The number of discrete projects finding completion continues.
In the last two days all of the electronics installation was completed, the color coat was painted on the dinghy, the dinghy centerboard well repaired and the stove installed.
For the electronics installation I used a terminal block rather than splices into wires, cleaned out some old wiring – will this never end? and cleaned up some existing wiring.
The VHF radio will sound a fog horn if I connect it to the hailer tucked away in the starboard cockpit cubby. Sounds intriguing. According to the manual, between fog horn soundings, the radio changes to ‘‘listen’’ mode. In the fog horn section of the manual there is no explanation, but in the description of another option using the hailer, the manual says the radio uses the hailer as a microphone listening for a response. I don’t know what it does if it hears one.
The color coat on the dinghy looks very good, but I got some overspray on the concrete blocks at the marina. It looks like the aerosol rolled under the dinghy and colored the blocks. I will make good and have it repaired.
Stove installation went well but required modifying the ¼” plywood liner in the space. The new stove, with gimbals, is exactly the width of the space without the liner. I cut a rectangle out of the liner.
The burners take a long time to light – they must be preheated with alcohol. This will take a little time to get used to, but they boil water very quickly, in our ad hoc test.
This, I believe, ends the large tasks. The compass will get calibrated this week. I still want to replace the steaming light — Jennifer will hoist me top the mast.
Though not critical path, I am looking forward to connecting the shower drain to diaphragm pump, which will pump the water overboard rather than allowing it to drain into the bilge. The pump is in place but running the hoses is a chore.
Tomorrow, we’ll be at 12 days. If you’re near by, join us for breakfast at the boat. Arrive about nine am.
4-APR-2016 The AIS has a flashing green light indicating an antenna issue. The code is VSWR, which is Voltage Standing Wave Ratio. This seems to be an indication of impedence. The connector for the mast-top antenna looked a bit corroded. I’m hoping that’s it.
VSWR implies that there is an impedance mismatch somewhere between the radio and
the antenna. All the components should be designed for the same impedance (52, 75,
or less common 90 ohms). So the corroded connection is the likely culprit.
Standing waves happen when signal is reflected at the point of a mismatch.
Opens, shorts, and corroded high-resistance connections all cause mismatches.
VSWR relates the strength of that reflection to the strength of the original signal.
Signal reflected in the transmission line is not signal getting out the antenna.
For receive, it means less signal makes it from the antenna to the radio.
I heard of a case once where the VSWR was good but signal wasn’t getting out.
In that case the long coaxial cable was weathered, causing it to be lossy. So
instead of reflecting the signal, it merely absorbed it. In this case, replacing the
transmission line actually made the VSWR worse, until the weathered antenna
was repaired as well.
Erwin and I continued our conversation via email. Here are some highlights:
John: I was mistaken about the connection being corroded. It looks pretty good. I must have been thinking about some other connections somewhere.
I am wondering whether I should replace the connector.
Jennifer is supposed to take me up the mast so I can take a look at the connections at the antenna. We’ll see if she actually does that.
The repeaters are finding us, so there is a signal going out.
Erwin:
I tried to find documentation on the Transmitter Status display.
I looked at the Vesper AIS documentation, and didn’t get very far.
What I saw in the picture is that the VSWR display has 10 boxes. 6 of them are “lit”.
If full scale is VSWR=3.0 (very bad), then 6 boxes is probably VSWR=2.0 (marginal).
But it may logarithmic.
The badness could be corrosion, dirt, an improperly tightened (cross-threaded) connector,
an improperly assembled connector, a kink in the coax somewhere, even a bend with
a radius smaller than 10X the coax diameter.
If you’re seeing a VSWR of 2.0, that’s probably not the end of the world, after all,
you said it was working. Just keep an eye on it from time to time. Note if it differs
in wet vs. dry weather.
If there’s nothing wrong with the connections, the antenna might be tuned too low.
Vesper’s website mentioned that they use 162 MHz, which might be too high a frequency
for some antennas. In that case you MIGHT be able to shorten the element 1-2% (is there
a setscrew?) If there is a setscrew, take note of your starting point before you loosen
the screw!
Shortening the antenna by CUTTING might fix things, but once you cut you can’t glue
it back. I suppose you could lengthen it 1-2% temporarily with a piece of wire or foil,
and see if VSWR gets worse, but that isn’t an absolute test. That would make it worse
whether or not the antenna was tuned to the proper frequency to begin with.
John:
I’m think this might be a great project for when you are here.
😉
Erwin:
I found a good practical discussion of VSWR at:
VSWR
Just go to the bottom of the page and read the paragraphs before and after Figure 3.
That should give you a better idea of the effect of VSWR on your AIS transmitter
and effective power output.
Some transmitters (especially transceivers) can be damaged by reflected power
coming back at them. I don’t know the extent to which it would affect your device.