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[Articles Home]  [Add Article]  

The 20-Meter J-Pole

(N2KMF) on August 14, 2006
View comments about this article!

The 20-Meter J-Pole

Recently, I decided that I was getting bored with my sloping longwire antenna. It worked fine, to be sure, but there were some serious holes in the coverage outside of the major lobes. I decided that I wanted to put up a vertical antenna, but the thought of burying a ton of radials in my backyard left me cold. A vertical dipole seemed to be called for, but I didn't have enough supports to run the coax perpendicular to the dipole for any appreciable length.

After some thought, I hit upon the idea of making a 20 Meter J-Pole. I've been a fan of J-Poles for years, having built them out of copper pipe for use on 2 Meters. Obviously, I wasn't going to be able to use copper pipe on 20. I settled on a design that used insulated 12 gauge stranded wire. After modeling it several different ways on EZNEC, I was prepared to mount the antenna with the matching section runs parallel to the ground a few feet up, whit the radiating 1/2 wave section vertical. That allows for a lower support, and only lowers the gain slightly. It also 'de-circularizes' the pattern, but only by about 1 dB.

I modeled the antenna with a 1-foot gap for the matching section. When I went to build it, I found that the scrap Lexan I planned on using as spacers wasn't long enough, so I re-modeled it with a 6-inch gap. The change had little effect according to EZNEC. I made the spacers by cutting the lexan into 7 inch by 3 1/2 inch rectangles, then drilling four holes, one near each of the corners. I made 5 spacers this way. I had planned on making 6, but one of them broke a corner while I was drilling it.

I took my measuring tape and staked a 16.5-foot distance on the ground. I wrapped the wire around it four times, then cut it about a foot past the stake to allow for the 6 inch gap and to allow for some extra at the top of the antenna for the insulator. I then staked the antenna down in the final shape. I cut and stripped the wire at the bottom, then started sliding the spacers up the matching section. The wire is stiff enough that they don't move once locked in place (the wire is threaded up through one hole, then down through the other on both sides of the spacer). If I ever need to use open wire line for a different antenna project, this is how I am going to make it. I put a small knot in the wire at the top of the matching section to keep it from sliding out of the spacer.

Feeding the antenna is dirt simple: An SO-236 chassis connector soldered on the bottom, with the center conductor going to the longer wire and the ground going to the matching section. I was concerned that I might need to have some kind of elevated feed, like I have done in the past with copper J-Poles on 2 Meters, but modeling suggested that it wasn't necessary. That was borne out in practice. I now think that the elevated feed seen in "Plumber's Delight" J-Poles is just a more convenient way to feed them, given their construction.

I first tried to get the antenna up into the tree I was going to use for support using a wrench tied to a rope, swinging it around my head. After several tries, all of them either failing to get enough height or missing the tree entirely, I gave up and gave my friend Jack KE7NL a call. He brought his slingshot/fishing reel antenna raisin' contraption, and a good deal more experience, with him. After a few tries, we were able to get the antenna high enough so that all but the very top of the antenna is vertical. Since the length of the antenna is about 50 feet, that is no small accomplishment. The bottom of the antenna is about 1 foot off the ground. I am indebted to Jack for his assistance and advice.

The antenna seems to work quite well on 20 Meters. The VSWR is between 1.5 to 2 for the entire 20-Meter band, and I have received good reports in the few short days it has been up. Contacts have been made from my Upstate NY QTH to FL, SC, WI, and OH on 20 Meters. Using a tuner, it will load up on 80, 40, 15, and 10 Meters. I have made a couple of contacts on 40 Meters with it, however I could tell that the antenna wasn't performing well. Since the 1/2 Wave section on 20 meters is 1/4 wavelength on 40 Meters, I am thinking about burying some 40 Meter radials underneath the antenna and possibly using a clip lead to connect them to the matching section of the antenna while I am on 40 Meters to see if that will help. On 15 and 10 Meters, modeling suggests that most of the radiation will go up, instead of towards the horizon. In any case, this is a much better 20 Meter vertical than you can buy, and since I already had all of the materials at home, it cost me all of zero dollars to put up.

73 de Bill N2KMF

Member Comments:
This article has expired. No more comments may be added.
 
The 20-Meter J-Pole  
by LNXAUTHOR on August 14, 2006 Mail this to a friend!
- sounds like a good idea for space-challenged folks... it would be nice if you could have some pics of your work available for folks to look at...

