~ J-POLE DOUBLE HEADER ~
I. A Brief History of the
By Richard Morrow, K5CNF
he J-pole antenna has been around for many years, in one form or another. It is a derivative of an end fed Hertz antenna, which is a voltage fed antenna. One of the disadvantages of this antenna is that it brings part of the radiating portion of the antenna into the shack. Another is that it will not discriminate against harmonic radiation, which can generate interference on other frequencies, TV sets being among the leading contenders for problems, along with any other solid state device. So there must be a provision for harmonic filtering if you use an antenna of this sort. Figure 1 is a version of this antenna. There are variations of feeding this antenna, however it is not in general useage anymore due to the interference problems that they can and will create.
However, there is a derivative of this antenna called the Zepp, which is short for Zeppelin antenna. This antenna was used on the famous lighter than air Zeppelin airships of many years ago. The antenna was either used as a trailing wire antenna, where it was allowed to trail the aircraft, being fastened at one end and at the feed point. When the aircraft was coming in for a landing, the antenna was reeled back into the ship. The alternate method had one end of the antenna attached to the lower vertical stabilizer and the other to a folding mast that extended down from the bottom of the radio room cabinet. Figure 2 shows how this was done. It was desirable to keep the radio room away from the main body and cabins of the Zeppelin due to the explosive nature of the hydrogen gas that was used to supply the lift for the airship. Since the radios of that time had great spark generating capabilities, it was usually isolated from the rest of the airship in a separate cabin. The cabin could only be reached by walking out to the shack via an open catwalk, which was a thrill in bad weather at normal flying altitude. The antenna was also kept as far away as it could be from the airship. Figure 2 is of a helium filled airship, which is non-explosive, so the radio shack was combined with the rest of the airship.
Now any single wire resonant antenna fed with a two-wire transmission line is called a Zepp and at one time the Zepp was a very popular antenna. Now how does this apply to a J-pole you say? Well it is due to the way the feed system evolved. Figure 3 shows how the original antenna on the Zeppelin was fed. Figure 3A is of the antenna with the feed point represented by the tx point on the antenna. Note that it is fed at a current maximum, or at the center of a half wave portion of the antenna. In this case, the antenna is one full wavelength long. The current flowing in the antenna is represented by the light purple lines. Now if you take the 1/4 wave antenna element to the left of the tx input and fold it around to parallel the other 1/4 wave element, the current flow as illustrated in Figure 3B, will be out of phase with the current in the other 1/4 wavelength section. This will effectively keep these sections from radiating. The remaining 1/2 wave section will radiate very well since there is no counteracting field present to keep it from radiating.
I realize that this is a very brief explanation of the radiation characteristics of this antenna but a detailed one is not required.
Figure 4 is a better illustration of this. In this diagram, the transmitter impedance is matched by tapping up and down on the 1/4 wave stub until the correct impedance is found that matches the transmitter. This antenna will be a mono-band antenna only.
The J-pole in the form that we normally use is made of tubing, either copper or aluminum. There are versions made of 300-ohm TV twin lead, which can be rolled up and put in a pocket. Figure 5 shows the version that we normally install at home or other permanent or semi-permanent operating positions. The J-pole is an antenna that has been around a long time and will probably be around for a long time. It is easy to build and inexpensive as well. It can be built for any of the higher amateur bands and has a low angle of radiation. Next month after the testing period is over, the last section of the 3 element stacked J-pole will be presented. -30-
II. Double Your Power!
By Old George Sharp, KC5MU
Ive been a Ham since I was 16 (now 83). I have experimented with antennas, especially small ones for 80 and 160 for the last 15 years. There have been a few successes and many failures! My best successes have been with 10-foot diameter loops made of 3/4 inch hardline using their internal capacitance for resonance on 80 and 160.
I am a retired Navy Captain (Submarines) USNA Class 1939 with some extra electronics courses back in the tube days. I have the usual Ham gear powered by Solar charged batteries. My best and well-used test equipment is an MFJ Analyzer.
George F. Sharp (Old George!)
ow, here is a J-pole antenna for 2 meters that will give you a definite increase in signal strength to the tune of 3 dB. It is a 2-5/8 wave collinear that will be easy to build and tune.
The materials list is as follows.
2 ea 10-foot (3.04m) lengths of 1/2 in. (1.27cm) electrical conduit.
1 ea 3x7 (7.62x17.78cm) sheet metal splicer for 2x4 (5.08x10.16cm).
1 ea SO-239 chassis connector - open one mounting hole with a 5/32 (4mm) drill and solder in a 4 in (10.16cm) wire to center conductor.
3 ea #8x 1/2 self-drilling sheet metal screws.
4 ea #8x 1/2 sheet metal screws
1 ea dowel 5/8 (63mm) diameter, 12" (30.48cm) long. Wood is ok, but fiberglass or plastic would be better
1 ea +/- 24in. (60.96cm) long wire, #8-12 wire size for phasing hairpin.
Putting A and B together
Force dowel C into A up to the mark. Measure 3/4inches (1.91cm.) from the end of the dowel and drill a 1/8inch (.32cm) hole. Screw in a #8-1/2 inch (1.27cm) sheet metal screw. Force the other end of the dowel up to the mark. Be sure that there is a 1/2 inch (1.27cm) space between A and B. Exactly opposite the screw in A measure 3/4inch (1.91cm.) up on B and drill another 1/8inch (.32cm) hole. Screw in another #8-1/2 inch (1.27cm) sheet metal screw. These two screws hold A and B together and provide for holding the hairpin in place.
Editor's Note: The J-pole has been around for years in one form or another. I like to use copper pipe for my J-poles as copper can be easily soldered and it is more resistant to corrosion than other materials particularly if you paint it. This is a good inexpensive antenna with gain that is easily built with a minimum of tools.
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Last modified: December 31, 2010