2m/70cm dual band j-pole antenna project

[wesm1957]

I am going to build a dual band j-pole antenna. I have two different plans that I found on the internet. Both call for 1/2" EMT conduit. I plan on using 3/4" copper pipe. Both plans call for 3/8" stainless threaded rod which I will be using. I am not sure if it will make any difference using 3/4 copper over the 1/2 conduit. I already have the copper on hand to use. I made two small Yagi antennas a few years ago to cover 800mhz when I lived in an apartment. I made them with brass tubing and brass rod and had good coverage for my scanners. I am hoping to have success with the j-pole. The J-pole will be mounted outside 15 to 20 feet in the air, using top rails for chain link fencing.

Wes

 

[milkman21218]

From what you said everything's going to be fine! I've been using top rail myself for several years. I find it to be cheaper than antenna mast. And stronger than some mast too! Just use the best coax that you can afford.

 

[zz0468]

...Both call for 1/2" EMT conduit. I plan on using 3/4" copper pipe. Both plans call for 3/8" stainless threaded rod which I will be using. I am not sure if it will make any difference using 3/4 copper over the 1/2 conduit...

Generally, when building something from published plans, it's best to stick with the original published plans. this is especially true of antennas. If you change the diameter and material of an antenna element, it WILL behave differently. How differently it behaves may or may not be significant. But don't be too surprised if it doesn't work.

 

[LtDoc]

You should come out okay with that change in diameter, it will be 'close'. Usually, increasing the diameter decreases the length very slightly. If element spacing plays a part (and it does) then that spacing is usually increased very slightly. Considering that the change in diameter isn't that much, that 'slight' increase/decrease is going to be -very- slight.
'zz0468' is right, it's generally a good idea to use the same 'sizes' of things when duplicating an antenna. All the 'tuning'/adjusting has been done by the original builder so usually you don't have to do -as_much- of it as he did. Expect to do a little tuning with any antenna, that's normal too, it's not going to be used in the same location so things change slightly.
The biggy in all this is that you've already got that copper tubing, don't have to go get the conduit, right? May not be a huge cost difference but it's still easier.
Have fun.
- 'Doc

 

[wesm1957]

Thanks for all the advise!! I constructed the VHF and UHF elements last night after work. Later this week I will finish the antenna. It will be a while until I can test the antenna for transmit. I am taking my test next month and still have to get coax. After that I have a friend that will be able to help set the swr.

Thanks again!!!

Wes

 

[k3cfc]

2m/440 j-pole

I am going to build a dual band j-pole antenna. I have two different plans that I found on the internet. Both call for 1/2" EMT conduit. I plan on using 3/4" copper pipe. Both plans call for 3/8" stainless threaded rod which I will be using. I am not sure if it will make any difference using 3/4 copper over the 1/2 conduit. I already have the copper on hand to use. I made two small Yagi antennas a few years ago to cover 800mhz when I lived in an apartment. I made them with brass tubing and brass rod and had good coverage for my scanners. I am hoping to have success with the j-pole. The J-pole will be mounted outside 15 to 20 feet in the air, using top rails for chain link fencing.

Wes

Look at this. i built one it works just great and it is lite weight also.

Open Stub J-Pole Project – Completed (Many times) « NT1K – Welcome

 

[NT1K]

Changing the diameter of the "elements" will effect the performance of the antenna.

For example a 1/2" Outside Diameter (OD) tube that is 48in long has an area of about 75.3 inches.
Going to 3/4" OD would have an area of about 113 inches. That's about 38in more of area.

Since RF travels on the "Outside" of an wire/tube (AKA Skin Effect), the length, diameter and the material of the element is factored into the design of the antenna. Changing the diameter of the tube will effect the resonate frequency and bandwidth which can give you bad SWR. Since you are increasing the area, you would either have to modify the length to be shorter.

If you trust the author and their design, I wouldn't stray to far from it as everything involved in the antenna has (hopefully) been calculated and optimized with it in mind

Look at this. i built one it works just great and it is lite weight also.

Open Stub J-Pole Project – Completed (Many times) « NT1K – Welcome

Thanks for linking my site!

 

[LtDoc]

If you change the word 'performance' to 'tuning', I'd agree with you.
- 'Doc

 

[wesm1957]

Changing the diameter of the "elements" will effect the performance of the antenna.