- thanks for sharing this!
 
The 20-Meter J-Pole  
by N2KMF on August 14, 2006 Mail this to a friend!
UPDATE: I did actually put down 4 shortened radials, each about 17 feet long, with a clip lead so that I can connect/disconnect it at will.

With the clip lead attached to the bottom of the matching section, the antenna matches better on 40 and 80 meters. I hear noticeable more band noise, and more signals, on both bands. Unfortunately, I really haven't had the time to try it out yet. When I do, I will report back. EZNEC suggests that it should perform about as well as a 1/4 wave vertical on 40.

Also, I will take a picture when I get the chance. There really isn't much to look at, however.
 
RE: The 20-Meter J-Pole  
by N2KMF on August 14, 2006 Mail this to a friend!
Forgot to add, the radials don't seem to affect the antenna one way or the other on 20 Meters.
 
The 20-Meter J-Pole  
by KA8VIT on August 14, 2006 Mail this to a friend!
>I was concerned that I might need to have some kind of
>elevated feed, like I have done in the past with copper
>J-Poles on 2 Meters, but modeling suggested that it
>wasn't necessary. That was borne out in practice. I now
>think that the elevated feed seen in "Plumber's Delight"
>J-Poles is just a more convenient way to feed them, >given their construction.

No, it is not a more convenient way to feed them.

The shorted stub used to feed J-Poles is a great way to
match two unknown impedences.

In the case of the J-pole, you have a value of about
2000 to 3000-ohms at the feed point. At the bottom of
the shorted stub, you have zero. Therefore, the 50-ohm
point is just up from the bottom of the shorted part of
the stub a little way.

Also, the J-pole or more specifically, the shorted
stub, is a balanced feed and should be feed with ladder
line or a balun (rf choke) should be used at the feed
point.

The J-Pole is nothing more than a half-wave vertical
antenna, end-fed with a shorted stub.

Tuning requires the adjusting of three things, the
radiator length, the stub length and the positon of the
feed point on the stub.

And I agree, the J-Pole is a great antenna!

How about some pictures!

73

Bill KA8VIT
ka8vit@ka8vit.com
http://ka8vit.com
 
The 20-Meter J-Pole  
by KA8VIT on August 14, 2006 Mail this to a friend!
>I was concerned that I might need to have some kind of
>elevated feed, like I have done in the past with copper
>J-Poles on 2 Meters, but modeling suggested that it
>wasn't necessary. That was borne out in practice. I now
>think that the elevated feed seen in "Plumber's Delight"
>J-Poles is just a more convenient way to feed them, >given their construction.

No, it is not a more convenient way to feed them.

The shorted stub used to feed J-Poles is a great way to
match two unknown impedences.

In the case of the J-pole, you have a value of about
2000 to 3000-ohms at the feed point. At the bottom of
the shorted stub, you have zero. Therefore, the 50-ohm
point is just up from the bottom of the shorted part of
the stub a little way.

Also, the J-pole or more specifically, the shorted
stub, is a balanced feed and should be feed with ladder
line or a balun (rf choke) should be used at the feed
point.

The J-Pole is nothing more than a half-wave vertical
antenna, end-fed with a shorted stub.

Tuning requires the adjusting of three things, the
radiator length, the stub length and the positon of the
feed point on the stub.

And I agree, the J-Pole is a great antenna!

How about some pictures!

73

Bill KA8VIT
ka8vit@ka8vit.com
http://ka8vit.com
 
RE: The 20-Meter J-Pole  
by WA2JJH on August 14, 2006 Mail this to a friend!
Looks good to me. I might try it. A PIC. would be good.
 
RE: The 20-Meter J-Pole  
by K0BG on August 14, 2006 Mail this to a friend!
For all of those who think a J pole is a great antenna, you need to read what's here: http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm

Alan, KBG
www.k0bg.com
 
RE: The 20-Meter J-Pole  
by K8MHZ on August 14, 2006 Mail this to a friend!
A wise move not using copper pipe.

A 10 foot stick of 1/2 inch blue is over 30 bucks now. 3/4 is over 44 bucks. For ONE piece.
 
RE: The 20-Meter J-Pole  
by N2KMF on August 14, 2006 Mail this to a friend!
Not to mention that 50 foot of copper pipe would be spectacularly unstable.
 
