J-pole antenna advice

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hibernian
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J-pole antenna advice

Post by hibernian » Wed Sep 28, 2016 6:48 pm

Hi - I've been mucking around with one of those Chinese cheapo transmitters (15w ish) and made a J-pole antenna to see how far out I could get with it. I'm out in the country so it's pushing out a good 6km radius with the antenna at about 15 feet off the ground - fairly flat terrain and I'm low down. Currently only using rg58 to feed it.

I'm thinking about getting a proper rig (from a builder here :?: or from one of the Dutch places) and I would like advice on how much power I can put into the antenna. How do I figure that out? It's made of 1/2 inch copper pipe. I'm assuming there are tonnes of variables - any advice is welcome and appreciated. I was thinking of eventually getting a 100w to 200w set up. Thoughts?

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Re: J-pole antenna advice

Post by thewisepranker » Thu Sep 29, 2016 10:07 am

The parts that limit the power are not the elements but whatever you've used to connect the coax to the elements.

If you've soldered the coax directly to the elements then your limitation is approximately whatever the coax can handle.
Use a blowtorch and a big, high-power soldering iron to assemble your J-pole. I use a Weller SP175, 175 W soldering iron meant for soldering windows and roofing together, which ensures the joints between the coax and elements (assuming this is how you did it) are nice and wet.

You'd be better off sticking to roughly 150 W output power and using better coax than aiming for 200 W as the output transistors get expensive.

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Re: J-pole antenna advice

Post by hibernian » Thu Sep 29, 2016 11:12 am

Thanks mate - that's exactly the advice I was looking for. For testing with the Chinese dinky rig, I just used jubilee clips. Googling the Weller SP175 now! (and I'll invest in proper co-ax too)

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Re: J-pole antenna advice

Post by thewisepranker » Thu Sep 29, 2016 11:55 am

I seem to remember writing a post about how I make J-poles on here but can't seem to find it. It might have been the old forum...

If you make it out of 22 mm pipe, it becomes a bit less wobbly in the wind and you can then do a few things to make tuning easier.

Get yourself a square flanged SO-239, like this:
Image

Three brass 22 mm clamps like this:
Image
http://www.screwfix.com/p/22mm-pipe-cli ... pack/2401j

And a 22 mm compression coupling:
http://www.screwfix.com/p/straight-coup ... pack/66818

Cut the three brass clamps on one side of the screw so that you're left with the screw and two semicircles. Like this:
Image

You can clamp the flanged SO-239 directly to two of the clamps with nuts and bolts, or perhaps the screws supplied.

Get a small length (6" should do it) thick (Ø2-3 mm) enamelled copper wire and tin one end (your iron must be at over 400 °C to do this properly - the Weller iron I suggested will do the job) and solder it to the centre conductor of the SO-239 (with a smaller iron).

Make sure your enamelled copper wire fits into the solder bucket of your SO-239.
You should now have a SO-239 bolted to two circles made of brass (the clamps) with a 6" piece of enamelled copper wire soldered to the centre of the SO-239.

Get some steel wool and rub the J-pole elements with it like that scene out of Ali G.

Now slide the SO-239 brass clamp assembly onto the short element of your J-pole. Or undo the screws, or do it in a slightly different order, it's up to you.

Take your remaining cut up 22 mm brass pipe clamp and slide it onto the long element of your J-pole. Line it up in an arbitrary position along the elements, but in line with the SO-239 assembly and tin the enamelled copper wire in the position above the lone 22 mm brass clamp. The wire should be fairly straight in all directions but with a small amount of slack, so that you can move the SO-239 up and down by about 5 mm in each direction relative to the other brass clamp.

Tin the brass clamp on the long element, but do not get any solder on the element itself, and solder the enamelled copper wire to it.

You now have a J-pole that you can tune up and down by tightening and loosening three screws.

When you're happy, solder the brass clamps to the J-pole. You should do this as soon as possible, but realistically within a few hours of doing your Ali G impression.

Be careful to not melt the dielectric in the SO-239, especially if you decide to solder the flange to the brass clamps.

This is where the blowtorch comes in handy to get it hot as quickly as possible.

