back up next
By Philip Karras, KE3FL, reprinted with permission
If you look at the antenna, it is a 3/4-wavelength radiating section attached to the matching stub by the shorting bar; all together it looks like the letter J, hence the name J-pole.
Read all of these instructions before beginning your construction project. Nothing is more frustrating than doing something, only to find a hint afterwards that would have made the project go smoother.
See below for a listing of parts and tools you'll need to make up this simple antenna.
Using "ladder line" is a bit different than using solid-dielectric TV twinlead. Before cutting, stretch out the wire so that you can position the proposed cuts at a position that has a center plastic support, and not at a position that has no center plastic. This may not be possible for both the 1/4-wavelength section and the total length position. If it comes down to a choice, I recommend selecting the support at the top.
This plastic melts well and can be melted back together. I have had to melt sections back together in both locations, and the antennas work just fine and hold up to field rigors.
Select the bottom of the antenna and strip off about 3 to 3-1/2 inches of insulation from both wires. Tack solder (temporary solder joint) a piece of wire as a shorting bar about 1 inch from the bottom of the antenna (this bar may need to be moved).
To start with, the coax will be connected about 1-1/4 inch from the shorting bar. This connection and the shorting bar connection may need to be moved in order to achieve the best SWR and frequency match.
Measure 17 inches up from the shorting bar on one end only and cut a 1/4-inch gap in the wire at this position. (You can melt the plastic back together at this location if needed.)
Now measure 52-1/4 inches up from the shorting bar. If this location has no center plastic support, try to remove as little insulation as needed in order to get at the wire and snip it. Cut out at least one inch of wire, then melt the plastic back onto the locations where you removed it.
I use a sharp knife to cut into the insulation and not into the wire. Then I pry the wire out with a pin and snip it or solder it at the correct location.
Bend the coax about an inch from the end, and score the insulation with a sharp knife. This cuts into the insulation without damaging the shield if done gently. Then rotate the coax so you can continue scoring the coax until it is cut all the way around. Cut the insulation from the new cut, up to the end of the coax. You should now be able to pull off the insulation with pliers.
Remember to always cut away from yourself!
Never use wire strippers on the large portion of the coax; it only damages the shield. If you have a tool designed for coax, use it.
Prepare the antenna end of the coax: Separate the coax shield and twist it together. Strip off about 3/4-inch of insulation from the center conductor of the coax. (Do not solder at this time.)
You'll install the appropriate connector (BNC, PL-259) at the other end of the coax. Follow the installation directions that come with the connector, or consult The ARRL Handbook for more information.
Wrap the shield 1-1/4 inch up from the shorting bar around the 17-inch side of the twin lead. Wrap it in such a way that the distance from the coax to the shorting bar is the same for both the shield and the center conductor. Solder the shield to the twin lead.
Wrap the center coax conductor around the longer twin lead wire up from the shorting bar (the same distance that the shield is wrapped to the other wire) and solder it.
Cut off the excess coax wire. Also, cut off all the excess twin lead at the top except for a loop or two. These ladder steps are great for hanging the antenna over a nail or hook, so leave at least one of them.
Your antenna is now ready to test.
Get your VHF SWR analyzer or meter. Hang the antenna away from all objects (I hang mine from the top of a window and this seems to work almost as well as from a tree).
For best SWR measurements, the antenna should be at least 2 wavelengths away from any object. (For 2-meters this is approximately 13 feet.)
Set your radio for lowest power and 146.000 MHz simplex. Test out the antenna for 144.000 and 148.000 as well. If all three are below 1.7 SWR and the SWR for 146 is about 1.3 or lower, you are done. If not, see Help for Lowering the SWR, Changing the Frequency, and Increasing the Bandwidth below.
Once you are done, slip the shrink tubing onto the antenna over the coax connections, squirt some electrical-connection safe RTV into the bottom of the shrink tubing, and then heat up the tubing from the bottom up. This should push (squeeze) some RTV all the way to the top of the shrink tubing. Wipe off the excess and hang the antenna for 12 to 24 hours to let the RTV dry.
