Tamara Wilhite is a technical writer, industrial engineer, mother of two, and published sci-fi and horror author.
What Are J Pole Antennas?
J Pole antennas get their name for their J shape. They’re often called J antennas for this very reason. The design is a modification of the early Zepp antenna. These antennas have a number of pros and cons.
What Are the Advantages of J Pole Antennas?
They’re easy to build. Simply cut and shape a piece of wire to the desired size. Furthermore, they can be made from a wide variety of materials.
They’re easy to tune. Simply adjust the length of curved wire. Some of the early versions were made out of common television twin lead antenna wire. Ladder wire is commonly used, as well. Today, it is common to use copper plumbing fittings when making the J pole antenna. Not only does this make it cheap, but there are versions you could roll up and put in your pocket.
As a length of bent wire, it gives you a lightweight half-wave antenna compared to a yagi antenna. You have low impedance at one end of the quarter wave line reflected as high impedance in the “J”. This results in a half-wave transmission line.
J-pole antennas like monopole antennas are omni-directional.
More on the J-pole Antenna (#163)
What Are the Disadvantages of J Pole Antennas?
They aren’t very robust. Wind or physical impact could alter the antenna’s geometry. This doesn’t matter much if you can go out and fix it.
Another problem with this design is that while there are many designs for J antennas, very few have been tested. Then again, the antennas are fed from an unbalanced coaxial cable. The power goes through the matching system, which is balanced, and then to the unbalanced radiator. This mixture of balanced and unbalanced often distorts the radiation pattern. It also makes matching a challenge.
End feeding any half-wave antenna is challenging due to the high and variable impedance. This is true for J-pole antennas, though this is true of other half-wave antennas, as well.
What Are Some Unusual Features of J-Pole Antennas?
You can adjust the J-pole antenna, though there are extendable monopole antennas, as well. You can move the feedline and the parallel matching section up and down. Unfortunately, you’ll need to do so if the weather conditions make it necessary due to the impendence issues we’ve already mentioned.
How Are J-Pole Antennas Used?
J-pole antennas are commonplace in amateur radio. They’re often used as vertical antennas for VHF and UFH. They’re rarely used in commercial applications. When reliability and performance are key, a quarter wave vertical antenna with radials is the better choice.
Is a Slim-Jim Antenna the Same Thing as a J-Pole Antenna?
A slim Jim antenna is 60 inches long, if you’re working on the two meter band, and it is very narrow. It uses the J-type integrated matching or JIM stub. That is the source of the name “Jim”. All J-pole antennas are slim and have an integrated matching antenna. This makes all slim-jim antennas J-pole antennas. However, not every J-pole antenna is a slim-jim. Many J-pole antennas have a take-off angle too great to make it a slim jim antenna. For example, you may be advised to have a 20 degree take-off angle for a J pole antenna when a comparable slim jim antenna on the same frequency only needs 8 degrees.
If the J-pole antenna uses a folded dipole as a half-wave radiator, it can be referred to as a 2BCX Slim Jim antenna.
How Do J-Pole Antennas and Slim Jim Antennas Differ?
They share the same general design. However, the slim jim antenna folds another half wave element over itself than the J-pole. That turns the J-pole antenna into a doubled half wave antenna.
One point in favor of the slim jim antenna is that it gives you more gain than a standard j-pole antenna. It may have up to twice as much gain.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2021 Tamara Wilhite