Fast Scan ATV Information:

There are many Amateurs running fast scan ATV. It's not as hard or expensive to do as it may seem. With ATV, you'll not only see the stations you're talking to but also be able to show them your station, videos, computer screens or that latest project on the bench. Balloons have launched cameras and transmitters, sending back spectacular high altitude pictures of the earth. Uses for ATV in the coverage of public service events are limitless. ATV stations, by FCC regulation, are also allowed to retransmit NASA Select TV, making mission coverage viewable via Amateur Television.

Much of the ATV activity is on 70-cm, which means you can use a standard analog, cable ready TV set for your initial tests. You will probably want to get fancier later on but here's how you can get started:

Taking the First Steps

Find out when and where there is ATV activity is in your area. Most ATV groups have a 2-meter frequency that they use for talking with each other while one station transmits on TV. In this area we use 144.34 MHz or 144.37 MHz FM simplex. Other frequencies used are 144.90 and 146.43. Get on and ask where the stations are located and tell them what you're doing. They'll be happy to help you out; point beams in your direction, run tests and give you more information. If there's a station or repeater close by, an analog, cable ready TV and simple antenna is all you may need to receive a signal. ATV signals use the same format that commercial, analog TV transmissions do.

To use an analog, cable ready TV for ATV reception you'll need to connect an antenna to the input where the cable is normally connected. Cable channels (not UHF TV channels) 57 to 60 fall in the 70-cm Amateur band. Their relationship to the ATV frequencies in this band are shown by the chart below:

CATV Channel	CATV Frequency	Closest ATV Frequency
57	421.25 MHz	421.25 MHz
58	427.25 MHz	426.25 MHz
59	433.25 MHz	434.00 MHz
60	439.25 MHz	439.25 MHz

Cable channels 58 and 59 do not correspond exactly with their respective ATV frequencies but a little fine-tuning should do the trick. Of course, an ATV downconverter such as those made by Hamtronics or P.C. Electronics with a preamplifier is the better way to go. These units convert the 70-cm band to a standard TV channel between 2 and 4.

The Antenna

The antenna and feedline are probably the two most important things in your ATV station. You should use good, low loss feedline like Belden 9913 as a minimum. The lower loss the better. The antenna should have good gain and VSWR characteristics across the entire band. Some antennas may have good gain but are too narrow for ATV work. Remember that you will be dealing with a TV signal that is a minimum of 6 MHz. If you're using a repeater, you may be transmitting at one end of the band and listening at the other. Antennas made by KLM (now out of bussiness) or of the K1FO design work well here. There are others and some research will tell you what is currently available.

ATV stations generally use horizontal polarization to gain some additional cross polarization isolation with the FM and packet stations that also use this part of the band. There are exceptions to this rule so check which polarization is used in your area.

Setting Up a Station

Now you're ready to seriously start thinking about setting up a station. There are a number of possibilities here and it's up to you to figure out what will be best for you.

A standard analog TV set will be used as the major part of the receiver. Ahead of it will be the ATV downconverter, which as I mentioned before, will take the 70-cm band and convert it to a channel between 2 and 4. Select a channel that is not in use in your area. The downconverter can be a separate unit or part of one of the many transceiver packages offered by companies like PC Electronics. These packages are basically self-contained ATV stations with a built in downconverter; transmitter, T-R switch and power supply in one box. The transmitter power output is 1 or 10 watts depending on what kind of higher power amplifier you wish to drive, if you choose to do so. Units like these make setting up an ATV station easy but you may wish to take the modular approach and gain some flexibility. I built the WA2FNQ-TV system up from modules and by looking at it you can see what each piece does.

The Transmitter

The heart of my transmitter is a PC Electronics TXA5-5 exciter/modulator. This board accepts standard 1-volt composite video from a camera or video source of your choice and also the 4.5 MHz signal from the audio subcarrier generator. It is crystal controlled on one of two frequencies in the 70-cm band, selectable by a switch. There are also controls to adjust this unit for driving higher power solid state amplifiers. Power output is 80 mw.

