"Bruce in Alaska" wrote in message
...
In article . com,
wrote:
Hi,
AIS operates on 162MHz which is in the maritime VHF band, I think. Can
I split the VHF aerial line to feed both a DSC VHF set and a stand
alone AIS receiver??? - this receiver in particular
http://www.allgadgets.co.uk/ag/produ...pf%5Fid=AG3933
TVMIA
Actually, if one were to do a bit of RF Engineering, it is possible
to accomplish the above relatively simply. Motorola built a Modar
Triton Bridge to Bridge that had a transceiver and a Ch. 13 Monitor
Receiver all in one package, that split the Receive Antenna line
after the Tx/Rx Switch. These are still one of the best VHF Radio's
ever built and still fond on a whole passel of commercial vessels.
What one would have to do is reengineer the receive path, after the
Tx/Rx switch, and bring out a Rg-174/u line for the secondary receiver.
this would surfice for a Passive AIS Receiver System. for an Active
System one would have to do a bit more and discribing it is beyound what
I would consider easy, for this forum, but it isn't impossible.
The govt. boys would use Larry's idea of Ferrite Ring Circulators
cause they have all out TAX dollars to spend.
One of the best resouces on AIS is Mark Johnson of ShineMicro near Port
Ludlow, Washington. He was one of the design engineers for SEA, and now
is BIG into AIS. Last time I chatted with him, he had prototypes out on
SeaTrials, and expected to be in production this spring, with some
consumer versions.
Just another note, if antennas were as narrow as Doug suggests most
of the Marine Vhf's would have given up their magic smoke long ago.
A Passive AIS Receiver will work very nicely with ANY VHF antenna.
Any VHF Transmitter that can't sustain full power into an open antenna
port, isn't engineered correctly, and needs more heatsink area on the
final Amp transistors. Never had a SEA156 or 157 give up any Magic
Smoke when transmitting continously into an open antenna port. That
was one of the design criterias for any SEA Radio System. Sure beats
paying for all the Warrentee Repairs when the sailing public get it
wrong with their fancy new radios.
Bruce in alaska
--
add a 2 before @
Bruce,
Regarding the antenna, when I first heard of the requirement at a Furuno
service seminar two years ago, I was as skeptical as your comments above.
However, the fact of life is, a new AIS antenna is required if a
transmitting AIS is used. Still skeptical I used an MFJ259B Antenna analyzer
to sweep some regular VHF marine antennas such as Shakespeare, Morad, etc.
Guess what! They were 3:1 or higher at AIS frequencies. Furuno even markets
ones with a hefty $ 316 price tag for their AIS units, but they are quick
to point out there are just as good AIS antennas available from other
sources at a cheaper price, such as the Comrad AV7 or Morad 159HD.
By the way, the MFJ unit has been a real jewel to sweep those top of the
line VHF marine antennas with built in filters for RFI from land mobile
public safety frequencies. We have customers who have areas on the Columbia
River where police and fire departments just blast through on a lot of the
cheaper radios. The SEA 156 was the old solution for them as it had a great
receiver that was immune to the IF image/front end over load problems. The
customer does not like the SEA 157 front panel operation and we are
currently testing other brands now on a few of their tug boats. The antennas
with filters started showing SWR problems right after install a year or more
ago. I now pull them out of the shipping tubes as soon as they arrive, sweep
them with the MFJ and weed out the bad ones. At first the manufacturer did
nothing but after our pile of rejects had built up to a dozen or more and
manufacturing date codes did not reveal a pattern, they had a technically
qualified guy come out and see how I was testing them. Bottom line, we got
free replacements and their production testing was changed to include a
final test after the antenna was placed inside the fiberglass tube. Before
that, their testing was done on the innards only at the factory and then it
was placed inside the tube later.
Regarding VHF transmitters designed to transmit into an open or shorted
port, the cheap VHF marine recreational radios have none or little
protection. We replace a lot of the VHF amplifier bricks in these radios due
to poor owner installed antenna connectors with no solder or the braid and
center wire twisted together at the PL259!
Back in the good old days, even ham gear had protection built in and some
service manuals such as Kenwood required checks with dummy loads with one
half or one third the normal 50 ohm impedance and again at 3 times, etc. I
have a set of the slotted push on PL259s with these various mismatch
resistors color coded for quick ID when working on ham gear.
73
Doug K7ABX