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 first thing needed is a very sensitive radio receiver.
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.

audioguru said:

The third thing needed is an absolutely HUGE directional dish antenna at both ends that might be 1 mile in diameter.

Click to expand...

Or potentially an array of dishes?:

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

audioguru said:

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.

Click to expand...

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...

cr0sh said:

Or potentially an array of dishes?:

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

Click to expand...

Now THAT is something I hadn't come across. Thanks for introducing me to another possibility!

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!

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.

Additionally, where you mention:

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).

Click to expand...

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

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

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.
Receiver sensitivity -130dbm
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.

RJAWriter said:

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.

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10 billion miles is perhaps what you meant by 7+ million miles (https://voyager.jpl.nasa.gov/mission/weekly-reports/index.htm).

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

John

I found this paper handy to get up to speed, at least up to 2005 technology: https://www.electro-tech-online.com/custompdfs/2012/04/DanielW.pdf

JimB's path calculations are correct by my reckoning.

It might be obvious, but worth a mention, that the nature of this transmitter that must be built is entirely dependent on the receiver that is up in space. That receiver dictates the frequency to be used, the modulation type to be used, and the data encoding necessary to communicate. If the thing in space is an American government craft of some sort, I would think that it more likely that the control frequency would be somewhere in S band.

I quote from another paper (see ref 2) "existing deep space receivers must phase-lock to the uplink signal and generate a data clock for command reception. These processes limit the sensitivity of present-day receivers to about -157 dBm for carrier lock and -145 dBm for reception of the minimum bit rate of 7.8125 bits per second. " The example mentions an uplink frequency of 7.2 GHz, which I take to be used for the command radio. This is X band rather than S band. The reference also provides some interesting example data that might be considered realistic including:

For interplanetary contact:
ground station dish 10m
ground station tx power 1KW
ground station antenna gain 55 dBi
path loss at 7.2GHz 289 dB
atmospheric loss 0.2 dB
spacecraft antenna gain 7 dBi (typical low gain of a command antenna)
total received power at spacecraft -170 dBm
spacecraft receiver noise temperature 324K
received P/No 3.5 dBHz

In order to bash together a transmitter capable of transmitting in this scenario one would have to have access to some reasonably good instrumentation, something a serious hobbiest might have. Items might include a signal source such as an old HP 8672A signal generator (bare bones microwave generator well within the reach of serious hobbiest budget at around $1000) to which a BPSK or QPSK modulator (or something simpler. what kind of modulation is typical for spacecraft low rate command channels? I don't know) and a power amplifier could be added to form the transmitter. A modulator would be built on a circuit board (by a skilled RF engineer such as our heroine). A power amplifier is tricky, so I would use a bit license (and the fact that we are only communicating 2 million miles which isn't a lot in space) and target an output power of, oh, say about 100 watts. I think this is do-able at 7GHz, but a bit of a stretch. Its much easier to get 10 watts from a few good semiconductors in a carefully designed PA. I'm not sure how I would get a hundred watts. The antenna would be a dish of course, and probably a surplus older TVRO dish (or one of those neat ones that you see at the local cable company head-end compound) modified to put our power amp and a feed horn up at the feed point.

To get everything to work, a skilled RF engineer would need, at a bare minimum, a spectrum analyzer and probably a vector network analyzer to build the necessary hardware. Serious hobbiests have such things, usually older HP gear such as HP 8593E 22GHz spectrum analyzer and the venerable and much loved HP 8753ES vector network analyzer. These two can be a bit pricey at, say, $4000 for the SA, and $10K for the VNA, but a handy engineer would borrow such things for a single project. (EDIT: just remembered the 8753 series only goes up to 6 GHz. Better use an E5071C instead.)


ref 2: https://www.electro-tech-online.com/custompdfs/2012/04/A0067Paper.pdf

If you're going to "hijack" an antenna, highjack the big dish at Arecibo, Puerto Rico. It's remotely located and has already been used as a big prop in "Goldeneye" where they magically flooded it under water. Anyhow, this 1000+-foot dish is buried in a sinkhole and is steerable and was reoutfitted many years ago with panels to increase the dish's resolution. It also has a built-in transmitter (or did have, anyway) for use in the SETI program and had broadcast our whereabouts to all kinds of nasty lifeforms who'll now come visit us for our raw materials and exterminate the vermin on this planet.

Ham's have been doing satellite and moonbounce work for scores of years using less than 1KW. Why does our genius heroine have to scratch-build anything when there is plenty of stuff already built to do most of what she needs, whether amateur, professional, scientific or governmental?

And what's with all this million miles or so for deep space? The sun's 93 million miles away and that's not even close to being out of the solar system. Better do some more "stellar" research.

jpanhalt said:

10 billion miles is perhaps what you meant by 7+ million miles (https://voyager.jpl.nasa.gov/mission/weekly-reports/index.htm).
The best science fiction is based on real facts, which are not hard to find.

