Author Topic: DIY portable wire J-pole CB antenna  (Read 9924 times)

Offline Alan Georges

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DIY portable wire J-pole CB antenna
« on: November 23, 2013, 09:23:16 PM »
    A couple of years ago I built a go-bag dipole antenna for a CB radio.  It was easy to build and tune and it worked fine, but it was a pain in the butt to hang.  Driven end... up.  Shield end... down.  And feed coax... into the center and perpendicular!  That last one was always the problem.  Whatever height you hoist the top to, the feed line has to come in sideways at almost exactly eight feet lower.  Just try that in a pine tree sometime, with bugs chewing on you to add to the hilarity.  (Here’s that plan, for what it’s worth:
http://thesurvivalpodcast.com/forum/index.php?topic=31265.0 )

There had to be a better way, something fed from the bottom end, and good riddance to that awkward midpoint feed line.  But you can’t just feed a wire from one end, the impedance is all wrong.  Well the Germans had this same problem with their Zeppelins in World War I, and they solved it by attaching a very simple, clever matching network between the feed line and an end-fed wire antenna.  Here’s the general idea:


BTW, that’s from the Wikipedia article, http://en.wikipedia.org/wiki/J-pole_antenna .  That 1/4-wave U-shaped bottom is called a tuning stub.  It matches the impedance of the coax cable (50 Ohms) to the end of the 1/2 -wavelength wire business end of the antenna.  And yes, it really is shorted across the bottom. But it’s a DC short, RF currents don’t quite see things the same way.

There are about a gazillion J-pole antenna plans on the web, but most of them involve ham UHF frequencies and copious quantities of soldered rigid copper pipe.  Not what we’re looking for in a roll-up CB antenna.  This page, http://www.qsl.net/dk7zb/J_Pole/wiremanjpole.htm , had the best, most applicable design.  Still, there are some quirks to that design, and he doesn’t give numbers for U.S. CB’s 27 MHz center frequency.  Here’s the general layout:


Borrowing again from that site, here’s another layout figure:


You can see the wire antenna sticking out of the top, the ladder-line tuning stub at the bottom, and the XX points where the feed line attaches.

I’ve tried to avoid weird electronics parts here, but the two that make this project easy are 450 Ohm ladder line and a SO-239 coax connector.  The ladder line keeps the width of the bottom of the “J” a consistent width, and it’s cheap and easy to find.  Here’s what I used ($0.37 per foot, you’ll need about 9 feet): http://www.hamradio.com/detail.cfm?pid=H0-009557  For the coax connector, I used this http://www.amazon.com/Falcon-Products-So239-Female-Ground/dp/B004432DKO/ref=sr_1_75?s=electronics&ie=UTF8&qid=1385254518&sr=1-75&keywords=so-239 because it’s a little less bulky, but this http://www.hamradio.com/detail.cfm?pid=H0-009213 will serve as well and ordering it together with the ladder line will save you some shipping charges.  Everything else you can get down at the Radio Shack, or maybe even from Wal-Mart. 

Here are the rest of the parts needed:
 - 19’ of wire, in any size in the range from 12 to 22 AWG.
 - one wire nut (or similar way to join two wires)
 - half-dozen zip ties, or bag twist-ties.
 - coax cable, at least 20’ long.
 - RFI choke big enough to go around coax (optional, more on this later)[/li][/list]

For tools you’ll need are a knife, wire cutters, soldering iron, and solder.  Also, you’ll need a way to hang this from a tree limb.  I recommend paracord, and a fishing rod, sinker, and slingshot.


Here are the length numbers for CB frequencies:

The single wire, L1: For 22 AWG wire like I was using, it comes to 17’ 6”.  For heavier wire in the 12 - 14 AWG range, this would be a little shorter, about 17’ 3”.  If you have an SWR meter, cut it 6” long and tune it down (more on this in a bit).  If you don’t, just cut to these numbers and cross your fingers!

The ladder line, L2: 8’ 2.3”.  Just cut the ladder line to this length and go with it.  At the bottom end, leave “legs” so that you can twist them together (see picture).  At the top, leave one leg so that you have something to attach the wire to (see picture).






The feed point XX is 6” from the bottom of the ladder line.  This measurement is surprisingly uncritical, but try to hit 6” when you solder things together.  When you cut the ladder line, try to cut it so that the feed point is in one of the “windows” and it’ll be easier to attach the feed lines.

