GMRS Radios for Boat-to-shore

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
Posts: 6463
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

GMRS Radios for Boat-to-shore

Postby jimh » Mon Nov 02, 2015 10:55 am

The General Mobile Radio Service or GMRS is a UHF radio service available to all citizens. A good summary is provided by the FCC on their website.

Previously I wrote about using the General Mobile Radio Service (GMRS) as a legal method for establishing boat-to-shore radio communication for recreational boaters. I re-introduce the topic in this new forum to point to the Midland MTX100 radios now being sold at a major outfitter. The Midland MTX100 GMRS radio is now available from Cabela's and discounted to only $120 from the MSRP $150 price. See the Cabela's On-line Catalogue for details.

Unfortunately, use of a VHF Marine Band radio on shore is not permitted under the rules unless a special shore station license is obtained, and generally a recreational boater cannot obtain a shore station license. Use of GMRS radios is permitted on shore and on the water, and by getting a GMRS license and using two Midland MTX100 radios, one on the boat and one on shore, recreational boaters can get the boat-to-shore communication they often desire. One GMRS license covers all family members use of the GMRS radios. The family is defined to be quite large, and includes spouse, children, stepchildren, parents, step-parents, siblings, grandparents, aunts, uncles, nephews, nieces, and in-laws. The FCC recently announced it would eliminate the regulatory license fee for GMRS radio licensing, although it will retain the application fee of $65. The cost of a license and two radios should be about $305, giving users in the USA a legal boat-to-shore communication method.

The Midland MTX 100 radio is supplied with an included quarter-wave ground plane antenna designed for use on a magnetic metal surface, such as the roof of an automobile. Using the Midland MTX100 GRMS radio on a boat with the included magnetic mount antenna may not be convenient or optimum. A GMRS-band UHF antenna (460-MHz) that does not require a metal ground plane will be more effective on the boat. Here are some links to antenna options:

https://www.rfparts.com/antenna/antenna ... 00sp1.html
The above is a very good value for a GMRS antenna. You will need to change the connector to a UHF series or buy an adaptor. Mounting is left to the installer to fashion.

http://www.dpdproductions.com/page_gmrs.html
The vertical outdoor base antenna would be suitable for your shore station, or perhaps for the boat, with some mounting adaptation.

http://www.sinclairtechnologies.com/catalog/resources/pdf/SC323-HF2SNM(D00-G03)-DI.pdf
This antenna is probably expensive, but I include it for reference.

Here is an inexpensive base station antenna: for $25 you can't beat this one. Mounting can be as simple as some large hose clamps.
http://www.jpole-antenna.com/shop/462-m ... e-antenna/

Another possibility for the boat antenna is a GAM ELECTRONICS antenna, such as their USS-2 model. GAM Electronics also makes an appropriate mount for this antenna, their ADAP-II. The small antenna could be mounted on an extension mast of four feet (or more), which would improve the range of communication.

The magnetic-mount gournd-plane antenna that comes with the Midland MTX100 could be used at the shore station if you can find a large steel surface to place the antenna onto. For example, the top of a refrigerator would be a good ground plane for the mag-mount antenna. If longer range is desired, an external antenna should be use. See the choices above. Unless you have a steel boat, the magnetic-mount antenna won't be very useful on the boat.

Because these GMRS radios operate at UHF (460-MHz), loss in the antenna transmission line should be considered. For a shore station installation, use lower loss coaxial transmission line like RG-8/U or the modern equivalents like LMR-400 or LMR-400UF. Boat antenna transmission lines should be kept as short as possible and use good quality small coaxial cable.

jimh
Posts: 6463
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: GMRS Radios for Boat-to-shore

Postby jimh » Tue Nov 03, 2015 8:55 am

Many claims for range are made for radio devices. Of course, no single radio device possesses an inherent and intrinsic range by itself. All radio communication circuits consists of two stations, and the range is a function of the capabilities of the two stations added together. We examine a range for communication between two stations in the GMRS service, one a mobile and one a base.

