marine radio antenna range

[Enids Hubby]

I have a 25 watt Humminbird marine radio with a Shakespeare 5101 8 ft. antenna 6db. It's been in my brother in laws boat and plan on putting in in mine since I don't use his boat much . I was planning on changing the antenna to a 3ft. antenna because the 8 footer would look funny on my small boat. I was looking at the Shakespeare web site and it says to calculate range you take the square root of the height , in feet , above the water and multiply that by 1.42 , calculate that for both boats or stations and add them together and you get total range. That does not sound right because I gotten better than that . I can talk to resorts in Crane Lake while at Kettle Falls hotel which is probably 25 miles . My questions are : are there other factors that affect range ? what is the db measurement mean? what is the difference between 1/4 wave or 5/8 wave ? Can signals "relay" from other antennas in it's path ? Being I could be by a resorts 100 foot antenna could I be "stealing" that signal? . If for example you had 2 boats with 8ft. antenna mounted at a height of 10 feet each that would only give you a total range between the 2 boats of 9 miles , thats seems pretty poor . Does anybody get better than that ? should I put the 3 footer on or look funny with the big 8 footer?

[kwkfsh]

VHF range is "height of eye" what can be seen from the top of the antenna can be heard or talked to. An 8 ft antenna will give you better range than a 3 ft one.

The resorts most likly have theirs mounted as high as possible for the best range.

Some areas have relay stations to send the signal along. I am not sure if there are any in MN though.

[chunkytrout]

You will decrease your range dramatically with the three footer. To my knowledge there are no Marine band repeaters in MN. There are a handful of factors effecting your range when dealing with VHF frequencies. The key is get the antenna high and keep your signal clear. When you talk about waves, think of it like this. A full wave antenna is the length of the antenna determined by the frequency. A twelve foot antenna would really be a hassle on a small boat so, 5/8 wave (eight foot) 1/4 wave (3 foot) and so on. dB is actually a ratio of noise to signal strength. Thats where I get kind of lost so I'll end it at that.

chunk

[chunkytrout]

Darn double posts!

[This message has been edited by chunkytrout (edited 04-10-2004).]