Amateur Radio

How-To

How To Build and Tune a Roll-Up 2M J-Pole

The Problem

So you need a good 2M antenna that doesn't take up alot of space and you don't wanna spend alot of money. A Roll-Up J-Pole is a great antenna that fits this bill as it can be made out of 300 Ohm Twin-Lead wire. You remember 300 Ohm Twin-Lead, it's the wire you used before Cable TV existed. This is a pretty simple antenna to build as long as you have a few parts and tools.

You will need:
  • 60 Inches of 300 Ohm Twin-Lead
  • A few feet of RG-58 or similiar Coax
  • A connector for your radio installed on the Coax
  • Soldering Iron
  • Rosin Core Solder
  • 3rd Hand Tool or Vice
  • Utility Knife
  • Wire Stripper
  • Wire Cutters or Scissors
  • Tape Measure
  • Heat Shrink Tubing or Small Zip Ties

To tune your antenna you will need the following:
  • SWR Meter
  • 2M Transmitter
  • Plastic or Wood Clothes Pin
  • String or Zip Ties




Measure and Strip Base

I usually leave my 300 Ohm Twin-Lead in a coil with Velcro to hold it while I start my antenna. Pull out enough to work comfortably with and get your tape measure. Measure in 1/2 Inch and remove all the plastic so only the wires are exposed. Do this by using your Utility Knife and slicing the center insulator out as one large chunk. Then when you have two insulated wires use your wire strippers to remove the insulation from both wires.

Cross Conductors

Now that you have stripped 1/2 Inch of insulator, cross the two conductors and twist them together tightly.

Twist Conductors

Twist the two conductors tightly together. You want the wire to go from one side straight across to the other. Then place the cable into your 3rd hand or vice.

Solder

Using your Soldering Iron, solder the two conductors together. You want to make sure both leads are well secured and the wire is tinned just to the edge of the insulation.

Cut Excess Conductor

Using your wire cutters trim off the spike of soldered wire so the conductor is more or less straight across. Remove the wire from your 3rd hand or vice.

Measure and Score Cable

Lay the cable flat on your work surface and measure 1 and 1/4 inches from your solder joint and remove 1/8 inch of material from both sides. Your cuts should be from 1 and 1/8" thru 1 and 3/8". Be careful to not cut the conductors on both edges.

Remove Spacer

Flip cable as necessary to cut through the thick spacer and remove the 1/4 inch of material.

Remove Insulation

Very carefully using your utility knife remove the insulation from the conductors. I usually score around the insulator until I just hit the conductor. Once I have done that to both ends and can spin the insulation I will slice carefully down the middle so I can peel it off.

Strip Coax

Using your utility knife strip between 1/2 and 1 inch of outer jacket off of your coax. Be sure not to cut the conductors.

Seperate Shield

Unbraid and seperate the outer shield conductors of the cable and pull them to one side then twist them tightly together. Next strip the center conductor insulator about 1/8 inch from the outer shield. You need both conductors exposed but unable to touch each other.

Fold Conductors Straight Apart

Fold both conductors parallel to each other but in opposite directions. They should be as equal to each other as possible so they will attach at the same points on the antenna.

Tin Twin-Lead Conductors

Place your 300 Ohm Twin-Lead back in one side of your 3rd hand and lightly cover your two exposed conductors with solder (called tinning). This will aid attaching the coax in the next step.

Align Coax and Twin-Lead

Place coax on opposite side of 3rd hand and align the exposed coax with the center of the notch on your 300 Ohm Twin-Lead. Place the coax conductors so they touch both leads of the twin-lead. Ensure the coax is centered down the short end of the twin-lead.

Solder Coax to Twin-Lead

Solder the Twin-Lead and Coax together. You want this connection to be strong so make sure you get solder all around and inside the joint.

Trim Excess Wire from Joint

Remove cables from 3rd hand and use your wire cutters to remove all the excess wire from the edges. Do not remove so much that you weaken the joint, but ensure that the remaining joint is not sharp. Use your tape measure and verify you have 1 and 1/4" between the joint to the base of the J-Pole.

Measure Overall Length of Twin-Lead

Now it's time to roll out your twin-lead flat and measure off the long part of your antenna. I am starting with 60 inches of cable, so I now have 59 and 1/2 inches left. If you do not have an SWR Meter to tune your antenna you may trim shorter. Use the data from when I tune the antenna to guess at what length to use.

Measure the Short Side of Antenna

We need to measure the short leg of the J-Pole. Do this by measuring from the base of your antenna about 17 Inches up on one side of the antenna. I have made this antenna with the short side connected to the shield and center conductor of the coax, so it should not matter which side you choose.

Cut Notch for Short Side

Measuring from the base of the antenna cut a 1/4 inch notch at 16 and 3/4 inches. This notch should be deep enough to break the conductor but no deeper. The insulator of the twin-lead needs to remain intact for physical strength of the antenna.

