More on the J-pole Antenna (
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- Опубликовано: 23 июл 2024
- A close look at how the J-pole antenna works with special attention to how it's fed. Lots of nice diagrams. Discussion of using quarter-wave transmission line stubs. The end-fed dipole as a special case of the J-pole.
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By far the best description of a J Pole I have seen. I am also impressed by the number of comments you have answered! If only I had a Maths Teacher like you back in the 50s! M0iiZ.
I 100% agree !!!
I did all this at tech-college decades ago. I wrote it all down and drew it all, but it didn't click.
With your explanation the light-bulb has just clicked 'on', and now it all makes sense. Thank-you! :-))
After making my J Pole, I took plastic carpenter clamps to the shield and center wire to the arms of the J Pole to find the sweet spot in the middle of the 2 Meter band. Then sweated the feeds to the copper. 144 -148, the SWR is pretty flat across the band and 1:1 at 146mHz.
Brilliant!! Great graphics and well presented. Looks like Ill be building a J pole for the RV ladder. Thanks!!
Got my first J pole after a hiatus. Of fifteen years. Placed on a 20' pipe. And tested on one watt. I went the full 4 watts. That baofeng 5r could give. And thrugh the summer growth (trees and such) I made the run from pine wood to white bluff TN. In fb fashion. Wating for fall. bet I could get "DX" so to speak. Tks de kv4li
Dave my thanks to you for some interesting videos especially about the j pole as I use a 2mtr j pole and a 70cms slim jim version. The videos are well presented and you make things understandable Regards 73 and Happy New year to you johnbob
Conductive masts CAN radiate. Watch out for resonant half wave mast lengths that CAN be close to the design frequency just by chance. Best to use an insulative joiner just below the "T" fitting.
I was extremely puzzled as to why the coax is connected this way but your explanation makes perfect sense. I will be building mine soon. Thanks alot!!!
Thank you for clarifying how the feed point is connected to the antenna. I spent hours a few weekends ago trying to remember which side I removed the hardware and co-ax from. I guess I got it right regardless of myself.
Doesn't matter. Either way works the same.
Excellent explanation, great.
Nicely eloberated.i never seen video on antenna like this.
Nice explanation of theory and practice.
Thank you Dave. Your knowledge is appreciated. Thank you for sharing it!
Going to try making one. Hope all is well in Ridgway.. I used to eat at the True Grit all the time when my daughter lived there.
Thanks Dave, an excellent description. Very helpful indeed. 73, Mike, M0MTJ
Excellent explaination
I thought I understood J poles you cleared the fog , thanks Dave. David Byrd KN4BHS
Dave, do you have a video covering cable lengths for home brewed antennas? I plan on building this j-pole for my brand new shack this weekend and want to know how long the cable needs to be to achieve maximum efficiency. Thank you!
I'm about to go out right now and add my sleeve choke to my recently built 220 j-pole and put the coax right at the 50 ohm point then add a cap with a brass bolt on the half wave section to see if I can achieve a perfect match.
Great video!
Very nice video on the J Pole
learnelectronics I am loving your channel and the videos! Keep on the good work, will there be more radio stuff?
Lots! the goal is a new video every week
Well explained.
Thank you sir
You are the real Master
appreciate the clear explanation, however I believe the use of RG-8X for a choke balun would be a mistake - after all the big drawback of 8X is the need to avoid sharp bends because the foam dielectric will lose it's shape and not hold the center conductor in the center, under those circumstances. And, I have begun using N connectors everywhere I can, as they are easier to install, are weatherproof, and have the same impedance as the cable. Even when I must use a UHF connector I'll install the N and use an adapter.
Dave, would dielectric grease a connection points on the Jpole, or other antenna connections (base,HT antenna threads) help prevent corrosion? Would it inhibit performance in any way? I’m just thinking here, I’m curious!
Thank you
Thanks!
Terima kasih atas pembelajaran.
Semoga menjadi kebaikan untuk semua.
Salam.
thank you for this video, I have so many question thanks to it! First: I just got a j pole for my GMRS HT yesterday, at about 8:40 into the video you mentioned putting a choke balance in the line. What is that for and does diameter matter of the 8 or 9 twists?
Choke balum. The diameter is usually 9 to 12 inches.
If I have a J-pole tuned to a VHF frequency, do I still do the coil of coax cable under the antenna?
tks Dave nice video... did you comment db gain of the jpole??
Gain numbers available on the well referenced Wikipedia article.
en.wikipedia.org/wiki/J-pole_antenna
how can simule this antenna j pole whith eznec , can you help me , for give the parametr.
I built a j pole once for 6 and 2 meters, but never got it up as i moved house, think it is time to get it out again and add 220 and 440 mhz to it, look good all polished up on the roof.
Awesome thanks
About the Choke Balun, how many wraps and how big around should it be? And, how far down from the feed point?
There are only rules of thumb, usually six to eight turns. Put it close to the antenna feed point.