RE: The 20-Meter J-Pole  
by K6AER on August 14, 2006 Mail this to a friend!
For those of you who would like pictures and direction on building a 20 meter J pole you can get the details at: http://www.vcars.org/tech/20_Meter_JPole.htm

I built one of these when I first moved to Colorado and had not put the tower up yet. It performed very well and I was consistently able to break bad pileups. I also had a dipole to compare it with and it was about 3 dB better performance than the dipole at 40 feet.

I made mine out of a 50 foot Rohn push up mast with some additional aluminum at the top. The matching section was a wave 12 gage wire which shorted to the main mast wave form the top of the pole. The only requirement for the mast is it have good solid conductivity.

The antenna was very quiet, required no radials and was all a DC ground which is a must in Colorado.
 
RE: The 20-Meter J-Pole  
by N2KMF on August 14, 2006 Mail this to a friend!
KA8VIT:
"Also, the J-pole or more specifically, the shorted
stub, is a balanced feed and should be feed with ladder
line or a balun (rf choke) should be used at the feed
point."

I had actually intended to put an RF choke (composed of several turns of RG-8), but unfortunately due to the distance from the operating position in my house to the tree where the J-Pole is hung, I didn't have enough coax left. So, I tried it as it was, and found it worked well enough without it. When I get the chance, I will probably put one on. I've done that with all the copper pipe 2 Meter J-Poles I have built in the past.

I don't think I am getting much, if any, feedline radiation. I'm not getting any RF in the shack that I can detect, and the feedline snakes past several wires and copper pipes, and I haven't noticed any strange occurances. The feedline is buried from where it exits the house to just under the antenna.

By the way, the bottom of the matching section is only about a foot or perhaps a 1.5 feet high, so the pattern has little to no chance of being distorted.

In any case, while feedline radiation can be a problem if it is causing mischief in the house or at the operating position, for most casual operating it isn't a problem, and for horizontal antennas like the G5RV can help fill in 'holes' in the pattern. If I were building a precise instrument I would be worried about it, but I built an antenna for casual operating.

My options for trying a vertical on 20 Meters (given the materials at hand) were basically a 20 Meter ground mounted vertical, or the J-Pole. An elevated ground plane was out of the question, for while I have the height, the tree is near the edge of my property. The ropes guying some of the radials would have to be staked down in my neighbors yard. We're on friendly terms, but not *THAT* friendly. While I did end up putting down 4 radials, that would have been unacceptable for a ground mounted antenna. Their main purpose is to provide me with at least some ability to use the antenna on 40 and 80 meters, with the known caveat that this is a 20 meter antenna, and can't be expected to perform as well as a purpose built antenna for those bands.

In sum, while it might not be the best theoretical antenna, it was easy to make and somewhat easy to put up, and it does seem to work.

I will put some pictures up when I get the chance.

I will also, again when I get the chance, try it on 20 Meters with the radials connected, and then disconnected, to see if it makes a difference.
 
RE: The 20-Meter J-Pole  
by N2KMF on August 14, 2006 Mail this to a friend!
Oh, by the way KA8VIT, thanks for keeping the USS Cod on the air. Look forward to working you every year during SOTA and Museum Ships.
 
RE: The 20-Meter J-Pole  
by KA8VIT on August 14, 2006 Mail this to a friend!
>Oh, by the way KA8VIT, thanks for keeping the USS Cod on
>the air. Look forward to working you every year during
>SOTA and Museum Ships.

Hey... thanks Bill N2KMF !

The USS COD Amateur Radio Club tries to keep the boat
on the air as much as possible, not just for special
events.

The COD is on the air most weekends, both CW and SSB,
mostly 20 and 40-meters.

If you'd like to see what the radio room or what the
inside of the boat looks like, check out these video
clips, (you'll need broadband)!

http://ka8vit.com/codvids

http://ka8vit.com/subqso

We really enjoy operating from the sub.

It also gave me more respect for the DX operators.

Being on the business end of a pile up is very
intimidating at first whether phone or CW and takes
a while to learn "control" it.

Glad we had a chance to work you.

73

Bill KA8VIT
ka8vit@ka8vit.com
http://ka8vit.com
 
RE: The 20-Meter J-Pole  
by KC8VWM on August 14, 2006 Mail this to a friend!
RE: The 20-Meter J-Pole
Reply by K6AER on August 14, 2006

For those of you who would like pictures and direction on building a 20 meter J pole you can get the details at:

http://www.vcars.org/tech/20_Meter_JPole.htm

------------------

Nice pics! Looks like a fun project.

