Hi all,

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? Could be
the clear cable plastic not good for the application?

I have looked for black ladder twin lead, but I found it only online in
very long reels and with expensive shipping charges for Roma, Italy.

Thx
--
Gianluca

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

On Fri, 29 May 2009 10:44:45 +0200, "LAB" wrote:

SWR is very high and it seems to be not adjustable to the same 1:1
of the first antenna. Why?

Different diameters of conductors, different dielectric relationships,
that is why. This leads to different dimensions for the same
resonance. Larger conductors resonate at lower frequencies (or at
shorter lengths for same frequency). More dielectic also shortens
resonant length. Just how much, is for you to discover through
experimentation.

73's
Richard Clark, KB7QHC

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? Could be
the clear cable plastic not good for the application?

The impedance of "300 ohm" twinlead is often not very well
controlled... it can vary quite a bit from one manufacturer / batch to
the next. It's quite common to find it necessary to adjust the
lengths of the radiator and matching section, and the position at
which the feedline is attached, in order to get a good match. The
distance from feedpoint attachment, to the bottom of the matching
section, seems to be quite critical (this appears as a shunt
inductance across the feedpoint, and small differences in length can
make a bit difference in the reactance seen at the feedpoint).

Once you figure out the exact segment lengths you need for a
particular lot of twinlead, you can usually reproduce this design
quite quickly... but only as long as you use that exact type of
twinlead. Switch to a different type and you'll have to do another
round of trimming-and-tweaking.

I have no reason to believe that the clear-dielectric twinlead is any
worse than brown- or black-dielectric twinlead, in terms of your
ability to achieve a good match. They just have slightly different
imepdances and velocity factors, and so need different trimmings.

It may also help to add a choke to the coax feedline, just below the
point at which it's attached to the twinlead. This will isolate the
outside of the feedline from the antenna, reduce RF currents on the
feedline, and make the antenna's effective impedance less sensitive to
the position and length of the feedline. Try a snap-on ferrite
two-part core.

A few years ago I made a J-pole for my bicycle, by taping wires to the
side of a fiberglass bicycle-flag mast... no twinlead was used. The
lengths of the various wires ended up being a *lot* different than
what I would have needed for a twinlead J-pole in free space... the
relatively high dielectric constant of the fiberglass had a very big
effect! Figuring out the correct length for the below-the-feedpoint
inductive shunt section was the trickiest... I used bare copper wire
for the two sides, wrapping a turn or two of bare stranded wire around
the pole to form the short at the bottom, and moving the short up and
down until I got a good match. It ended up being quite a bit shorter
(if I recall correctly) than a standard twinlead J-pole would have
required.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!