On Wed, 3 Mar 2004 07:37:03 +0100, "Meindert Sprang"
wrote:

"Rodney Myrvaagnes" wrote in message
.. .
On 2 Mar 2004 18:22:07 -0800, (Gw) wrote:

Most VHF antenna's have 0 ohms to ground The resistance you are
reading is the round trip up and down the mast of the coax.

Interesting. What connects the shield to the center at the top? And
how does the shorted cable transfer power to the antenna?

For instance, an impedance matching circuit. RF and DC resistance ar
different beasts. A nice example is a folded dipole as seen with television
reception: the folded dipole is a perfect short for DC, if you measure it
with a multimeter, you'll measure almost 0 Ohms. Still, to RF this folded
dipole has a 'resistance' of 300 Ohms. To prevent confusion, resistance for
RF is called impedance. And with RF, resistance is not always equal to
impedance.

With marine VHF antenna's, the antenna itself is often a 1/2 radiator which
is fed at the end. The impedance at the end is high, a few kOhms. To match
that to the 50 impedance of the cable, a coil of, say, 10 windings is
connected between the antenna end and the ground. The cable is connected
between ground and a turn or two from the ground. This way, you measure 0
Ohms with an ohmmeter, while perfectly matched to 50Ohm at VHF frequencies.

Thanks, Meindert.

I found that out this morning when I went to the yard. I had (for no
good reason) thought the transformer was just a loading coil. It
indeed has 0 DC resistance across the primary.

However, this one is DC open to the antenna itself, which suggests it
is a transformer with isolation rather than an autotransformer as you
describe, or else that it has a series capacitor. I suppose different
makes may vary in that respect.

I apologize to all for my previous hasty answer, and acknowledge that
everyone else was right, and I was wrong.

As for the wish that I only speculate about things I know about,
wouldn't that be contrary to the spirit of the ng? :-)



Rodney Myrvaagnes NYC J36 Gjo/a

"Happy is he that taketh thy little ones and dasheth them upon the stones." __Psalm 137

If your ham friend has an MFJ-259B or similar antenna analyzer, use it
first, before the Bird 43 meter. The MFJ will measure impedance, show the
frequency response, and the SWR, all without requiring your radio to
transmit. I use it whenever someone asks me to check an installation, as
many new boat owners have no idea how to put an antenna connector on the
cable properly. IF the MFJ shows a short, it is shorted at RF. Once I have a
good reading on the MFJ, I remove it, insert the Bird 43 inline with the
radio and measure the actual power output and reflected power from the
antenna.
Doug K7ABX
"Ron Patterson" wrote in message
om...
"garry crothers" wrote in message
Ron

If you are worried about damaging the vhf, then contact a local ham
radio
guy who has a vhf swr meter, e.g a bird43, and ask him to measure the
swr
of the aerial. shud be less than 1.5.

regards
garry

Garry, I am going to do exactly that. A guy I work with is a ham and
happens to have one. I am going to test it next week.

Thanks
Ron

Ron,

Realize that the sail track for your mainsail, as well as spreader-mounts,
standing rigging mounts, etc, all have razor-sharp ss-screws that penetrate
the open space area inside the mast. Cables inside this space are subject to
a law that says constant degradation will result, normally as soon as the
mast is stepped.

In almost 30 years of stepping masts with radio feedline, masthead
instruments etc, only plastic conduit protecting the cable runs has ever
really helped. The taller the mast, the more difficult that is to fit inside
between the protruding ss-srew array. But is is a better long-term solution
and protects the coax from near-instant degradation from moisture
penetration.

Jack

"Ron Patterson" wrote in message
om...
I have a 34 foot sloop which I bought six years ago on Kentucky Lake
near Paducah. I motored it up river to Guntersville Lake in North
Alabama. I sailed her on Guntersville Lake on the very few days that I
had off and the wind was blowing. That however, was no more than a few
days a year. Now I am retiring in June and am going to take her down
the Tenn-Tom Waterway to her new home in Pensacola, Florida. There we
will sail and fish for the rest of our days.

When I bought the boat it had a pretty good VHF radio with the antenna
mounted at the top of the mast. It worked fine when we were bringing
the boat down to Alabama. But a few months back I tried the radio for
the first time in six years. It would not even power up. Not to worry,
I bought a new Icom VHF radio.

Before installing it I decided to check the antenna resistance from
the center of the coax to the shield. I got about two and one half
ohms. I was thinking it should be an open. What should the resistance
be, measured from the radio end, from the center conductor to the
shield? Someone please help me because I don't want to hook up the
radio and blow the transmitter. Is this a danger? Or perhaps I have
nothing to worry about.

Thanks in advance

Ron Patterson

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