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Re: J-pole antenna advice

Post by thewisepranker » Thu Sep 29, 2016 12:59 pm

I forgot to mention the compression coupling. This is a way of making the J-pole just a bit shorter, so you can get it into places like an elevator or up a stairwell without having to resort to a rope over the side of a building.

The J-pole typically uses two plumbing joints - a tee piece and a 90° elbow. The bit that sticks out the bottom of the elbow will be where you mount the J-pole to a scaffold pole or whatever you're mounting it to. If you cut this quite short, say 200 mm from the tee, you can fit the nut and olive of one end of the compression coupling to the end. You then fix another, much longer piece of Ø22 mm copper pipe to the rest of the compression coupling and attach that to whatever you want to fix it to, however you want to fix it. Simply put the J-pole into the compression coupling and tighten the big nut.

You are required to own to fairly large spanners (the nuts are 32 mm and the flange is 34 mm - you need both) but you can buy adjustable spanners that are big enough for very little from Screwfix.

One of my J-poles lasted over 9 months on a 15-storey building using this construction and only came down when we switched over to a gamma matched dipole. It never needed straightening despite the entire bending moment being on the compression coupling. If you space it right, you can get half of the J-pole tube and half of the support member in the rigid section of the coupling, which adds to therigidity.

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Re: J-pole antenna advice

Post by hibernian » Thu Sep 29, 2016 11:12 pm

That's really cool... it'll take me a while to get everything sorted, but when I do, I'll drop back to give an update on how it works out for me. The easy bit about being out in the country in Ireland is that we have tonnes of open frequencies. I'll have access to a high site with power soon (high compared to local terrain - not really all that high) so it'll be a fun project (even if the music I'm playing isn't to everyone's taste!)

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Re: J-pole antenna advice

Post by hibernian » Thu Sep 29, 2016 11:38 pm

Hi wisepranker - I was looking at images so I could see your handy way of connecting the co-ax to help me visualise it. I saw that a lot of the j-poles online have the centre conductor attached to the stub and the shield to the longer element. Is that right? I have mine the other way around (shield to stub, centre conductor to the long element).

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Re: J-pole antenna advice

Post by Spokes » Fri Sep 30, 2016 7:25 pm

hibernian wrote:I have mine the other way around (shield to stub, centre conductor to the long element).
Stay as you are dude :tup

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Re: J-pole antenna advice

Post by thewisepranker » Fri Sep 30, 2016 7:40 pm

It doesn't matter which side of the coax you connect to which element, as it should be driven as a balanced load.

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Re: J-pole antenna advice

Post by Albert H » Fri Sep 30, 2016 9:43 pm

Pranker - the J-poles that I've used have been an asymmetrical load, which is why they'll match coax without a balun. I've always fed the long side of the J from the coax centre; I agree that you could - theoretically - reverse it, but I'd rather have the high Voltage anti-node at the top of the aerial, away from me!

A carefully matched J-pole has a performance that is very marginally better than a dipole. In the Real World ™ you'll find that a J-pole does much better than a dipole because the match will be better, and you're not losing any energy in a wasteful balun! You'll also find that the J-pole is truly omni-directional - a dipole always has a slight null where the mast is! I've also used the mast as part of the aerial, providing a reflector for an improvised "H".

Once you've made your first J-pole that accurately matches for your frequency of choice, it becomes very easy to reproduce exactly the same dimensions over and over again. I used to run the coax inside the mast and inside the copper pipe that made the aerial. I drilled small holes at the feed points and the connections were fished through from inside the pipe, and soldered to the outside. The whole aerial was spray painted with pale grey acrylic paint, so that the aerial was even more invisible from the ground against an English sky! I used to bung up the ends with hot-melt glue. There are still a few of these intact on high sites in Yorkshire from the early 90s!
"Why is my rig humming?"
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Re: J-pole antenna advice

Post by hibernian » Wed Oct 12, 2016 10:00 pm

been away working for a bit - thanks for all the advice guys. I'm keeping all this info for when I get my 'proper' tx rig

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Re: J-pole antenna advice

Post by pirateaddict » Thu Oct 13, 2016 7:16 am

Lovin this thread as I'm interested in these j poles.. It's what I'll be using when I'm ready to go.. @Albert H, where abouts in Yorkshire are these J poles you mentioned? Are they still in use or are they surplus to requirements? Sorry for all the questions bud..

