Choosing a WiFi solution is not simple. This tutorial is designed to explain basic WiFi concepts and apply them to available solutions. This is the first update to the tutorial. Readers of the original version have commented that they enjoyed and appreciated the tutorial, but it was too technical. I hope that this updated version will be easier to read and understand by the layman. Once again, comments, suggestions and criticism are welcome.

A few things to note, although WiFi is based on science with accurate formulas, real life installations have many aspects outside of our control. Technical specifications often do not translate into actual experience due to the complexity of the specifications, lack of detail, and specifications of components which will not take into account your mix of antenna types, cable lengths and other aspects of your installation. So, in comparing solutions, specifications are helpful, but basic understanding of concepts is required to ask questions so as to understand how a solution will meet your expectations and needs.

Click Realistic Expectations of WiFi to understand what our WiFi products can and cannot do.

This tutorial uses information freely available on the Internet and includes links for those of you who wish to dig deeper. I believe  that one must always question and try to understand information gleaned from any information source.(Even this tutorial) If it doesn't make sense and can't be understood, it may be wrong. Trust your own judgement.

As an engineer who “graduated” from the corporate world into being a small business owner and addicted to sailing, I felt the need to be on board as much as possible during the short sailing season of New England. Unfortunately I have to work, but luckily much of my work is done via the Internet or phone and I can work from anywhere that has Internet access. Hence my need for Internet access while onboard. Initially, I regarded Internet access in a similar way to onboard TV (as a distraction from the world), but given the choice of more time onboard with some work activity, compared to working on land out of an office, Internet access won. Others feel the need for access to weather data, staying in touch via email or even low or no cost phone such as Skype. All valid reasons to get Internet connected.

There are a limited number of ways to connect to the Internet while on your boat. These include WiFi, Cellular Modem (Using a cell phone network), Satellite and SSB (Ham Radio or Marine). This article concentrates on WiFi which is probably the first choice, as it has the lowest hardware and service cost and because most of us need an Internet connection when we are at a anchor,  on a mooring or in a slip where WiFi is often available for free or payment by use.

Cellular connections have become more and more popular and is my second choice to WiFi. Cellular data plans change often, almost weekly, but they are still $40 to $60 per month for "unlimited" data for a laptop. Cellular technology has moved from sparse cells with high powerful towers that limit the number of conversations, to dense low power cells that carry enormous amounts of conversations. This is great for the typical consumer but bad for boaters in that the shoreside antennas "spill" far less over the water, and range offshore is a lot less than it has been in the past. In general, in areas with lower population density, the cellular data speed is significantly slower than WiFi and cellular service away from land is not available. In addition, cellular carriers can, and do restrict what you can do with a cellular data connection. One of the large carriers prevents use of Skype and is currently being sued for this restriction. In some cellular contracts' fine print, you can also find data quantity limits for "unlimited" service and other restrictions. Breaking these rules can result in service termination without notice! Inconvenient to say the least once you depend on a connection.

Of the options available for connecting to the Internet while onboard, WiFi provides the fastest communications speed and the lowest ongoing cost, which in many situations is free. Can’t beat that. The downside of WiFi is that you need to connect to a “Hot Spot” or some network that connects to the Internet. This can only be done wirelessly unless you have a physical connection at your slip.

So essentially, you have to be in the vicinity of a “Hot Spot” or wireless network for a WiFi connection. The upside of WiFi is that more and more marinas, hotels, restaurants and other organizations are providing Hot Spots that in many cases provide free access and residential areas abound with unsecured wireless networks. We won’t go into the ethical and legal aspects of borrowing access on these networks, but from a technical perspective Hot Spots are preferable as they have stonger signals. Later on you will understand that a successful WiFi connection is limited by the weaker of the Hot Spots (Access Point) and your computer or WiFi solution. See also  Realistic Expectations of OmniBox and other WiFi Solutions

So why aren’t we sitting on our boats with our wireless enabled laptops enjoying Internet access? Well, many are, albeit often with a sense of frustration where only the cost of the laptop has prevented it being thrown overboard.