Audio is added to the picture by a subcarrier module. The module outputs a 4.5 MHz signal, which is then injected in to the video modulator circuit of the TXA5-5. The 4.5 MHz signal is frequency modulated by the audio information. Deviation of this unit is set to +/- 25 kHz with a 75-microsecond pre-emphasis curve, the same as broadcast TV. On the PC Electronics unit, there are both microphone and line-level inputs.

Next is a PC Electronics PA5 amplifier. This unit uses a Motorola MHW-710-2 or, in my case, an Amperex BGY-41B hybrid power module. It takes the 80-mw signal generated by the TXA5-5 and amplifies it up to 10 watts. The output impedance of this unit is 50 ohms so it may be used to drive a higher power amplifier or feed the antenna directly.

The final part of the transmitter is a Mirage D1010 amplifier. The output of the amplifier feeds a pair of horizontally polarized KLM 10 element beams. T/R switching is done at the input of the amplifier at the lower power level.

The Receiver

The input of an Advanced Receiver Research P432 VDA preamplifier is fed from the normally closed contacts of the T/R relay. The output of the preamplifier is connected to the input of a PC Electronics TVC-4 downconverter. Signals are converted to standard TV channel 3, then sent to the TV sets/VCRs/DVD recorders in the station or to the cable distribution system for viewing anywhere in the house.

Adjustments

There are no adjustments to be made to the receiving system other than tuning in the signal you want to watch. The transmitter is another story.

If you are using one of the already assembled transceiver packages as a stand-alone system, you will have little to do. Maybe just set the microphone gain, tweak the video level and you'll probably be on the air. If you put your transmitting system together yourself or plan on driving a higher power amplifier, there are a few more adjustments you'll have to make.

Video

A characteristic of high power transistor amplifiers is they suffer from gain compression in the last 3 dB of their available power. As they approach maximum power they loose linearity, something very important for ATV work. This is true for amplifiers made by Mirage, RF Concepts or any of the other brands. In AM television, here in the USA, we transmit a negative picture. White is at 12 1/2%,black is at 70%, blanking at 75% and sync is at 100% modulation. The sync tip is the point at which the transmitter is at maximum power. The gain compression of a transistor amplifier can destroy the sync amplitude of an ATV picture causing it to tear and roll uncontrollably at the receiver. One way of solving this problem is to run the amplifier at half its power rating. Doing this you won't hit the 3 dB compression point but you'll never be able to use the full power available from the amplifier. Another approach is to pre-distort the sync amplitude to compensate for the gain distortion in the amplifier. Adjusting the blanking pedestal using the modulator bias pot does this. On the TXA5-5 exciter there are 2 of these pots. This allows you to set up the transmitter for 2 different power levels at the flip of a switch. To adjust my transmitter and 100-watt amplifier, first the video is disconnected from the input of the exciter. The transmitter is then turned on and adjusted for 80 watts or a DC input of around 15 amps at 13.8 volts. The video is then reconnected and the video level adjusted for the best picture, just before white limiting. Remember, a picture with a lot of white in it will cause the transmitter to draw less current and show less power on an average reading wattmeter due to the negative modulation.

Another way of solving the sync compression problem is to use a tube type amplifier. A properly designed tube amplifier is a very linear device and does not have the gain compression that a transistor amplifier does. If you're in to home brewing, it may be worth it to build a tube amplifier or convert an old surplus unit for ATV use.

Audio

Now we want to add sound. Before we discuss how it's done in the WA2FNQ-TV transmitter, let's look at the several methods available for transmitting audio.

On-carrier audio -

Up to now, all we have mentioned is transmitting audio by means of a subcarrier. Another method you will hear about is called "on-carrier" sound. With on-carrier sound, the picture carrier is narrow-band frequency modulated with the audio information. Using this method, as long as there is a picture present you should be able to recover the audio. This makes on-carrier sound very good for DX work. The disadvantage is that it's incompatible with the audio system in a standard analog TV set. You must use a separate FM receiver tuned to the frequency of the picture carrier to hear the audio. Transmitters like those made by PC-Electronics have to be modified for on-carrier sound. Other transmitters, such as those made by Wyman, have this as a standard feature. You should check with your local ATVers to see if they are using on-carrier audio.