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Re: J-pole antenna advice

Post by teckniqs » Thu Oct 13, 2016 11:26 am

Haha, thing is if he tells you there's a lot less likelihood of it still being there in another 20 years....

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Re: J-pole antenna advice

Post by pirateaddict » Thu Oct 13, 2016 12:26 pm

Yeah, i know what you mean teckniqs lol..I wouldn't steal it but would to know if he put one up in Sheffield tbh. Even though I've never seen a j pole on me travels.. I remember the pirates in Sheffield always used a dipole which moved back n forth in the wind coz they weren't on a boom i think. And others used what looked like a 3 element beam with half a dipole on top, used to love working out who's aerial was who's...A couple of mates of mine knew who ran a few of em but i never got to visit or owt though... Pirate addict indeed..The money and a place to set up never came at the same time, ne'er mind.

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Re: J-pole antenna advice

Post by Albert H » Thu Oct 13, 2016 6:15 pm

The aerials were further up the county, up in the higher parts of the Pennines. Last time I was up near Queensbury, I spotted one on a big scaffold pole on top of a barn, hidden from the road (mostly) by trees. I remember that one being installed - with the knowledge of the son of the farmer! The rig ran off a couple of car batteries and would last about 15 hours on a charge. It was linked on UHF and had remote switching so that we could use it for a few hours on a Saturday, switch off late at night, and switch on again on Sunday afternoon. From up there it really got out!
"Why is my rig humming?"
"Because it doesn't know the words!"
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Re: J-pole antenna advice

Post by pirateaddict » Thu Oct 13, 2016 8:42 pm

Thanks for the reply Albert.. Yep, not round my way.. Was that in the eighties Albert, and how far if ya know, did ya get out from there? Sorry I'm getting a bit brave now asking questions lol.

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Re: J-pole antenna advice

Post by Albert H » Fri Oct 14, 2016 7:13 am

As I remember, that was the early 90s. One rig that was used up there did around 120 Watts and was heard in Manchester and Preston to the west, Stoke and even Lincoln to the south, Darlington to the north (and even as far as Newcastle on a couple of occasions). It was weak into Sheffield and inaudible in Nottingham because of the topology of the hills to the south. It really proved that "height is everything" when it comes to getting out a good Band II signal.

Just to further illustrate that, my station in the USA was only running 40 Watts from a site in the Hollywood Hills. There was a Latino pirate in downtown LA, coming off a 10 storey building with 200 Watts, and we had much better coverage than them - bigger area and less patchy. Their signal was huge in their neighbourhood (which is what allowed the FCC to track them down), but we were everywhere! Our site was eventually found by the FCC, and they left their usual "cease and desist" paperwork under the rig, which they left switched on! I moved the gear about half a mile uphill and it was never found again - except by some curious hikers who left a note requesting a couple of obscure records!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: J-pole antenna advice

Post by Maximus » Fri Oct 14, 2016 3:16 pm

It's good that the FCC left you alone Albert. I've been thinking about doing something similar with a solar panel and some leisure batteries. But I'm unsure how to link from 15 miles away, seeing as there isn't any internet apart from 3G/4g?


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Re: J-pole antenna advice

Post by pirateaddict » Fri Oct 14, 2016 5:09 pm

That is a big area Albert.. I didn't receive it as always been a frequency skipper since around 1989 but lived in a fairly low dip in Sheffield back then (flipping hills, 7 of). Do you recall what format the music was? That's me last question Albert for now anyhow.. Thankyou Albert for your replies as i find it all very interesting. :tup

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Re: J-pole antenna advice

Post by Sietedj » Tue Dec 06, 2016 5:09 pm

Hello, I am building this antenna and I have read that it is better to weld the cables to the tubes of the J Pole. If I leave the cables as in the photo could withstand powers of up to 100W? There is only one test cable with a round terminal that would go with a nut to the clamp.
A greeting!!
Image
I made this little plastic box to protect the cables.
Image

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