Rail mounting a VHF arial?

[SteveTibbetts]

I will be renewing my VHF this winter and upgrading the arial from its current 40 year old home made status as the current set up doesn't receive signals that the handheld does. (and isn't DSC, no external speaker etc) I know there would be a deterioration in signal strength if I mount the arial on the pushpit v the mast top but by how much? I only do coastal stuff and running the co-ax through the boat, using a connector/gland and up the mast looks like a right royal PIA.


Thanks in advance!

 

[rob2]

As VHF is line-of-sight, you can calculate whether the aerials can see each other from dipping tables. As an example, assuming the aerial on the rail is 3m above sea level and communicating with a mast head aerial at 10m above sea level, the distance is 8.7M. If both were at 10m, then the distance is 13.2M. To put that in context, the first example means that if you can see the aerial of the other station, it will work, if you cannot, then the masthead mount will extend your range by 4.5M. Of course, if you are communicating with a land station (CG, etc.) their antenna will be much higher than any vessel can achieve, so the range is far better.

Rob.

 

[pvb]

It'll essentially have the same sort of range as the handheld. Putting the aerial at the top of the mast increases the range, because VHF signals effectively work on a "line of sight" basis. In most circumstances it probably won't make much difference.

 

[chuzzlewit]

Compromise? I stuck mine on a ss pipe fixed to the pushpit with hose clips.

 

[Salty John]

As an example - two yachts with 49' high antennas will communicate at approx 20nm. (1.4 x root 49 + 1.4 x root 49). If one of the yachts moves his antenna to the rail, 9' above sea level, range will drop to about 14nm. (1.4 x root 9 + 1.4 x root 49). If both had rail mounted antennas range would be just over 8nm.

 

[William_H]

It'll essentially have the same sort of range as the handheld. Putting the aerial at the top of the mast increases the range, because VHF signals effectively work on a "line of sight" basis. In most circumstances it probably won't make much difference.

IMHO opinion it will have much better range or performance than the HH. Firstly the transmitter power will be 25 watts more than your 1 ,2or 3 w of HH. Secondly on receive the longer antenna/better groundplane will give better reception sensitivity. The longer antenna may also give better transmission.
As said the mast top antenna will give better range by some miles but I have found a stern rail mounted antenna very successful. You should be able to get it a metre or so above the rail. Every bit helps. The stern rail antenna certainly works better than mast top when the mast falls down.
While VHF is essentially line of sight we do get a kind of spill over beyond line of sight. In this mode more power and better antenna do become critical which is where even at the same height fixed will outperform the HH.
good luck olewill