GMRS J-Pole Antenna

2/27/2005 marc 57 comments

J-Pole design

A j-pole antenna is a half wave radiator with a quarter wavelength matching stub. The full length ends up being three quarter wavelengths long. The two lengths are electrically connected at the bottom forming a “J”. There is a feed point some distance up from the bottom of the “J” where the shield connects to the matching stub and the center conductor of the coax connects to the radiating element. The distance between the bottom of the “J” to where the feed point attaches can be adjusted up or down to achieve a Standing Wave Ratio (SWR) closest to 1 however, this distance can be calculated to some extent.

J-Pole diagram

Antenna designs for the most part, work very well across the frequency spectrum. While there are special considerations that can be taken at extremes, most antenna designs can be scaled and built for a certain frequency. All we need to do is find the right frequencies and do a few calculations…

The next page covers calculating the right lengths and other dimensions of the J-Pole.

Related posts:

PL576Ws goes back, WRT54G comes homeUnfortunately the ProView PL576Ws LCD monitor, as nice as it...

Pages: Page 1 Page 2 Page 3 Page 4 Page 5

Shop Work antenna, communications, DIY, electronics, ham radio, projects, radio, technology

Comments

Leave a comment Trackback

Written by David about 6 years ago. Reply

You antenna geek. Nice j-pole. When are you going to get your ham license? You have ham written all over you… Then you could get a vanity call sign, like W7MEC. Yes! It’s available!
Written by Marc about 6 years ago. Reply

Hey David! Good to year from you. Wen’t to lunch with Ed Orcutt last Friday. We both thought you should have been there with us. So, do you have your licence yet? I had thought you had mentioned getting it once… Yes, I think I should get it – one of these days! 8^) Thanks for the complement on the antenna…more projects to come.
Written by David about 6 years ago. Reply

Yes, I do have my license. It’s really not that hard. You have enough background that you probably would have a very easy time of it. And it’s only $14. The ARRL’s Now You’re Talking is a great way to go. Then take the technician (or general if you want to go for that) practice exam on QRZ a bunch of times and you’re ready to go. (All of the exam questions are in the book, too, but it’s nice to take some practice exams.)

I was really interested in getting involved in some of the emergency response stuff, as well as volunteering at the various events. But now I’m all getting into the technology side of it a bit more. It’s great fun.

Seriously, go for it. You’d love it!
Written by brian about 5 years ago. Reply

hey great antenna
i was wondering if any one has theought about making a yagi antenna for frs/gmrs. ive tried and haven’t had much luck. if any one can figure it out please email it to me at shadowx_49 (at) hotmail.com
thanks
Written by marc about 5 years ago. Reply

Hey, thanks brian.

I have also thought about building a GMRS/FRS yagi. Maybe this summer as i get some more time. Right now I’m pretty busy with work but that would be a good project to do and I have thought about it a little – so when I get some time maybe I’ll put some work into it…
Written by Bruce Hodges about 5 years ago. Reply

Marc,
I’m interested in building your J-Pole antenna. My Midland GTX 400 does not have a removable
antenna. I opened the radio and removed the antenna by desoldering it from the circuit board
and inserted a male plug in its place for a coax connection. My question is that since there is no connection for ground (braided sheild of coax) will the J-Pole perform properly? I’ve looked inside three different GMRS/FRS radios and they all just have a single connection for the antenna.
Thanks for your help,

Bruce
Written by marc about 5 years ago. Reply

Hi Bruce,

OK, remember my warning about modifying a radio…since it’s illegal (from what I understand), I have to stand my that statement…but if you’ve already done it, I cant stop you 8^).

For any antenna to work best, it needs to be properly grounded so I think you’ll need to find a ground. That typicaly is the same as the ground from the negative battery terminal. However, I am unfamiliar with your radio. So, I’d recommend checking with some other ham/radio types to make sure. I’d hate to give you advice that toasts your radio so this is just an idea for you to look at. I have not checked to see if my G-28′s ground and negative terminal are connected. If I get some time, I’ll check and post here. If you build the antenna, I’d like to hear about it. I think it’s a great project and I love mine. Best of luck!
Written by marc about 5 years ago. Reply

Bruce,

After checking my radios, as I thought, the negative terminal on the battery is ground for the antenna which makes complete sense. Most likely, its the same for your radios but check for sure. Let me know how your antenna project goes.
Written by R. Martin about 5 years ago. Reply

I just wanted to say: excellent stuff. I also have a question: is it posisble to build a smaller version of the J-Pole antenna for use with portable GMRS radios (the Cobra/Motorola type), anything say less than 10 inches long?

