VHF Range Question

[pjard]

I read everywhere that VHF radio is "line of sight". However, I have a scanner at my house and I know that I've heard boats on parts of the lake that I am 100% certain are NOT line of sight. Also, same thing on the boat. I was once in Smith Cove and on a handheld VHF and heard a fishing guide who was in Meredith Bay....no way was that line of sight. I also read that antenna height is a very important factor. However, outside of a sailboat mast, how high can you really install an antenna. Maybe 20' on a BIG boat. Would 20' vs. 4' really make much of a difference on Winnipesaukee? There has to be some signal bend or something else at play. Any thoughts/comments?

[ishoot308]

Pjard;

Antenna length is the biggest factor in determining transmission distance. Going from a 4' to an 8' antenna will double your db gain or transmission distance. Here's a link that explains it better... http://sierracgaux.org/vhfradios.html

As an fyi I have an 8' antenna mounted in the center of my rod holder tube on the top of my Eastern so effectively I am approximately 15' above the water. This gives all the transmission distance I need on the lake and then some. If you do get an antenna, make sure to mount it with a quick release style mount to lower and remove quickly that is how mine is mounted. See pics below.

Good luck!

Dan

[MAXUM]

Great write up on VHF antenna design and considerations. Granted this is written specifically for HAM radio but the basic concepts apply. In fact the marine bands are just above the 2 meter amateur band. Thought you might find this to be interesting.

VHF Gain Myths

Far too many amateurs purchase VHF antennas based solely on their advertised gain. Adding insult, the published gain figure typically doesn't have a quantifying designator. That is to say, they just list the dB, and not the dBi or dBd. The i stands for isotropic, and the d for dipole. Without one of these designators, the figure is meaningless.

One enterprising Pacific Rim antenna company's brochure, states in fine print that their gain figures are based on a comparison to a standard hand held antenna. That's a rubber ducky! What's more, the phasing coils used are miniscule, and any real gain they might exhibit, is all but lost in the phasing coils.

There is more to the story. Doubling your effective radiated power (+3 dB) will not magically double the distance you can communicate over, especially when using FM. Further, there is a good case to be made about using unity gain (Ø gain) antennas in a metropolitan area. The reason is simply this. The HAAT (height above average terrain) of the repeater, versus that of a mobile, requires radiating power at higher angles. Gain antennas achieve their gain by compressing the pattern, thus their high angle radiation is reduced. As a result, 1/4 wave, unity gain antennas perform better in these situations, than their higher-gain counterparts do. This article by Danny Richardson, K6MHE, (http://www.k0bg.com/images/pdf/mobile_vhf_ant.pdf) compares the differences between 1/4, 1/2, and 5/8 wave VHF mobile antennas, and their effect on radiation patterns.

Be advised that some Pacific Rim antennas are so poorly designed, and made, that one good slap from a errant tree limb will render them useless. Again, you need to ponder more than just a gain figure when making an antenna selection, especially a VHF one. By the way, that includes the way they're mounted. In short, if you want the best performance, you have to drill a hole!

Antennas don't achieve gain in the usual sense. If you feed an antenna with 50 watts, the radiated power is still 50 watts. What does happen is the radiation pattern is changed. As a result, more power is radiated in a specific direction, and reduced in others. It is the differential which is expressed as gain.

That referenced article is a very good read - reason being it describes the different antenna choices and breaks down the performance by what they are used on. It's a great illustration that even if you do buy an antenna that advertises a certain amount of gain, that is in a best case scenario and choosing where to put the antenna and how good the ground plane is will have a drastic impact on overall performance.

Nice boat Dan!!

[Rich]

Pjard,

Ball park and for planning purposes, it's line of sight.... but that's radio line of sight, it doesn't mean that you can actually see, meaning, nothing in your way, type of line of site.

As an example, this weekend, I was at the docks at Meridith, and I heard someone talking from the West Alton Marine area. That's almost from one end of the lake to another. ;)

As stated, the type of antenna being used on both ends, as well as the height of the two antenna's can make a great difference. Also the quality of the installation (type and length of cables used, etc.

Also, sometimes atmospheric and other conditions can occur to cause signals to travel further or shorter than you expect. Some signals can bounce off of things, but mostly this doesn't happen very often with VHF marine frequencies.

As an example, sometimes, when I'm on the south end of the broads, about by Rattlesnake Island, I sometimes can weakly hear a Coast Guard station somewhere on the coast. I can only guess that they have that antenna quite high on a tower.

[Phantom]

... I was once in Smith Cove and on a handheld VHF and heard a fishing guide who was in Meredith Bay

Comment#1
Also, keep in mind that you will hear (Receive) chatter from all over the Lake on your handheld, but you will not (in most cases) be able to transmit back due to the distance/ power limitations of the handheld.

Comment#2
I have a 4' Shakespeare mounted on a 26' bow rider (i.e. not high up on an arch or anything) & have absolutely no problems xmitting/receiving across 90% of the Lake. We routinely talk from Meredith or Center Harbor with my daughter in WAM. The sometimes sketchy distance would be Center Harbor to the Alton Public docks area. Mount Major gets in the way for that !



.

[Descant]

Line of sight seems to be more a simplification than a true descriptor in this case. There's a good article on en.wikipedia.org/wiki/Line-of-sight_propagation, but it boils down to radio range (miles) is 1.23 x the square root of the altitude (feet), which is also what sticks in my mind from government issue electronics classes in sliderule days. Something that could be easily and quickly calculated in your head. This gets modified by antenna type (broadcast or focused), power, etc. as discussed earlier as well as frequency. Think of the radio signal as a lobe or tear drop shape. You could be in the center or on the edge of the lobe and moving a little up, down or sideways may improve or degrade the signal.

[Rich]

yes, and the height of your antenna (and type of antenna) will greatly affect your range.

Just getting the antenna up 6 or 8 feet more will make a difference. So if you have a small boat, get the antenna as high as possible. They even sell antenna extensions to help with the height. Some people don't like the extensions as they may mean you need to lower the antenna when going under a bridge, etc. but if you want better radio range, the higher the antenna, the longer distance you will be able to communicate.

Some antennas have more 'gain' than others. The higher the gain 'gain' basically means that the antenna focuses the energy more than one with a lower gain. Higher gain antennas tend to focus the radio energy to the horizon (and works both for receiving and well as transmitting), but on a boat, the higher the gain, the more fading you can get as the boat rocks. This may be more of a problem on the ocean with swells and such, but just keep it in mind. A high gain antenna may be bad for a sailboat as it heels over a lot, but not so bad for a power boat as they tend not to spend much time on their side. ;)

This page at Westmarine has a pretty good description along with good illustrations to demonstrate what is going on. It also has a good table that illustrates how much height can affect your radio range:
http://www.westmarine.com/WestAdvisor/Selecting-a-VHF-Antenna