All described, here are the assembly steps:

(1) Cut the ladder line to the specified length, leaving two legs at the bottom to twist together, and one leg at the top to attach the antenna wire.  Before cutting, try to line things up so that one window in the ladder line is centered at 6” from the bottom end.

(2) Twist the two bottom legs together so that they make good electrical connection.  Solder them if you feel like it.  (I didn’t bother.)

(3) Drill two pairs of holes in the center insulating part of ladder line to attach the coax cable as seen in the final assembled picture.  You want the coax to run straight down the middle of the ladder line.

(4) Shave about 1/2” of insulation off the two sides of the ladder line at the feed point, centered 6” from the bottom of the line.

(5) With some of the extra antenna wire (you know, the 19’ piece), cut two 3” pieces.  Solder them to the coax connector and to the ladder line attach points.  Be sure to get the center connector wire soldered to the antenna wire side of the ladder line!

(6) Cut the antenna wire to the proper length discussed above, or to a little more than 18’ if you have an SWR meter and are going to tune this the right way.

(7)Attach the antenna wire to the single leg on the ladder line, with the wire nut or any other way you wish.  You may need to trim back some of the ladder line’s center insulation, as seen in the pic.

Here a picture of the main part all completed:


Done!  Well, almost.  Arrange for a piece of paracord to drape over a convenient limb, 20’ or higher, in your yard.  Attach the coax to the antenna’s connector, and zip-tie it down to the ladder line, keeping it centered on those holes you drilled earlier.  Clip on the RFI choke just below the antenna; you may need a zip-tie to keep it from sliding down (mine did).  Hoist the antenna.  If you can’t get all of the stub off of the ground, stretch it out horizontally well above the ground.  It’s not critical, but don’t leave it laying on the ground.  (This may involve paracord, duct tape, and/or strong language.)  Try to get the main antenna wire vertical.

If you couldn’t find an RFI choke, wrap a few turns of the coax around something like a piece of 3” PVC pipe or a peanut butter jar, just below the bottom of the stub, and tape them down to keep it all from moving.  Either of these methods will keep RF power from creeping back down the feed line.  It’s not enough energy to worry about for cooking you, but it can affect tuning.  Stopping it at the bottom of the antenna keeps things consistent, no matter which way your coax cable is flopping around.

If you’re going to tune, hook up your CB & SWR meter.  It should give the best (i.e., closest to SWR = 1) reading at Channel 1, and ramp up as go up to Channel 40.  Tune the antenna by chopping off an inch or two at a time of the wire’s end.  (drop, untie, chop, re-tie, re-hoist; wash, rinse, repeat)  Stop when things are about centered on the CB band, or when it’s good enough for you.  Mine was able to tune down to SWR = 1.2 on Channels 1 & 40, dropping to almost exactly 1 in the middle.

Other general notes:

This antenna doesn’t have to go up.  You can hang it upside down from a (non-conducting) broomstick out of a window, if that suits your needs.  The ladder line doesn’t need to hang straight down, it can run along the broomstick.  The SWR may change, especially if the window frame is metal, so prop up the broomstick. 

If you use unlockable zip-ties (or twist-ties) you can detach the coax from the antenna for easier, tangle-free storage.

There was something weird in that other J-pole design at one of the above links, with the coax connector soldered onto the bottom where things are shorted out.  Just didn’t sit well with me.  But if it works, it works.

I originally tried to do this with 300 Ohm TV antenna cable, but it was much harder to work with, and I couldn’t get it to tune down consistently.  Too bad, because that stuff is much more common than 450 Ohm ladder line.

It rolls up sort of compactly, but the ladder line doesn't roll up into a diameter much less than 10".  Small price to pay for being able to end-feed this thing.

The real mystery in designing this thing was “what is the distance to the feed point XX?”  All of the design calculators assumed you could customize the width of the “J”, but here that’s set by the ladder line’s width.  I found it by building a test mule antenna and running up and down the stub with a clip-on coax fitting.  (see picture)  Reminds me of that bug Morpheus’ crew pulled out of Neo’s belly button, except this one has lobster claws.  (http://www.youtube.com/watch?v=X08SJFEBSxA)


Enjoy!

