What do I need to make a properly tuned j pole antenna ? I’ve used dipoles cut to length (nearly) using the calculation on jugglingpirate but they are utter shyte.

I used a jpole that had a balun and was tuned using some kind of meter. The performance compared was ridiculous. 30w easily coverd an area that the dipole would need 80-100w to achieve the same.

Goes to show you don’t need massive amounts of power for a nice solid signal.

The honest answer is unfortunately a network analyser, or to know someone that has one.

Rig expert antenna analyser.
rigexpert.com


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You need a big wallet.

I dont get it about dipole. Why such a bad experiances with it? I assume you did not had some matching, like pawsey stub? Albert wrote few days ago on another topic that you loose about 30 percent of power and cant achieve really good SWR. It also makes a radiation pattern worse.
That is what makes a difference between good and poor dipole.

So am I right in thinking that even if I use a rig expert tuner to get a dipole dead on the frequency, it still won’t perform that great without correct matching ?

Yes correct. You need to get the matching right, but the rig expert will help you do that correctly.

I’ve had to build my aerials through trial and error until everything was right. I don’t build enough aerials to warrant buying an analyser.


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OldskoolPirate wrote: ↑Thu Nov 02, 2017 8:39 pm What do I need to make a properly tuned j pole antenna ? I’ve used dipoles cut to length (nearly) using the calculation on jugglingpirate but they are utter shyte.

I used a jpole that had a balun and was tuned using some kind of meter. The performance compared was ridiculous. 30w easily coverd an area that the dipole would need 80-100w to achieve the same.

Goes to show you don’t need massive amounts of power for a nice solid signal.

A J-pole doesn't need a balun - the bottom "U" section is actually a balun in itself (effectively). The centre of the coax goes to the long side, and the outer goes to the shorter arm. The distance up from the bottom of the "U" can be calculated, but it's better to get it roughly right (I connect using alligator clips for alignment purposes) and then tweak the feed-points up and down for minimum SWR. When you find the magic positions, you make the permanent connections.

A J-pole puts your power where you want it. Stephen Moss got a similar result using a ½-wave vertical, with a capacitive and inductive matching circuit. The J-pole is a ½-wave vertical on top of a ¼-wave matching section. I used to build my J-poles out of microbore plumbing pipe, and painted them matt grey with acrylic paint to stop them corroding (and to make them less visible from the ground). The radiation characteristic of the J-pole is omni-directional and flat - not much is wasted upwards. The "Super-J" uses the same principle, but adds an extra ½-wave radiator above, fed by a matching stub. The two radiating sections are in phase and reinforce each other. Best of all - neither the J-pole or the "Super-J" need ground radials and are really easy to match. They're also DC-short circuit, so they offer a measure of anti-static protection.

So what’s better? A jpole or dipole ? Does a dipole waste a lot of power up in the sky ?

Albert H wrote: ↑Sat Nov 04, 2017 4:55 am
A J-pole doesn't need a balun - the bottom "U" section is actually a balun in itself (effectively). The centre of the coax goes to the long side, and the outer goes to the shorter arm. The distance up from the bottom of the "U" can be calculated, but it's better to get it roughly right (I connect using alligator clips for alignment purposes) and then tweak the feed-points up and down for minimum SWR. When you find the magic positions, you make the permanent connections.

I disagree Albert. If you have tuned several J poles then you will know how shit the tuning is. Put a j pole on a network or antenna analyser and tune to sweet spot, keep the tapping point the same place and then move the coax, lift it of the ground,move it a metre away etc and it throws the readings all over the place, not forgetting how tricky it is to try and tune.

Now add a balan and then move the coax, its much much much more stable and the reading hardly changes.

Ive build building Jpoles for the last 12 Months and have mastered them. I use a transformer to 200ohm so my tapping point is much higher, i then earth the bottom of the matching stub ' the U section' and they perform awesome!

"" A J-pole doesn't need a balun - the bottom "U" section is actually a balun in itself (effectively) "" ???
No its not?
The U section is just a quarter wave section, BUT a balanced quarter wave section!! Your coax is unbalanced! Therefore add a balun

A J Pole has a better radiation pattern to a dipole, more of a doughnut shape. A super J is great performer to but you must match it correctly. Remember as your metal gets thinner further up the velocity factor changes and therefore the top halwave section may be a different length to the bottom halfwave. Its virtually impossible without an analyser

Banus - the "U" section is a balun - it works as a transmission line transformer.

Just for grins, I put a 98 MHz J-pole up in my garden earlier on. It was up a four meter pole, with the nearest building 26m away (I have a big garden). I fired 1 Watt into it at 98 MHz and set the feed point using croc clips.. No matter where I moved the coax, the SWR remained stubbornly at 1.04 : 1. Adding a short bit to the radiator (by screwing in a piece of studding) brought the match to exactly 1 : 1. I can't understand why you have problems with connecting your coax straight to the J-pole unless you have such a bad mismatch that you have RF on the outer of the feeder!

Still - if you're getting great, repeatable results using your balun, stick with it.

