30M Jpole and antenna analyser question.
 

Hi All,
I've built a 30M Jpole using dimensions from the Jpole calculator in
Hamcalc and using 300 ohm twin lead for the matching section.

It seems to resonate about right, but the SWR is 2.2. I'm using an
Autek RF-1 analyser, and want to know if there's any way to determine
what changes to make to get a lower SWR? I don't have a lot of the
twinlead left, and don't have enough time to get more, so cut-and-try
isn't the best solution.

73, doug

Doug Faunt N6TQS +1-510-655-8604 wrote:
I've built a 30M Jpole using dimensions from the Jpole calculator in
Hamcalc and using 300 ohm twin lead for the matching section.
It seems to resonate about right, but the SWR is 2.2.

Probably not a J-pole shorted at the bottom but a J-pole
(Zepp) open at the bottom? The trouble with a Zepp is
there are no adjustments except length. With a J-pole
shorted at the bottom, you move the feedpoint until you
find the 50 ohm point. With the open ended J-pole (Zepp)
the 1/4WL bottom section is a fixed transformer.

Your feedpoint impedance is probably 110 ohms (don't think
it would be 23 ohms but it might be). Installing a 1/4WL
piece of 75 ohm coax between the feedpoint and your 50 ohm
coax should give you a near perfect SWR. For a piece of 75
ohm coax with VF of 0.66 (RG-11) that would be about 12.7
inches.
--
73, Cecil http://www.qsl.net/w5dxp


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"Cecil Moore" wrote in message
...
Doug Faunt N6TQS +1-510-655-8604 wrote:
I've built a 30M Jpole using dimensions from the Jpole calculator in
Hamcalc and using 300 ohm twin lead for the matching section.
It seems to resonate about right, but the SWR is 2.2.

Probably not a J-pole shorted at the bottom but a J-pole
(Zepp) open at the bottom? The trouble with a Zepp is
there are no adjustments except length. With a J-pole
shorted at the bottom, you move the feedpoint until you
find the 50 ohm point. With the open ended J-pole (Zepp)
the 1/4WL bottom section is a fixed transformer.

Your feedpoint impedance is probably 110 ohms (don't think
it would be 23 ohms but it might be). Installing a 1/4WL
piece of 75 ohm coax between the feedpoint and your 50 ohm
coax should give you a near perfect SWR. For a piece of 75
ohm coax with VF of 0.66 (RG-11) that would be about 12.7
inches.
--
73, Cecil http://www.qsl.net/w5dxp


Hey, Cecil -

Did you slip a digit or two? 1/4WL at 30 meters with .66 V is just shy of 5
meters, no? I get about 16 feet.

John
KD5YI

John Smith wrote:
Hey, Cecil -

Did you slip a digit or two? 1/4WL at 30 meters with .66 V is just shy of 5
meters, no? I get about 16 feet.

Egads John, I thought this was a J-Pole for 146 MHz. You are
certainly correct for 30m. The principle still applies but
the fact that it is for 30m may mean the feedpoint impedance
is indeed 23 ohms and the 75 ohm coax solution won't work.
If it is 23 ohms, one would need 34 ohm coax and I don't
have any. :-)
--
73, Cecil http://www.qsl.net/w5dxp


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In message , Cecil Moore
writes
John Smith wrote:
Hey, Cecil -
Did you slip a digit or two? 1/4WL at 30 meters with .66 V is just
shy of 5 meters, no? I get about 16 feet.

Egads John, I thought this was a J-Pole for 146 MHz. You are
certainly correct for 30m. The principle still applies but
the fact that it is for 30m may mean the feedpoint impedance
is indeed 23 ohms and the 75 ohm coax solution won't work.
If it is 23 ohms, one would need 34 ohm coax and I don't
have any. :-)

2 x 75ohm in parallel?
Ian.
--

On Sat, 29 Jan 2005 02:56:46 -0500, Doug Faunt N6TQS +1-510-655-8604
wrote:

Autek RF-1 analyser

What does your Autek RF-1 analyser say the resistance and reactance is?

If the reactance is capacitive, lengthen the 1/2 lamba section.

On Sat, 29 Jan 2005 02:56:46 -0500, Doug Faunt N6TQS +1-510-655-8604
wrote:

Hi All,
I've built a 30M Jpole using dimensions from the Jpole calculator in
Hamcalc and using 300 ohm twin lead for the matching section.

It seems to resonate about right, but the SWR is 2.2. I'm using an
Autek RF-1 analyser, and want to know if there's any way to determine
what changes to make to get a lower SWR? I don't have a lot of the
twinlead left, and don't have enough time to get more, so cut-and-try
isn't the best solution.

73, doug

Thee are two things you have to take into account:
1. tuning your radiator to resonance (reactance = 0 on Autek)
2. matching to your feedline.

What does your Autek say?

If the reactance is capacitive, lengthen the 1/2 lambda radiator. If it is
inductive, shorten it.

Now that the antenna resonates, you have to match it. The antenna handbook
details calculating the proper length of 75 ohm coax to transform
impedances from 50 to something else. Your Autek will tell you what the
"something else" is.

Why a J-Pole?

Would it not be easier to simply string a vertical dipole? A J-Pole is
merely an end fed vertical dipole with a tapped shorted 1/4 lambda
matching section.