"J pole" or "Slim Jim"

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"J pole" or "Slim Jim"

Postby VK4WDM » Wed Apr 21, 2010 8:47 pm

Hello 6m family

I am going to put up a vertical for band monitoring purposes. I am contemplating either a "J pole" or a "slim jim" made out of copper tubing.

I am leaning towards the slim jim because of its very low angle of radiation and the claimed 6 db gain. I would be interested in hearing other people's viewpoint on the merits of each.

The other wild idea (yep, its a good crop of weed this year :P ) is to have verticals for 6m, 2m, and 70cm with a single feed line and triplexers so I can monitor all three bands.

73

Wayne VK4WDM
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Re: "J pole" or "Slim Jim"

Postby VK2ZRH » Wed Apr 21, 2010 9:12 pm

Both antennas are an end-fed half-wave, otherwise known as an end-fed Zepp (short for Zeppelin - see http://en.wikipedia.org/wiki/J-pole_antenna). The half-wave section is fed by a quarter-wave transmission line, which is not part of the radiating section. 8)

The Slim Jim is a variation of the J-pole, simply using a folded half-wave section. There's no special magic in that. However, it means you can build it from balanced feedline (like 300 Ohm ribbon or ladder line). Great for /P and field days - you can make a roll-up antenna. :wink:

Compared to a vertical dipole, both have 0 (zero) gain; ie. 0 dBd. Compared to an isotropic antenna, they have 2.16 dB gain (ie. 2.16 dBi). Any other claims about X dB gain over X-Y-Z antenna are just . . . claims. Not fact. That includes the one about "vertical radiation angle about 8 degress towards the horizon". :roll:

I have built and used J-poles for 6m in years past. They worked. So did a standard (quarter-wave) groundplane. Compared A-B (antennas at same height), it was much of a muchness. J-poles are easier to build than ground planes. However, you must choke the coax below the feedpoint, or use a balun ! One trick I found worked well with a J-pole made with Al tube was to feed the coax down the tube and down the inside of the metal tube support mast.

Good luck with the project.

73, Roger Harrison VK2ZRH
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Re: "J pole" or "Slim Jim"

Postby VK4WDM » Wed Apr 21, 2010 9:56 pm

Hi Roger

Yes, I always take the "claims of gain" in some of these designs with a grain or two of the white stuff!

During the last SS peak I used a dipole for monitoring, but sod's law meant that the incoming signal that indicated a band opening to Ga Ga land or somewhere was in the null zone. That is why I decided to go for a vertical this time around. One way to check both designs out would be do as you suggest, make them from ribbon and tape them to a bit of PVC pipe.

73

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Re: "J pole" or "Slim Jim"

Postby VK4WDM » Thu Apr 22, 2010 11:53 am

Halo Matt

I had a halo once but it slipped and I kept tripping over it. I also found it very tricky to tune, but it is certainly worth considering.

73

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Re: "J pole" or "Slim Jim"

Postby VK2ZRH » Thu Apr 22, 2010 12:56 pm

The big advantage of the J-pole is simplicity of construction, whether made from wire feedline or from metal tubing.

Halos are all very well, but you have to keep polishing them. :mrgreen: Real halo antennas aren't too challenging constructionally. As in yoga, it's the bending that's hardest. However, halos exhibit less gain-over-isotropic than a dipole and matching them across a useful bandwidth is challenging (ie. don't use a gamma match). In general, most of the 6m DX is horizontally polarised . . . but ionospheric propagation can/will give the signal a bit of a twist (and you never know how much).

For a bit of a challenge, check out the Big Wheel. Here, for example: http://www.wimo.de/cgi-bin/verteiler.pl ... nas_e.html

A Big Wheel for 6m is spatially challenging, at nearly 4m in diameter. :!: Best attributes are omnidirectional pattern, horizontal polarisation, useful gain and a bit of bandwidth. A bit of a "twist" on the Big Wheel is the Cloverleaf, where the three elements of the Big Wheel are twisted from the horizontal by 30-45 degrees so as to get dual polarisation ~ useful for ionospherically propagated signals. However, gain for a given polarisation is reduced. As Mick Jagger sang: You can't always get what you want. :wink:

73, Roger Harrison VK2ZRH
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Re: "J pole" or "Slim Jim"

Postby VK2CSW » Fri Apr 23, 2010 5:59 pm

Excuse my ignorance,

If you need horizontal polarisation and J-Poles are easy to build, could you mount the J-Pole horizontally?

Colin
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Re: "J pole" or "Slim Jim"

Postby VK3AUU » Fri Apr 23, 2010 7:37 pm

Of course, but you will probably get a null off each end.

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Re: "J pole" or "Slim Jim"

Postby VK4WDM » Fri Apr 23, 2010 7:57 pm

Hi Colin

Mounting the Slim Jim or J pole horizontally would work (an HF "Zep" is in effect, a horizontal J pole) but that defeats the purpose. What you need to monitor for band openings on 6m is an omni-directional antenna. Signals can come in from any direction and can be very narrow. It is not uncommon for instance to have signals coming in from south Italy and nothing at all from mid way up the Italian coast only a hundred km away, or you can be working stations around Paris but hear nothing at all from the south of the UK. The same goes for the "foot print" at the receiving end. Signals heard in Townsville may not be heard in Charters Towers 100km away.

I don't really buy the horizontal versus vertical argument, especially on long-haul dx. The ionosphere can cause signals to twist and turn between transmitter and and receiver. Last SS peak I worked plenty of JA's and quite a lot of Europeans on a very simple vertical, and sometimes with verticals at the other end. Sure, signal strength was down a number of S points than on my 7 element beam, but we get fixated on the "S bit" of the report. The "R bit" is the important one. There are plenty of R5 S1 reports in my log, but not a lot of R2 S9 :)

73

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Re: "J pole" or "Slim Jim"

Postby VK3PF » Fri Apr 23, 2010 11:37 pm

Hi all,

You could try the "Hentenna", which purports to be horizontally polarised, but with Figure-8 pattern, IIRC.