Your antenna uses 3/4l + 1/4l, can we use proportional values (say 1/2 of those and still have a (sub-optimal) good antenna? The current antenna on my radios is helicoidal, about 2 inches long, it’s a 1A radio and the range is not very good, I am just trying to improve on it.

Comments would be most welcome.

Thx.

r,.
Written by marc about 5 years ago. Reply

R, Martin,

Thanks for the feedback. For the j-pole, you probably can’t go that far away from the measurements and have it affective. However, you could easily do a coaxial di-pole antenna which would end up being 1/2 wavelengths long. Other than that, your stub is the best option. The J-pole and such antennas work best on vehicles and for base stations because of their size. I have made a GMRS “pocket j” of the same dimensions but using 300 Ohm twin lead. Do a google seach for “pocket j” and then use the same measurements from this j-pole to make that. A pocket j-pole can be folded up and put in a backback and then unroled for use and hung from a tree, etc. While the size stays the same, it’s certainly more portable.
Written by Mario about 5 years ago. Reply

Hi Marc!
I’ve been thinking on building a J-Pole antenna using 1/2″ rigged copper tube. Will it make any difference from the material you used?
Written by Mario about 5 years ago. Reply

Sorry, I didn’t mention to use RG-58 Coax.
Written by marc about 5 years ago. Reply

Mario, It shouldn’t matter at all if you use the 1/2″ copper tubing. The higher the frequency, the smaller the gaps between the J segments so you might have to trim the 90 degree bends shorter so that the proper spacing between the segments can be made. But the tubing should work fine IMO. Let me know how your project goes. Good luck!
Written by Brian about 5 years ago. Reply

Very nice! I was looking for ideas for a cell phone antenna and think that this might work, do you have any idea if a jpole or super jpole would work for 900mhz or 1800mhz? In any case I think that I am going to have to dig the handhelds out from the closet!

Brian
Written by marc about 5 years ago. Reply

Brian,

It should work just fine. Just scale the design for the frequency as noted in this article. Let me know how it goes – sounds cool!
Written by Shawn about 4 years ago. Reply

Great read. I was wondering do you have any ideas how to increase trans and receive distance on a handheld? Would like to get a greater range between two parties, especially while hiking in mountainous terrain. Your antenna would be great for a car or stationary camp, but I was looking for something more mobile. Any ideas on a better antenna construction that would just screw into the existing mount? Thanks again!
Written by marc about 4 years ago. Reply

Hey Shawn,

Almost any antenna will be better than the stub antennas that are on most hand helds. If you have a handheld with a removable antenna, a 1/4 wave whip is the easiest antenna to make and will really increase your range by quite a bit. Using some piano wire, a matching connector for your antenna, and silver-solder from a hobby shop should work. For these radios, a 1/4 wave whip for the 70cm band may work fine.

If you want to test it, follow the directions for figuring lengths in this article make a 1/4 wave antenna and try it out!

Good luck!
Written by Jan Hoover about 4 years ago. Reply

Does anyone know where to find an antenna connector for the Motorola distance DSP
FRS/GMRS radios? I want to use the J-pole with this unit.
Written by Don about 4 years ago. Reply

I read postings about the J-pole and. But article wasn’t clear at all about how to make this ant. Anyone can help?? thanks. donstv@aol.com
Written by Phone Guy about 4 years ago. Reply

Can you post a list or email some of the other models with removable antennas that might be available.

TIA!
Written by john about 4 years ago. Reply

does anyone make base station antennas for those of us who are all thumbs? i’m talking about gmrs antennas? how about mobile antennas? thanks,
john
Written by Your Cuz Craig about 4 years ago. Reply

Hey Marc I have a wireless camera that also has audio
is it possible to make a jpole for the reciever it has a removable antenna
and if so I may need your help in making it. it is 1.2 gighz
Written by marc about 4 years ago. Reply

Hey Craig, sure – it should be easy enough to make. Let me know if you need help designing or working on it. Sounds cool.
Written by Michael about 4 years ago. Reply

If you are looking for a good GMRS J-Pole antenna and don’t want to build one yourself, I highly recommend the KB9VBR J-Poless at http://www.kb9vbr.com/jpole.php They are high quality and quite affordable.
Written by Jeff about 4 years ago. Reply

Could a design like this be used as a “backpack” sort of setup? I’d strap the antenna to one side of my backpack, and have it stick up above my head. Or could you totally conceal it in a backpack so it wouldn’t be subjected to damage as you walked?
Written by marc about 4 years ago. Reply

Jeff,

You could certainly use this j-pole antenna with a backpack. It could possibly be attached to a fiberglass dowel or something similar. I wouldn’t conceal it inside the backpack as your body would reduce a great deal of the signal (both reception and transmission) in that direction. It would be best to get the j part at least as high as your shoulders, higher if possible. If you do this, I’d be interested in hearing about your experience.