I have an old basketball pole (the kind the kids use in town portable) can I hook a J-pole to that it's about 10 feet tall
Great video. One question. Can a Ferrite Choke (with correct mix for frequency) be used instead of turns of coax? Especially if using thick lmr 400 for 70cm? Please and thank you for your time and advice.
Yes, but not usually just one ferrite choke. Maybe 15 or 20, all pushed up as close to the antenna as you can get them.
@@davecasler Thanks! I've been enjoying J-pole since we last corresponded. I have another question: Is there a way to calculate a j-pole with K factor (element Diameter)? Please and Thank you!
Forgot to mention I will be using a vector analyzer to see where the 50 ohm impedence is.
hi, can i use a j-pole antenna for a marine vhf?
May some please help or advise....THANK YOU SO MUCH.
I just put up a 20 foot tower with 3 legs and one mono pole about 20 feet up.
On the 20 foot tower i have a Imax2000 antenna to talk on 27 MHZ which that alone is another 23 feet up in the air...so it's up 43 feet from the ground a bit higher then the peak of the roof.
On the 20 foot mono pole i have just a discone antenna for my scanner.
On the 20 foot tower i grounded all legs together with 3 gauge ground wire, and i also spliced the coax on top of the tower and took the shield and grounded that. Is that wrong?
Guys in the ham radio shop said nah don't do it, no reason to shave the coax and ground the shield to the tower, other 50 % of the stuff i read sais to do that and at the bottom of the antenna tower.
any advice on that? I also have 3 ground rods at 10 feet depth. One rod at the tower where all 3 legs are grounded then go to the first ground rod, then 20 feet away another ground rod and another 20 feet away at the electrical panel another 10 foot ground rod.
Can you ground the Imax antenna even though it's a 5/8 antenna? That was the argument with the ham guy in the shop, he said if i ground it, i introduced a new wavelength in the equation...Then i returned and said but if the mast of my Imax is touching the tower, that is also grounded together, so what is the difference if the coax shield is grounded on top of the tower?
I seen professional installations on line where they ground the coax on top of the tower and at the bottom...Any advice?
I just installed the mono pole and antenna on top...i will attach a ground wire on the mono pole and bring the ground wire to the tower which is only like 7 feet away to connect all grounds together.
More or less when i listen to HF i hear a buzzing sound above 10 mhz, not sure if it's conditions or because i grounded the shield of the coax on top of the tower. A few times i hear no buzzing, so i assumed it must be conditions.
Any ideas if it's ok to ground an Imax2000 antenna even though it's a 5/8 antenna?
Thanks to all, please help..
Theo
I wouldn't recommend shaving the coax covering because then you have to worry about keeping it waterproof. Better to put in a connection like a barrel connector, and grounding that to the bottom of the tower, or better yet, put a lightning surge protector there.
Theo
Don‘t mix up HF ground and DC ground. Your antenna is not a type that should be DC grounded, even if the metal mount at it‘s base looks like it, it mustn‘t be grounded. If you ground the mounting point or ground the coax shield lead, you will produce an additional radiator, that acts barely calculateble.
Hey Dave how about doing a 220 mhz j-pole
Thanks a Stack Dave, Greetings from Christchurch, New Zealand.... I just could NOT Understand a J-Pole and how it works .... Thanks to this Very Old vidclip.... AND a 6 Year Old one on the J-Pole, ...... I Now understand .... Many Thanks, Very Well explained .... Please tell me, Is there ANY Advantage of a 'Slim-Jim' over a J-Pole .... ????? Also what is the Gain for Either/Both .... Best to You and Yours .... Cheers from ChCh, NZ
How would this be for a variation: Take a full-wavelength horizontal wire, cut it in the middle, feed that with a quarter-wave open wire/ladder line, short it at the bottom.
Could you then feed it the same way as a J-pole, at the 50R point up the stub, and effectively get a 2-element dipole with ~3dB gain?
Yes, it's a double zepp or collinear. See ruclips.net/video/nyTn4_2xwTk/видео.html
What can we say about this antenna that it is a 1/4 wave or 1/2 wave?
Great video Sir. Question: I have seen plans for a dual band 144/440mhz J-pole in which there is an insulating joint between the two bandwidths joining the two there is a solid heavy gage copper wire bent in a strange manner, if i would describe it, it's bent around the j-pole in something similar fashion that looks like the "WHEEL-O" wire and wheel magnetic toy gizmo from the 70's, hard to describe but it's wrapped around in almost a 360° Deg radius and then bent back to its original starting point, its some sort of inductance load, I hope something here made some sense???. I'am new to ham radio, so please bare with me. How does the dual band J-pole work???.
Kind regards, Eric Dee.
The bent wire is acting as a quarter-wave stub. 70cm frequencies won't go past this stub, which presents a high impedance at those frequencies. The 2-meter bands happily incorporate this stub into the antenna.