Click to expand...

Dean Huster said:

And what's with all this million miles or so for deep space? The sun's 93 million miles away and that's not even close to being out of the solar system. Better do some more "stellar" research.

Click to expand...

John (and Dean), "million" was a typo for "billion" (large numbers are not my strong point, and I easily get them muddled in my head) and when I said "7+" I was going by an information source which I now see was very outdated. But for the purposes of my question the precise distance to Voyager was not important. I apologize for not being more accurate in stating my question; but rest assured I mean to be careful about getting my facts right when writing the book. That is why I'm here, after all.

JimB, RadioRon and Dean Huster, thank you very much for your detailed and helpful replies. (I will keep in mind that distinction between "transmit" and "broadcast", JimB -- my apologies for using the wrong term!)

To give a few more details for those wondering, the device receiving the transmission is located on the other side of a wormhole through which the transmission will first have to pass, and it is a highly sophisticated and very sensitive alien device attached to a space station which could well act as one giant receiver. So I am not worried about that end of things, only about my heroine's ability to send the transmission in the first place...

RJAWriter said:

To give a few more details for those wondering, the device receiving the transmission is located on the other side of a wormhole through which the transmission will first have to pass, and it is a highly sophisticated and very sensitive alien device attached to a space station which could well act as one giant receiver. So I am not worried about that end of things, only about my heroine's ability to send the transmission in the first place...

Click to expand...

There is your problem, do we know what frequency this device will receive, do we know what type of modulation it will respond to, do we know if any coding is required in the "message".
Without knowing these things, all the rest of this stuff is moot.

JimB

The Moon is a quarter-million miles away, and the reflection coefficient is only 7%. So an amateur radio "moon bounce" rig ought to serve OP's protagonist transmitting 2 million miles away.

RJAWriter, welcom to this forum. You have given a number of the members a real challenge. I think the thing to use as a signaling device from earth to the distant location would be a laser beam, as a very powerful one could be energized with a discharge from a Tesla coil. I also think you could use a device that has not come to the public eye as of yet and therefore it could still be near fiction. I say this as I believe there are numerous things that have been created by the scientific world at different governments expense that are not public, hence we do not know of them.
Good luck with your book. It sounds exciting to me!

Aiming a laser would be a little more difficult than aiming a radio antenna. Although at that distance, the beam will have spread a lot, the aiming won't be as fine as aiming at our moon or even a receiver across the room. However, remember that the laser light will be attenated by dust, nebula, etc. and totally blocked by hard forms such as planets, stars, etc. And stars, emitting their own light, may introduce other problems. Dust will scatter the beam.

The same will happen with a radio wave, but will not be as severely affected since it isn't coherent like the laser. A radio wave won't be blocked by a solid object as will a light beam ... my teensy mind has temporarily forgotten the wave propagation terms ... but if the receiver is far enough behind a solid mass, it will still receive the signal.

However, there's still something about taking over an established transmitter site that sounds more adventurous to me. The research for the site, getting to the site, breaking in .... all that seems more heady than sitting in a basement soldering together a circuit. You know, ET did it with a Speak-n-Read, umbrella, etc. ... a big research antenna sounds more plausible.

If I was doing this, I'd approach it more from the hacker angle. Rather than designing some super sophisticated transmitter, I'd be designing something that would be just capable of getting that single message transmitted and nothing more.

Let's say that the required control message is a digital sequence of 20 bits, and let's say for argument sake, that it's FSK (frequency shift keying). The heroine builds 20 flux compression generators, one for each bit of the message. Those that generate zeros are of one particular design, and those that generate ones are another variation of the design. Then, she locates these in front of naturally occuring reflecting surfaces. They could be all in one location, or they could be spread out over many miles. The reflectors won't be parabolic, but even a flat reflecting surface will double the output power. Then, under computer timing control, the generators are fired off in proper sequence to get the correct signal sent out.

A flux compression generator is a device that combines a coil, a power suppy, and a high explosive charge to generate an intensely powerful pulse of energy. It's a one shot device that destroys itself in the process. See here:
https://en.wikipedia.org/wiki/Flux_compression_generator

Because of the immense amount of power that these can generate, the need for a high quality parabolic reflector would be somewhat mitigated.

Fascinating phenomenon.

Points up the fact that energy, in whatever form, rules.

Your heroine using this concept would not affect my "willing suspension of disbelief" in the least.

As an avid reader of the genre, all I can say is: RJAWriter, "get crackin'".

Let us (or at least me) know when the book is published. Perhaps you would allow me to proof read it (a task I have performed before)??!!??