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 slim jim on 70cm 
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Post slim jim on 70cm
I made a lash up job of a slim jim for 70cm the other day and it worked quite good indoors, i intend to put one up outside in the next few weeks depending on the weather.
Anyone else tried one for 70cm ? i used to use the indoor version for 2m when i was first licensed and it worked unbelievably well, i will try and find the old article for the indoor 2m version and post it on here.

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Sat Jan 01, 2011 8:18 pm
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
I have built "slim jims" or J poles as they are sometimes referred to, over many years, and alway had good results on 2 & 70.

I have made them from copper pipe and elbow joints etc, 300 ohm ribbon, and even just simple wire round a brush handle, and always find if you get the swr minimum around centre of desired band they seem pretty good over a fair spread.

When I lived in Heywood many years ago, I had one (the brush handle with wire), and inserted into some plastic tubing. Had a reasonable take-off there and Midlands was always availble to work.
It also acted as a power monitor, becasuse if I gave it 100W of RF a puff of smoke would emanate from the end of the pipe!.

They cost so little to construct, and the results can be very surprising.

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Sat Jan 01, 2011 9:21 pm
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
I should have put this in antennas so if someone could move it that would be great.

I have an idea in my head of what i'm going to make it out of and hopefully make it easy for anyone to copy , i'll post some photos when i make a start on it ,i expect it to cost me nothing to build as i should have most items to hand.

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Sun Jan 02, 2011 8:53 am
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
g7pan wrote:
I should have put this in antennas so if someone could move it that would be great.

I have an idea in my head of what i'm going to make it out of and hopefully make it easy for anyone to copy , i'll post some photos when i make a start on it ,i expect it to cost me nothing to build as i should have most items to hand.

This is one thing I can't give an opinion on. :)
Other than to say that, over the years the J pole/Slim Jim has achieved some sort of mythical status that facts don't uphold.
That's not to say it doesn't work, of course, just that there are some very dubious claims that have been made.

Other than 4M (not in use for some time) I've not had any verticals for many years.
When I did they were always colinears, groundplanes or dipoles.

Good luck with the project.
Hopefully you will post the results and some photos.

I'll try and move it now.
I'm a beginner at this so standby for a mess up. :lol:

Cheers

Tony

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Sun Jan 02, 2011 9:35 am
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
Moved perfectly.
I found this very helpful site http://www.m0ukd.com/Calculators/Slim_Jim/index.php which i will be using to get the dimensions from.
I have a small dualband ,similar to a x30 ,older and made from aluminum which i will be comparing it against which i'm oping it will out perform.

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Sun Jan 02, 2011 10:19 am
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
I'm obviously getting the hang of the moderator lark. :lol:

I think Steve did some calcs after an "interesting" thread on another site. :D
I can't find it at the moment so I'll leave it to Steve.

Cheers

Tony

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Sun Jan 02, 2011 11:55 am
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
I think Tony is referring to some extraordinary claims that were made a while ago for the Gain of J-pole derivatives.

The J-pole and "Slim Jim" are end-fed half-wave antennas; as such they can't have any more gain than a vertical half-wave dipole. They may show marginally more gain than a quarter-wave ground-plane - about 0.6dB.

Their main attraction seems to be ease of construction, but like any half-wave end-fed, it's tough to stop the coax feedline or mast becoming part of the radiating structure.

A quarter-wave ground-plane is a much more "repeatable" design, which is one reason professionals fit them rather than J-poles or Slim-Jims.

Steve G3TXQ


Sun Jan 02, 2011 12:25 pm
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
G3TXQ wrote:
I think Tony is referring to some extraordinary claims that were made a while ago for the Gain of J-pole derivatives.

The J-pole and "Slim Jim" are end-fed half-wave antennas; as such they can't have any more gain than a vertical half-wave dipole. They may show marginally more gain than a quarter-wave ground-plane - about 0.6dB.

Their main attraction seems to be ease of construction, but like any half-wave end-fed, it's tough to stop the coax feedline or mast becoming part of the radiating structure.

