# Electronic Circuits and Projects Forum

1. ## Author here, need help "building" a fictional long-range radio transmitter

Hi --

I'm a professional author working on a contemporary SF novel with a heroine who is an electronics and engineering whiz, and she needs to build a radio transmitter capable of sending a signal some distance into space (say, 2 billion miles). She's limited by budget constraints, so she can't afford expensive pre-fab equipment, but she is capable of building just about anything she needs from scratch.

This is all being done in secret over the course of the book, and she's only going to need to transmit the signal for a few seconds, so I'm not worrying about the legality of it, the need for a radio license, etc. She just needs to send a command to another device in space to tell it to perform a simple command (i.e. turn itself off).

For the record, the book is set in the present day and on Earth, so ideally I want my heroine to use real, existing materials and equipment in order to build her radio transmitter. That being said, she is a genius so she could potentially come up with a Tesla-like stroke of genius to do something nobody has done before, but I'd like what she's building to be at least theoretically possible.

So what I need to know is, what equipment would my heroine need, or need to build, or possibly hijack temporarily (again, this is fictional!) in order to send a radio signal a fair distance into our solar system?

My heartfelt thanks in advance to anyone who can help me with this. If you are really helpful I will gladly mention your name in the acknowledgments when the book is published, later this year!

Gratefully yours,
--
RJA

The second thing needed is A LOT OF POWER for the transmitter maybe from a nuclear generator.
The third thing needed is an absolutely HUGE directional dish antenna at both ends that might be 1 mile in diameter.

See, fiction is silly.

3. Originally Posted by audioguru
The third thing needed is an absolutely HUGE directional dish antenna at both ends that might be 1 mile in diameter.
Or potentially an array of dishes?:

http://en.wikipedia.org/wiki/Very_Large_Array

4. Originally Posted by audioguru
The second thing needed is A LOT OF POWER for the transmitter maybe from a nuclear generator.
The third thing needed is an absolutely HUGE directional dish antenna at both ends that might be 1 mile in diameter.
Hm, that seems a little extreme given that NASA is still communicating with Voyager at a distance of 10 billion miles and Voyager's dish antenna is only 2.7 meters wide. Also, I've just found a web page stating that "Both Voyagers are so far away from Earth that only the largest DSN antennas — 230 feet in diameter — can send commands to the spacecraft. To do that, they use a 20 kilowatt S-Band transmitter. That's about one-half to one-quarter of the power transmitted by an ordinary commercial AM or FM radio station on Earth."

So I still hold out hope that a nuclear generator is unnecessary, though I am starting to think my heroine may need to remotely hijack one or more of the antennas used by NASA's Deep Space Network to get the job done...

5. Originally Posted by cr0sh
Or potentially an array of dishes?:

http://en.wikipedia.org/wiki/Very_Large_Array
Now THAT is something I hadn't come across. Thanks for introducing me to another possibility!

6. Thanks CrOsh,
My TV screen is too small to show NASA's huge array of many huge antennas. I think they all turn as the Earth turns.
The heroine in the made-up-story (fiction) must be a millionaire!

7. Keeping fiction in mind, during the 50's the old magazine Popular Electronics ran monthly stories about two kids named Carl & Jerry, actually adventures. This is an example of one such story. Keep in mind this material for this story was written in 1956. Fast forward the story and into the future. You are the writer so get creative.

She just needs to send a command to another device in the deep reaches of space to tell it to perform a simple command (i.e. turn itself off).
Also look at the first Star Trek movie as part of it has a similar theme. Vega is Voyager. Bottom line is fiction is fiction and for this just keeping it real.

Ron

8. Most fiction movies are just a bunch of lies by "researchers" who don't know anything about the items.

9. First point, please do not use the word "broadcast" in this context, that really blows your credibility for a technically competent reader.
Broadcasting is what the BBC and other similar organisations do.
This application is a point to point link, use the expression "transmit" rather than broadcast.

Now whether a nuclear reactor is necessary for the power supply depends on several things.

What is the remote "device", is it intended to receive signals for Earth at this sort of range?

Let us assume that it is.
Let us assume that it is intended to receive a signal on a frequency of 145 Mhz which is in the two metre amateur band.
Let us assume that it has an antenna with a gain of 10db pointed to earth. Such an antenna would be about 4 or 5 metres long and 1 metre wide.
Let us assume that the receiver is of good sensitivity (0.1μV or -130dbm).

The path loss for a freespace path of 3 million kilometers is 205db.

Back here on earth, let us assume that the transmitting antenna is similar to the one on the remote device with a gain of 10db. Or our heroin could hijack something like the Jodrell Bank (UK) radio telescope which will have lots** more than 10db antenna gain at 145Mhz.
Again, the antenna is very directional and must be pointed toward the remote device.

The transmitter power required, assuming the 10db gain antenna would be of the order of 330watts.
To allow for feeder cable losses, let make that 1000watts.
A 1000watt transmitter at 145Mhz is quite easy for our technically savvy heroin to build.

The mathematical bit.

At 145Mhz, the free space path loss over 3000000km is 205db.
Receiving antenna gain is 10db

The ERP (Effective Radiated Power) required by the transmitter is (205 - 130 - 10) = 65dbm
Transmitting antenna gain = 10db
Transmitter power required is 55dbm which is equivalent to about 330 watts.

So it is feasible, nuclear reactors and antennas the size of a planet are not required.

JimB
(Who hopes that he has not dropped a decimal point or two, or added where he should have subtracted, otherwise he will look a right prune!)

** using a 75 meter diameter parabolic reflector antenna, typically used for radio telescopes would give a gain of about 40db at 145 Mhz.
This would bring the transmitter power requirements down to around 0.3 watts, which could easily be provided by a hand held radio.

Points to note.

As the frequency is increased, so the pathloss increases, but this is offset by increased antenna gain for a given size of antenna.

Polarisation loss. If the elements of the antennas at each end of the link are not in the same plane, there will be greatly increased loss. Which is why spacecraft tend to use circular polarised antennas, typcally in the form of a helix.

10. Originally Posted by RJAWriter
Hm, that seems a little extreme given that NASA is still communicating with Voyager at a distance of 7+ million miles and Voyager's dish antenna is only 2.7 meters wide.
10 billion miles is perhaps what you meant by 7+ million miles (http://voyager.jpl.nasa.gov/mission/...orts/index.htm).

The best science fiction is based on real facts, which are not hard to find.

John

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