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Author Topic: Slim-Jim made from DC cable?  (Read 2781 times)
G7MRV
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« on: March 17, 2013, 04:26:28 PM »

Im in need of a very quick and dirty 2m vertical. I dont have any solid rod or sufficiently stiff wire for a groundplane antenna, nor do I have at present any ladderline/twin feed suitable for a quick J-pole or slim-jim.

What I DO have, is lots and lots of high current 'DC' twin cable. This is twin 2.5mm2 flex, in a PVC jacket. Online calculators ive tried suggest that this cable, which has a spacing between the conductors of about 3mm, would make a 2m slim-jim resonant at 145.5MHz (the UK calling frequency) about 1.5m long. However, the calculator seems to take no account of velocity factor (at least, it didnt ask for one, perhaps it uses a ballpark figure, say 0.8, built into the calculation)

This is really just a mad-cap experiment. But I wonder if anyone on here has tried using such cable as this, and if so what were your results?
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N3JBH
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« Reply #1 on: March 17, 2013, 05:09:01 PM »

Well if it is a experiment your wanting to do. I say just do the experiment and tell us what your success was. Asking other's there thought's  is not same as actually putting your experiment to work.
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ONAIR
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« Reply #2 on: March 17, 2013, 11:51:41 PM »

Just make a vertical dipole by taping wire to a broomstick!
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G7MRV
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« Reply #3 on: March 18, 2013, 03:01:06 AM »

Well if it is a experiment your wanting to do. I say just do the experiment and tell us what your success was. Asking other's there thought's  is not same as actually putting your experiment to work.

Indeed, but as I posted that at around midnight UTC (actual time here in the UK) I wasnt going to do so at the time. I asked as if theres likely to be a major difficulty due to the very narrow spacing, then its not worth the effort.
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G7DMQ
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« Reply #4 on: March 18, 2013, 04:38:10 AM »

I don't think there will be an issue with the close spacing - it's certainly worth a try!

Do you have a plan for tuning it - moving the connection point up & down the bottom stub?

I've made a few just out of coathanger wire & connector blocks to provide the spacing - which have been pretty good all things considered.

Just use a connector block for the feed in point which is then easy to adjust.

One made out of 'shotgun' DC cable would be good for taking on holiday!

Si
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G7MRV
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« Reply #5 on: March 18, 2013, 05:06:51 AM »

I don't think there will be an issue with the close spacing - it's certainly worth a try!

Do you have a plan for tuning it - moving the connection point up & down the bottom stub?

I've made a few just out of coathanger wire & connector blocks to provide the spacing - which have been pretty good all things considered.

Just use a connector block for the feed in point which is then easy to adjust.

One made out of 'shotgun' DC cable would be good for taking on holiday!

Si

The plan for matching is to use pins to pierce into the wires (being stranded) and move it around checking with an analyser. I need to find a 1/2w length of coax with a suitable plug on as the temporary feedline (halfwave so i dont disturb the impedance with the feeder!)

The cable im using is very heavy duty twin, but is sheathed, like thick mains flex. DC shotgun cable would be much easier, as the one issue ive realised is that the cores in this are twisted! Its not a rapid twist, so i'll be able to straighten out the feed point section, but ive no idea yet how it might affect the performance!

My first ever antenna, some 21 years ago, was a slim jim made by stapling stripped mains wire onto a long trip of cardboard (the back of an A4 notebook!), hung on my bedroom wall with a pushpin, and fed from first my KT-22 handheld, then later an IC-226E. It was also my first RF burn when the thing fell on my bare arm whilst tx'ing on the Icom!!! But I worked all around my local area on that, plus some DX into Holland, before upgrading to an outside antenna!
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N4CR
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« Reply #6 on: March 18, 2013, 01:30:12 PM »

The plan for matching is to use pins to pierce into the wires (being stranded) and move it around checking with an analyser

The problem with this is that the "loop" is the length of all of the conductors. So if you add anything to the circuit, it will change the characteristic impedance.

Build a 1/4 wave ground plane and be done with it.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
G7MRV
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« Reply #7 on: March 18, 2013, 02:22:07 PM »

The plan for matching is to use pins to pierce into the wires (being stranded) and move it around checking with an analyser

The problem with this is that the "loop" is the length of all of the conductors. So if you add anything to the circuit, it will change the characteristic impedance.

Build a 1/4 wave ground plane and be done with it.

Not sure what you think im adding to it? Im using a pair of pins in soldered to the end of the feed coax to make the connection, before soldering. Im not 'adding' anything?

