Jimmie D wrote:
Thanks Cecil This was my first plan but I dont have room to run the stub
straight out so I kind of tabled the idea for a while. Now that I have
thought about it there is no reason I couldnt wrap it around the antenna.

There's no reason why you couldn't spiral it around
the antenna like a pancake inductor. If you kept each
spiral about a foot away from the next one, the area
occupied would be pretty small. Good luck.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
om...
Jimmie D wrote:
Thanks Cecil This was my first plan but I dont have room to run the stub
straight out so I kind of tabled the idea for a while. Now that I have
thought about it there is no reason I couldnt wrap it around the antenna.

There's no reason why you couldn't spiral it around
the antenna like a pancake inductor. If you kept each
spiral about a foot away from the next one, the area
occupied would be pretty small. Good luck.
--
73, Cecil http://www.w5dxp.com

Cecil, I got the antenna up Sunday, for now the stub is straight out from
the antenna. Is it normal for the 50 ohm point to be a little hard to find?
About the best VSWR I can get now is 1.7:1 This is pretty close to the point
where my radio starts limiting power. I m thinking either the stub or the
antenna may be a little long is there an easy way to tell or is easier just
to try something and see what happens? Checking the VSWR at differerent
freqs doesnt help much. Maybe a slight indication that it is too long.

Jimmie

Jimmie D wrote:
Cecil, I got the antenna up Sunday, for now the stub is straight out from
the antenna. Is it normal for the 50 ohm point to be a little hard to find?
About the best VSWR I can get now is 1.7:1 This is pretty close to the point
where my radio starts limiting power. I m thinking either the stub or the
antenna may be a little long is there an easy way to tell or is easier just
to try something and see what happens? Checking the VSWR at differerent
freqs doesnt help much. Maybe a slight indication that it is too long.

You've discovered the problem with SWR meters. They
don't read phase. If you were using an antenna analyzer,
the solution would probably be obvious. With only an
SWR meter, your tuning algorithm either converges or
diverges but you may not know which. The easy way to
tune the antenna is to use an antenna analyzer to
determine where the impedance is purely resistive and
then adjust the stub tap point to 50 ohms. An antenna
analyzer is a good investment.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
...
Jimmie D wrote:
Cecil, I got the antenna up Sunday, for now the stub is straight out from
the antenna. Is it normal for the 50 ohm point to be a little hard to
find? About the best VSWR I can get now is 1.7:1 This is pretty close to
the point where my radio starts limiting power. I m thinking either the
stub or the antenna may be a little long is there an easy way to tell or
is easier just to try something and see what happens? Checking the VSWR
at differerent freqs doesnt help much. Maybe a slight indication that it
is too long.

You've discovered the problem with SWR meters. They
don't read phase. If you were using an antenna analyzer,
the solution would probably be obvious. With only an
SWR meter, your tuning algorithm either converges or
diverges but you may not know which. The easy way to
tune the antenna is to use an antenna analyzer to
determine where the impedance is purely resistive and
then adjust the stub tap point to 50 ohms. An antenna
analyzer is a good investment.
--
73, Cecil http://www.w5dxp.com

I think before I start cutting I will try adding a little series capacitance
or inductance to the antenna and see how it reacts to that. My work week
just started so it may be a week or so before I get back at it again.

In order to guarantee the ability to obtain a 1:1 SWR, you need to be
able to adjust at least two independent or partially independent things.
This can be two variable lumped components; any two of the following:
stub length, position, and Z0(*); and so forth. Adjusting, say, a stub
length and the value of a capacitor across the input end of the stub
won't do it because they're basically adjusting the same thing and
therefore aren't independent. If you've only been varying one item,
you'll get a perfect match only if lucky.

If you're as close as 1.7:1, it's likely as easy or easier to find the
solution by trial and error rather than calculation.

The reason that two adjustments are required is that 1:1 SWR requires a
particular value of R and a zero value of X. If you vary only one item,
it'll change only R, or only X, or, more likely, both R and X but in
some fixed relationship.

(*) Because you can practically adjust Z0 over only a relatively narrow
range, this is a good method only for fine tuning or as a way to set the
range of other adjustments.