 

Ready to Tune

Now you are ready to start tuning your antenna. If you do not have access to an SWR Meter I recomend you read this following section as you can get the lengths of the finished antenna. Any antenna will work better when properly tuned so I always recomend tuning your antenna prior to use. It is illegal to transmit without a license and without identifying yourself even when tuning an antenna. When tuning use as little power as is necessary to get an accurate reading on your meter.

Tuning Clip Information

To tune my antenna I am using a wooden clothesline clip with two small zip-ties and a hook in the middle of my living room ceiling. This antenna can be tuned anywhere that it is away from metal. Use a plastic or wooden clip to hold the antenna while tuning so you can quickly pull the antenna out to trim off cable.

Baseline SWR

Before I start tuning I will take a base SWR reading of my antenna. SWR is a measure of how effectively the antenna radiates the power it receives. A badly tuned antenna will radiate badly and can damage your transmitter. My SWR meter is a Dual Needle which means it shows the power going to the antenna and the power returning from the antenna. By looking at where these needles meet I can measure the SWR. This looks like a 1.9:1 SWR. Most amateur radio operators will tell you anything under 1.5:1 or even 2:1 is acceptable, but I like to get as close to perfect as I can get.

Tuning Procedure

One of the problems I ran into when building this antenna for the first time was that none of the instruction pages I found had instructions on how to tune the antenna, only the schematic of how to build it. The instructions on how to tune a copper j-pole helped some, but half of them involve moving the feedline point up and down which is not feasable with this antenna. As a result I am writing this page as an explination of how to tune, and I will be tuning this same antenna here.

Hang the end of your antenna from a clip attached to the ceiling or a tree. You want a plastic or wood clip so there is as little metal as possible to interfere with the antenna as well as an open area to do your tuning. I used a wooden clothesline clip attached to a hook hanging from the ceiling in my living room. Attach the coax to your SWR meter and radio. I am tuning this antenna to be centered on the US 2M band (146MHz). For each SWR adjustment I will take a reading at 144MHz, 146MHz and 148MHz. When tuning an antenna, you are finding the sweet spot where the antenna radiates the best. As the frequency moves away from that sweet spot the SWR starts to go up. Since we know this, we can move that sweet spot by changing the length of our antenna. It is always easier to remove material than it is to add material so we start with a slightly long antenna and cut it down to size. Wavelengths get longer as they get lower in Frequency, so if our antenna is long it should be tuned lower. By shortening the antenna we raise the tuning point or sweet spot.

The following is a chart of the SWR at each frequency and how much length I trimmed from the tip or stub of the antenna. Read it, "At 144MHz the SWR was 1.6 and the SWR was 2 at 148MHz so I trimmed 1/2 an inch from the tip and none from the stub."

SWR @ 144MHzSWR @ 146MHzSWR @ 148MHzRemoved from TipRemoved From Stub
1.61.823/4"-
1.61.823/4"-
1.51.823/4"-
1.451.751.951/2"-
1.41.71.951/2"-
1.31.71.91/2"-
1.251.61.6-1/8"
1.21.451.6-1/8"
1.31.31.65-1/16"
1.41.21.6-1/16"
1.451.11.55--

As you can see by the lengths of material I cut, it took a few inches to get close to the sweet spot. Once I got close to the sweet spot I did not have to remove as much material to make the changes. I start with the long side first when tuning as it seems to move the sweet spot the most. Once you get the low end close to where you want it, start trimming the stub very slowly and watch as it brings the tuning into focus. I tuned this antenna to be centered on the 2M band, however this procedure will work for just about any frequency.

Final SWR

As you can see, now the SWR is much closer to what we want. This will more efficiently transmit our power and will reflect less back into the radio.

Secure Coax

Now that your antenna is tuned and you know your connections are good it's time to secure your coax to the bottom of the antenna. You can use Electrical Tape (I don't recomend this), Zip Ties or Heat Shrink Tubing. I am using 3 inches of 1/2 inch diameter tubing. Slide your heat-shrink tubing from the end of the antenna down until it covers the solder joint and strengthens the entire area.


Shrink your Tubing

Use a lighter, torch or heat gun (preferred) to shrink the tubing fully. The 1/2 inch tubing I used closes nicely around the coax and the twin-lead. Optionally you can place tubing around the notch for the short leg, but for this antenna I am not. I have found the heat causes a kink in the thin twin-lead so I would rather leave it bare and let it hang flat.

Final Measurement

After tuning the notch has grown from 1/4 inch to just about 5/8 inch. This means the short side of this antenna is 17 minus 5/8 for a total of 16 and 3/8 inches long.

The overall length of this antenna as tuned is 55 and 1/2 inches long. This means we trimmed 4 inches off the total length. This amount could have changed if we were tuning it for the low or high end of the band. I tuned this antenna for 146MHz.


Antenna Hanger

The final step is to attach some method to hang the antenna. I used a short zip tie and cut a hole into the insulator, but string would work just as well.

Finished!

Now you have a portable antenna that offers gain, can be hung just about anywhere, can should be able to run 25 to 50 watts of power. All in about 7 inches space!