 
The 20-Meter J-Pole  
by N1LO on August 14, 2006 Mail this to a friend!
I enjoyed your article. HF 'J Pole' antennas have worked very well for me.
I built mine a little differently, all vertical wire, that I pull up into trees or on a side arm on my tower. I have plans for 10m and 20m versions at:
http://www.qsl.net/n1lo/antenna.htm

Happy Building,

--...MARK_N1LO...--
 
RE: The 20-Meter J-Pole  
by N2KMF on August 14, 2006 Mail this to a friend!
Here is the feed of the antenna:

http://img154.imageshack.us/img154/7745/im002462bt5.jpg

Here is a close-up of one of the spacers:

http://img138.imageshack.us/img138/829/im002463gg8.jpg

And here is a long view of the antenna:

http://img85.imageshack.us/img85/4431/im002465vi6.jpg

Note, you are only seeing about half of it in the long view, as the foliage of the tree obscures the top of the antenna and quite frankly, only the matching section is interesting.
 
RE: The 20-Meter J-Pole  
by K9FV on August 14, 2006 Mail this to a friend!
"The feedline is buried from where it exits the house to just under the antenna. "

Does this buried feedline not help get rid of the common mode currents on the outside of the coax?

Ken
 
RE: The 20-Meter J-Pole  
by KC8VWM on August 14, 2006 Mail this to a friend!
N1LO,

I visited your website (excellent by the way) and I have a question about:

http://www.qsl.net/n1lo/20mjpole.pdf

The radiating element seems to be exactly 33'. Have you ever considered modifying this design into a 10/20/40 meter band vertical antenna?

It would seem that merely changing the feedpoint might make it work out that way. I say this because it's possible to make this antenna into a very "light package" for QRP portable work without the need of carrying around a seperate antenna tuner.

I also did some calculations and found it comes very close to the idea of using the ladder line section as a 10 meter 1/2 wave radiator.

Perhaps a small relay will do the job of switching around the feedpoint for operating different bands.

73 KC8VWM
 
RE: The 20-Meter J-Pole - multiband?  
by N1LO on August 15, 2006 Mail this to a friend!
The 20m version seemed to perform reasonably well on bands above 20 using a tuner (17, 15, 12 and 10), but not below.

Sounds like a fun experiment to try alternate feed configurations. You might want to use simple clip on jumpers instead of relays for your experiments. Give it a try, then come back and tell us how it worked.

--...MARK_N1LO...--
 
The 20-Meter J-Pole  
by K9MI on August 15, 2006 Mail this to a friend!
Hi Bill... It's early so if I missed this part, I'll have to blame it on that, ok?

You mentioned 6 inches of seperation between the antenna and the matching stub and that the matching stub was now horizontal. Were you able to use your 6 inch spacing, and bring the tuning stub back up vertical, like one would do on a copper j pole for vhf?

With it coming so close to the ground, and attaching radials, it sounds like an "almost" 1/4 wave vertical for 75 meters (if the 50 ft section was 60ft).

Anyway you look at it, Bill has experimented with this antenna in building it and using it.

Good job Bill!

73, Mike K9MI

 
RE: The 20-Meter J-Pole  
by N2KMF on August 15, 2006 Mail this to a friend!
Mike K9MI:
"You mentioned 6 inches of seperation between the antenna and the matching stub and that the matching stub was now horizontal. Were you able to use your 6 inch spacing, and bring the tuning stub back up vertical, like one would do on a copper j pole for vhf?"

My original fear was that I wouldn't be able to get enough height to run the entire antenna vertical, so I was prepared to run the matching section horizontal, parallel to the ground. I was able to get the antenna pretty much completely vertical (there is a small section at the top which is at an angle, not enough to significantly effect the radiation pattern). I modelled it with a horizontal matching section in EZNEC, and I found that you don't really lose much. If you don't have the height to string it completely vertical, you can make the matching section horizontal.


"With it coming so close to the ground, and attaching radials, it sounds like an "almost" 1/4 wave vertical for 75 meters (if the 50 ft section was 60ft)."

The way I have it now is the radials, which are only 1/4 wavelength on 20 Meters, attach to the matching section with an alligator clip. That allows me to remove them easily for testing. My main thinking was that 33 feet (the non-matched section of the vertical) is about 1/4 wavelength on 40 Meters, and having them would improve my efficiency on that band. It also seems to have helped on 80 Meters, but I haven't had enough time to truly test it. It does seem to make the tuner happier, though.
 