In my case, the marina where I rent a mooring, put in a Hot Spot. Or rather, a company that specializes in Hot Spots at Marinas installed the Hot Spot. I don’t know the details, but as part of my mooring rental I am provided with an access code to not only my marina’s Hot Spot, but which will also work with all the Hot Spots installed by this company. My mooring is about 1500 feet from the Marina antenna in a tidal river. Sitting in the cockpit with ebb tide, my laptop found the marina Hot Spot, 2 bars of signal, and I could actually connect. Unfortunately the connection was flaky (intermittent) and as the tide turned I found myself perched on the bow pulpit. I realized that although I got a strong signal from the marina hot spot, a good connection requires a strong signal back to the marina from my laptop. So the marina Hot Spot provides a great Internet solution when at a slip close to the marina antenna or more accurately, when you can talk back!
It's not intuitive, but getting a good signal from a Hot Spot means exactly that! You are receiving! You have no idea of how well the Hot Spot is receiving your signal apart from the lack of or quality of your connection.

Additional factors that I considered are the marine environment, the available power sources on the boat, power consumption, ease of installation, my family’s request that each of their laptops be Internet connected, and having access from anywhere on the boat without wiring a local area network, or having wires running all over the boat.

In other words: low cost, great performance, multiple laptop access, and all with a minimum of labor. And of course, remaining within the FCC legal requirements for unlicensed transmitting.
The one thing I initially left out is ease of use. While some folks enjoy the battle (I mean" sense of accomplishment") in making technology work, most of us simply want to use and enjoy. So ease of use has become top of the requirements list.

Now my needs may differ from yours, so lets start with understanding the basic terms used to specify WiFi, add some basic concepts, then develop some solutions that will hopefully cover a variety of needs.

The power output from a WiFi system is described in a few ways, which is confusing. There are milliwatts (mW), Watts (W) and decibels (dB). Decibels also come in a few flavors: dBm, dBi and others. Basically decibel (dB) measures a power ratio.

dBm is an abbreviation for the power ratio in decibel (dB) of the measured power referenced to one milliwatt (mW).
dBm is used in communications as a measure of absolute power values. Zero dBm equals one milliwatt. A 3 dBm increase represents roughly doubling the power, which means that 3 dBm equals roughly 2 mW.
Note that the decibel scale is a logarithmic scale. A little confusing but important. For example a 400mW transmitter has double the power of a 200mW transmitter, an increase of 3dB, whereas adding a 6dB antenna increases the power by 4 times.
The formula to convert mW to dBm is
dBm = log₁₀(mW) x 10
The formula to convert dBm to mW is
mW = 10^(dBm/10)

dBi is used to describe antenna gain or amplification. It is the effective gain of an antenna compared to an isotropic antenna. Isotropic means in all directions, like a ball around a single point. An Omni directional antenna differs from an Isotropic antenna in that it radiates in 360 degrees in the horizontal plane and only a certain angle in the vertical plane. Similar to a flattened ball, or a donut. Its amplification or gain comes from redirecting the signal above and below the vertical angle, to only within the vertical angle.
Have you ever wondered how an antenna that is not powered manages to give such enormous amplification? It's directing energy rather than actually amplifying!
So, when looking at transmit power, the Transmitter Power Output (TPO) plus the antenna gain less coaxial cable and connector losses gives the (EIRP) Effective Isotropic Radiated Power.
The FCC legal Maximum Transmitter Power Output (TPO) is 1.0 watt or 30dBm and the maximum EIRP power allowed is 36dBm (4 watts).

1. Your boat's transmitting (EIRP) power or that of the Hot Spot or Access Point is often the limiting factor.

Hot Spots typically transmit at close to the maximum legal power and do have sensitive receivers, but WiFi is bi-directional and for a good connection, you have to transmit with enough power so that your signal is received. This is much higher power than the typical laptop is capable of transmitting, so something must be added onboard.  Conversly, even with a powerful transmitter such as the OmniBox on board, connecting to an Access Point such as an indoor residential wireless router will be, if at all possible, difficult, slow and frustrating unless you are getting a signal of -70dB or better.

