[aprssig] antenna help + voice alert.

Steve Noskowicz noskosteve at yahoo.com
Wed Sep 3 20:22:29 CDT 2008

I'll try this one.

--- joe wrote:
> On my boat I have 4 radios ...marine VHF, APRS radio, FT-2800M 2 meter, and AIS receiver. I am going nuts trying to get antennas set up
> correctly for all of them. Right now I have one on the
> masthead, about 60 feet up, one on the rail about 6 feet up,
> and one on an extension about 14 feet up. Some experiments
> show the range increase even by just going from 6 to 14 feet
> is huge. Question is, if I put the 3rd antenna up on an
> extension it will be only inches or maybe a few feet from
> the other one. Will this cause any ill efects? 

   I don't know what this extension looks like, nor the antennas.
  It's not clear which antenna is where, but in general:
  Get one co-located antenna above the other -- as much as you feel comfortable with (or can do).  You want to get these spaced *vertically*.  If at all possible, get one *directly* above the other.  There is a null below/above a vertical (ideally),  Side-by-side, one will be transmitting the most power into the other and this could blow the receiver front end worst case.  Best case, nothing, or they can desense each other, when transmitting.

> I have also
> thought of making an AIS antenna by simply stripping back
> the shield of the coax and sealing the wire. This is a
> receive only antenna I can put on the extension [f]or one of
> the other ones. 

  Not familar with AIS, but if this is a vertical, you can make a vertical, coaxial dipole.  The center conductor of the coax extends upward 1/4 wave.  I believe this is what you are saying.  However, an equal amount of braid should be "folded" back over the outside of the coax outer insulation.  This makes the other half of the 1/2 wave dipole.  The proximity of the regular part of the coax shield to this (lower) end of the dipole will produce some capacitive loading and make it necessary to make the length of this lower part shorter, but I can't suggest a specific amount except to SWAG 5-10%.

>  One last thing - I have my 2 meter set to
> 144.39 MHz on CTSS of 100. I should be able to hear anyone
> using voice alert, right? 

   Yes.  you will then be "using" Voice ALert it as well.  
I like to call it "Voice Alert CQ" to help understand the purpose.

> I can't SEND with that tone on
> the APRS radio unless I want to hear myself AFAIK.

   Your question is not clear. You can't set the radio to *hear* Voice alert ("CT") without also sending that tone when you transmit -- and, therefore, be heard as a Voice Alert station by others.  All modern 2M Ham radios, that I am aware of, also send Tone when "CT" is set for tone squelch.
   On the other hand, if you set the radio to only send a 100 Hz. Tone - "T" on radios I am familiar with - you will be heard by other Voice Alert users, but you will hear *ALL* APRS packets, not only others using Voice alert.
   That help?

--  73, Steve, K9DCI


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