r/PirateRadioPlus

Welcome to PirateRadioPlus. I think Mike u/KG7M had started a wonderful Subreddit that will attract a lot of interesting people. He's already seeded this Subreddit with great content.

So, let me introduce myself, I'm Stan. Mike asked me to participate as a Mod for this Subreddit. I'm very happy to do so not only to moderate but to contribute. I'm an EE and CS amongst other things. I'd like to do some interesting projects and share them with you. More on that in other posts to come.

That's me in the picture when I was 11. I can tell because I got my Heathkit GR-64 SW receiver when I was 10 and my hand is on the Q-Multiplier that I got the next year. You can see the QSL cards for I believe Radio Moscow and Radio Prague.

In 1968 the Soviets invaded Checkoslovakia and Radio Prague went pirate for 8 days. They evaded the Soviets by using mobile transmitters playing a game of cat and mouse that the Soviets had taught them thinking that should NATO invade the Czechoslovakians would be able to broadcast resistance. Kinda ironic how the tables turned.

So for 8 days this 14 year old and his GR-64 tuned into these brave privateers elude the Soviets. The next year I got my first after school job as a technician in a TV repair shop.

This information is geared towards experimenters that would like to use a vintage tube, amateur radio transmitter for broadcasting. The most suitable AM Transmitters will use Plate Modulation. Heising Modulation is a close second - the advantage is lower initial cost for a transmitter. This article is not intended for techs and engineers as it leaves out a lot of information that may interest this type.

Plate, Heising, and screen modulation are historical and modern AM techniques used in Amateur Radio transmitters. They differ by where the audio signal is injected: Plate Modulation alters total power at the anode, Heising uses a constant-current choke, and Screen Modulation controls gain at the low-power screen grid.

1. Plate Modulation (High-Level)

How it works: The audio signal is injected in series with the DC voltage supply of the final RF amplifier using a heavy, iron-core Modulation Transformer. The audio voltage alternately adds to and subtracts from the high-voltage DC, varying the plate voltage and output power.

Advantages: Produces pristine, high-fidelity 100% modulation with a highly linear envelope. It is the most efficient and rugged method for vacuum tube transmitters.

Drawbacks: Requires a large, heavy, and expensive modulation transformer capable of handling the full DC input power of the final amplifier.

2. Heising Modulation (Constant-Current)

How it works: A specialized sub-type of plate modulation. Instead of an audio transformer, a high-inductance Choke is placed between the power supply and both the modulator and RF final amplifier. The total current drawn from the power supply remains constant, with the current simply shifting back and forth between the modulator tube and the RF tube.

Advantages: Simpler and lighter since it eliminates the heavy modulation transformer.

Drawbacks: The plate voltage can never reach zero during negative audio swings, meaning it typically tops out around 80% to 90% modulation capability without causing distortion.

3. Screen Grid Modulation (Low-Level)

How it works: The audio signal is applied directly to the screen grid of a tetrode or pentode final amplifier, rather than the plate. The screen grid voltage controls the gain of the tube, altering the RF output as the audio fluctuates.

Advantages: Requires very little audio power. The modulator can be a tiny, inexpensive audio amplifier, completely eliminating the need for bulky iron transformers.

Drawbacks: Inherently non-linear ("S-shaped" curve). It requires careful tuning to avoid distortion and typically limits your maximum carrier output power.

It is illegal to use a transmitter on the HF Bands in the US, without an appropriate license, except under Part 15 of the FCC Regulations.

Clandestine Caster

Vintage Underground Publication

We often use vintage amateur radio transmitters for AM in the shortwave bands. Unfortunately, there are more transmitters available that don't work well for this purpose, than those that do work well. Many vintage transmitters either don't have a Modulator built-in, or the Modulator used is what is known as "Screen Modulation". This is a poor form of AM Modulation that is weak and is not suitable for broadcasting.

We are interested in either Plate Modulation or Heising Plate Modulation. Both methods are suitable for AM Broadcasting. True Plate Modulation is available on transmitters like the Globe King. Unless we have very deep pockets, the Globe King id usually out of our price range. An affordable transmitter is the Globe Scout Deluxe with Heising Modulation. I own two, my last one was purchased for $70 USD.

Transmitters like the Heathkit DX-35, DX-40, DX-60A, Globe Scout 680, and the Globe HG-303 are not suitable for use as an AM Broadcast Transmitter.

Unless you purchase a vintage transmitter that has been restored, you will likely need to replace all of the electrolytic capacitors.

There are 10 slides in this article.

I posted a short video a few days ago, when I first received the AM Transmitter. Here is a bit more detail.