For example a 1/2" Outside Diameter (OD) tube that is 48in long has an area of about 75.3 inches.
Going to 3/4" OD would have an area of about 113 inches. That's about 38in more of area.

Since RF travels on the "Outside" of an wire/tube (AKA Skin Effect), the length, diameter and the material of the element is factored into the design of the antenna. Changing the diameter of the tube will effect the resonate frequency and bandwidth which can give you bad SWR. Since you are increasing the area, you would either have to modify the length to be shorter.

If you trust the author and their design, I wouldn't stray to far from it as everything involved in the antenna has (hopefully) been calculated and optimized with it in mind



Thanks for linking my site!

I bookmarked your site! I may have to rethink my antenna construction. Here is where I found plans that said to use 1/2 inch EMT conduit -- http:/www.eastalabamaarc.com/2m70cm_jpole_project.htm

Thanks again for all the help!!!!

Wes

 

[NT1K]

I bookmarked your site! I may have to rethink my antenna construction. Here is where I found plans that said to use 1/2 inch EMT conduit -- http:/www.eastalabamaarc.com/2m70cm_jpole_project.htm

Thanks again for all the help!!!!

Wes

To be honest, that project seems to be a waste of money. Not dismissing it but you can do much better for the price, material and labor involved.

The use of threaded rod and EMT is asking for trouble if you're mounting this outdoors. Exposing raw steel (threaded rod) and galvanized tubing is asking for rust. You could spray paint but I am not sure how it will affect the antenna (if anything). It's also asking you to buy a "CB Antenna Mount" which you're not even using the entire mount, Just the SO-239 with plastic spacers and a coupler that can be purchased by itself for much cheaper (Radio Shack 21-961).

If you have a 3/8"-24 threading die ($6 at SEARS), you can use 3/8" Aluminum Rod and 1-1/2"X1-1/2" Aluminum Angle for just as much, if not cheaper than the link you suggested. Aluminum is a better conductor and will last much longer outdoors.

I would avoid big box/chain stores for the aluminum as you will be paying almost double than what you can get it for at a local metal supply store or metal fabrication shop. If you play your cards right and depending on the shop you go to, you could get it scrap value (or free) as some shops don't need a small cut of angle and don't want to hassle with paper work. Just don't take advantage. Worst case is I would order online from places like McMaster Carr

The only reason I would build that antenna you linked is If there is no way I can obtain aluminum angle or aluminum rod and only had access to big box stores.

- NT1K

 

[wesm1957]

I have the SO-239 connector. We use them on our trucks where I work, and I salvage what I can that way. I did buy the stainless all thread and the bracket the other day. The all thread is 3/8 x 16 so I tapped the top of the 239 to take that. I had the copper from another project and thought it would be better for outdoors than the conduit. I can check with a local metal supplier for the 3/8 aluminum and the angle. I know they have cut offs that can be I can purchase by the pound. I know there is always a problem with bi-metal corrosion. I do have a tap and die set so cutting threads is no problem. I had planned on covering the original antenna with clear coat, but I know in the elements here in Michigan that would be a temp fix.

I am really thankful for all the help and advice!!!!

Wes

 

[dksac2]

There are also far better antennas that can be built than the J Pole. They have almost no gain and copper is getting expensive these days.
I'd look at a bunch of other plans before going to the J Pole.
Be sure to put a good rolled up bunch of coax just below the antenna also (a balun). The J Poles are famous for putting a lot of RF on the jacket of the coax.
They are OK for a quick to put up EMCOMM antenna, but for your QTH, you want something with gain to help get your signal out further. You'll notice quite a difference between some of the better verticals (Even home made) and the J Pole as to the distance they are usable at.
I live in a valley with mountains on 3 sides. With a J Pole, I can hit the repeaters on top of one of the mountains, but cannot get a signal out of the open end of the valley. With a vertical with 8.3 DB of gain on 2 meters, I can open 4 repeaters that I can't even hear with the J Pole. There is a huge difference. VHF/UHF need as much gain and power as possible to get distance.
Whatever you decide, use LMR-400 coax and keep the run as short as possible. 50 feet or under is best. If you are using anything less than LMR-400 (less than a .405 inch diameter coax) and have a 75 or 100 foot run, you are almost getting negative power to the antenna, cutting the power of a 50 watt radio down to next to nothing.

Don't want to rain on your parade, just telling the facts.

