Creating a Home TV Transmitter


Now that TV broadcasts have gone digital, what is a television collector to watch? Of course, you can hook up a DVD or VHS tape player to watch recorded material on channel 3 or 4. You can also use a converter to watch digital TV broadcasts on an analog set.

I prefer to watch television the "old fashioned way," with a rabbit ear antenna. I also like to use my vintage TVs anywhere in the house, free of wired connections.

My solution is a home TV station. It covers the whole house and can be received by as many old TVs as I feel like powering up.

Let's Have a TV Party!

This impromptu "TV party" shows my transmitter in action. The color set is my restored RCA CTC-11H. The black and white set at lower right is my DuMont RA-103. On top of the DuMont are two handheld TVs, the type which can only receive through an antenna: a Panasonic Travelvision and a Sony Watchman.

All four TVs are receiving through their antennas, from my home broadcasting station on the other side of the house. The content is a live movie from a satellite channel.

The second party features my Sharp 3LS36 mini color television along with the two handhelds. In this case, I hooked up a DVD player to the transmitter.

Many mini portable TVs lack an external input, so if you collect handhelds, a home transmitter offers the only way to use them at all.

Here are a couple of space age JVC sets receiving a movie through their built-in rod antennas.

Agile Modulator as Transmitter

Setting up this home station was easier than connecting a typical DVD player. The heart is a device called an "agile modulator," specifically a Blonder-Tongue model AM60-550A.

I paid $26 for a surplus unit on eBay. It has ample power to reach the entire house, using a set of rabbit ears as the transmitting antenna.

Connecting Your Video Source

Hooking up the transmitter couldn't be simpler. It has one input and one output.

For input, I often feed the transmitter from one of our satellite TV boxes. You can use any source that produces standard audio/video output: a DVD or VHS player, digital converter, video camera, and so on. Whatever you choose will be broadcast to any TV in the house, on the channel you select.

On the back of the Blonder-Tongue are jacks labeled AUDIO IN and VIDEO IN, where you plug in the audio and video cables from your source device.

Note the short length of coaxial cable connecting the IF IN and IF OUT connectors. My unit came with that cable. If yours lacks the cable, you'll need to supply one. Nothing special is needed; use the same coaxial cable as with any audio/video device.

The VIDEO IN jack is an "F" type. Most video sources will use phono cables. If needed, you can get an inexpensive F-to-phono adaptor from Radio Shack or similar sources.

Broadcasting From an iPhone, Streaming From the Internet

If you have an iPhone or iPod, you can easily connect that to your transmitter. All you need is a cable that produces composite audio/video output. In this photo, my iPhone is feeding video into the agile modulator, to be received by TVs all around the house.

A smartphone or similar Internet-enabled device vastly expands the content that you can broadcast. Whatever you can stream from the Internet or load onto your handheld player can be viewed on your vintage TVs. I used to watch recorded tapes and DVDs on an old TV while working in my shop. Now I can also watch movies or TV shows streaming from the Internet.

Broadcasting From Your Computer

If your computer has an S-video output jack, you can also plug in an inexpensive S-video-to-composite adapter like this one, to send video to your transmitter.

You'll also need a cable that plugs into your computer's audio-out jack, with an adapter to convert its stereo output to mono for the transmitter.

Thus equipped, you can transmit any video that you can view on your computer, whether playing a DVD movie or streaming from the Internet. On my computer, going to the Control Panel and choosing Connect To a Projector tells the computer to enable the S-video output. The procedure may differ on your computer.

Connecting Your Transmitter Antenna

Like every transmitter, the agile modulator needs an antenna. I use this funky old pair of rabbit ears. Any cheap antenna from a thrift store or flea market should work as well.

Notice the little adapter on the end of the antenna cable. This lets you plug it into the transmitter's coaxial output jack. Radio Shack and many other sources sell these adapters for a buck or two.

The next photo shows the RF OUT jack in back of the modulator, where you will connect your antenna. To place your antenna some distance from the modulator, connect it with an ordinary coaxial cable.

The type and length of your transmitting antenna is not critical if your modulator is close to the TV. However, a little bit of tuning may improve your signal farther away.

