The S/V Just Us’ design and refit doesn’t start when we buy the vessel.  It starts long before we’ll know what we’re looking for.  The S/V Just Us will be many things to us: home base, lock up, locomotion, shelter, and a floating amateur radio station.

Spectral Purity

A week before mother’s day, I sent out an email to Joe, Asa’s dad, simply titled, “building a tube radio.”  Maybe I should have titled it, “don’t quite know where to start with amateur radio, so let’s start with a tube radio.” He quickly set me straight that tubes and radio designs for on-land aren’t the best for a radio station out in the ocean. So after our first phone discussion and questions from Joe, and an email for clarification, he laid out the basics for receiving and transmitting to the world.  Here are the facts:

  1. 100W RF Output Power in HF Bands (1.8Mhz thru 30Mhz)
  2. 18A @ 12VDC (nominal) Transmitter Power
  3. 50% Efficiency operating in Class B mode

Item #3 about 50% efficiency is very important, as it means we’ll need twice as much power to ensure we don’t get or send out garbage,

“spectral purity when amplifying an amplitude modulated waveform such as single sideband or AM, will require about twice the power going in for what comes out”

A quick note about spectral purity, its one of the key characteristics we’ll be focusing on for transmitting and receiving.  (Found the following in this article from Lab Notes, What Rig Should I Buy?)


  • Sensitivity
  • Dynamic Range
  • Cleanliness


  • Output Power
  • Spectral Purity

The spectral purity, I’m guessing, means transmitting in the band that you intend to while minimizing any transmissions outside that band. If you think of someone wiggling a string as the signal, then how hard you wiggle the string would be the amplitude, and the way you wiggle the string would be the frequency.

Amplitude Modulation (AM) radio carries its information as changes in amplitude. That’s why you hear pops and cracks in AM radio when you turn on the lights or if a fan is running nearby. You will need enough power to transmit a signal where the changes in amplitude are clean so that people far away can pick us up without excessive noise.

Electromagnetic Waves

If you want to see what I mean, let’s use some math.  RF waves are in essence an electromagnetic wave that’s being sent out from your transmitter.

Using the same string wiggling analogy, your transmitting antenna is your hand doing the wiggling. On the other end of the string is your buddy holding the loose end and trying to figure out how hard you’re wigging the string from what he feels.


But as we all know form experience, by the time the wave and wiggles you put into it gets to your buddy, the size of the wiggles is much smaller. We have equations that describe this, but let’s just simplify it down to power drops off proportional to the inverse square of the distance from the source.  Or in other words, what can be detected and received by anyone will drop off very quickly.

Pr = Pt / d^2

Yes, I know this isn’t the correct equation, but this is for simplicity.  Pr is the Power at the receiving station, Pt is the Power at the transmitting station, and d is the distance between the two.

Imagine 3 people, each spaced equally apart.  The 2nd person is half as far away as the 3rd person, and the 3rd person is 4000 miles away on the opposite side of the planet.  The middle person will receive a signal that’s 4 times stronger than the person at the end.


The world is about 7,918 miles in diameter.  So the furthest that you can possibly be is on opposite ends of the earth or about 4000 miles.  If we just look at a simple situation with a perfect clear line of sight and a flat earth, our 100W signal will turn into a 62.5nW signal.  

That’s nanowatts, as in .0000000625 Watts.  As you can guess, there isn’t much a difference between 62.5nW and 63.125nW (or a 1% change in amplitude).  A 0.625nW change will require a 1W change in output signal.

This means that our transmitter must be able to transmit a signal in clean 1W increments, or 250mW for added margin.  That’s the challenge.  I’ll go through signal losses in a later post.  But I found this neat article here describing Path Loss for RF transmissions.

Power Modes

From that bit of information, we can starting doing done estimates for powering our vessel.

I had a discussion with Jim last week about how to drive the prop in our vessel.  Jim was an engine specialist in the Navy, so he knows all about what works and doesn’t work for spinning a prop in the water.  His suggestion: hydraulic engine.  I’m sure we’ll get into the disadvantages of using a hydraulic engine, but in the beginning phases, it was all about the advantages.

Design Requirements to Turn the Propeller

  • Operates without fuel
  • Easy to Maintain
  • Tolerates Marine Environment
  • Fallbacks for all fluid requiring systems
  • Runs on AC or DC

And a hydraulic engine serves this quite well:

Hydraulic Engine

  • Runs on pressure from an accumulator
  • Accumulator can be charged with anything, even a bike
  • We can run this thing on urine if it comes to that
  • No petrol required on board to run

So with a hydraulic engine, a compressor charges the accumulator, which provides the pressure to run the engine. The compressor can be run on battery power, solar power, generator power, or manual power. If we lose hydraulic fluid, we can use sea water or even our own urine if needed.  I’m liking this design… multiple fallbacks! But… back to the power design for our radio.  Since the engine and prop or completely separate systems, we can now focus on making sure that we have enough batteries to power everything as well as enough solar to make up any power we use. I’ve identified 4 different power modes we’ll be operating in:

  1. AC Power (120V to 240V Main) = lotsa power
  2. DC Power (24V Battery Bank @ 20Ah) = 480W
  3. Solar Power (18.9V @ ~5A) = 300W to 500W (cloudy to full sun)
  4. Generator Power (120V @ ~40A per ~5 gallons of fuel or about 6 hours) = ~4800W depending on make/model

An article I found searching for solar powered transceivers informed me that Tx and Rx power are different.  In a given hour, you’re not transmitting and receiving the entire time.   Instead, they gave a rough ratio of 1:3 for Tx:Rx.  So for every hour we transmit, we can expect about 3 hours of receiving.

Assuming we operate the station for 4 hours a day, we’ll be transmitting for 1 hour and receiving for for 3 hours.  Transmitting power is the 100W that Joe gave me above.  That’s a nominal 18A at 12VDC when transmitting.

For receiving, the power requirements are much less- essentially keeping the power on and to amplify and output the signal.    So when receiving, I’m estimating 2A at 12VCD.  That means we’ll need 20Ah at 12VAC = 240W of power.

Solar power output will be our weakest source of energy.  A typical 500W solar array consists of 5 100W panels, each putting out 19V @ 5.3A for full sun.  Amazon reviews of the panels indicate that on cloudy days, it’ll put out about 3A.  Therefore we can expect 3Ah out of our solar array for a power output between 285W and 500W.

But we’ll use the 285W figure for our estimates as nothing every runs at what its specified to do.  So 285W from the solar panels on a cloudy day >= 240W operating power.  45W to spare!  I like it.

We’ll need a 500W solar array which equals about 6-8 hours of sunlight a day to top up our batteries.

For the batteries, that means we’ll need capacity (wire batteries in parallel).  That means wiring up an array of a combination of deep cycle and multi-purpose marine batteries in parallel.  Good marine batteries are rated at about 40A @ 6V, so we’ll need to wire them up in series in 4’s for 24V, with 2 banks of them wired in parallel. So my strategy will be to go from the 4 power modes I’ve identified, and to work my way from the outlet to the antenna.