VHF Antenna Resistance, Center Conductor To Shield
 

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

That's yahoo.com

VHF Antenna Resistance, Center Conductor To Shield
 

On 2 Mar 2004 05:47:51 -0800, (Ron Patterson)
wrote:

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.

That does not sound right. It should be open to DC. I expect that you
might damage the new xmitter with that cable.



Rodney Myrvaagnes NYC J36 Gjo/a

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

VHF Antenna Resistance, Center Conductor To Shield
 

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.

Don't worry about it. I would expect to see a lot of post here
refuting the bad advice that Rodney gave you. sorry Rodney but if you
don't know what you are talking about maybe you should not say
anything and learn .


GW



(Ron Patterson) wrote in message . com...
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

That's yahoo.com

VHF Antenna Resistance, Center Conductor To Shield
 

Rodney Myrvaagnes wrote in message . ..
On 2 Mar 2004 05:47:51 -0800, (Ron Patterson)
wrote:

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.

That does not sound right. It should be open to DC. I expect that you
might damage the new xmitter with that cable.



Rodney Myrvaagnes NYC J36 Gjo/a

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

These VHF antennas very often appear as as short circuit, which is
normal. What you should do is disconnect the andtenna from the coax
and then measure the resistance. It should appear as an open circuit
(infinite resistance). If it does not, it is because the sheath of the
coax drinks moisture like a spong... So you should instakll a new
coax; I have had very good results with "MM 8" from "The Wireman" who
sells mostly to radion hams. But even his coax is not eternal and I
have replaced mine after 10 000 miles of offshore sailing. If the
antenna connector is in good condition, your antenna probably is also
so that after renewing the coax, you can operate your VHF in full
confidence and will see a better range
G´luck
john

VHF Antenna Resistance, Center Conductor To Shield
 

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?





Rodney Myrvaagnes NYC J36 Gjo/a

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

VHF Antenna Resistance, Center Conductor To Shield
 

My understanding is that it has to do with impedance as opposed to
resistance.

Rodney Myrvaagnes wrote:
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?





Rodney Myrvaagnes NYC J36 Gjo/a

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

VHF Antenna Resistance, Center Conductor To Shield
 

"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.

Meindert

VHF Antenna Resistance, Center Conductor To Shield
 

"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

That's yahoo.com

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

VHF Antenna Resistance, Center Conductor To Shield
 

"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

VHF Antenna Resistance, Center Conductor To Shield
 

In article ,
(Ron Patterson) wrote:

snipped for brevity

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

That's yahoo.com

Most, if not all, commercial Vhf antennas are designed to be DC
Grounded from the center conductor to the shield, and have a 50 Ohm
RF Impedance at the antenna connection. The 50 Ohm Impedance
is because that is the defacto Standard Output Impedance for
Commercial Twoway Radio's, worldwide. The DC Grounded connections
are used, so that any Static Charges that the antenna picks up are
shunted directly to ground thru the feedline connection, and, or
the DC Mounting System of the antenna.

Bruce in alaska Hopeing, Rodney Myrvaagnes of NYC, would only
speculate on things he actually has knowledge of
--
add a 2 before @