It's easy to experiment if you are using rabbit ears to transmit and receive. Simply shorten or extend the antennas for best reception on the channel that you're using. Higher channels (such as 12) need a shorter antenna length than lower ones (such as 3). The diagram in the next section shows you the optimum length for any VHF channel.

Building a Tuned Antenna for a Specific VHF Channel

If you don't mind using the same channel all the time, you may be able to extend your range a bit by making a dipole broadcasting antenna tuned to a specific channel. This video tutorial by shango66 explains how.

In a nutshell, your antenna can be a simple dipole (two ordinary wires) connected to the agile modulator's RF output with a coaxial cable. This antenna is a lot like the familiar FM radio dipole antenna. The type of wire used for the dipoles is not critical. The coaxial cable is a standard 75-ohm cable, the same type used to connect DVD players, etc.

This diagram shows the correct dipole lengths for VHF channels 2-13:

For example, to build an antenna for broadcasting on channel 3, cut each dipole wire to 44.2 inches in length. To build an antenna tuned for channel 12, cut each dipole wire to 13.5 inches, and so on.

Note that higher VHF channels use shorter antennas. In case you are curious, the formula for calculating the dipole length in inches is: 468 divided by the channel's center frequency (in megahertz) divided by two (since you want two dipole wires) multiplied by 12 (to convert feet to inches).

Solder one dipole wire to the center wire on a coaxial connector and the other dipole wire to the connector's shield (outer) wire. Attach the antenna's coaxial cable and then connect the cable to the RF out jack on the agile modulator.

Hang your antenna as high as practical, with the arms (dipoles) spread out as in the diagram. Antennas are directional, so experiment by turning your antenna one way or another in relation to your receiving TV.

Choosing Your Broadcasting Channel

The agile modulator transmits on one channel at a time. You pick the channel by setting little DIP switches in the front, following instructions on a plastic card that slides out from the panel.


A DIP switch can be flipped up or down with a pencil point or small screwdriver blade. Flip your switches to match the desired channel's diagram in the chart. You can use any of the standard VHF channels 2-13.

It's prudent to check whether any stations are still broadcasting in your area. Although many digital stations now use the UHF channels, a handful of stations still use a VHF channel. You don't want to pick a channel that's already in use, which might create a jumble of noisy interference.

To check for area stations, enter your address and zipcode in the TV Fool station locator. You'll get a list of stations with their channels and distance from your home.

When I checked, I found several VHF channels that are not used for broadcasting in my area. As long as I pick one of those unused channels, there is no chance that I'll interfere with other signals.

If you have problems with RF interference inside your house, you'll find that the higher VHF channels (7-13) are less affected by RFI than the lower ones (2-6). Modern homes have many potential interference sources, such as computer power supplies, "wall wart" AC adapters, fluorescents, light dimmers, and so on.

These photos show serious interference on channel 3 from my laptop's power supply:


When my computer was plugged in anywhere nearby, the video was full of moving diagonal lines and the audio was drowned out by loud buzzing. The TV was unwatchable!

When I switched the modulator and television from channel 3 to channel 12, the interference disappeared:


For this test, I chose a room at the farthest end of our house from the transmitter, where the signal is faintest. If choosing a higher channel eliminates RFI here, where the signal is weak and the RFI is strong, reception will be even better in rooms closer to the transmitter.

I noticed the same improvement inside our garage, which sits at the opposite end of our property. When I used channel 3, the garage's fluorescent lights would essentially blank out TV reception. The picture was a mess and the sound was unintelligible. After I switched to channel 12, the video and audio cleared up dramatically.

Blonder-Tongue Owner's Manual

If you buy one of these modulators, it's worth getting the owner's manual, which explains how to set the output controls. This allows you to get maximum transmitting power without audio or video distortion.

The manual is free upon request from Blonder-Tongue. Phone the service department at 1-800-523-6049 and give them the stock number and model number stamped on the rear panel. For example, mine is stock number 59414 and model number AM60-550A:

Legal Transmission Limits

The Federal Communications Commission regulates broadcasts in the radio and TV frequencies. I'm not good at deciphering legalese, but as far as I can make out, a home transmitter's signal strength must not exceed 100 milliwatts. My transmitter doesn't reach beyond our property, and we live on three acres, so there's no danger of interfering with neighbors' reception.

I happen to have an old TV field strength meter, which was used years ago when servicemen installed roof antennas in customers' homes.

