|Building a VHF or UHF Ladder Line J-Pole Antenna|
|Articles - Technical|
|Written by Javen O'Neal|
|Sunday, 06 March 2011 14:39|
For those of us who live in RF canyons or have difficulties hitting a local repeater with a rubber duck, a better antenna is a must. I live at the bottom of Poly Canyon on the California Polytechnic State University campus on the first floor of apartments that have the same electrical properties as a Faraday cage. It should not be necessary to lean against the window transmitting a full 5 watts to to hit a repeater that’s only half a mile away.
As Marcel Stieber, KI6QDJ, helped me discover using a RF service monitor (a.k.a. awesome), the CTCSS coming out of my HT is quite unstable, and partially explains why I can't make it into the campus repeater very well. After my recent FT-60R purchase this has been less of an issue, but I still wanted a way to hit the repeater on 50 mW and come through like I was in the same room.
After an initial suggestion from Marcel this past summer, I decided to host a portable J-Pole antenna build session for club members and anyone else in the community who was interested. So on Saturday, January 22, 2011 we made history. The turnout was great. There were 15 signups with a cost of around 15 dollars for antenna materials.
Marcel KI6QDJ, John Chen KI6QDF, and I had a difficult time finding reasonably-priced SMA coax cables at any usable length, which is why we opted for BNC cable with an SMA adapter. For the truly brave, you could try crimping your own BNC or SMA cables.
Building the Antenna
Measurements from Ham Universe were used as a general guideline, and the antennas were tuned from there. (The measurements from this site start the antennas around 138MHz and allow plenty of room to tune it up to band to 144-148MHz for repeater use). Therefore the antenna will be longer than needed with the lengths given in the instructions and will need to be trimmed shorter during tuning.
Preparing the Coax Cable
We bought the longest affordable BNC coax cable we could find, which was 10 feet. If you can find 12 or 20 foot cable, you’ll be better off. Remember, J-Pole antennas are very sensitive to nearby objects, so your performance will increase the farther you and your radio are from your antenna.
Making the Staple
Remove about 1 inch of plastic from the end of your ladder line. Fold over the exposed copper and solder together. For the strongest joint possible, the wires should overlap the entire width of the ladder line. Be sure this is symmetrical and a nice clean solder joint. Contaminates and uneven connections make RF behave erratically and can decrease the lifetime of your device.
Connecting the Coax to the Ladder Line
Measure 1.25 inches above the staple. This is one of the crucial measurements on your antenna; a millimeter makes a difference. Remove the plastic covering from both sides of your antenna, leaving a gap between the feed point and ladder line to avoid melting the plastic when soldering. You may need a knife to remove the plastic. The 1.25 inches is measured from the base of the staple to the point where the coax cable first contacts the antenna. Ensure that both the left and right feed points are the same distance from the staple.
When soldering, avoid melting the dielectric insulator or ladder line, so minimize the time the iron spends on the joints. To reduce strain on your feed points, zip tie the coax to the antenna to restrict movement. This will reduce the number of broken wires and increase the lifespan of your antenna. There are several attachment options, as pictured to the left.
To help with durability, strain relief, and weather proofing, we used liquid electrical tape on the coax solder joint at the base of the antenna. You could also use normal electrical tape, but would not have as much water resistance. Regular silicon would do the trick as well.
Wrapping the RF Choke
I decided to wrap my coax 5 or 6 times around a 1.25” long, 0.50” diameter PVC pipe segment that I cut from a longer piece of pipe. Ham Universe added their RF choke about 18 inches from the base of the antenna and omitted the pipe. Secure with zip-ties.
Cutting the J
Measure 18.5 inches from the staple and cut a piece out of the side of the ladder line connected to the shield. Using wire cutters, remove an inch of ladder line above this point. If you keep the upper half of the now electrically disconnected U, it will affect your radiation pattern and SWR. One of our members had luck tuning his antenna by removing about 2 inches of ladder line above the tip of the J. Other members had luck removing the entire parasitic element. If you go this route, a razor blade and wooden board will result in a cleaner result. IMPORTANT: leave a couple rungs at the top of your antenna so that you can hang your antenna. We advise against using a ring terminal connector as these can detune the antenna and are a hassle to remove and reattach when tuning your antenna. Optionally you can use zipties to fabricate a hanging mechanism.
Tuning Your Antenna With a SWR Meter
Disclaimer: Let me first make it clear that an antenna can have a low SWR but still not be resonant at that frequency. By tuning an antenna purely on SWR, we hope that it is resonant within the ham bands which it will likely be. This topic however is beyond the scope of this article.
As emphasized previously, each dimension can dramatically affect the performance of your antenna. Adjusting the feed points at the base of your antenna can move your SWR around and affect its magnitude. Leaving the parasitic upper half of your J-Pole intact can affect your results. Justin Kenny, KJ6KST, left his parasitic element on his antenna, but just by increasing the gap between the J and the parasitic element, his SWR dropped from around 2:1 to 1.2:1. If your feed points and measurements aren’t precise, now is your opportunity to check and fix them before you permanently shorten your antenna.
It makes sense that a longer antenna performs best at longer wavelengths, so as you trim length off your antenna in the tuning process you will be increasing the frequency of your lowest SWR (likely your resonance too). To simplify the process, let me define SWR which is your "Standing Wave Ratio" (the ratio of transmitted vs reflected power). A SWR is created when part of the transmitted signal is bounced back towards the transmitter from an unresonant antenna, idealy an antenna has a 1:1 (pronounced one to one) SWR but it is more realistic to obtain a 1.3:1 or lower SWR.
It becomes tricky when tuning the J-Pole for use on two bands as an untuned J-Pole may have a SWR of 1.8:1 at 435 MHz (70 cm band) yet when measured on VHF (Very High Frequency) in the 2 meter band, you may discover that that antenna has a SWR of 2.3:1 at 145 MHz. If this is the case, shortening your antenna will increase the frequency of your SWR on both 2 meters and 70 cm. Care must be taken if you are tuning this J-Pole for dual-band use. For this reason, tuning a J-Pole antenna for dual band operation is very difficult.
For the curious, hamuniverse.com also has dimensions for a 5/8 wave J-Pole, which requires an extra foot of ladder line but provides you with more gain. Good luck! Post your own J-Pole adventure in the forum!
I would like to thank Justin Kenny (KJ6KST) and Marcel Stieber (KI6QDJ) for the photos within this article.
|Last Updated on Tuesday, 12 April 2011 15:46|