70cm ATV – Amateur Televison

By KE3GG

What began in 2009 as a casual conversation between two Bobs, KE3GG and KB3KOW, has become a big project.  Actually, it has become a big project of many projects and adventures.  Most importantly it has been a huge learning experience.

The conversation, details of which are long lost in my memory, revolved around my interest in doing ATV for the challenge and fun of it.  The other Bob, KB3KOW, was interested in the potential of ATV to provide a valuable service in emergency communications and sparking interest in amateur radio within the Boy Scouts.

Thus began a journey of learning, building, researching, and experimenting.  While there are a few places that sell complete ATV transceivers in a box they are not plug and play units.  Until recently all of them required assembly from multiple kits and alignment with specialized test equipment.  The prices were not cheap!  That changed just a few months after we began our journey when a local company began producing a plug and play video transmitter.

We had read lots of material stating how easy it was to get going with ATV.  Documents abound that told us all we needed was a simple antenna, a cable ready TV, a low power transmitter and we could send beautiful, color TV signals for miles and miles.  So while in Dayton I picked up one of the 5-watt ready made transmitters.  Bob in turn purchased a smaller ready made transmitter that had an output level of 100 mW.  We were ready!  Or so we thought.

Caveat emptor!

Those that do ATV and want more participation DO NOT publish any useful how-to guides.  Even the equipment sellers are somewhat secretive about what is really needed for success.  Except, of course, to say that success can only be obtained using the equipment they sell, not what their competitors sell.

Because ATV is such a very small niche market, there are very few manufacturers of equipment for this mode.  Thus the road to getting on the air with ATV involves lots of kit building, equipment modifying, and system designing from lots of individual functional blocks.

The British publish some useful how-to and equipment construction books but they are sorely out of date.  Many of the equipment projects in their publications use parts that are not readily available in the USA or are no longer even made.  To compound the trouble with the British publications they often do not operate ATV on the same frequencies as here in the US.  Further, some of their designs are not compatible with the NTSC television format used in the US.

Even our normally helpful ARRL has no practical info with regard to ATV in spite of their publication of a book titled Practical Image Communications Handbook.  Interestingly, that book is no longer in print after a very short publication run.  Furthermore there is very little info beyond the simple basics in the ARRL Handbook.

Our first attempts at transmitting video were entirely less than spectacular.  In fact, dismal might be a better description.  We did not get the anticipated 8 miles of range.  We didn’t even get a half mile.  In fact, we barely got a viewable signal beyond 1/8 of a mile!

After much research we finally stumbled upon a hint that we might need better receiving equipment.  As it turns out the average TV set capable of directly receiving an ATV transmission lacks the required receive sensitivity.  So we purchased high performance receiver frontend devices that used TV channel 3 as an IF frequency.  We also built and purchased higher performance antennas than the simple antennas we previously used.  Simple antennas, which I might point out, were proclaimed as adequate for successfully getting started.

Armed with the new receiver frontend devices and improved antennas we actually were able to send acceptable live video over a distance of 3/4 of a mile.  One full mile was a distance that still eluded us.  But with our success we were able to provide live video for the March of Dimes Walk for Babies in 2012.  A short clip of which can be seen on KE3GG’s website

After the event I went back and reviewed the video that we recorded.  I noted many problems with the video signal that we were transmitting.  Each of these problems could contribute to a lack of range.  Primarily these problems meant that even if the receiver was receiving an adequate strength signal, the necessary data for the television to properly display the picture was too damaged for the television to decode.  Thus I went on a search to find out why.

It turned out that there were a multitude of problems, most related to the automatic features built into the video camera and the video transmitter module that are designed to make life simple.  All these automatic functions were in competition with each other.  Then there were some simple shortcomings of the transmitter itself.  Unfortunately these automatic features can not be disabled.  Since the transmitter module is sealed it also does not lend itself to modification to improve its basic operation.

So the search was on for a way to remedy the transmitter issue.  Thanks to the demise of analog TV with the conversion to digital there was a flood of analog cable TV equipment on the market.  I found some high quality cable TV modulators capable of outputting a signal in the 70 cm ham radio ATV band.  The units were available for practically nothing.  The only trouble was that they have an output power of less than 10 mW.

To my amazement, connecting one of the modulators directly to a simple ground plane antenna gave better performance and distance than the same antenna connected to the previously used video transmitter.  That’s right; I was getting further distance on less than 10 mW than I was getting with a 5 watt transmitter on the same antenna at the same test location!

Now the only trouble was to somehow get that meager 10 mW signal up to a few watts of power.  Again, another near scratch build project was in order.  The folks at PC Electronics offer a ready made circuit board for use with RF power amp modules manufactured by Mitsubishi.

We purchased the boards from PC Electronics, the amp modules from RF Parts, heat sinks with cooling fans at a hamfest, chassis connectors from Digi-Key Electronics, and metal boxes to build it all into from Mouser Electronics.  I built our prototype amplifier and connected it to the modulator.  I adjusted everything for approximately 30 watts peak envelope power, or about 7 watts of carrier power.  At that power level it is very close to the same output as the 5 watt carrier power video transmitter module previously used.

I am happy to report that we are currently receiving a useable picture at distances up to 4 miles away from the transmitter.

Where do we go from here?

We have a few intermediate goals.  We would like to be able to provide multiple video “channels” for events like the March of Dimes walks so they can see multiple places along their walking route.  We would like to be able to extend our range in order to provide similar service for some of the 100 mile bike rides that the club supports.

Our ultimate, long term goal is to establish an ATV repeater within Charles County.  This would serve multiple functions including:

  • Provide live video coverage to the County EOC during times of emergency.
  • Provide a means of transmitting amateur radio related educational videos to club members.
  • Provide the ability to support broad area events with live video.
  • Provide an exciting option for members to converse both audibly and visually.

We are still in the fledgling stages of our ATV project.  We have come a long way since we started, we’ve learned an awful lot along the way, and we have a long way to go.

We are currently also experimenting with 2.4 GHz FM ATV.  We are using a 200 mW transmitter circuit board from Comtech and matching receiver circuit board.  Using a pair of MFJ 15-element yagi antennas we have successfully transmitted a clear video signal one mile.

Future plans also include experimenting with 900 MHz FM ATV, and perhaps AM ATV on the same band.