McGuffin wrote in news:alias-F69096.09082513092008
@news.xs4all.nl:
Hi Larry
Thank you for your thorough explanation.
My boat is made from aluminium, I forgot to add in my first post.
So just attaching the tuner to the backstay is not possible.
A 23"/7 mtr whip would be possible, but consider it as very
vulnerable.
Waves, wind, boat being knocked-down (it is a sailboat afterall), etc.
Losing the antenna seems very realistic.
So basically an insulated backstay comes out as the best solution.
(I don't like the idea of cutting my stays though...)
h
Wow....I'd kill for an aluminum hull seawater ground....very nice!
Plastic boats suck for HF radio comms.
Let's play a trick on it before we cut the backstay all up for
insulators....
Your backstay is so well grounded at its aft end to the aluminum ground
monster, I'd recommend trying a SHUNT (parallel) feed to the backstay.
Deck Backstay Mast
(GROUND) 1/3 2/3
__________________________________________________ ______________________
|
_______ |
| | |
| TUNER |_______________|
|_______|^tuner output
|
GROUND
What we're going to do is to hang a small wire on insulators UNDER the
backstay at a distance of 30-40cm up along the backstay about 1/3rd the
length of the backstay. At 1/3 up the backstay, the wire is then
connected to the backstay. The distance from the wire to the backstay
doesn't really matter as long as it's as far away from it as you can
stand to look at.
1/3 the way up the backstay is NOT RF ground, it's some complex
impedance on any frequency. If this distance isn't far enough up the
backstay, the tuner won't be able to tune it on the lowest frequency
band you use. If this happens, make the wire longer and hook it higher
up the backstay, say 1/2 the way up. The tuner has a certain range of
impedances it can tune up....we've got to put it up far enough so the
tuner can tune it on the lowest band. Upper bands are easy to tune this
far up the backstay. Don't forget the silly sail or boom lift has to
miss the feed wire hanging down from the backstay, so that is just about
the limit how far we can put the feed wire from the backstay. Any kind
of plastic that doesn't snap easily makes a good insulator to hang this
wire from the backstay. Small diameter PVC works great. If the boom is
supported by wire rope, change it to nylon or some other line that's not
conductive. The boom lift wire rope is way too close to the backstay
when the sail is close hauled, eating your signal.
Experiment with the actual feed point to find a point that will tune on
all the channels you use.
If you get good signals fed this way, you now have what is called a
shunt-fed, Delta Loop. The Delta greek symbol, is a triangle which
consists of the backstay, down the mast and across the deck
ground...Delta loops radiate great if you can load them. Loading the
mast also loads the forestay, shrouds and all. Some power will be lost
to the wiring inside the mast backing up down those wires. You may find
wind instruments go crazy if the RF screws up the signal from the mast.
It doesn't bother ours on the Jeanneau that I can see. There's an
indicator light over the hatch to the forward cabin we installed so we
can see the masthead light is on. It glows with RF energy if the light
is off...(c;
Because it costs almost nothing to try shunt feeding that nicely
grounded backstay on your aluminum hull, you have almost nothing to lose
except a little time. Put a crockadile clamp on the backstay end of the
wire so you can move it around and find a good spot before installing it
permanently....
http://www.vias.org/radioanteng/rae_02_03_04.html
we've been shunt feeding towers like this since the 1920's.
http://www.qsl.net/w9rb/webdoc9.htm
http://www.fybush.com/sites/2007/site-070223.html