J-pole tub radiates about 20% of RF energy  
by K6AX on August 15, 2006 Mail this to a friend!
Actually the stub radiates about 20% of the RF, so it is important with a 20mtr J-pole to get the entire length of the antenna + stub as high as possible. I built a home brew 20mtr j-pole and it worked ok, even with the stub running horizonal to the main radiator, but I think it would have worked a lot better with the stub pulled up in a vertical orientation. I only had a 40' tree to work with.
 
RE: J-pole tub radiates about 20% of RF energy  
by W4VR on August 15, 2006 Mail this to a friend!
I keep hearing about these J-poles but have never tried one. I think I'll stay with my DI-pole.
 
RE: J-pole tub radiates about 20% of RF energy  
by NL7W on August 15, 2006 Mail this to a friend!
In a perfect world, the stub wouldn't radiate at all, I believe. Someone just mentioned approximately 20% of supplied power radiates from the stub. How does one come up with this figure? Is there any science surrounding this? Have texts in years gone by discussed this?

Tnx and 73.
 
The 20-Meter J-Pole  
by KE4ZHN on August 15, 2006 Mail this to a friend!
Isnt a J pole nothing more than a base fed vertical halfwave? Perhaps it may have a slight dx advantage as far as low take off angle. I dont see any other advantage to using one unless its simply a matter of not having the 33 ft available to string up a horizontal dipole for 20. I would imagine it would play alright but you can do just as well with a dipole strung between a couple of trees with alot less headaches.
 
RE: The 20-Meter J-Pole  
by KC8VWM on August 15, 2006 Mail this to a friend!
"but you can do just as well with a dipole strung between a couple of trees with alot less headaches."

-------------

Some Advantages to using a J-pole instead of a dipole.

The J-Pole is easy to erect requiring only 1 point to hold it up, not 3 points like a dipole antenna.
The J-Pole needs no radials or counterpoise system.
The J-pole uses up much less real estate to erect than an dipole constructed for use on the same band.
The J-Pole has low angle radiation.
The J-Pole has greater bandwidth than a dipole.
The J-Pole is a DC grounded antenna and has greater immunity to terrestrial noise thatn a dipole resulting in lower noise for weaker signal work.
The J-Pole exhibits more gain than most dipole and ground plane antennas.
The J-Pole meets most "stealth" antenna and field portability requirements.
The J-Pole is a DC grounded antenna which means your equipment is less susceptible to damage from nearby lightning strikes.
The J-Pole uses a versatile and uncomplicated trombome like matching system.

73
 
RE: The 20-Meter J-Pole  
by N2KMF on August 16, 2006 Mail this to a friend!
You missed a couple KC8VWM:

Unlike the horizontal dipole, the vertical J-Pole is omnidirectional.

Unlike a conventional 1/4 wave vertical, you don't need gobs of radials for it to radiate efficiently (at least on it's design frequency).


And yes, the J-Pole in essence is just an end-fed half wave antenna.
 
RE: J-pole tub radiates about 20% of RF energy  
by K6AX on August 16, 2006 Mail this to a friend!
Actually there was an article in QST about 6 months ago about building an alternative matching system for a J-pole that does not radiate any energy from itself. This was for 2 meters. The alternate matching system had a base metal plate with 4 vertical elements that paralleled the main element. Looking at it from the top:

x x

O

x x

As I remember, one of the points of the article is that a J-pole using a stub radiates about 20% of the RF energy, and this was a proposed solution.
 
The 20-Meter J-Pole  
by W1YW on August 16, 2006 Mail this to a friend!
End-fed vertical dipoles can be a pain to match, and that is the only true advantage for a J-Pole at HF.

Remember: a J-Pole is a type of dipole.

The 'u' section is not tiny, however, and this makes it a pain to mount in a tree, for example.

If you are getting gain at high elevations you didn't make your J-Pole right.

73,
Chip W1YW
(ex N1IR)
 
RE: The 20-Meter J-Pole  
by KC8VWM on August 16, 2006 Mail this to a friend!

Remember: a J-Pole is a type of dipole.

-------

Actually the J-Pole antenna is a direct descendant of the windom and zepp antenna design, not a center fed dipole antenna design.

The J-Pole is basically a modified windom antenna with an off center feedpoint which was originally based on the idea of an off center fed windom antenna design.