Using a separate audio transmitter -

What I consider the best method for transmitting audio is to use a separate transmitter. In analog broadcast TV, audio is transmitted by an aural transmitter, which is 4.5 MHz above the picture carrier. The same thing can be done in ATV by using an FM transmitter keyed in parallel with the ATV transmitter. The deviation should be adjusted to +- 25 kHz with a 75-microsecond pre-emphasis curve. This is not standard for Amateur FM radios and may require a little modification. I did this for a while when our repeater had it's input on 439.25 MHz. A Kenwood 401-B was modified to accept external audio through a passive pre-emphasis network. The transmitter was put in the low power position (5 watts) and tuned to 443.75 MHz, 4.5 MHz above the picture carrier at 439.25 MHz. A separate 14-element KLM beam was used, mounted below the beams for the visual transmitter. Using this method, I was able to deliver full quieting audio even when the picture was marginal.

Subcarrier audio -

The most common method of transmitting sound is by a subcarrier. The output of the subcarrier generator, such as the PC-Electronics FMA5, is a 4.5 MHz signal, which is frequency modulated by the audio. Again, 75-microsecond pre-emphasis is used and the deviation is set to +- 25 kHz with the input level control. If your using the line level input and lack any means of measuring deviation, you can use the same simple method that I've used. Using a VCR for your source, adjust the audio of the downconverted ATV signal until it matches the audio from the modulated RF output of the VCR.

The injection level of the subcarrier generator also has to be adjusted. This level is normally about -15 dB below the picture carrier. Too little injection will cause the audio to be noisy. Too much injection will produce interference in the video, maybe even capture out the color. An off frequency subcarrier can also create interference in the video. The beat frequency produced between the color subcarrier and an audio subcarrier that is on frequency cancels out between the adjacent horizontal lines of video. Interference caused by a subcarrier that is off frequency may appear as diagonal lines in the video. Improperly set up video will do the reverse, create interference to the audio subcarrier. Too much video level will result in a buzz in the audio on high whites. Backing off on the video level will correct this.

WA2FNQ FMA5 modifications -

The subcarrier unit originally used in the WA2FNQ-TV transmitter was an old PC Electronics FMA5 board. The pre-emphasis curve in these older units wasn't even close to the standard 75-microsecond curve and had to be modified. PC Electronics seems to have fixed this in their newer units. I also removed the diode clipper. Audio is fed to the transmitter from the studio compressor/limiters, eliminating the need for the diode clipper. The diode clipper is a potential source of audio distortion.

Adding stereo -

I was able to obtain a Scientific Atlanta model 6380 TV stereo generator, which I wanted to see if I could-make work on the ATV transmitter. The unit was configured for left/right stereo plus a secondary audio program (SAP) channel. There is also input for program video because the stereo pilot must be phase-locked to the videos horizontal line frequency. Outputs are BTSC composite baseband out, a 4.5 MHz subcarrier modulated by the BTSC information and a video output with the 4.5MHz subcarrier mixed in. This is for use with video modulators that don't have separate subcarrier inputs.

Stereo audio from the station compressor/limiters was fed to the left and right inputs of the 6380. Audio from the 2-meter receiver was hooked up to the SAP channel input to make the co-ordination frequency audio available there. All audio levels were adjusted using the front panel pots and bargraphs on the unit.

A second subcarrier input was added to the TXA5-5 exciter board. This allows switching between the 6380 and the FMA5 if needed. Subcarrier injection was adjusted on-air for best performance. This is also a front panel adjustment on the 6380. Tests of the received signal on a stereo TV showed presence of both the left/right stereo signal and the SAP channel with the entire transmitter in operation.

Conclusion

Hopefully, this has given you some idea of how to start setting up an ATV station of your own. There's lots of good consumer video equipment available today. With a little creativity you'll be able to do what your local TV station does. Have fun. You'll find it interesting to see the people you're talking to.