A quarter-wave ground-plane is a much more "repeatable" design, which is one reason professionals fit them rather than J-poles or Slim-Jims.

Steve G3TXQ

That's the one. :)
I meant to add that the professional antenna systems have never, as far as I know, involved J Poles.
Assuming your antenna is similar to the X30 - 2m/70cm band. Length 1.3M , Gain 3.00/5.5dB - and the figures are correct, I suspect there will not be an improvement.

Not, in itself, a reason for not having a play. :D

Cheers

Tony

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Sun Jan 02, 2011 12:59 pm
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
the Sirio 4m antenna is advertised as a J-pole.

http://www.sirioantenne.it/prodotti_ant ... 189&idp=90

as is the 70cm version.

http://www.sirioantenne.it/prodotti_ant ... 1011280845

and 2m.

http://www.sirioantenne.it/prodotti_ant ... 189&idp=89

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Sun Jan 02, 2011 2:22 pm
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
Well there we go.
As I've said before, something to learn all the time.
I actually own one :oops: but I don't remember it being described as that when I got it.
Mind you it is some years ago and it's not been in use for ages.

Its construction doesn't seem to shout it out but, if they say it is then fair enough.

I always thought they were a conventional, gamma matched 3/4 wavelength vertical.

Still not what I would describe as a "professional" antenna. :)

Cheers

Tony

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Sun Jan 02, 2011 2:52 pm
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
gw8asd wrote:
Well there we go.

Still not what I would describe as a "professional" antenna. :)

Cheers

Tony



fair comment!

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Sun Jan 02, 2011 3:16 pm
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
I see that Sirio refer to them as Gamma-matched 3/4 waves and as J-poles. I wonder which they really are, and how they produce 2dBd Gain ;)

Steve G3TXQ


Sun Jan 02, 2011 3:23 pm
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
G3TXQ wrote:
I see that Sirio refer to them as Gamma-matched 3/4 waves and as J-poles. I wonder which they really are, and how they produce 2dBd Gain ;)

Steve G3TXQ


are they mutually exclusive, Steve?

their 27mhz vertical has the same description, and I always thought it correct?

http://www.sirioantenne.it/prodotti_ant ... 169&idp=66

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Sun Jan 02, 2011 3:27 pm
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
G3TXQ wrote:
I think Tony is referring to some extraordinary claims that were made a while ago for the Gain of J-pole derivatives.

The J-pole and "Slim Jim" are end-fed half-wave antennas; as such they can't have any more gain than a vertical half-wave dipole. They may show marginally more gain than a quarter-wave ground-plane - about 0.6dB.

Their main attraction seems to be ease of construction, but like any half-wave end-fed, it's tough to stop the coax feedline or mast becoming part of the radiating structure.

A quarter-wave ground-plane is a much more "repeatable" design, which is one reason professionals fit them rather than J-poles or Slim-Jims.

Steve G3TXQ

Isn't the slim jim more than a half wave ,not sure how it works out but one side is 3/4 wave and the other 1/2 wave ,anyway it will be interesting to see the difference better or worse between the the 2x 5/8 wave.

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Sun Jan 02, 2011 4:51 pm
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 slim jim on 70cm 
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Post Re: slim jim on 70cm
bonehead wrote:
are they mutually exclusive, Steve?


I would say so.

A J-Pole is a half-wave end-fed radiator matched with a quarter-wave transmission-line section. Currents in the TL section are predominantly Differential-Mode and contribute little to the radiation.

A Gamma-Match is a quite different matching arrangement; for one thing you would expect a classic Gamma Match to have a series capacitor element. Currents along the two conductors of a Gamma Match are part CM and part DM; so you should expect there to be significant radiation from all parts of a 3/4 wave Gamma Matched antenna - not just the end 1/2 wave.

If it really is a Gamma-Matched 3/4 wave, there is some justification for it having more gain than a dipole.

73,
Steve G3TXQ


Sun Jan 02, 2011 5:37 pm
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