Good luck!
Written by Jim about 4 years ago. Reply

what gmrs radios are available for base station use with external antennas?
Written by marc about 4 years ago. Reply

Hey Jim, Not sure what base stations are actually available. However, I think I saw a Kenwood that had a removable antenna. It’s been a couple years since I looked at radios so I don’t really know what’s out there now. If anyone else knows, maybe they can post here.
Written by ROY about 4 years ago. Reply
We have a set of cobra frs/gmrs radios with fixed antennas.. Is there a way to get better reception LEGALLY of course.. We live in a hilly area and can only talk to each other 7/10th of a mile one way and maby a mile the other way the other.. I guess you can say we use it for security sence she has a voilent ex who has threatened us…

Thanks Roy & Mary
Written by marc about 4 years ago. Reply

Hi Roy,

There may be a way to legally (IMO, however, IANA!!!) get better reception. If you build two of these antenna, and connect the two antennas together with 50ohm coax (gnd to gnd, center to center) and place one as high as you can – on a tower or on a near by hill top and run the other one close to you. The first one will pick up the signal and the second one will re-radiate it. This is called passive re-radiation.

OK, that being said, there will be significant signal loss, especially with a longer run of cable. You could also make a jpole that then connects to a parabolic antenna built for the same frequency that is aimed at your residence. It will then re-radiate a directional beam to you. Again, significant signal loss will occur. Anyway, you may want to try it to see if it helps out with reception. YMMV.

It may be cheaper and/or easier to just get a radio with a removable antenna for home and place the antenna as high as possible. Be sure to use proper grounding of all antennas! They make great lightning rods!!!
Written by Skip Sanders about 3 years ago. Reply

A bit late, but to note, you will never find any legal FRS unit with a removable antenna, because FRS regulations REQUIRE a fixed, non-removable antenna, to prevent people from using high-gain antennas. The service was designed for SHORT range, and increasing it increases interference for all.

GMRS, on the other hand, does allow external antennas. Of course, any modification to a GMRS/FRS unit to allow replacement antennas voids its type acceptance for EITHER service.
Written by cbfanatic about 3 years ago. Reply

hey there. i’m really looking for a gmrs radio with external antenna.
i can’t seem to find any midland g-28 or memorex mk 1995.
are there any other items that have NOT been discontinued.
any help would be appreciated.
showroomcondition2004@yahoo.com
thanks,
Written by marc about 3 years ago. Reply

cbfanatic, unfortunately I don’t know of any available. I know that Kenwood made some. There are some that are GMRS only I think (these are meant more for commercial use and with a license can legally be use d with an external antenna) so a license and one of these might be the only way to go right now. Last I looked they were a lot more expensive though at around 2 – 3 hundred dollars a piece. Not exactly in the same range as the Midland g28s. Have you looked on e-bay yet?
Written by whitman about 3 years ago. Reply

let’s just say I opened up my frs/gmrs radio and replaced the internal antenna with a bnc plug with a 3/4″ lead of copper wire.. would my antenna size for 1/4 wave be 15.4cm minus the 3/4″ wire lead and minus the 1/2″ or so for the male bnc plug? or would you make the antenna exactly 15.4, disregarding the distance of wire and metal from the actual electronics and the antenna connector? thanks for help on my theoretical project!

whitman
Written by marc about 3 years ago. Reply

Whitman,

If the internal portion of the antenna is unshielded and it’s base is the start of the connection, then yes, the whole thing becomes part of the antenna. However, this is suboptimal since that portion of the antenna will be covered by your hand sometimes and or shielded by other electronic components in some portion of the radiation pattern.

The best thing to do is get some small diameter 50-ohm coax (like RG-174), solder the center connector to the internal antenna lead on the circuit board and the shield to ground. Then connect that coax to your BNC plug (you can get them with a small RG-174 end on them). That way, the antenna would start at the connector you could then buy a real 1/4 wave antenna or any other antenna meant for those frequencies – or make your own. However, doing it the other way would limit you to only DIY antennas.

