Entries from December 2012 ↓

HF J-Pole

The J-pole antenna has been around for decades and most commonly finds use on the VHF and UHF bands, most commonly on 2 meters. Its dimensions at 2 meters are convenient and easily implemented. The long leg is about 5 feet with the short leg being one third that length. Space the two radiators about 1.5 inches apart, strap them together at the bottom and connect that junction to ground. The bottom quarter wave length serves as a matching section. In the 2 meter example the 50 ohm tap is 1.5 inches above the grounded strap. The signal is radiated from the free standing half wave section above the quarter wave matching section. In the vertical orientation we have an omnidirectional vertical radiated pattern. This is a very special vertical. It does not need ground radials to be effective. It does not need to be insulated from ground which prevents static build-up and allows the antenna to be part of the upper section of a simple antenna mast installation. With all these advantages there is just one major disadvantage for application to lower frequencies, size.

What about lower frequencies? Well, since the long leg of this vertical is 3/4 wavelength long a practical length of 70 feet would be needed for the 30 meter band. Or is that necessarily the case? The 1/4 wave lower section is merely an impedance transformer. This impedance transforming section would transform impedance if it were horizontal as well as it would work being oriented vertically. In fact it would also work if the impedance transforming section was vertical with the 1/2 wave radiator being horizontal. That configuration is more commonly known as a zepplin antenna. Evidently antennas of this type were trailed behind old world war tweo lighter than air craft.

Seventy feet is not an unrealistic height. Such a mast might be useful as a 30, 20, 17, 15, 12, and 10 meter efficient vertical antenna. Perhaps all bands could be driven using a single feed line. It certainly seems worth a try.

Another possible application might be feeding a dipole at one end instead of in the middle of the half wavelength. This avoids the need to support the weight of the feed line at the center of the antenna. It also avoids the need for a balun since the quarter wave matching section is a balance feed. The only possible problem would be that this would be a single band antenna unless an antenna tuner is used. If an antenna tuner is used there is no need for the quarter wave matching section.

An end fed half wave horizontal antenna with a quarter wave vertical matching section would also result in a fairly inexpensive antenna by reducing the need for a long coaxial transmission line run. The vertical matching section requires that the horizontal portion of the antenna be at least 1/4 wave above ground which is the ideal minimum distance. Even so, there is no reason why the quarter wave matching section needs to be all vertical or all horizontal. Finally, the quarter wave matching section can be home made as an open wire line. A parallel run of wire seperated by homemade insulators.