The original windom antenna on the other hand was based on early zepp antenna designs in which the feed point was moved from the very end of the antenna to slightly off center.

The dipole was based on early Marconi antenna designs in which a 1/4 wave radiator was used.

Ref: 1923 (version) Windom (Article by Loren G. Windom) September 1929, QST magazine.

73 Charles - KC8VWM
 
The 20-Meter J-Pole  
by W1YW on August 16, 2006 Mail this to a friend!
Remember: a J-pole is a type of dipole. Period.

End of factual discussion on the matter.

--SK--

Chip W1YW
(ex N1IR)
 
RE: The 20-Meter J-Pole  
by KC8VWM on August 16, 2006 Mail this to a friend!

The term "dipole" regardless of the which context the term "dipole" is applied in electronics, chemistry or physics, alway refers to the idea of "2" inherently equally balanced and bilaterally symmetrical reference points.

A J-Pole is neither a center fed antenna nor can it be considered an equal but opposite balanced antenna system.

Charles - KC8VWM

 
The 20-Meter J-Pole  
by W1YW on August 17, 2006 Mail this to a friend!
A J-pole is a type of dipole.

There are an infinite variety of dipoles that are not balanced and not center fed.

The most popular antenna in the world right now--you may have one in your hand--is not balanced and not center fed--and it is a dipole.
 
RE: The 20-Meter J-Pole  
by KC8VWM on August 17, 2006 Mail this to a friend!
Alright, can we agree on this statement:

A Jpole incorporates an perfectly symmetrical 1/2 wave dipole "into" the antenna's overall construction and design but adds a 1/2 wave radiator stacked on top of a folded dipole.

The 1/2 wave Jpole construction and feedpoint is based on a 1/2 wave windom antenna design.





 
The 20-Meter J-Pole  
by N6TZ on August 18, 2006 Mail this to a friend!
Just to straighten out a little terminology for KC8VWM, the J pole is a 1/2 wavelength radiator sitting on top of a 1/4 wavelength matching stub.

The stub is zero ohms impedance at it's shorted bottom end, and very high impedance at its top end where it joins and matches the high impedance end of the 1/2 wavelength radiator.

The coax feedpoint is tapped up on the matching stub to a point at 50 ohms impedance to match the coax.

And that is all there is to it. All said, All done !!

I would love to see all new hams buy the ARRl Antenna book and read it from cover to cover.

Hal, N6TZ
 
The 20-Meter J-Pole  
by K4UUG on August 20, 2006 Mail this to a friend!
The J-Pole antenna is an omnidirectional antenna that can be used for base, mobile and field day stations. It does not need a ground plane, radials or a complicated matching system.
THEORY
The J-Pole antenna consists of a half-wavelength radiator fed by a quarter-wave matching stub. Effectively, the antenna is an end fed dipole. The antenna has an omnidirectional pattern with a low take-off angle. The quarter-wave stub is a transformer that provides a means of transforming the high impedance of the antenna to that of the transmission line.
The ARRL Antenna Book describes two common configurations of the J-Pole antenna: an open-stub version and a shorted-stub version. Figure 1 shows the two configurations.
According to the ARRL Antenna Book, the open-stub version, Figure 1A can be connected directly to low-impedance, 50 ohm, coax lines with good results; however, the lack of a movable balun allowing impedance adjustment may make this version of the antenna difficult to adjust for minimum SWR. Alternatively, the shorted-stub version, Figure 1B, which is usually fed with 200-600 ohm open-wire line, or 50 ohm coaxial cable using a 4:1 balun, allows easy adjustment for minimum SWR. The ARRL Antenna Book does not recommend feeding the shorted-stub version directly with 50 ohm coaxial, citing less than optimum results, a lack of reproducibility and heavy coupling with nearby objects. Despite this, many J-Pole antenna designs are based on feeding the shorted-stub version with 50 ohm coax;The diameter of the radiating element affects the bandwidth and the physical length required for a given operating frequency. As the diameter of the radiating element is increased, the useable bandwidth increases and the physical length of the radiating element required for a given operating frequency decreases with respect to the free space half-wavelength.by using a larger diameter radiating element a larger bandwidth can be realised with a physically smaller antenna. In reality this is a velocity factor effect. This is described at length in the ARRL Antenna Book. The antenna diameter and the physical length, and the dimensions of the J-Pole antenna can be calculated using These measurements.

