LAB wrote:
I have tried the Slim Jim antenna on the 2m. I used a 1/4" copper tube,
antenna is very good. Then I made another one using 300 Ohm TV clear twin
lead, but SWR is very high and it seems to be not adjustable to the same 1:1
of the first antenna. Why? Does anyone know where is the problem?

How can one move the feedpoint up and down the
matching stub with insulation on the wire?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

CECIUL MOO
How can one move the feedpoint up and down the matching stub with
insulation on the wire?

I started at the same distance of the copper tube antenna. About 1cm
apart SWR seemed to be about the same: not better, not worse. It seemed to
be the best... I've had no time in the week for other tests; I'll continue
today or tomorrow. If someone has an idea it will be appreciated.


--
Gianluca

LAB wrote:
CECIUL MOO
How can one move the feedpoint up and down the matching stub with
insulation on the wire?

I started at the same distance of the copper tube antenna. About 1cm
apart SWR seemed to be about the same: not better, not worse. It seemed to
be the best... I've had no time in the week for other tests; I'll continue
today or tomorrow. If someone has an idea it will be appreciated.

The velocity factor of the twinlead might be as much
as 20% different from the copper tubes. If you use
the same dimensions, the resonant frequency will be
too low. Did you compensate for velocity factor?
Did you check the resonant frequency with something
like an MFJ-259B?

By changing the characteristic impedance of the
matching stub, you change the optimum tap point
on the stub so you cannot use the same dimensions
as the copper tube antenna.

Seems you need to make the adjustments using a
throw-away version of the antenna. Once you determine
the proper dimensions, you can build the final version.

I would start by shaving the insulation off both
stub wires so they can be soldered to the coax and
chopping off 10% of the length of the 1/2WL section.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

"Cecil Moore" wrote in message
...

snip

The velocity factor of the twinlead might be as much
as 20% different from the copper tubes. If you use
the same dimensions, the resonant frequency will be
too low. Did you compensate for velocity factor?
Did you check the resonant frequency with something
like an MFJ-259B?

snip

Seems you need to make the adjustments using a
throw-away version of the antenna. Once you determine
the proper dimensions, you can build the final version.

I would start by shaving the insulation off both
stub wires so they can be soldered to the coax and
chopping off 10% of the length of the 1/2WL section.

Yes! All of the above!

Also, it has been my experience that the recommended quarter-inch gap cut in
one of the twinlead conductors [to create the stub section] is insufficient.
I make it at least a half-inch.

I have been surprised for the twin lead problems because i had already
made another Slim Jim using 2mm bare copper wire on the sides of a black
"electric" plastic tube (d=about 2cm) and I get the same good performances
of the copper tube...

--
Gianluca

LAB wrote:
I have been surprised for the twin lead problems because i had already
made another Slim Jim using 2mm bare copper wire on the sides of a black
"electric" plastic tube (d=about 2cm) and I get the same good performances
of the copper tube...

The velocity factor of the insulated twinlead is much
lower than bare wires. If you don't understand velocity
factor, please study up on the subject. Velocity factor
can make as much as a 20% difference in element lengths
between bare wires and insulated twinlead. The
characteristic impedance is also different causing the
50 ohm feedpoint position to change.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

On May 30, 10:33*am, Cecil Moore wrote:
LAB wrote:
* * I have been surprised for the twin lead problems because i had already
made another Slim Jim using 2mm bare copper wire on the sides of a black
"electric" plastic tube (d=about 2cm) and I get the same good performances
of the copper tube...

The velocity factor of the insulated twinlead is much
lower than bare wires. If you don't understand velocity
factor, please study up on the subject. Velocity factor
can make as much as a 20% difference in element lengths
between bare wires and insulated twinlead. The
characteristic impedance is also different causing the
50 ohm feedpoint position to change.
--
73, Cecil, IEEE, OOTC, *http://www.w5dxp.com

Whats the advantage of the Slim Jim over a plan old J antenna.

Jimmie

I had already made another Slim Jim using 2mm bare copper wire on the
sides of a black "electric" plastic tube

The velocity factor of the insulated twinlead is muchlower than bare
wires. If you don't understand velocity factor, please study up on the
subject.

I know what is velocity factor. It is determined by L and C. In the
"other" Slim Jim a plastic tube was between the 2mm wires, but performances
was similar to the 6mm tube antenna, in which only air is between the tube

--
Gianluca