2. Transmitter Signal Strength (EIRP)

The signal strength depends on the power of the transmitter and the gain of the antenna. An antenna is normally a passive device that concentrates the signal in a particular pattern and direction. Antennas can be directional or omni-directional. The maximum legal EIRP power allowed is 36dBm (4 watts) for point to multipoint, which is where our interest lies. Point to point has higher limits but is not applicable until we are stationary, in a slip, or parked in an RV parking area, and where we choose to use a directional antenna. 
Antenna gain does not consume power, nor generate heat. Think of an antenna as concentrating the signal in a particular way. Replacing a 2dBi antenna with a 5dBi antenna will double the power of the signal. The equivalent of replacing a 200mW transmitter with a 400mW transmitter. There are limits to antenna amplification in terms of the signal angle. This especially is of concern with omni-directional antennas where high gain means narrow angle and a potential for drop out when your boat rocks and heels. Close Hot Spots on high buildings or cliffs may be difficult to use due to the narrow angle.

3. Coaxial Cable Losses

At WiFi’s 2.4 to 2.5GHz frequency, the signal attenuation (defined in 5 below) caused by the coaxial cable is very high. Unless you use heavy, large diameter, difficult to work with, and expensive coax, cable lengths between the transmitter and antenna can significantly reduce signal strength. For example LMR100 coax has a loss of 0.4 dB per foot at 2.4Ghz and 0.09 dB per foot at 150Mhz (Marine VHF frequency). Running this cable up a 50 foot mast for a VHF antenna results in an acceptable 4.5 dB loss, especially as the power for VHF is up to 25 Watts. With WiFi, the loss is 20dB. As a very powerful WiFi transmitter could be only up to 1 Watt or 30dB, 20dB would be lost in the cable, leaving only 10dB plus the antenna gain. LMR-400 cable, a high quality low loss cable 0.4" in diameter has a 6.8dB loss per 100 feet. A 25 foot cable would have a loss of 1.7db. So if you have an antenna cable, you should know the type, length and loss.

4. Powering a Remote Transmitter

Avoiding cable loss by mounting a WiFi antenna at the top of the mast requires that the transmitter and associated electronics be mounted within a few feet of the antenna. This requires a weatherproof enclosure and running both power and a cat 5 communications cable to the unit. A fairly new standard called power over Ethernet, PoE, does allow a single cat 5 cable to be used for both power and communication. Theoretically, the cable length can be up to 300 feet. But in practice, if the PoE runs at 12V DC, which is very convenient, then the 24 gauge wire for a low power consuming transmitter (8 watts) will limit the length to about 60 feet. Power guzzlers of 15 or 25 watts will require a higher voltage for 50 feet or more and will typically require a 48V DC or a 110V AC power source. Not convenient for boats unless you put up with an invertor and its inefficiency. Electrical power is commonly available onboard at 12V DC and sometimes 24V DC. Other voltages can be provided, but the conversion adds to the cost and complexity, and there is energy loss in the conversion. Unless we are on shore power, energy consumption is always an important factor. The ideal is a PoE injector that can be powered directly from 12V or 24V DC boat power and a transmitter that consumes 8 watts or less so that the transmitter and antenna can be mounted as one unit anywhere on a typical power or sailboat.

5. Range

Range depends on Transmitter Power, Antenna Gain, Attenuation, Interference, and Line of Sight.
With enough power and sensitive receivers, WiFi can travel long distances if the connection is line of sight and there is no interference.
Range estimates are estimates, as there are a few factors that cannot be controlled such as Attenuation, Interference, and Line of Sight. 

Transmitter Power
Easy to understand. Your laptop may have a 50 or 100 milliwatt (mW) WiFi Modem. Long range transmitters are 200mW to a maximum of 1000mW with typical units being 400 or 500mW. So surely 1000mW is the best? Not necessarily. To output 1000mW you typically need to provide 10 times or more power, and the power that is not radiated is released as heat. Running hot can affect the reliability of electronics. If powered via USB, expect to need 2 USB ports or a separate power supply.
An increase from 500mW to 1000mW is a factor of two or 3dB. A 3dB gain is a lot simpler to achieve with passive antennas and minimizing losses. Remember that the FCC legal limit that limits EIRP power to 36dB is normally the determining factor. (500mW transmitter = 27Dbm + 9 dBi antenna = 36dB) 