Dimensions
Figure 2 shows a diagram of a shorted-stub J-Pole with the significant design dimensions.

hR = (149.9 fMHz) k in metres

Where k is related to the antenna diameter in degrees and can be found from Figure 3. k is the fractional decrease of the antenna compared to the electrical length.

hS = (74.6 fMHz) v in metres

Where v is the velocity factor of the stub, which is an air-insulated transmission line. For this situation the velocity factor is 0.975.

s 6400 fMHz in mm

The feed points need to be found by trial and error. If co-axial cable is used the centre conductor is attached to the radiating (l /2) element.


1.CONSTRUCTION
Almost any material may be used to construct a J-Pole. The considerations being where the antenna will be located, the effects of weather, construction of the short-circuit between the two elements and the method of attaching the transmission line to the antenna.Some materials are aluminium tubing, 300 W twin lead, wire, copper tape and copper tubing. 300 W twin lead J-Pole antennas are light and very portable.

2.Shorting Element
The shorting element between the two elements may be constructed in a variety of ways depending on the type of material used. The simplest is possibly to weld a plate between the two elements. But this does not provide for any adjustment. A bracket may be made which allows the elements to be moved up and down to simplify tuning.

3.Attaching Feedline
The feedline can be attached by welding it to the elements or by making or using commercially available clamps. Lugs can be soldered to the end of the transmission line to simplify attachment. The locations of the transmission line feedpoints usually needs to be determined by trial and error and adjusted to give the lowest SWR.

4.Sealant
If the antenna is too be installed outside for prolonged periods of time then all joints and the co-axial cable connections should be sealed using a sealant to prevent water ingress and corrosion.

5.Balun or Choke
Co-axial cable is an unbalanced feedline, and it will radiate from the outer shield and affect the radiation characteristics of the antenna. Therefore, if co-axial cable is used as the feedline then a choke or balun is recommended.A choke wound from four to six turns of coaxial cable with a diameter of 125 mm can be used. Alternatively, depending on the type of co-axial cable used, a ferrite bead balun or other current type balun could be used. One suggestion is to use a 50-ferrite-bead sleeve-over-coax balun taped to the base of the antenna.

6.Mounting the Antenna
The antenna may be fastened to any supporting structure including grounded metal. Ideally, the antenna should be mounted at least a quarter-wavelength above any metal structures.


I still think the double bazooka dipole at 20' to 30 ' above ground in the inverted vee out performs any HF vertical antenna.
 
RE: The 20-Meter J-Pole  
by K8MHZ on August 22, 2006 Mail this to a friend!
"I still think the double bazooka dipole at 20' to 30 ' above ground in the inverted vee out performs any HF vertical antenna."

I am using a double bazooka for 20 in the inverted V configuration.

It seems to work VERY well. What I use as an indicator is the fact that I can run w/o an amp @ 100 W and make a call to a DX station. Out of the din of a huge pile up the words 'M H Zed only, M H Zed only please' become music to my ears.

However, the experimenter in me and the near perfect location (a small lot with trees) makes me want to take a shot at building one of these. Maybe I will be able to give a side by side comparison.

Now that the serious part is behind me.....

What about a FAN J-POLE????
 
RE: The 20-Meter J-Pole  
by AE6RO on August 22, 2006 Mail this to a friend!
Any vertical antenna needs a good or great ground if it is on the ground. As many radials as can be spared plus coldwater plumbing, etc.
Otherwise it is just an earthworm cooker. Also, the radiation angle is less than optimum because the overall antenna is 3/4 wavelength. Better to use a 5/8 vertical with appropriate matching at the base. Plus it is "only" 41 feet tall as opposed to 50+ feet.
73ss, AE6RO
 
RE: The 20-Meter J-Pole  
by N6TZ on August 22, 2006 Mail this to a friend!
Comments to AE6RO, the reply just above this one...
I guess you are refering to the J pole when you mention that it is 3/4 wavelength and won't have a good angle of radiation. Your comments are not very easy to interpret.

Well, sorry but you are wrong, wrong, wrong. Please get an ARRL Antenna Book, or at least read the previous posts. The J pole is a 1/2 wavelength radiator. The bottom section is only a 1/4 wavelength matching stub and it DOES NOT radiate.

Where do these wrong theories come from, and why do not more fellows get out a solid text and read the darn things. Stop the wives tales, there is no black magic.

It is all in the books guys. Do something different, turn off the Simpsons on the boob toob, and crack open a book.