For both stations we use the transmitter power of 5-Watts, the rated power of the Midland MTX100 radio, and an antenna gain of 3-dB, a tranmission line loss of 2-dB (due to the higher loss at 460-MHz of the typical transmission lines that will be used). With this data we can predict a transmitted signal level:

Transmitter power in dBm = 5-Watts = 10log(5) + 30 = +37dBm
Transmitter antenna gain = 3dB
Transmitter line loss = -2dB
TOTAL TRANSMITTER SIGNAL = +38dBm

For path loss we first investigate a 10-mile path distance in free space. The formula for distance in miles and a frequency of 460-Mhz is:

PathLoss (in dB) = 36.6 + 20log(460) + 20log(d) where d is in miles

Thus for a distance of ten miles at 460-MHz, the free space path loss is predicted to be

Path loss = 36.6 + 53.3 + 20 = 129.9dB

With transmitter signal of +38dBm and a path loss of -129.9dB, the received signal will be 38 -129.9 = -91.9dBm

The receiver antenna will have a gain of 3dB and a line loss of 2dB, for a net gain of 1dB, presenting a signal to the receiver of -90.1dBm

Now we compare the received signal level to the receiver sensitivity. For the Midland MTX100. I could not find any clear statement of receiver sensitivity for the radio, but let us assume it is a typical modern communication radio with a sensitivity of at least -110dBm for 10dB signal to noise and distortion demodulated signal. (This is very reasonable assumption as it only assumes a sensitivity of about 1-microvolt, and most modern radios have better sensitivity.) This gives us a signal margin above minimum sensitivity of

-90.1dBm - -110dBm = 19.9dB

This calculation suggests that at a range of 10-miles the received signals will be about 20dB above the receiver minimum sensitivity. That suggests good communication between radios, suitable for voice transmission and human reception by ear.

In some of the literature for the MTX100 a range figure of 40-miles is mentioned. Let us examine the communication link at that range. The only change in the parameters used above will be path length. This increases to 40 from 10, making the path loss increase by 12dB. The received signal strength is down to -102.1dBm, which is now just above the receiver minimum signal sensitivity of -110dBm by 8.1dB. We should be able to copy a signal at that level, but the fade margin has decreased to only 8dB from 20dB.

In the above, the most optimistic assumption made in the calculation was for path loss. The model used was for free space propagation. A more realistic model is to use a path loss calculation with a factor of 40log(d) for the effect of distance. Recalculating the 10-mile path loss at 460-MHz with that factor gives us a path with 20dB greater loss. This suggests that even with a 10-mile path with line-of-sight between stations, if the path is over typical earth terrain, the signal from a 5-Watt 460-MHz transmitter at a distance of 10-miles will be arriving at the receiver at around -108dBm. A signal at that level is just above the rated sensitivity of the receiver (-110dBm), and although that signal level should be good copy, it would have almost no fade margin.

In summary, a reasonable estimate of the reliable range of communication for a 5-Watt 460-MHz transmitter using a 3dB gain antenna with 2dB of transmission line loss, with a line-of-sight path to a receiver with a sensitivity of -110dBm will be about 10-miles. Under ideal conditions, an ultimate range of 40-miles might occur.

jimh
Posts: 6463
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: GMRS Radios for Boat-to-shore

Postby jimh » Tue Nov 03, 2015 9:06 am

In the above analysis of range, the two stations are assumed to have a line-of-sight path, that is we assume their radio horizons overlap. For this to happen the height of the antennas must provide each with a radio horizon distance that can reach other other station's signal. If we assume a symmetrical height, that is both stations have antennas of the same height, each must have a radio horizon distance of 5-miles.

The formula for radio horizon is

d = (2h)^0.5 where d is in miles and h is in feet

Solving for h as a function of d we get

h = d^2/2

For d=5, h = 12.5-feet

This is the height of antenna that both stations need for a radio horizon of 5-miles each, allowing their signals to be in a line-of-sight path to each other at a range of 10-miles.

If we assume the mobile station in the circuit has a lower antenna and is limited to an antenna height of only 6-feet, its radio horizon distance would be only 3.5-miles. The base station would then need a radio horizon of 6.5-miles to communicate to the mobile. The base station antenna height would then have to increase to about 21-feet. We still need no intervening terrain between the stations to maintain the communication circuit.

As usual, height of antenna will be a dominant factor is radio range for a line-of-sight path. In a base-to-mobile communication system, increase height at the base station helps all mobile stations improve their range.