Be careful that you get the right ground! WARNING! with any modifications like this you can toast your radio! Be careful! YMMV.

Does that help?
Written by whitman about 3 years ago. Reply

thanks for the quick response! okay I’ve looked up the rg174 and I’m understanding that as long as the connector and/or cable is shielded, it’s not added in the antenna length calculation. Next step is making a jpole for it! figure I could have the actual connection point be a downward facing male bnc and have the low “D” length just bend around the connector. If I get all of this working, I’ll post a picture (computer rendered of course) of what it would look like if I had done it. thanks for the help!

whitman
Written by Brian about 3 years ago. Reply

Hi Marc, some interesting info here. I have constructed a j-pole from your design & my reception has increased greatly. I need to purchase an swr meter to check the match before I start using the antenna. Maybe you can answer this question for me. Everything I read states that PL259 / SO239 connectors are not useable above 300 mhz due to unstable impedance. Yet all the swr meters I could find, that go to 500 mhz, use them. I’m puzzled

Brian K.
Written by marc about 3 years ago. Reply

Hey Brian,

Thanks for your comments! I’m glad you found my guide useful and I’m even happier that you actually have made an antenna. Very cool! I’d be interested in seeing some pics of your finished work.

So, unfortunately I don’t have the answer to you question. Maybe another reader with more experience can add an answer as a comment here. Best of luck with your new antenna!

Marc
Written by Mihailo about 3 years ago. Reply

Hello Marc!

First sorry if my English is bad!

I was read almost every post here and i was build this antenna. It isn’t the prettiest antenna, but work just fine. ;-) I decide to build antenna because my midland GXT635 didn’t have good range in city conditions. I was remove non removable antenna :-) and insert a BNC connector on walkie-talkie but also on original midland antenna so i can put it back. On 5m 50ohm cable a insert another BNC, connect cable to antenna and range was increased pretty much. I build one more antenna for my friend and everybody was happy.

Then I wish to build a mobile (whip) antenna. I took wire for bass guitar :-) and now i don’t know how to connect this. I understand measurements for whip and everything else, but you told that antenna must have ground connection for best performance.

Where i must sold ground on my new whip? It looks just fine, but don’t work properly. :-( My original stub antenna (helical) had only one (positive) connection.

Thanks
Mihailo
Written by Chris about 2 years ago. Reply

Hi folks .. I read with interest this thread. I found a web page that gives all the dimensions for the J-Pole with the frequency of FRS/GMRS being
around 462.5 Mhz. The long radiator is 18.24 inches and the short on is 6.12 inches. The spacing between the two is .6 inches and the coax connection is also .6 inches up from where the long and short connect. However, the point I want to make is how did the makers of this J-Pole get .6 inches spacing between the long and short. Think about it, if you use standard copper plumbing the T connector and 90 degree angle will never get that close. It’s impossible. I have no idea on how you all did that, but I would be interested to know how. On anther note, I have a VHF/UHF vertical antenna for my Ham radio. The UHF range for Ham is 440 to 448 Mhz. I thought about how good the SWR might be at 462.5 Mhz. Luckily, I have an antenna analyzer from MFJ and
guess what, the SWR is 1.2 to 1. It’s even better than my Ham band. I said wow! So I promptly modified my Midland GXT-450. I took the rubber duckie antenna and installed a piece of coax in it’s place. The other end of the coax, I installed a PL259 connector. I had to have the batteries external to the radio as it would not all fit in for my radio. I tried it and wow, does it ever work. I just tested it with my sister at her house and it goes like crazy. I just did it today and I have yet to get any real distance tests but we’ll see how it goes. Let me know if you have any questions. I’ll keep an eye on this page. Maybe I should get a web page or something. If you all ask, then I’ll do something like that as I took pictures of my modify. Cheers!
Written by Chris about 2 years ago. Reply

Okay .. it’s Chris once again. I always thought J-Poles were made of 1/2 inch copper plumbing. I have seen some hams use them but of course the frequency is much lower than 462.5 Mhz. I did not see the J-Pole links on this web page. I just noticed them. Now I see that this J-Pole can be made from plain wire and with that said, one can get a half inch spacing between the long and short parts of the J-Pole. Any road, the vertical that I am using is working like a charm still. Cheers!
Written by Kendall about 2 years ago. Reply