@@davecasler: Thank you for your time in response, I understand now, thank you very much, you have a very interesting channel, keep up the great work sir, We viewers appreciate it.
Kind regards, Eric Dee.
What cable should I use as a feed line for a 5 w handheld radio ?
For a run of more than a couple feet, I'd use RG-8X. If it's really long, like over 50 to 100 feet, use a better coax like RG-213 or LMR400.
Can the long side be longer 3/4. Or full wavelength. Just thinking that maybe if it was a wavelength it may do better. I am new and not sure about what I think I know. Just that I have been told that the longer the radiation part is the better it antenna is. But I know that there is a limit to this. Just thinking that if it was a wavelength there would be more getting out or total of 1.25 wavelength to make 1 wavelength to radiate the signal.
The basic antenna is the half-wavelength dipole. A full wavelength is hard to feed properly.
help me for 88-108mhz j-pole antenna making n also how to make balun of half dipole antenna...
The J-pole is too narrow-banded to cover the entire FM broadcast band. You need a more log-periodic-like antenna, like TV antennas. Some TV antennas are sold specifically covering the FM broadcast band.
Could you make a jpole at 5/8ths.
73 from PU4BIO, Brazil
It is important to understand there is no mechanism to prevent currents from flowing down the mast. Therefore the mast cannot be simply ignored as so many do including this video at about 1:42 where you suggest it can be most any length. The Wikipedia article summarizes the J antenna quite well with ample primary references to help understand why the mast conducts and how to mitigate undesirable mast currents.
en.wikipedia.org/wiki/J-pole_antenna
There is a school of thought that special arrangements should be made to keep the mast from radiating. But most of us just use that piece as something to connect to the mast, which is often itself metal, and good amateur practice says to ground the mast. MFJ makes a couple end fed Zepps that use the J-pole technique for matching and they work well. With them, there is no extra metal at the feed end.
There is also the school of myths including the notion the bottom of the J antenna somehow has immunity to flowing currents down the mast while upward currents are unrestrained. The reasoning given is it's a point of zero voltage and can therefore not force a current. What many miss is this is really the point of not zero, but minimum voltage and, critically, "maximum current" resulting in power flow necessary to resonate the entire assembly. If the top of the mast presents anything other than a very high impedance, it has just as much right to some of that current as any of the other portions of the antenna.
The simulation nerds have sounded the alarm for some time concerning grounded J antennas...
- www.hamradio.me/antennas/j-pole-antenna-should-i-ground-it.html
I confirmed their simulation results and later confirmed the issues with real measurements...
- www.hamradio.me/antennas/j-pole-antenna-grounding-have-your-j-pole-and-ground-it-too.html
As the title of the latter article attests, you can ground your J antenna without potentially spoiling its performance with just one more stub... or radials if you like, but the mast decoupling stub avoids induction to the mast below and just looks cooler.
The Zepps are certainly good examples where the isolated J concept works just fine by virtue of its disconnect from other conductors. Those spiffy rollup ladderline J antennas also benefit from this and work surprisingly well despite some of the manufacturers not totally understanding how their antennas work.
- www.hamradio.me/interests/slimjim
I've spent a considerable amount of time on the topic of the J antenna, but it has been worth learning the real deal and denouncing the various myths...
- www.hamradio.me/interests/j-pole
A hopelessly boring discussion is available in my patent application...
- patents.google.com/patent/US20170201002A1/
can you paint these things to help them stand up against the weather?
I spray painted mine flat black so the nosy neighbors wouldn't notice ;-) Antenna still works well, no change in SWR
Do you personally consider the J-Poile Antenna for the 2 meter as a directional Antennae?
No. Its horizontal pattern is equal in all directions.
A 6m copper J pole is to spindly using only 1/2 in copper pipe, better to use 3/4 in and a reducer to an adjustable 1/2 in stinger, it covers about 1mhz
That's exactly how I built mine. It's cumbersome... about 17ft. long. I'm able to hit a 6 meter repeater about 90 miles away with between 5 and 10 watts without even fine tuning. Mine steps down from 3/4 to 1/2" copper in the upper portion. It looks very streamlined for that reason, with lower center of gravity and a reduced windload.
I want to try this jpole on a mobile radio and a HT radio for fun.
Dave, Is there any mods to the 2m J pole to also work 70cm? Thanks Mike KM6KWG.
I have seen multiband Jpoles, but the antenna is really best used to operate on one band.
J-pole . kalau di buat di indonesia apa bisa? david.c
Thanks Dave! Is there a way using an ohm meter to measure the 50 ohms needed to find the connection point?
Unfortunately no. The 50 ohms is at RF. A regular ohmmeter will not measure that. So the point has to be calculated, or found by trial and error.
One can use an antenna analyzer such as the N1201SA to find the 50 ohm point and see a graph of the SWR.
you uploaded the video 10 minutes after i expained dad what a j pole is (i forgot to comment then)
I have no idea what he's saying
Clear as mud. sorry
Thank you, again. N0QFT