Antenna Gain
Note, I did not say "antenna power". Antennas boost the signal by directing the available signal into a particular direction. Sort of like a magnifying glass. Antennas, like magnifying glasses, are typically passive and don't require a power source. So why don't we choose a high gain 27dBi directional antenna?
Antennas fall into 2 categories: omni directional antennas that radiate in a 360 degree horizontal plane with a maximum vertical angle above and below the horizontal; directional antennas that radiate in a fixed horizontal as well as vertical angle. If your antenna is mounted on a stationary object such as a boat in a slip or pole on a dock or on the side of your RV, then a high gain directional antenna may be the ideal solution. If, however, you are on a mooring, at anchor or rocking or moving around, then an omni directional antenna is most suitable.
So why not go with a high gain (15dBi) omni directional antenna? Well, remember, you don't get anything for nothing. A higher gain means that the energy is directed into a narrower vertical angle. Your Hot Spot must be within this angle. If you boat is heeling or rocking, with a narrow beam you may lose your signal. Or, your Hot Spot may be on the top of a building or cliff and not within the angle of your beam. An omni directional 8dBi antenna has a typical vertical coverage of 25 degrees (12 degrees above and below the horizontal). The highest gain omni directional antenna I could find is the Hawkings Technology HA015sip 15dBi antenna. Strange, the vertical coverage or antenna pattern does not seem to be available. My generous guess is a maximum of 8 degrees (4 degrees above and below the horizontal). To serve both the in slip and at a mooring situation, there are units that support 2 antennas. When set to diversity mode, the unit selects the antenna having the best signal. 

Attenuation
Attenuation is the loss of signal due to the signal moving through the air. It is also measured in dB which you subtract rather than add.
To calculate attenuation through the air , also known as free space loss, we need to know the transmit strength and the receiver sensitivity in dB. Let's assume that the receiver has –97dBm sensitivity and the transmitter is 36dBm, the legal FCC limit. This gives a total of 133dBm. (Receive sensitivity is given with a minus sign) This gives a theoretical range of 30km or 18 miles with a 3dB margin. We cannot control and do not know the receiver’s sensitivity , so with our known transmit power (FCC Limit), this enormous range should only be assumed when connecting to a Hot Spot having the assumed receive sensitivity and maximum transmit power.
The above assumes line of site through air. Obstacles such as buildings, trees,and hulls will greatly increase attenuation.
Although line of sight is important, the signal is not a single line, like a physical line joining two points. The antenna's radiation patterns determine how the signal travels. Think of the signal as a flattened balloon between two points. Surprisingly, at my mooring, where my line of site to the marina Hot Spot is through a forest of masts, I don't see any measurable attenuation. The marina Hot Spot is at about 50' (height) and my antenna is rail mounted at 8 feet high and I am at 1500 feet away. Presumably the masts are narrow and part of the "balloon" passes around and above the masts. There is also an effect called "Fresnel Zone blockage" which will be discussed later, which in my case as we will see in the calculation later, has minimal or no effect. 

Interference
Interference at 2.5GHz comes from other WiFi networks, microwave ovens, cordless phones and other interference sources that we can’t control. We should adjust our own WiFi systems to use just enough power so that we minimize interfering with other systems. We can also do some tuning by selecting Hot Spots that may be weaker but use a channel (frequency) where there is less or no interference. There are other more advanced tuning parameters that can be used to limit interference, but these are beyond the scope of this tutorial and may not be supported by the Hot Spot or your system. 

Line of Site
The range is limited by line of site due to the curvature of the earth. For example, an antenna at a marina mounted at a height of 27’ (9meters) and a boat antenna mounted on the rear rail at 9 feet (3meters) above the water gives a line of site of approximately 9.5 nautical miles. Solid walls, trees, and other objects close to, but not in, the line of sight can also cause interference if they are in an area called the "fresnel zone".
Remember the "balloon" we mentioned above. This is the fresnel zone. Solid objects protruding into this area can deflect part of the signal causing it to be out of phase with the main signal, causing something called diffraction which reduces the signal strength. Over 9.5 nautical miles, the maximum beam width radius (thickest part of the squashed balloon) is approximately 40 feet. Fresnel interference is insignificant where objects protrude 20% into this area but becomes significant if they protrude 40% into the area. As this squashed balloon is widest at its midpoint, masts close to my Hot Spot have a minor affect on my signal.

Marine Environment
NEMA 6 Standard - Can be submersed in 6 feet of water for up to 30 minutes

Conclusions

What can we learn from the above concepts?

