Tube and Twin lead Slim Jim
 

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

Tube and Twin lead Slim Jim
 

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

Tube and Twin lead Slim Jim
 

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

Tube and Twin lead Slim Jim
 

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

Tube and Twin lead Slim Jim
 

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

Tube and Twin lead Slim Jim
 

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!

Tube and Twin lead Slim Jim
 

"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.

Tube and Twin lead Slim Jim
 

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

Tube and Twin lead Slim Jim
 

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

Tube and Twin lead Slim Jim
 

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

Tube and Twin lead Slim Jim
 

On Sat, 30 May 2009 12:38:08 -0700 (PDT), JIMMIE
wrote:

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

It has this coooool name that gives it at least 3.78dB advantage over
the name J-Pole (which, for the same nominal advantage (nominal
meaning name), has about a 5dBd gain over a rubber ducky).

If you want another 3dB nominal advantage, tag the word fractal to it:
Slim Jim Fractal J-Pole Antenna
of course, this being crass and a populist slant toward CBers, you
could tart the name up with academic trappings:
Slim Jim Fractal Gaussian Particels J-Pole Antenna
without bringing so much as 14.2dB loss for your effort.

Through the simple addition of a 2 meter sized dish, you can make it
omindirectional on the 160M band.

73's
Richard Clark, KB7QHC

Tube and Twin lead Slim Jim
 

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

Apparently, the take-off-angle:

http://www.para.org.ph/membersarticl...Slim%20Jim.htm
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Tube and Twin lead Slim Jim
 

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

Tube and Twin lead Slim Jim
 

Apparently, the take-off-angle:

http://www.para.org.ph/membersarticl...Slim%20Jim.htm

I have *serious* doubts about the analysis in that article.

I believe that the author's assumption that splitting the radiator
current in half, and running it through two parallel elements, has the
effect of increasing the gain and dropping the takeoff angle is
incorrect.

The analysis I've read on Cebik's web site of J-poles of various sorts
seems to make no mention of this alleged effect. Nor have I seen it
discussed in writeups of folded dipoles.

If increasing the gain of a half-wave dipole were as easy as that,
it'd be a lot more popular a technique, and much better known.

I believe that a "slim jim" might have a slightly wider bandwidth
and/or lower resonant frequency than a single-wire J-pole of the same
dimensions, but more gain? Unless somebody's got independent evidence
of this (measurements or a good NEC model) I'd take it with a
good-sized grain of salt.

--
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!

Tube and Twin lead Slim Jim
 

LAB wrote:
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

The plastic tube had mostly air between the conductors.
The insulation on 300 ohm twinlead or window line is solid
dielectric around and between the two wires. VF is easy
enough to measure with something like an MFJ-259B.

I measured the VF of 300 ohm window line at 0.8
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Tube and Twin lead Slim Jim
 

When I made the antenna I thought the plastic tube could do bad things.
I also read that black tubes have carbon in the plastic, then white ones
should be better...

--
Gianluca

Tube and Twin lead Slim Jim
 

Now it's very, very late here. I go to sleep. Have a good day!

--
Gianluca

Tube and Twin lead Slim Jim
 

On May 30, 8:30*pm, (Dave Platt) wrote:
Apparently, the take-off-angle:

http://www.para.org.ph/membersarticl...Slim%20Jim.htm

I have *serious* doubts about the analysis in that article.

I believe that the author's assumption that splitting the radiator
current in half, and running it through two parallel elements, has the
effect of increasing the gain and dropping the takeoff angle is
incorrect.

The analysis I've read on Cebik's web site of J-poles of various sorts
seems to make no mention of this alleged effect. *Nor have I seen it
discussed in writeups of folded dipoles.

If increasing the gain of a half-wave dipole were as easy as that,
it'd be a lot more popular a technique, and much better known.

I believe that a "slim jim" might have a slightly wider bandwidth
and/or lower resonant frequency than a single-wire J-pole of the same
dimensions, but more gain? *Unless somebody's got independent evidence
of this (measurements or a good NEC model) I'd take it with a
good-sized grain of salt.

--
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!

Dave,That was my thoughts on it also. Two wires very close togethether
with in phase and more or less equal currents just adds up to a fatter
wire.

Jimmie

Tube and Twin lead Slim Jim
 

"JIMMIE" wrote in message
...

snip

Dave,That was my thoughts on it also. Two wires
very close togethether with in phase and more or
less equal currents just adds up to a fatter
wire.

Not a fatter wire with twice the current?

Recall the colinear J-pole, The two currents are stacked and fed in phase
by means of the phasing section. Yes, I realize the currents in the Slim
Jim would appear to cancel.

I'm going to have to build one of these and see what it does. I have a lot
of twinlead and I have a precision RF generator. I always wanted my own
antenna range; tomorrow's the day. Maybe Tuesday.