And I DO NOT have the materials or the inclination to build a groundplane, NOR do I want to build a vertical dipole! This thread is quite clearly about building a slimjim. Please keep the discussion to that.
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N4CR
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« Reply #8 on: March 19, 2013, 03:18:21 AM »

The plan for matching is to use pins to pierce into the wires (being stranded) and move it around checking with an analyser

The problem with this is that the "loop" is the length of all of the conductors. So if you add anything to the circuit, it will change the characteristic impedance.

Build a 1/4 wave ground plane and be done with it.

Not sure what you think im adding to it? Im using a pair of pins in soldered to the end of the feed coax to make the connection, before soldering. Im not 'adding' anything?

And I DO NOT have the materials or the inclination to build a groundplane, NOR do I want to build a vertical dipole! This thread is quite clearly about building a slimjim. Please keep the discussion to that.

So instead of building an easier to load and tune, better behaved antenna, you'll build a J-Pole with a picky to load high impedance feed point, longer antenna with the same gain that wants to couple to everything in it's vicinity, alter it's pattern and have a lower overall height. Ok. I'll butt out. Have fun.

I sure don't understand people's obsession with J-Poles considering all the negatives against them compared to much easier antennas to build and operate, such as a basic quarter wave ground plane or a half wave vertical dipole.
« Last Edit: March 19, 2013, 03:24:26 AM by N4CR » Logged

73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
G7MRV
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« Reply #9 on: March 19, 2013, 03:50:47 AM »


I sure don't understand people's obsession with J-Poles considering all the negatives against them compared to much easier antennas to build and operate, such as a basic quarter wave ground plane or a half wave vertical dipole.

Hmm, lets see. I have a 1/4w groundplane made from 5 lengths of welding rod as my VHF general receive antenna, I have a dual band colinear as my main VHF/UHF antenna, A 2m halo under that, a 6m 3 el beam and a 2m 9 el beam, a doublet for HF, various fan and link dipoles for portable use, plus a few slim jims. I have a very wide band solid sheet discone, plus various small portable beams. ALL home made.

I dont want to build anything else, other than the slim jim. Why? because I want to. I want to see if it can be done in this form and how it works.

What I dont understand, is why people come onto a thread that is quite specific about what its asking and what the subject is, just to make bone comments and push their own ideas of what someone should do. I see this on every thread on this site. Someone asks a specific question, often about something they have (often an expensive rig!) and people just come on and say 'you dont want that, get this'. Perhaps this website needs to diffrentiate its sections like another im involved with, which has a serious, extremely heavily moderated section, and the 'NAAFI' section, where anything goes.


In case anyone else hasnt grasped it yet, im building a SLIM-JIM. Please only post if you can add something useful about that particular antenna.
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G0VKT
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« Reply #10 on: March 19, 2013, 05:32:53 AM »

I am not sure how close is too close. I have certainly seen versions for 10m where, if taking the wavelength into account, the elements are very close.
Try it and see as it will effectively be free. Might be tricky to tune? Worst case is that you end up stripping the conductors out and using those individually.
Common mode may be an issue, so a choke may help. It certainly has with my end-fed antenna experiments.

I do agree with your comments though. Too many nay sayers!
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G7MRV
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« Reply #11 on: March 19, 2013, 06:30:01 AM »

I also have one for 10m, which works very well. That one is a J-pole from a length of conduit wire and a length of 450 ohm window line, and is used with a 10m fibreglass telescopic pole. Yes in terms of fractions of wavelength its very small a seperation.

Its the twist in this cable that im concerned with now, never seen a twisted slim jim before!

Its all built save for matching and securing the coax. Im just hanging it at the moment to drop out the curve of the cable and make it straight (it came straight off the drum yesterday)
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G0VKT
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« Reply #12 on: March 19, 2013, 07:28:31 AM »

I guess by twist you mean it is bit like a helix. Can't see that making much of a difference as the spacing of the wires stays the same.

Twist it enough and you will have a helically loaded J-Pole  Grin
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G7MRV
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« Reply #13 on: March 19, 2013, 07:49:28 AM »



Twist it enough and you will have a helically loaded J-Pole  Grin

 Grin I did wonder if it would come out circular polarized!  Cheesy

The twist is the built in twist of the pair within the outer sheath. Probably something like one turn in 6inch. I believe this is put in to aid flexibility.

I can predict the effect of the twist in rifling on the passage of a bullet, but not the effect on radiation patterns!
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N3OX
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« Reply #14 on: March 19, 2013, 08:19:09 AM »

Twist will lower the characteristic impedance of the matching section, but 1 turn/six inches will not have an appreciable effect. It will have no effect on the radiation pattern at all. The lower impedance section (compared to a wide spaced wire) will probably tend to have higher losses, though maybe nto so bad, and might be a bit hard to find a 50 ohm match point (though on that latter point I'd have to do a little more analysis)

I have additional advice about your project if you care about radiation pattern but you've made your feelings on that quite clear.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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