A better option is probably the turnstiled Quads design by LB Cebik (6-Meter Quad-Turnstile, QST May 2002, pp. 42-46, A omni directional, horizontally polarizes 6-meter loop antenna) - from memory very close to omni pattern, horizontally polarised and around 10 dBi gain (IIRC). ARRL members can download the article from the ARRL website....

A further option would be to scale a Cebik 2 m antenna - 3 dipoles appropriately phased to give an antenna equivalent to a "Big Wheel", but it is getting BIG at 6 m: "A New Spin on the Big Wheel" in QST March 2008. Basically, 3 dipoles arranged in a triangular pattern, fed appropriately. Dipoles are spaced at 120 degrees to each other. At 6 m, the radius from the centre out to each dipole centre is around 1.13 m (quick scaling). On 2 m, at 6 m above ground, the antenna gives about 7.2 dBi omni. Therefore, one might expect similar performance on 6 m at around 17 m above ground..... The article includes information on turning the 3 dipoles into "a circle".

Good luck,

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Re: "J pole" or "Slim Jim"

Postby VK4WDM » Sat Apr 24, 2010 12:48 pm

Hi Peter

Height limit here is 10m and I want to keep a low profile or "semi-stealth" situation, that is why I am opting for a black-painted vertical that is difficult to see against the trees. My beam will be below the level of the roof unless raised for use.

As I said earlier, I don't really think there is a need for horizontal polarization. This antenna will be used for monitoring the band using spectran or a similar program and a few db loss won't be much of a worry.

I was drawn to the Slim Jim because of the low angle of radiation claim but Roger VK2ZRH dismisses that as being false (although a number of others support it). :?

73

Wayne
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Re: "J pole" or "Slim Jim"

Postby VK2ZRH » Sat Apr 24, 2010 4:38 pm

Hi Wayne,

The J-pole and low angle of radiation: here's the story.

The J-pole is a half-wave end fed antenna. It's free space radiation pattern is the familiar donut (as associated with a half-wave dipole). The radiation pattern achieved in the real world, where the antenna is mounted at some height above ground, depends on how many wavelengths it is above ground. Ground reflection at a distance from the antenna, when added to the radiation pattern, determines the radiation angle at the peak of the radiation pattern. This is always at an angle higher than the horizon. Even for a 10-element beam, the same principle applies. :roll:

You need to keep in mind that the ionospherically-propagated DX arrives at your station at an angle above the horizontal; for 6m that's often only a few degrees. So how come you hear S9 signals but the peak radiation angle of your antenna is up at 10-15 degrees ? Ionospheric propagation on 6m often experiences signal focusing by the ionosphere ~ Es and F propagation alike.

The Hentenna:

The Hentenna (Hen = "weird", or "strange", in Japanese) is two half-waves mounted vertically, 0.14~0.2 wavelengths apart and fed in-phase (via the "connections" at top and bottom). This gives it some gain by compressing the radiation pattern in the horizontal plane. The resulting rectangle is then fed by "tapping up" from one end of each half-wave element (ie. a balanced feed). The curious result of the physical arrangement is horizontal polarization. However, the free space radiation pattern resembles the donut of the horizontal dipole; ie. there are nulls either side of the antenna, as you look at the 'loop' front-on.

A turnstile made from two Hentennas mounted on the same mast at right angles would make an omnidirectional antenna with useful gain over a dipole. Mind you, a Hentenna for 6m is just shy of 3m tall (taking 'end effect' into account).

Today's 4 cents-worth.

What the heck, Wayne. Don't agonise. Chuck up a J-pole :lol:

73, Roger Harrison VK2ZRH
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Re: "J pole" or "Slim Jim"

Postby VK4WDM » Sat Apr 24, 2010 6:13 pm

Hello again Roger

No agony mate, just interesting discussion. Its not so much about what I finally put up, its more about getting technical discussions going for the edification of the masses, especially the new converts, or near converts, to the AR ranks, so thank you for your valuable input - much appreciated.

We might not all be comfortable playing around it the inside of our equipment, or building home brew transceivers, but antenna building and experimentation is something we can all become involved in, and very often at very low cost. :)

73

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Re: "J pole" or "Slim Jim"

Postby VK2JDS » Sat Apr 24, 2010 7:00 pm

My J pole 'array' is made from waterpipe for 10 feet or so , then lengths of aluminium tube scrounged from old 27 meg verticals with additions on the top, it does 6m and 10m all on the one pole.
its mounted against the wall of the shed and is grounded at the bottom which is handy when you are in a lightning prone area.
The one single long pole becomes the radiating element for both bands. Very handy when i am out in the shed working to monitor the bands.
There are 2 J sections on this pole, the bottom one for 10 metres and the top one for 6. as the band opens you can sweep 10 and see whats happening then flick over to 6 and sit on 50.11 etc. When it gets interesting I wander over to the house and fire up the 746 with the 6 element yagi.

Its got separate feedlines and the 6m one is just taped to the side of the pole which doesnt affect 10m. It seems to work really well and once they were both adjusted its been fine ever since i installed it.
As for gain, no. its not like a 2 element or 3 element yagi , not even remotely like them so gain is as a dipole.
The other onmi i use is a 80 metre loop up on the mountaintop, it works surprisingly well on 6 and the hunter valley beacon is always there as a reference.
73's Dave vk2jds
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