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Antenna for Marine VHF



 
 
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  #31  
Old Yesterday, 11:00 PM posted to rec.radio.amateur.antenna
John S
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Posts: 535
Default Antenna for Marine VHF

On 4/23/2017 2:34 PM, highlandham wrote:

When all is done ,the exposed SO-239 centre pin to which the 1/4 wave
radiator is soldered (or the complete SO-239 socket can be covered with
Epoxy or Bluetack (giving excellent protection)


Frank , GM0CSZ / KN6WH in IO87AT


Frank - Thanks for the Bluetack reference. It looks quite versatile so I
have ordered it to try.

Cheers,
John N1JLS
  #32  
Old Today, 02:17 AM posted to rec.radio.amateur.antenna
Jeff Liebermann[_2_]
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Posts: 1,280
Default Antenna for Marine VHF

On Sun, 23 Apr 2017 20:34:31 +0100, highlandham
wrote:

It is useful to initially make the vertical radiator and radials ,say 10
mm longer than a quarter wave for the mid band frequency (somewhere
between 157 and 162 MHz).


Nope. You want to tune the antenna for lowest VSWR in the transmit
range (156.0 to 157.5MHz) and take whatever you can get in the 162MHz
receive only region. If you try to tune the antenna for the middle of
the 6 MHz split, you'll end up with lousy VSWR at both extremes where
you want to operate, and good VSWR where you're not operating.

Frank , GM0CSZ / KN6WH in IO87AT

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #34  
Old Today, 05:06 AM posted to rec.radio.amateur.antenna
John S
external usenet poster
 
Posts: 535
Default Antenna for Marine VHF

On 4/23/2017 8:17 PM, Jeff Liebermann wrote:
On Sun, 23 Apr 2017 20:34:31 +0100, highlandham
wrote:

It is useful to initially make the vertical radiator and radials ,say 10
mm longer than a quarter wave for the mid band frequency (somewhere
between 157 and 162 MHz).


Nope. You want to tune the antenna for lowest VSWR in the transmit
range (156.0 to 157.5MHz) and take whatever you can get in the 162MHz
receive only region. If you try to tune the antenna for the middle of
the 6 MHz split, you'll end up with lousy VSWR at both extremes where
you want to operate, and good VSWR where you're not operating.

Frank , GM0CSZ / KN6WH in IO87AT


How lousy? Enough for the transmitter to reduce power to protect itself?
Do you have SWR numbers?
  #35  
Old Today, 06:52 AM posted to rec.radio.amateur.antenna
Jeff Liebermann[_2_]
external usenet poster
 
Posts: 1,280
Default Antenna for Marine VHF

On Sun, 23 Apr 2017 23:06:37 -0500, John S
wrote:

On 4/23/2017 8:17 PM, Jeff Liebermann wrote:
On Sun, 23 Apr 2017 20:34:31 +0100, highlandham
wrote:

It is useful to initially make the vertical radiator and radials ,say 10
mm longer than a quarter wave for the mid band frequency (somewhere
between 157 and 162 MHz).


Nope. You want to tune the antenna for lowest VSWR in the transmit
range (156.0 to 157.5MHz) and take whatever you can get in the 162MHz
receive only region. If you try to tune the antenna for the middle of
the 6 MHz split, you'll end up with lousy VSWR at both extremes where
you want to operate, and good VSWR where you're not operating.

Frank , GM0CSZ / KN6WH in IO87AT


How lousy? Enough for the transmitter to reduce power to protect itself?
Do you have SWR numbers?


Of course I have numbers. I never go anywhere without a handy NEC
model of a marine VHF band ground plane. For your viewing pleasu
http://www.11junk.com/jeffl/antennas/Marine%20VHF%20Ground%20Plane/index.html
If you want to play with it, the 4NEC2 model is at:
http://www.11junk.com/jeffl/antennas/Marine%20VHF%20Ground%20Plane/slides/Marine%20VHF%20Gnd%20Plane.html

Argh, you're right. With a Ground Plane, the VSWR is low enough at
the band edges (156 - 163MHz) for transmit, if I tune the antenna for
mid-band (159MHz). TX reverse power protection usual kicks in at 2:1.
Looks like the VSWR at 156 and 162 is about 1.2:1 which is quite safe:
http://www.11junk.com/jeffl/antennas/Marine%20VHF%20Ground%20Plane/slides/VSWR.html

Lame Excuse: I'm used to dealing with higher gain marine band
antennas, where the bandwidth is much less. With these antennas, it's
common practice to tune the antenna for the transmit range, and take
whatever you can get at the higher receive only frequencies. However,
it seems the bandwidth of a common ground plane is sufficiently wide
to have low VSWR over the entire VHF marine band.

Grumble...
--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 




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