If we are going to be less than 5 miles from a hot spot, which is most cases, there is no reason to mount the antenna, or antenna and transmitter at the top of a mast. If we do mount the antenna at the top of the mast, the antenna must protrude clear of the mast for a 360 degree view and then it becomes attractive for lightning strikes. If we mount the antenna at the top of the mast, with the transmitter below deck, we incur cable losses, which are significant unless heavy, expensive coax is used. If the transmitter is mounted up the mast, then servicing the unit requires a trip up the mast. For example, a factory reset may require accessing the unit. We also assume that most hot spot antennas are high enough to avoid line of site interference even with your antenna on the rail. Bottom line, no real reason for mounting antenna or antenna and transmitter up the mast, even if you enjoy the view and thrills of going up the mast.

Get the maximum legal EIRP power by carefully choosing the transmitter and antenna. Your transmit power is one of the few variables that you can control.

You cannot control range, but understanding the concepts allows you to make decisions as to where to put your boat, your installation and what equipment to purchase.

Available Product Types

These fall into 3 categories:

These units connect to your computer's USB port for communications as well as power. Some units require 2 USB ports or an external power supply. Two variations are available. An indoor unit with an outdoor antenna connected via 25 feet of coax with the resultant coax losses that are compensated for by a more powerful transmitter.
An outdoor unit, basically an indoor USB unit in a weatherproof box, with an attached antenna. Distance between the computer and box is limited to a 15 foot USB cable, although an active indoor USB cable can be added. You can make this yourself with an off-the-shelf Alfa USB unit, a pelican waterproof box and an antenna.

Pros

• A simple solution for a laptop that requires no external power
•  Lowest cost
•  Portable

Cons

• Drivers are not always available for Apple OS, Linux.
•  Won’t work for WiFi enabled PDA’s .
•  Drivers need to be installed. You must install the driver on every computer that may use the unit.
•  Supports only one computer
•  Cable length limits where you can work, especially if you have a midsize or large boat.
•  Your laptop, which powers the unit, is probably charged by an inverter or run from an inverter, so there will be additional energy losses. 
• Not rated for marine use. No conformal coating on electronics and no outdoor ratings
•  Software installation and setup may require technical skills

Summary

A portable solution for a single windows based computer with limits on the distance between the antenna and user.

Basically, the unit converts the wireless signal to a wired local network. Outdoor units are regular commercial units in a weatherproof box with attached antennas, or a semi weatherproof unit that includes a built in directional antenna. (Example EnGenius EOC2610). Indoor units require an antenna cable and external antenna. The power ratings range from 200mW to 600mW.

Pros

• Maximum power, up to the FCC limit
• Multiple users (via added hub) and support for any computer and operating system which has an Ethernet port
• No drivers or software required
• Some powered by 12V DC and PoE
• Setup is via a web browser

Cons

• Difficult to setup and use. Configuration can be really complex. The interfaces are designed for networking technicians. The documentation is often written in non-native English and simply repeats what you see is on the screen. The device is normally a general purpose unit that can be configured as an access point or bridge which adds to the complexity.
• Not rated for marine use. No conformal coating on electronics and no outdoor ratings
• Wired connection to your computer
• Reliable solutions can be expensive
• Some solutions require long coax cables
• Some solutions require 110V AC

Summary

A flexible powerful solution if installed properly and if the user is able to understand and take advantage of the features inspite of the complexity.

This solution is similar to the Client Bridge above with the following differences:

• Designed for boaters. The user interface is simple to use and specifically designed for the boater without removing powerful features
• Portability. The repeater function allows computers to be connected via WiFi through the Repeater/Bridge to the Hot Spot. 
• PDA support and multiple device support. Numerous computers and WiFi enabled PDAs/Cell phones can connect simultaneously
• Designed for marine and outdoor use. The electronics has conformal coatings and is installed in a NEMA6 certified waterproof (submersible) aluminum box
• Less battery drain. Low power CMOS technology minimizes power consumption and heat produced.
• No antenna losses. Antenna directly connected to the motherboard 
• Runs on boat DC power 
• Waterproof through-deck connector simplifies installation 
• Latest WiFi technology

Pros

• Easy to use, designed for boaters and the marine/outdooor environment
• Maximum power, up to the FCC limit
• Multiple users and support for any computer that has WiFi or an Ethernet port
• No drivers or software required
• Powered by 12V DC and PoE
• Setup is via a web browser
• Supports omni directional and directional antennas concurrently
• Reasonable price

Summary

A flexible, powerful solution designed for boaters and the marine environment by boaters!