Tube and Twin lead Slim Jim
 

In message , Cecil Moore
writes
JIMMIE wrote:
Whats the advantage of the Slim Jim over a plan old J antenna.

Apparently, the take-off-angle:

http://www.para.org.ph/membersarticl...Slim%20Jim.htm

I'm sure that W4RNL did a comparison. It might be in here.
http://www.scribd.com/doc/8511817/Some-JPoles-That-I-Have-Known-4
http://www.cebik.com/
If it's not in there somewhere, a Google will be necessary.

Essentially, the answer was 'not a lot', except that the Slim Jim
version (the one with the folded-over top) had a slightly wider
bandwidth / flatter SWR plot.
--
Ian

Tube and Twin lead Slim Jim
 

On Jun 2, 4:51*am, Ian Jackson
wrote:
In message , Cecil Moore
writes

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

Apparently, the take-off-angle:

http://www.para.org.ph/membersarticl...Slim%20Jim.htm

I'm sure that W4RNL did a comparison. It might be in here.
http://www.scribd.com/doc/8511817/Some-JPoles-That-I-Have-Known-4
http://www.cebik.com/
If it's not in there somewhere, a Google will be necessary.

Essentially, the answer was 'not a lot', except that the Slim Jim
version (the one with the folded-over top) had a slightly wider
bandwidth / flatter SWR plot.
--
Ian

Tube and Twin lead Slim Jim
 

"Sal M. Onella" wrote in message
...

"JIMMIE" wrote in message
...

snip

Dave,That was my thoughts on it also. Two wires
very close togethether with in phase and more or
less equal currents just adds up to a fatter
wire.

Not a fatter wire with twice the current?

Recall the colinear J-pole, The two currents are stacked and fed in phase
by means of the phasing section. Yes, I realize the currents in the Slim
Jim would appear to cancel.

I'm going to have to build one of these and see what it does. I have a
lot
of twinlead and I have a precision RF generator. I always wanted my own
antenna range; tomorrow's the day. Maybe Tuesday.

Hi "Sal"

I share your thoughts about a pattern range. I recently talked my buddy
into writing a program that plots Elevation Plane patterns of an antenna
when scaled to a frequency sent from any Polar Orbiting Satellite. I use
it alot using the 137 MHz beacons sent from NOAA satellites. Some HAM
satellites could be used. There are also L-band beacons sent from the NOAA
satellites.

Jerry KD6JDJ

Tube and Twin lead Slim Jim
 

"Jerry" wrote in message
...



snip


I share your thoughts about a pattern range. I recently talked my
buddy
into writing a program that plots Elevation Plane patterns of an antenna
when scaled to a frequency sent from any Polar Orbiting Satellite. I use
it alot using the 137 MHz beacons sent from NOAA satellites. Some HAM
satellites could be used. There are also L-band beacons sent from the
NOAA
satellites.

That's an interesting approach. Thanks.

Tube and Twin lead Slim Jim
 

"Sal M. Onella" wrote in message
...

"Jerry" wrote in message
...



snip


I share your thoughts about a pattern range. I recently talked my
buddy
into writing a program that plots Elevation Plane patterns of an antenna
when scaled to a frequency sent from any Polar Orbiting Satellite. I
use
it alot using the 137 MHz beacons sent from NOAA satellites. Some HAM
satellites could be used. There are also L-band beacons sent from the
NOAA
satellites.

That's an interesting approach. Thanks.

Hi "Sal"
If you ever get involved with that Pattern Range, Patrik Tast publishes
information on his program on the web. Everything Patrik does is *hobby
related* and available free. Nothing I do is sophisticated or difficult to
reproduce or to improve upon.
I was using Excel to plot the RSSI level before I asked Patrik to make a
program to display the signal strength in polar form. I can get actual
patterns that Very closely resemble EZNEC plots.

Jerry KD6JDJ

Tube and Twin lead Slim Jim
 

"Jerry" wrote in message
...

"Sal M. Onella" wrote in message
...

"Jerry" wrote in message
...



snip


I share your thoughts about a pattern range. I recently talked my
buddy
into writing a program that plots Elevation Plane patterns of an
antenna
when scaled to a frequency sent from any Polar Orbiting Satellite. I
use
it alot using the 137 MHz beacons sent from NOAA satellites. Some HAM
satellites could be used. There are also L-band beacons sent from the
NOAA
satellites.

That's an interesting approach. Thanks.

Hi "Sal"
If you ever get involved with that Pattern Range, Patrik Tast publishes
information on his program on the web. Everything Patrik does is *hobby
related* and available free. Nothing I do is sophisticated or difficult
to
reproduce or to improve upon.
I was using Excel to plot the RSSI level before I asked Patrik to make a
program to display the signal strength in polar form. I can get actual
patterns that Very closely resemble EZNEC plots.

Thanks. Whatever I set up will probably be for signal strength A/B testing
between antennas at a fixed distance.