The SWR at 146.0 should be close to and below 1.3 to 1; for 144.0 and 148.0, it should be 1.7 to 1 or lower. If you have difficulty obtaining these results, see Help for Lowering the SWR, Changing the Frequency, and Increasing the Bandwidth, below.
At 445.0 MHz, the antenna should read below 1.5 to 1. I have not checked it out as thoroughly as I have 2 meters, but I do know that it is not a nice one-dip curve; rather, it is a multiple dip/peak curve.
Philip Karras, KE3FL, lives in Mt Airy, Maryland. An ARRL Life Member, he holds a field appointment as Assistant Emergency Coordinator in Carroll County, Maryland. He's also an OES, ORS, and a volunteer examiner. He may be contacted via e-mail to firstname.lastname@example.org. Visit his Web site at http://www.qsl.net/ke3fl.
5 feet of 450-ohm ladder line
20 feet of RG-58 or similar coax
2 inches of heat-shrinkable tubing
Soldering iron (20-30 W)
VHF SWR meter or antenna analyzer
RTV silicone sealant
Heat gun or hair dryer (for heat-shrinkable tubing)
If your antenna did not have a nice low SWR at the desired center frequency, try moving the shorting bar down about 0.1 inch at a time until you get the lowest SWR you can--even if this is nowhere close to 1:1. You may have to move it back up if you go too far. Normally I find that I have to move the shorting bar down, ie, away from the feed-point, but it's always possible that it will need to go the other way too.
If you have already cut the extra wire off the bottom of the antenna, you will need to add some back if moving the shorting bar closer to the feed-point only makes the SWR worse. Add about two inches to both the matching stub and radiator at the bottom of the antenna.
Once the position of the shorting bar to the feed point that produces the lowest SWR has been found, move the coax contact points and the shorting bar together until you can get this lowest SWR match at the desired frequency. The important point to remember here is that the distance between the feed-point and the shorting bar determines the lowest SWR. This distance must not change while trying to get the lowest SWR at the desired center frequency.
If the lowest SWR you can get by moving the shorting is not 1:1, it will turn out to be closer to 1:1 once you move both the shorting bar and the coax feed point so that the lowest SWR is at the desired center frequency.
If you need to shift the frequency and moving the tap point doesn't change it enough, you can cut the J-Pole. You should not have to do this for this antenna since the dimensions for this antenna have been worked out over years of experience by many different people.
Here are the two rules of thumb for changing the center frequency of any antenna:
LLL: Longer antenna = Longer wavelength = Lower frequency
SSH: Shorter antenna = Shorter wavelength = Higher frequency
When cutting the antenna shorter, I recommend making only one-half the change you calculate. In this way you may be able to prevent making too large a cut and having to undo it.
All changes are interactive, some more so than others, but expect to see SWR changes for length changes, and frequency shifts when moving the shorting bar/feed-point up and down. (Remember to move both the feed-point and the shorting bar in tandem, keeping the distance between them constant when trying to re-center the lowest SWR at the frequency you want.)
Once again you should not ever have this problem with the 2-meter J-pole since the dimensions have been worked out by calculation and by trial and error by many people. However, if you are trying to design for a new frequency, you might need to be able to change the BW.
A very narrow BW may be an indication that the radiator is too long, or it is too long in relation to the matching stub. I have only performed one experiment so far. In this experiment I added one inch of wire to the top of a good working J-pole antenna for 2-meters. The bandwidth dropped to about 0.6 MHz. When I removed the extra wire, the BW returned to about 3.8 MHz between 1.7:1 SWR points.
Other things I've tried made such small changes in the bandwidth that I was never sure the data was significant. Was the change due to the method tried or did I do something else a bit differently that caused the change?
back up next
Nick Meacher, N3WWE
Northern SE AK ARES