• The AM Transmitter is available on AliExpress for about $25. In order to have a decent sounding station, you need a few extras. A Mixer is the most important part next to the transmitter. Inexpensive models are available, but don't go too cheap - the lowest cost ones have a lot of hiss. Mine works fine and cost a bit over $50 USD. Next, some kind of antenna tuner is needed for any range greater than 500 feet. If you are only interested in running a Part 15 station, a 10 foot piece of wire is all that is allowed for an antenna.
• Antenna Tuners can be built from a Variable Capacitor and a coil. For power levels under a watt, an old variable capacitor from a junked AM Tube Radio can be used. The coil can be wire, wound on an empty oatmeal box. Or you can buy a used ham radio antenna tuner. Make sure that it can tune down to 160 Meters.
• A Compressor/Limiter is a great asset. It adds punch to your audio and prevents over-driving the audio if adjusted properly. I use a Behringer Composer MDX 2100. I bought it used for $15 USD.
• A decent Microphone is needed to announce songs and interact with listeners. I use a Shure SM-58. I am a musician, and this is what I have for my vocals, so it's what I like - and use.
• Laptop and Desktop Computers are perfect for setting up your playlists. Almost any older computer can be repurposed to use at your station. I use Linux Mint as an Operating System. It's totally free. I use Pulse Audio to direct the audio from my laptop to the Mixer. I can select Bluetooth using "SQ-MIXER', or my laptop 3.5 mm in/out jack with a splitter. I also use Audacity for recording promos and music tracks. All of the mentioned software is free.
• An SWR/Power Meter is a needed item for adjusting transmitter output and making sure that your antenna is tuned, We are dealing with power levels under 1 watt so I am building an SWR Meter using a board from AliExpress, and a vintage meter found on eBay. I plan to install everything in an enclosure.
• I hope this information is useful. I started out wanting to broadcast when I was a kid and saw the Remco Transmitter Kit for children. I made a lot of mistakes along the way, but finally figured it out.

Running more than 100 milliwatts, and using an antenna greater than 10 feet is not legal in the USA. Proceed at your own risk if you are in the USA.

The background music is "Beyond the Northern Line" by Les Miserables, my aging band. If you find it annoying, just stop playing the video after the radio info is finished. I ran it through an AI Detector program for the AI haters in this community. I tend to overproduce as I provide promos for some stations. It probably would have scored better if I hadn't remixed it.

A small but effective AM Transmitter from China. It's putting out around 3/4 of of a watt. Frequency is quite stable for a transmitter with an L/C Oscillator. It shipped from China and arrived broken. I had to remove, and replace the broken input gain control by modifying a standard part.

Range is about 2 blocks. Using an outside wire antenna, it covers my 5-story apartment building. An outboard amplifier can be added to boost the range. Of course it's not operating under Part 15, so it is not a legal device for users in the US.

Station

• Name/Call: CODAR WROZ722 & WRPD298
• Location: Oregon, Washington, and California Coastline
• Language: RADAR Oceanographic
• Target: Western USA Coastline

Time

• 0426 0500 UTC | 08 MAY 2026

Receiver and Receiving Location

• Radio: AirSpy HF+ R.4.0.8-CD
• Antenna: K-480WLA | 35' AGL
• Software: SDR# Studio v1.0.0.1921 64-Bit
• Location: Portland, Oregon | CN85qn

Signal

• RS(T) = 57 and 54

My posts on Operating an AM Radio Station call for an Antenna Tuner. This is a very important piece of equipment for the AM station (not needed for FM). Because a resonant antenna at 1.6 MHz is very long, we must come up with a matching device when we use a shorter antenna. A quarter wavelength antenna would be 146 feet. Most of us will barely be able to find an area for a 50 foot wire antenna. So we use an Antenna Tuner to match our shortened antenna.

You will need 22 gauge solid, enameled copper wire. You can find scrap wire in old transformers and electric motors. You also need a 365 pf Variable Capacitor. Old, junk AM tube type radios all have this variable capacitor. The coil form is an empty Oatmeal Box. A couple coats of shellac should be applied to the bare oatmeal box.

Wind 75 turns of your wire, closely spaced,.on the oatmeal box. You will be "tapping" the coil. To do this you can twist a loop in the winding at the specific points shown in the diagram. The enamel coating will need to be scraped off of the wire to get solder to stick for the connections.

Follow the diagrams and you should be good to go. Remember, any antenna longer than 10 feet is not legal in the US. Neither is declaring war without Congressional approval, so I guess we are fine!

There are 2 slides in this post.