Roy Lewallen, W7EL

Jimmie D wrote:
"Cecil Moore" wrote in message
...
Jimmie D wrote:
Cecil, I got the antenna up Sunday, for now the stub is straight out from
the antenna. Is it normal for the 50 ohm point to be a little hard to
find? About the best VSWR I can get now is 1.7:1 This is pretty close to
the point where my radio starts limiting power. I m thinking either the
stub or the antenna may be a little long is there an easy way to tell or
is easier just to try something and see what happens? Checking the VSWR
at differerent freqs doesnt help much. Maybe a slight indication that it
is too long.
You've discovered the problem with SWR meters. They
don't read phase. If you were using an antenna analyzer,
the solution would probably be obvious. With only an
SWR meter, your tuning algorithm either converges or
diverges but you may not know which. The easy way to
tune the antenna is to use an antenna analyzer to
determine where the impedance is purely resistive and
then adjust the stub tap point to 50 ohms. An antenna
analyzer is a good investment.
--
73, Cecil http://www.w5dxp.com

I think before I start cutting I will try adding a little series capacitance
or inductance to the antenna and see how it reacts to that. My work week
just started so it may be a week or so before I get back at it again.

On Tue, 12 Sep 2006 18:20:02 -0400, "Jimmie D"
wrote:


"Cecil Moore" wrote in message
t...
Jimmie D wrote:
Cecil, I got the antenna up Sunday, for now the stub is straight out from
the antenna. Is it normal for the 50 ohm point to be a little hard to
find? About the best VSWR I can get now is 1.7:1 This is pretty close to
....
I think before I start cutting I will try adding a little series capacitance
or inductance to the antenna and see how it reacts to that. My work week
just started so it may be a week or so before I get back at it again.

Roy has given you some good advice.

Some further thoughts.

I assume that the coax is not decoupled, that will complicate
achieving your outcome.

Before cutting things, an observation of whether VSWR is better at a
higher or lower frequency will help guide you in the reactance
dimension of the problem.

Find the tap point for lowest VSWR, change frequency and do it again.
This will hint to you whether the antenna / stub combination is short
or long.

Keep in mind that there is not a correct length of the radiator
independently of the stub, small reactance due to the radiator being
off resonance can be compensated by suitable stub dimensions. That
says cut / lengthen the elements that are most convenient.

Temporary increase in length by clamping an additional conductor to
top of the radiator might less final that cutting bits off. Again, the
shift in point of lowest VSWR is a hint.

With knowledge, you should be able do this easy enough without an
antenna analyser.

Owen
--

"Owen Duffy" wrote in message
...
On Tue, 12 Sep 2006 18:20:02 -0400, "Jimmie D"
wrote:


"Cecil Moore" wrote in message
et...
Jimmie D wrote:
Cecil, I got the antenna up Sunday, for now the stub is straight out
from
the antenna. Is it normal for the 50 ohm point to be a little hard to
find? About the best VSWR I can get now is 1.7:1 This is pretty close
to
...
I think before I start cutting I will try adding a little series
capacitance
or inductance to the antenna and see how it reacts to that. My work week
just started so it may be a week or so before I get back at it again.

Roy has given you some good advice.

Some further thoughts.

I assume that the coax is not decoupled, that will complicate
achieving your outcome.

Before cutting things, an observation of whether VSWR is better at a
higher or lower frequency will help guide you in the reactance
dimension of the problem.

Find the tap point for lowest VSWR, change frequency and do it again.
This will hint to you whether the antenna / stub combination is short
or long.

Keep in mind that there is not a correct length of the radiator
independently of the stub, small reactance due to the radiator being
off resonance can be compensated by suitable stub dimensions. That
says cut / lengthen the elements that are most convenient.

Temporary increase in length by clamping an additional conductor to
top of the radiator might less final that cutting bits off. Again, the
shift in point of lowest VSWR is a hint.

With knowledge, you should be able do this easy enough without an
antenna analyser.

Owen
--

I am thinking about building an ANTENNASCOPE, see fig 1 at
http://digilander.libero.it/hamweb/r...tennascope.htm
.. This one doesnt allow you to measure impedance but i thought if I replaced
one of the capacitors in the bridge circuit with a variable cap of about
twice the value I could at least tell if an antenna was inductive or
capacitive by which way I had to adjust the cap to get a perfect null. I
think I have most if not all of the parts.

Jimmie

On Wed, 13 Sep 2006 20:30:14 -0400, "Jimmie D"
wrote:


With knowledge, you should be able do this easy enough without an
antenna analyser.
....
I am thinking about building an ANTENNASCOPE, see fig 1 at
http://digilander.libero.it/hamweb/r...tennascope.htm
. This one doesnt allow you to measure impedance but i thought if I replaced
one of the capacitors in the bridge circuit with a variable cap of about
twice the value I could at least tell if an antenna was inductive or
capacitive by which way I had to adjust the cap to get a perfect null. I
think I have most if not all of the parts.

It looks like you are going to develop an instrument to solve the
problem, when you should be able to solve it with a little ingenuity
and the VSWR meter that you have.

Owen
--