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