73's John KF7VXA

 

[k3cfc]

Experimenting is part of the fun. i built my j-pole and could bring up repeaters more then three counties away. i made three turnes 8 inch loop taped it up. swr 1.3-1 and i have no trouble with it. i hope the op builds the one i linked to him because this one works well and is portable.

K3CFC

 

[wesm1957]

Experimenting is part of the fun. i built my j-pole and could bring up repeaters more then three counties away. i made three turnes 8 inch loop taped it up. swr 1.3-1 and i have no trouble with it. i hope the op builds the one i linked to him because this one works well and is portable.

K3CFC

I am in the process of building the antenna you sent in the link. I have everything needed except the mast, ground rods and coax. That I will get in a few weeks. I will also have to get the coax into the house. That may be a job, since the area where my shack will be has concrete walls. There is one feed hold coming into the basement where the cable company ran in cable and that is where the coax for my scanner antenna is now. I am thinking about a junction box on the outside and cutting a new hole in the block to bring everything in that way.

Wes

 

[k3cfc]

I am in the process of building the antenna you sent in the link. I have everything needed except the mast, ground rods and coax. That I will get in a few weeks. I will also have to get the coax into the house. That may be a job, since the area where my shack will be has concrete walls. There is one feed hold coming into the basement where the cable company ran in cable and that is where the coax for my scanner antenna is now. I am thinking about a junction box on the outside and cutting a new hole in the block to bring everything in that way.

Wes

Good for you. i didn't use ground rods. i put it on 20 ft of military fiberglass pole used to assemble a tent.i can bring up a repeater in lycoming county from snyder county PA where i live. i built this j-pole as directed and it works just fine. going to build another one for portable use.

Have fun.

K3CFC

 

[wesm1957]

Good for you. i didn't use ground rods. i put it on 20 ft of military fiberglass pole used to assemble a tent.i can bring up a repeater in lycoming county from snyder county PA where i live. i built this j-pole as directed and it works just fine. going to build another one for portable use.

Have fun.

K3CFC

I am having a great time building this antenna! I will have to look on a map to see where you are. My parents were from Clearfield County. I still have relatives there.

Wes

 

[SCPD]

comments from reading the thread.

1. Make the j-pole out of copper. And solder it together, like a plumber would.
I can't see why you would want to make it out of steel. Like others have said, once it starts rusting, the threaded connections are going to blow the swr out the water.

2. The larger the diameter of the pipe, the greater the bandwidth with a j-pole. But the larger the diameter, the greater the weight of the thing.

3. I get tickled when people say it's not a good antenna. When I had one up, I was full quieting into repeaters 75 miles away any time of the day or night. I used to work a repeater 160 miles away most mornings.(by noon it would fade out).

 

[wesm1957]

I have finished the antenna. I did have a chance to try it on receive last week. I heard someone on my scanner talking on simplex, but was only hearing one side. I took the j-pole outside and hooked the handheld up to it and heard both sides then. And the antenna was only 8 feet up propped on my fence. I am ready to see how it works 20 feet up.

Wes

 

[LtDoc]

... I think you'll like it. Won't be the absolutest bestest antenna in the world, but it won't be bad!
- 'Doc

 

[dksac2]

1. Make the j-pole out of copper. And solder it together, like a plumber would.
I can't see why you would want to make it out of steel. Like others have said, once it starts rusting, the threaded connections are going to blow the swr out the water.

2. The larger the diameter of the pipe, the greater the bandwidth with a j-pole. But the larger the diameter, the greater the weight of the thing.

3. I get tickled when people say it's not a good antenna. When I had one up, I was full quieting into repeaters 75 miles away any time of the day or night. I used to work a repeater 160 miles away most mornings.(by noon it would fade out).

I'd have to see that for myself. Any other details, how high up, how much power, what is between your QTH and the repeaters. Were you on top of a mountain when doing this?

I know what I can work with a high gain antenna, set up with an antenna analyzer, almost perfect SWR in the range of several repeaters and running 80 watts from a Yaesu FT-2900, short coax, LMR 400.
No calling you a lier, but as I said, I'd have to see that for myself and have more information. J poles do work, but are in the negative gain zone by the time you run any length of coax to one. I also have made and used J Poles and never had anywhere near that kind of range from one. Good for local use or back up, but long range, you can keep them.
For soldering J Poles, you are supposed to use solder made for electronics, not plummers solder, two different animals.

73's John