j-pole questions

[26tm696]

i want to make a j-pole to use at home, im wanting to use copper tube and have a few questions.
i use a superstar 3900 and also uk 40 cb radios.
ive used a j-pole calculator and heres where i get confused.
it gives me sizes for multi-band but doesnt say
1= what tube to use, 1/2 or 3/4 inch?
2= where to connect the coax feed?
3= will the multi=band j-pole be ok for both cb radios?

any help appreciated

 

[The DB]

For the CB band a j-pole will be in the neighborhood of 27 feet long. Copper is a soft and heavy metal so if you don't want it to collapse under its own weight, much less with the effects of things like wind and ice, you should use as thick copper tubing as you reasonably can, and I would still consider supporting it with guys on the antenna itself. I wouldn't use anything less than an inch diameter copper tube, and would consider thicker.

There are antennas made out of aluminum tubing at those lengths that don't hold up well, and aluminum is both more rigid and much lighter than an equivalent copper tube, so make sure you take that into consideration.

The coax feed will be somewhere on the stub, pretty close but not right at the bottom. Moving that is how you tune the antenna. I would guess, offhand, that it will be between 6 inches and a foot up from the bottom of the stub.

Do you have a link to the antenna calculator you used? Most j-poles are a single band design.


The DB

 

[26tm696]

found new calculator

http://www.kb3kai.com/j-pole-calculator

 

[The DB]

A few things to point out.

1) That tool is designed for antennas in the 2 meter ham band frequency range. As you move away from said 2 meter frequencies, error is introduced. The further away from the band the calculator was intended for the more error will be introduced, especially when you are using lower frequencies.

The reason has to do with the width of the pipe. At 27 MHz, for example, the effects of a 1/2 inch diameter antenna element aren't overly drastic, but at 2 meter frequencies, whose wavelength is less than 1/5 the size of the CB band frequencies you are trying to build the antenna for, that 1/2 inch diameter will have a much larger affect.

It is possible to adjust for such, however, it is very rarely done. Those instructions are intended for making a 2 meter antenna, not an 11 meter antenna. The page is listed as a page for 2 meter antennas, I highly doubt it will scale properly to CB frequencies.

2) Even if it the site does properly compensate for the width of a frequency so far from its intended use, you have a structure problem. How do you plan to support 26 or so feet of 1/2 inch copper tubing? I can tell you that it is not anywhere near strong enough to support itself, much less the stub also made out of copper tubing, and even if it does, the first strong wind or ice conditions will knock the antenna over. As I mentioned above, copper is not a rigid metal compared to others, nor is it light. At two meters where the long side is about four feet long above where the stub connects is one thing, but 26 feet?

You will need to find some serious support that won't affect the RF coming from the antenna, good luck with that.

3) That small tubing at that length will be a safety issue. At minimum you will need to make sure you get some pretty thick walled copper tubing, but I doubt even that will help much. When it falls, (note, I did not say if) you can only hope that it doesn't hurt anyone or damage anything on its way down.


The DB

 

[vkrules]

you might want to reconsider what you want to build. That calculator is for 1/2 inch copper and is scaled from vhf. It wont stand up at 25 feet plus and by the time you scaled the diameter up the copper would be worth more than I would like to spend. You could build it with tapered elements out of aluminium, but those dimensions would need adjusting.

 

[vkrules]

Far better option is to look at other 5/8 vertical designs. As for bandwidth you should be able to get coverage for both the old and new uk bands. I recommend buying a fishing pole and making a gainmaster, With the SWR less than 1.4 to 1 from 26.5 to 29 mhz That would get you the NZ band as well as the DL and both UK bands and the 10 meter ham band.

 

[26tm696]

this is the first calculator i used

 

[vkrules]

.No good for cb. thats 440 220 and 144 mhz not for 27mhz

 

[26tm696]

thanks for info, il scrap that idea

 

[Henry HPSD]

http://cb-antennas.com/?page_id=399

Perhaps of use for those willing to construct a J-pole...for CB

Kind regards,

Henry

 

[bob85]

Excellent Henry,
a honest no voodoo antenna builder is a very rare find these days, one that shows you how to build the antennas he makes even rarer,

have you seen this one ?,
http://www.antennex.com/preview/Sep09/the_x-pole.pdf, a j-pole with gain-master style vswr curves.

 

[Henry HPSD]

Hello Bob,

Thank you! comming from you, it is valued and encouraging.

Yes, i have seen the X-pole and you probarbly have seen the Y-pole as well ..
http://www.antennex.com/preview/Oct509/y-pole.pdf
I think it was you who actually pointed out both antennas a couple years ago..

Sometimes it is awkward why some antennas are not that well known within the community as the right to be so seems to be obvious.

Kind regards,

Henry

 

[bob85]

I don't recall having seen the y-pole before Henry, I only remember the xpole cropping up in the past,
thanks.

 

[Marconi]

thanks for info, il scrap that idea

696, I would not so easily scrap the idea of a j-pole. Many of the ideas that have been discussed already do need to be consider however.

IMO the process of building your own antenna can be very instructive, and the J-Pole is not really that bad of an idea if you have any success at all. Most of the complaints are technical at best, and I have shown many cases where claims of big differences in antenna design and theory are often far less that most imagine, say less about what is often reported by technical articles we read on the Internet.

DB, warns about the scaling of the J-Pole calculator, and the fact that your calculator does not scale correctly except at 2 meters. Even my Eznec does not scale the element diameters to what might work in the real world if I make a big change in frequency. This is because Eznec and you caluclator uses a frequency ratio equal to the element lengths at one frequency to another, and this can make the element diameters get really out of whack, because the wire diameter is generally such a small relationship to the frequency. Common sense will guide you in some decisions using calculators, and they can get you close.

You noted in your original post that you would be considering 3/4" copper, and this is my point that common sense has to be applied in antenna making as well. Even if you calculate an antenna at a different frequency and use the results...and it was off a bit, you often need to make adjustments in order to get right on the money. Again, common sense.

These are things that you can learn in the process of making your own antennas, and what works and what does not. You may also find that all the old wives tales you hear about...as making huge differences or presents potential errors...are often only somewhat minor in the end if we just use some common sense.

 

[26tm696]

ive never made an antenna apart from a bamboo dipole in the am days. was just hoping to get into making antennas, being a heating engineer (retired ) i have tubing, fittings and time etc