I have no way to check this meter's accuracy. However, it was fun to see its needle move when I turned on my home transmitter.

A handheld TV like my Travelvision or Watchman makes a more practical tester for a home broadcasting station. Just turn it on and walk around the house!

You'll find that antenna orientation is important, for the transmitter as well as the receiving TV. Many homes are also full of devices that create unwanted RFI (radio frequency interference). These include computers, fluorescent lights, big-screen TVs, security systems, light dimmers, electric motors, and so on. If you have bad interference from such a source, your choices are limited. Either turn off the device or move your transmitter or receiving TV away from it.

Handy For Restoration, Too!

I have found my home transmitter very useful when restoring vintage TVs like the RCA CTC-11H and DuMont RA-103 seen earlier.

Restoring an old television means playing it over and over, to check this or that. In the old days, a serviceman could just connect rabbit ears and tune in a local station. But nowadays there are no local stations!

To test the TV on the workbench, you can connect a video player or digital converter box, but that adds to the crowding and rat's nest of wires on your bench. It also limits you to two channels: 3 or 4. What about all the others? Does your TV work beautifully on, say, channel 8, but poorly on channel 4? Knowing that kind of thing might save a lot of troubleshooting time.

When I'm working on a TV project, I leave the transmitter on at all times and connect rabbit ears when it's time to try my TV again. Quick and easy! By changing the transmitter's channel, I can test the TV's performance on any channel.

Receiving a "real" over-the-air broadcast is also an excellent performance test. Vintage televisions were designed to operate in a world of mostly-weak broadcast signals. The signal from a modern device like a DVD player can be quite different.

If a signal is very strong (often the case), it can overload a sensitive old TV, causing the picture to "bloom" and become unfocused or too-contrasty. If the video player is your only source, you might wrongly think that your TV has a serious problem, when in fact it's responding normally to an excessive signal.

Modern video players can also introduce problems that didn't exist decades ago. A VHS tape player may create horizontal smear or bending in the picture. MacroVision and similar copy protection schemes may cause ugly picture interference that vintage TVs can't handle.

This photo shows copy protection interference (columns of colored lines) on my CTC-11A. There's nothing wrong with the TV, as we saw in the earlier Shrek photo. The lines are caused by extra copy-protection information in the video signal. Such information didn't exist when the TV was built in 1961, so it causes garbage on the screen and buzzing in the speaker.

(An inexpensive video stabilizer will allow you to watch DVDs and tapes on your vintage TV without such interference.)

If I'm receiving satellite TV via my home transmitter, I know that I'm working with a relatively pure signal and won't have to waste time chasing false symptoms caused by a new device.

Servicing the Blonder-Tongue Modulator

The Blonder-Tongue agile modulator is a commercial grade device, designed to operate 24/7, year in and year out. Every device can have problems, however.

After a couple of years of continuous use, my modulator developed a faint horizontal band that moved slowly up the screen. This symptom often indicates a filtering problem in the power supply. Inadequate filtering allows a 60Hz AC signal to escape from the rectifier and invade the RF circuits, and since this 60Hz signal is out of sync with the 60Hz signals in your TV, an interference wave crawls slowly up or down the screen.

The power supply is not hard to service if you're comfortable with modern PC boards. In the next photo, I have removed the small power supply board and placed it atop the opened modulator. Also shown is the EDS Capacitor Analyzer that I use to test low-voltage electrolytic capacitors.

It took only minutes to confirm that the four 1000-uf electrolytics in my power supply were leaky. The next photo shows the board after I replaced them.

That cured the problem and my modulator has performed faithfully ever since. This unit does get rather hot when running. If you're concerned about extending its life (or simply saving energy), put it on a power strip and switch it off when you don't need it.

Note that 60Hz interference from other devices can also create faint traveling bars. The poorly filtered "wall wart" AC adapters used in many devices are frequent offenders. After repairing my B-T modulator, I still saw some slight bars, which magically disappeared when I moved my wireless router (and its wall wart) to a different outlet.

This radio construction project, including all descriptions, diagrams, photos, and the underlying electronic design, is published here for the noncommercial use of radio hobbyists. You may print and reproduce these project instructions for your personal use. Commercial use of this material is strictly forbidden.

©1995-2016 Philip I. Nelson, all rights reserved