Hal, N6TZ
 
RE: The 20-Meter J-Pole  
by AE6RO on August 23, 2006 Mail this to a friend!
The total length of the antenna would be 1/4 wave stub + 1/2 wave antenna = 3/4 wave. A 3/4 wave antenna has high angle radiation which isn't suitable for DX.
Most of the radiation will take place near the bottom of the antenna from the point on the stub where coax attaches. If that is near the bottom of the stub you have a 3/4 wave antenna.
But the main thing is you do need to use radials.
If you don't the antenna will be very good at heating the ground beneath the antenna. Or you could raise the antenna and use elevated radials.
I've looked into the "Antenna Book" and wasn't that impressed with the section on verticals. I appologize for a lack of clarity on my first post. I blame alcohol.
73sssssssssssssssssssss, AE6RO

 
RE: The 20-Meter J-Pole  
by AE6RO on August 23, 2006 Mail this to a friend!
I wanted to add that a basic principle of verticals is the length doesn't matter so much as how good the ground is. This was defined around 1937. In theory a short antenna can work better than a long one if it is properly matched and GROUNDED!
I have done my homework, you see.
I prefer "King of the Hill" to "The Simpsons," thank you very much.
73s, John AE6RO
 
RE: The 20-Meter J-Pole  
by N6TZ on August 23, 2006 Mail this to a friend!
For AE6RO,
You can lead a horse to water, but not make it drink.
None are so blind as those that will not see.
OK, for you the J pole is a 3/4 Vertical.... The remainder of us will let you believe what you want.
It don't matter to me.
73, N6TZ
 
RE: The 20-Meter J-Pole  
by AE6RO on August 24, 2006 Mail this to a friend!
It's too tall. If you don't believe that a short vertical antenna can work as well as a tall one, don't take my word for it. Look it up. The most important factor is the GROUND!
Let's say the J-pole is really a half-wave antenna.
Why not use a 5/8 wave and get a lower angle of radiation with less metal? It's still shorter than a J-pole at 41 feet. My 40 meter antenna is much better than a J-pole and much shorter. I think you need to remove the blinders.
73s, AE6RO

P.S. Built my first vertical (1/4 wave for 15 M in 1973.)
 
RE: The 20-Meter J-Pole  
by N6TZ on August 24, 2006 Mail this to a friend!
AE6RO
You are changing the subject. This thread is about J poles, not other verticals. The J pole works fine with no ground as it is a half wave independent of ground, much like a dipole is, but end fed, and sitting on it's end.

See my vertical for 30, 40, 60, 80, and 160 on www.earthsignals.com/N6TZ The url is case sensitive.

p.s.-put my first vertical up in 1957.

73, Hal N6TZ
 
RE: The 20-Meter J-Pole  
by N6TZ on August 24, 2006 Mail this to a friend!
AE6RO

Here is a little addition to my previous comment just above this one.

I agree that verticals on the ground can work just fine, as you will see when you look at my system. You can read by biography on QRZ, I have dealt with broadcast antennas for many years.

I will part with one exception to that rule. Most of us do not have the property to have a ground mounted vertical "out in the open".

I see you live in the Long Beach area. Unless you are very fortunate to have a half acre or so, you probably are like many of us and have a city lot. Not a good place for a ground sitting vertical.

Your power is going into houses, yard sheds, rebar in concrete walls, and a host of other unfriendly RF sucking obstructions. It is better to have the radiating part up in the air. I do not have the area, and as you will see, I lose a lot of power into my house.

A J pole for HF, especially below 20 meters, is not such a practical antenna, and I only use them for VHF, you can see mine on the url. But the J pole is probably applicable down thru 20 meters. The radiating part then is well above most of the obstructions. THAT HELPS !!

So let's agree to disagree on some things, but as the old adage about antennas goes - - "There is no one antenna for all applications"

Hal, N6TZ
 
RE: The 20-Meter J-Pole  
by AE6RO on August 24, 2006 Mail this to a friend!
N5TZ: Well I checked out your site and it was very interesting. Here in Long Beach we are only supposed to go 35 feet or less. I don't think we can buy permits to go higher.
I agree that it is much better to put the vertical antenna up high to get it away from obstacles and ground losses.
My 1/8 wave 40 meter has gotten me Hawaii, several east coast states, Cuba, Alaska, and Japan. With 35 watts of CW. Hence the joys of CW!
73s, AE6RO

When I was young I wanted to be a broadcast engineer.
 
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