Albert H wrote: ↑Sun Nov 05, 2017 12:00 am Banus - the "U" section is a balun - it works as a transmission line transformer.

Just for grins, I put a 98 MHz J-pole up in my garden earlier on. It was up a four meter pole, with the nearest building 26m away (I have a big garden). I fired 1 Watt into it at 98 MHz and set the feed point using croc clips.. No matter where I moved the coax, the SWR remained stubbornly at 1.04 : 1. Adding a short bit to the radiator (by screwing in a piece of studding) brought the match to exactly 1 : 1. I can't understand why you have problems with connecting your coax straight to the J-pole unless you have such a bad mismatch that you have RF on the outer of the feeder!

Still - if you're getting great, repeatable results using your balun, stick with it.

I dont have problems conecting coax directly to it, I can good a good swr easily but if you fire a j pole up on the analyser you will see all sorts changing as you move the coax, The impedance moves, X value changes loads and so on, thats the reason why the aerial companies dont sell them. you dont notice it to much on a vswr meter. I had exactly this conversation with sandpiper a couple of months ago.

Please can you explain how the U section is a balun or a transmission line transformer in itself. Its a transformer on a jpole aerial as you tap on on the 50ohm point on the 'balanced' transmissoin line then at one end of one of the lines you dump a halfwave on top at high impedance probably somewhere in the range of 3K - 5K therabouts upwards, thats why its a transformer, but its not a balun! It doesnt convert unbalanced to balaned hence why everybody has to put turns of coax on at the tapping point to try and prevent the mismatch returning.

Trust me on this, try a balun on your jpole and you will be amazed at how much easier it is to tune.

On a few watts I probaly wouldnt bother with a balun but when im running 800W + the balun makes a huge difference.

A J-Pole antenna is not an unbalanced antenna, even though it is fed at the end visually.
A good section of the coaxial cable will radiate unwanted energy potentially screwing up your radiation pattern if you don't use a Balun.
The bottom U section doubles as a phase match and a impedance match allowing the signal to be pumped up the dipole 1/2 wave section with phase shifted so that both 1/4 sections of the 1/2 wave pole don't cancel each other out and thus act just like the center fed dipole would

OK - what balun design do you use? The 4 : 1 coax balun looks like a good candidate.

Im seeing things... Ive just had to take a chair..... and sit down..... Albert has agreed he he

Yes I use 4:1 but it means the coax ground has no connection to the antenna so I earth the antenna and earth the feeder. The beauty of the 4:1 is you can tap higher up on the jpole. And the other great thing is you can stick a 200ohm resistor across your setup and check your balun is spot on! Ive found RG213 4:1 Balun has about 10mhz bandwidth with 1:5 on the outer frequencies and a perfect curve on the analyser.
Ill get some pics of my next one and upload here

Ha Ha Banus!

I've been messing about with a couple of balun configurations this evening, and nearly got frostbite in the process - it's -9C outside at the moment here!

I tried a ferrite balun (for low power) and found that I got an acceptable match for about 2½ MHz either side of the nominal frequency - not as broadly tuned as yours, but that's probably because I'm using thinner elements (microbore plumbing pipe in my case). The 4 : 1 coaxial balun worked really well, and I don't think that I'd have much issue with using it with a few hundred Watts. I'm going to arrange a test in the next few days. Again, I got a reasonable match for a couple of MHz either side of nominal. You're right - the feedpoint is much higher up the aerial - and an issue I had was coupling into the outer of the coax from the aerial elements....

LB suggested a double-J with one element going upwards, and a co-phased second segment going downwards. This would make a very big colinear, and would probably outperform the Super-J It would be centre-fed, and a little over 3m from end to end. If the phasing's right, it should be capable of 6dBd of almost omnidirectional gain, with a flattened bit where the supporting mast is. He's promised to prototype one in the next few days. I won't have time since I have a conference to attend....

Cool. I am following this story, so looking forward to see other results from you guys. Good job.

Albert H wrote: ↑Tue Nov 07, 2017 12:32 am
I tried a ferrite balun (for low power) and found that I got an acceptable match for about 2½ MHz either side of the nominal frequency - not as broadly tuned as yours.

Arrrr sorry that was just the balun that had a wide bandwidth, only about 2 1/2 mhz when on the jpole

Funny enough me and another user from here did exactly as your brother suggested, it works well. The 1/4 wave section is horizontal then 2 vertical halfwaves pointing up and down. Its on Harry Lythalls Website. We used 1" aluminium for the 1/4 matching section then stainless stainless steel rods for the radiators, it was surprisingly descreet. if you use 1" 18swg ally ive found the VF is around 0.97 for when doing your maths

Heres the link to Harrys website for that antenna http://213.114.131.21/antennas/6dbvhf0.htm

Ill find some photos

I would use a rig expert aa170 mind you it shows it on kHz rather mhz .but there good and accurate .

you can built an end fed dipole easyly and avoid having something stick out the boom, i've done it with 75 ohm co-ax, you just cut to 2 elements, tape the cable to the bottom element and then coil it at the bottom, 2 turns about 10 cm diameter worked.