Wow, over 3 years old and still a great piece of info. Hope Marc is still around to answer more questions. The base. Do I mount the base on the chassis of the vehicle, or do I mount it like the standard car radio, isolating the stub from chassis ground?
Written by Ty about 2 years ago. Reply

I had to make a post on your blog on the j-pole page. I was searching for designs and click on a link and would you know it. I know the author. I was like, hey hold on a minute. You’ll have to check out me j-pole I built for my ham shack. I am one of the Amateur Radio guys for our stake. The one I built is for 2 meter and 70 CM bands. 144 – 148 Mhz and 420 – 450 Mhz area, respectively.
Written by marc about 2 years ago. Reply

Kendal,

I’m not sure if it needs to be grounded to the vehicle but it is connected to the ground of the radio’s antenna feed so if your radio’s antenna feed is grounded to your chassis, then it won’t matter if your antenna is directly connected to the frame of your car either. If not, I’d keep them separate.
Written by marc about 2 years ago. Reply

Hey Tyler! Great to hear from you. Yeah, I’d love to see what you did for the ham shack. Very cool.
Written by dwl about 2 years ago. Reply

Remember, GMRS/FRS radios REQUIRE an FCC licence.

David
Written by J Fraley about 2 years ago. Reply

I would like to build your J Pole antenna. Can you give me the details?
Written by marc about 2 years ago. Reply

All information to build the antenna is in the article.
Written by Louis about 2 years ago. Reply

I have a Midland GXT950 witch I added a bnc connector. I have made this J-Pole antenna and my question is the section of B (1/4 y) is very long at the bottom, actually it’s the continuity of this section about 1 foot. I did not cut it at the bottom. Is this ok or it will affect the antenna.
Thks
Written by marc about 2 years ago. Reply

Louis, The section below the “j” should not affect the antenna. However, the continuation should be an extension of “A” rather than “B”. The extension is usually grounded. It may work fine if you extended “B” rather than “A” just as long as you have the ground and signal wires connected properly (signal to A, ground to B). Let me know how it goes!
Written by Louis about 2 years ago. Reply

That was i did ( It may work fine if you extended “B” rather than “A” )
Many thanks , all i need now is a SWR meter, would you by recommed this swr and power meter ?
http://tinyurl.com/SWR-Meter-MFJ-874
Written by marc about 2 years ago. Reply

I don’t really have a good idea of which SWR meter to recommend. Maybe someone else has a good recommendation?
Written by Andy about 1 year ago. Reply

Marc,
Any idea if this would work with something more flexible like piano wire or bass string so it would be more like a whip? If the “D” portion were something rigid and the top of the quarter wave secured with plastic like your design? Also, would it make more sense to tune the antenna to one of the higher GMRS frequencies to capture full power transmissions or is it just not necessary? Thanks,

Andy
Written by marc about 1 year ago. Reply

Hey Andy, No need to tune to any other GMRS/FRS frequency. The middle is where is should be and it’s broad enough to pick up the whole spectrum covered by the radios. A flexible design should work well as long as the total distance from the base of the j to the tip is not longer or less than 3/4 wavelength. Good luck!
Written by Andy about 1 year ago. Reply

Marc,
What happens if you don’t do a choke with the coax? If I am using 75 ohm coax would the choke be the same 4-5 turns? Thanks!
Written by Mike about 2 months ago. Reply

Hi Marc,

Great write up. I’m planning on building one of these soon.

I’m curious how you ended up attaching this to your car. Did you use some type of window mount?

Also, in lieu of finding a solid FM antenna, do you have any recommendations for other materials? As long as the material is a good conductor and can be formed into the J shape you describe are there any other properties you have to worry about?
Written by marc about 2 months ago. Reply

Hey Mike, I found an antenna mount that connects to a luggage rack. You can get one at any store that carries radio equipment (Radio Shack, a local ham shop, a truck stop, etc). If you don’t have a luggage rack, then you might want to look into a magnetic base.

You can use any metal for the material however, there are a couple things to keep in mind. The velocity factory will vary from metal to metal although, it may not matter much as the difference may not be that great. The nice thing about a car antenna as the base-material is that it is stainless steel and will last years, and is very strong. Copper and aluminum will bend easily. You should look for something that can be soldered for sure, either silver soldered like I did in this article or regular solder (copper, brass, etc).

Best of luck!

Leave a Comment

Previous Post Next Post

Recent Comments

Please support this site:

This site

Achives

Friends