Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
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. |
#2
|
|||
|
|||
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 |
#3
|
|||
|
|||
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 |
#4
|
|||
|
|||
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 |
#5
|
|||
|
|||
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 |
#6
|
|||
|
|||
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 |
#7
|
|||
|
|||
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! |
#8
|
|||
|
|||
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 |
#9
|
|||
|
|||
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 |
#10
|
|||
|
|||
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 |
Reply |
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
100 Ohm Twin Lead | Antenna | |||
Twin lead lightning arrestor? | Antenna | |||
300 Ohm Twin Lead Antenna Wire | Antenna | |||
300 Ohm Twin Lead Antenna Wire | Antenna | |||
Staples and twin lead | Antenna |