello Flux. Although not called a wave trap, I have a similar LC series, one piece "capacitor choke" connected from the B- to chassis on an AA5. I understand it's function, but I have a question on the choke portion. Is there any reason I cannot use a small, color coded inductor instead of hand winding a small coil? It's a small value in the 5uH range. I've used several turns of hookup wire in the past, but this is a tiny radio with limited space. Just wondering if I am missing something. Thanks.