Offline Alan Georges

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Re: DIY portable wire J-pole CB antenna
« Reply #1 on: July 05, 2014, 05:58:20 AM »
Two more tricks I figured out while building a similar antenna for 2m ham:

- Use blue painter's masking tape to control the window/ladder line when you're marking and cutting it.  Tape it down on a work surface to stretch it out, it makes the entire thing much easier and more precise.

- Instead of drilling through the ladder line for zip-tie holes, a hot soldering iron punches through more easily and neatly.

Offline Alan Georges

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Re: DIY portable wire J-pole CB antenna
« Reply #2 on: November 28, 2015, 01:54:29 PM »
In random surfing window shopping for multi-band trail ham antennas, I stumbled on what's effectively a pre-made version of this DIY antenna: http://www.lnrprecision.com/store/#!/EF-11/p/52657628/category=10468543

The upsides of this antenna are that it's pre-made, well made (probably; I have one of LNR's shortwave end-feds and that's well made), 8' shorter because the ladder line is replaced with a little transformer box, and would stuff more compactly into a pack than that 8' of ladder line which has to be carefully rolled so as not to kink.  The downside is of course that it costs $58.  TANSTAAFL.

Either the DIY or the pre-made antenna shown here, plus something like a Midland 75-785 hand-held CB along with a suitable length of coax, and somebody could have the 10 mile go-bag license-free two way comms problem solved.

Offline Carl

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Re: DIY portable wire J-pole CB antenna
« Reply #3 on: November 28, 2015, 03:10:36 PM »
Good deal Alan...you made a bigger stick...If I could suggest that paracord be used to hang and SUPPORT the antenna AND coax to eliminate stretch and breakage and with the paracord taped to the antenna and feedline it should be easy to deploy and support it's
weight without mishap.

Offline Alan Georges

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Re: DIY portable wire J-pole CB antenna
« Reply #4 on: November 29, 2015, 08:52:10 AM »
Mmmm, paracord.  It's like beer: the solution to and cause of most of life's problems.  One thing I'd worry about is how wet support paracord could change this thing's SWR.  Because Murphy.

Two side notes.  After cobbling together several to give as Christmas gifts a few years ago, I have let my own lie neglected in the parts bin.  Maybe next weekend.

Second note, I'm turning over the idea of building a 100% ladder line 11m slim jim.  It could have a broader bandwidth as well as a lower take-off angle, in the same way it does for 2m.  However, the 1" spacing of ladder line, which makes a difference on 2m, might not even be noticed by 11m waves.  The result might not be any improvement over this j-pole.  Any thoughts, Carl?

Offline Carl

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Re: DIY portable wire J-pole CB antenna
« Reply #5 on: November 29, 2015, 11:47:04 AM »
Mmmm, paracord.  It's like beer: the solution to and cause of most of life's problems.  One thing I'd worry about is how wet support paracord could change this thing's SWR.  Because Murphy.

Two side notes.  After cobbling together several to give as Christmas gifts a few years ago, I have let my own lie neglected in the parts bin.  Maybe next weekend.

Second note, I'm turning over the idea of building a 100% ladder line 11m slim jim.  It could have a broader bandwidth as well as a lower take-off angle, in the same way it does for 2m.  However, the 1" spacing of ladder line, which makes a difference on 2m, might not even be noticed by 11m waves.  The result might not be any improvement over this j-pole.  Any thoughts, Carl?

I think it will work ,though not as well as a ground mounted 1/4 wave with 3 or 4 of the 1/4 wave counterpoises as it really needs interaction with ground for propagation rather than height above ground as used for  a horizontal antenna.

I have made and used "J" poles like my Big Stick ,adjusted for frequency ,on 6 meters and many MARINE install on boats while I was in Washington DC area proved it will work with minimal ground.(we used a "T" with a shunt/stub of coax in those early days of the '70's)

One of many ON-LINE "J" Pole calculators may aide with starting length for the elements,  :)

Offline Carl

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Re: DIY portable wire J-pole CB antenna
« Reply #6 on: September 15, 2016, 07:51:23 AM »
I was passing by on a GOOGLE SEARCH and felt I should add that this is very similar to the ZEP or W3EDP antenna used for multi-band antenna , just it is cut to size for CB (11 meter band)