T&T: AIS antenna

[Ed Bruette]

Steven,

Installing any kind of radio antenna system on a boat is a compromise.
There are at least 3 things that come into play with your intended
installation.  Two have to do with the cable and the 3rd is antenna
placement.  The size of the cable (RG58, RG8X, LMR400, RG8/213, etc) affects
signal attenuation due to line loss.  The decision factors, if the coax
isn't permanently attached to the antenna, are cost and what size will fit
in the cable way.  Purists will opt for the lowest loss cable available but
boaters usually can't do that because of bend radius and size constraints.  

The 2nd factor in the attenuation equation is connectors.  Even though you
use good quality cable for the extension, it requires a connector in the
middle of the cable run.  Coax can not be spliced effectively.  The
generally accepted number for signal loss through each connector is 1 db.  A
3 db loss is half of the signal strength!  One db doesn't sound like much
but . . .

The general rule of thumb for antenna placement is to keep them at least 1/4
wavelength of the longest frequency band apart.  A wavelength for marine VHF
is roughly 18".  Many of us with HF installations (SSB) don't have the real
estate to keep the rest of our antennas far enough away from the HF antenna
to comply with the rule.  In that case stay as far away from the HF antenna
as is practical.  You may want to temporary AIS install to test the
interaction with other radio systems.

The most effective placement option for antennas in close proximity is to
have vertical separation between the antennas.  You may not be able do that
on the KK.

Antenna height for AIS is as important as it is for your VHF.  The higher
you can get it the farther it can "see".

Chuck is right on about the antenna splitters.  They are not all created
equal.  They also introduce signal loss into the system.

Good luck with the install.


Ed, N7NVP
PT-38  "Makin' Do"
Poulsbo, WA
 

-----Original Message-----

I am planning to install an AIS receiver.  The antenna and cable included
with unit has cable 15-18 feet.  This is too short for my antena cable
route!  Manfacturer says extension will affect performance.  Is this mostly
an SYA statement or does it really effect performance.  If so how?  What
will suffer.  Will it be that I simply do not see far away boats or what.

Also I have read about antenna spliters that allow the use of one's VHF
antenna to be used with thhe AIS.  That would possible be a good solution
since my VHF antenna is a the top of the mast.

Also what says the group about placing antenna with respect to other antenna
eg SSB , GPS or other VHF antenns.

Thanks for your comments.

Stephen Starling
 KK42