Help with swr point?
 

I just installed a Comet GP-6 and it's VHF low
swr point is 139 mhz. I called Comet and they
said there is no way to tune the antenna. I'd like
to raise the 1:1 SWR point up to 146 mhz. The
closest other antenna is an AEA 6m Halo 5'
below the GP-6. The mast supporting both antennas
is 30' high. I'm using non conductive guy ropes
just below the halo.

I remember there were discussions on this group
about altering "percieved" SWR by changing
feedline length. I'm using 30' of Flexi 4xl coax
from the antenna to my secondary SWR bridge
and a Radio Shack 2' rg58 patch cable from
the bridge to the rig.

I measured a 1:1 at the rig @ 139 mhz and
1.5:1 at the swr bridge (lowest swr seen there)
but the low point on the bridge was 138 mhz.
I was getting a SWR warning on my FT-857D
around 152 mhz and adding a 3' RG-213 patch
between the antenna and SWR bridge fixed that.

So, which way do I go? Shorter or longer?

--
73's es gd dx de Ken KGØWX
Grid EM17ip, Flying Pigs #1055,
Digital On Six #350,
List Owner, Yahoo! E-groups:
VX-2R & FT-857

On Fri, 8 Apr 2005 17:36:13 -0500, "Ken Bessler"
wrote:

I measured a 1:1 at the rig @ 139 mhz and
1.5:1 at the swr bridge (lowest swr seen there)
but the low point on the bridge was 138 mhz.
I was getting a SWR warning on my FT-857D
around 152 mhz and adding a 3' RG-213 patch
between the antenna and SWR bridge fixed that.

Hi Ken,

This just cries for feedline isolation through a 1:1 BalUn.

73's
Richard Clark, KB7QHC

Ken Bessler wrote:
I remember there were discussions on this group
about altering "percieved" SWR by changing
feedline length.

With an SWR greater than 1:1, you cannot use 50 ohm
coax to achieve a match to 50 ohms by varying feedline
length. The target impedance (50 ohms) must lie on
the feedline's SWR circle. With 50 ohm coax, the center
of the SWR circle is the target impedance and cannot be
reached by simply varying the length of the feedline.

You could put a matching network at the base of the
antenna to match it at the desired frequency.
--
73, Cecil http://www.qsl.net/w5dxp


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"Ken Bessler" wrote in message
news:xHD5e.3395$up2.2389@okepread01...
I just installed a Comet GP-6 and it's VHF low
swr point is 139 mhz. I called Comet and they
said there is no way to tune the antenna. I'd like
to raise the 1:1 SWR point up to 146 mhz. The
closest other antenna is an AEA 6m Halo 5'
below the GP-6. The mast supporting both antennas
is 30' high. I'm using non conductive guy ropes
just below the halo.

I remember there were discussions on this group
about altering "percieved" SWR by changing
feedline length. I'm using 30' of Flexi 4xl coax
from the antenna to my secondary SWR bridge
and a Radio Shack 2' rg58 patch cable from
the bridge to the rig.

I measured a 1:1 at the rig @ 139 mhz and
1.5:1 at the swr bridge (lowest swr seen there)
but the low point on the bridge was 138 mhz.
I was getting a SWR warning on my FT-857D
around 152 mhz and adding a 3' RG-213 patch
between the antenna and SWR bridge fixed that.

So, which way do I go? Shorter or longer?


Try Duct-Taping a piece of alum foil a few inches from the end of the driven
element.
or
whack off an inch and see if the swr improves.

Hal Rosser wrote:
"Ken Bessler" wrote in message
news:xHD5e.3395$up2.2389@okepread01...

I just installed a Comet GP-6 and it's VHF low
swr point is 139 mhz. I called Comet and they
said there is no way to tune the antenna. I'd like
to raise the 1:1 SWR point up to 146 mhz. The
closest other antenna is an AEA 6m Halo 5'
below the GP-6. The mast supporting both antennas
is 30' high. I'm using non conductive guy ropes
just below the halo.

I remember there were discussions on this group
about altering "percieved" SWR by changing
feedline length. I'm using 30' of Flexi 4xl coax
from the antenna to my secondary SWR bridge
and a Radio Shack 2' rg58 patch cable from
the bridge to the rig.

I measured a 1:1 at the rig @ 139 mhz and
1.5:1 at the swr bridge (lowest swr seen there)
but the low point on the bridge was 138 mhz.
I was getting a SWR warning on my FT-857D
around 152 mhz and adding a 3' RG-213 patch
between the antenna and SWR bridge fixed that.

So, which way do I go? Shorter or longer?



Try Duct-Taping a piece of alum foil a few inches from the end of the driven
element.
or
whack off an inch and see if the swr improves.

Clip off a half inch first...'ya can't glue it back on if you took off too much!

Butch KF5DE

Clip off a half inch first...'ya can't glue it back on if you took off
too much!

Butch KF5DE

I'll buy that.
Sounds like experience speaking. :-)

"Hal Rosser" wrote in message
.. .
Clip off a half inch first...'ya can't glue it back on if you took off
too much!

Butch KF5DE

I'll buy that.
Sounds like experience speaking. :-)



Experience is directly proportional to the amount of equipment ruined.

73
H.