"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
--

"Owen Duffy" wrote in message
...
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
--

Oh I know I can, this just seemed like it might be fun. I figure I can
cobble this together in a couple of hours and have a useful instrument when
I get finished.

Jimmie

Thanks all I got the antenna finished this past Tuesday. When I got finished
the stub seemed a little short, 5'11". Does this mean the antenna is a
little long?. The stub is still straight. I got the SWR down to 1.2:1, I
think I am going to leave it there. I built the ANTENNASCOPE but I am unsure
whether it is telling me the antenna is inductive or capacitive at this
point. Oh well I will set up a little groundplane antenna on 10 or 15 that I
know is too long and see how it reacts.