Principle of J Pole Antenna

J Pole antenna is quite popular among radio amateurs for 2 m and 70 cm bands, that is the VHF and UHF bands. It consists of two segments, one three fourth wavelength segment and a matching stub, which is a parallel shorted matching stub, of quarter wavelength. The portion above that, that is corresponding to the portion beyond the quarter wave, will be a half wave. This is a half wave, and another quarter wave is attached here. And, for this half wave antenna, up to this, when the coax is fed here, it can be considered as a end fed antenna. End fed antennas have high impedance and this requires a matching. So this arrangement of the J pole is mainly for matching. You have connections from the coax on either side, and the connection point has to be adjusted, by sliding up and down, to have the lowest SWR. That is how the J Pole antenna is tuned. J Pole has a gain of around 2 dB, above a ground plane vertical antennna. This region, between the two elements, between the shorting stub and the, matching stub and the main element, should be less than two percent of the wavelength. And, the whole assembly should be away from conductin material. You should not have metallic structures near this.
If you are using a galvanized iron pipe, as is usual among us radio amateurs, then, a section of non conducting material, like a wood piece or PVC pipe has to be attached, below the antenna, between the antenna and the mast and the length of that should be at least three times of the separation between the two elements. And there should not be any conducting material near this, like a window grill, or another mast, should not be there, very near to this antenna, for best performance. And, J Pole antenna has a little higher gain in the direction of the quarter wave matching stub. And, this antenna has been used for terrestrial communication through repeaters and simplex operations, as well as for satellite communications. In satellite communications, if you have a VHF antenna, usual amateur band for UHF, ultra high frequency, is on the third harmonic of our VHF band.
In satellite communications, usual amateur 2 m band and amateur 70 cm band, are like third harmonic, that is a 2 m antenna, will also resonate as third harmonic, on the 70 cm band. So this can be used as a single antenna for satellite communication for favourable passes, just like you use a 7 MHz dipole antenna for 21 MHz also, with a reasonably good standing wave ratio or SWR. Similarly, this can work for, as a dual band antenna also, though not optimal. And, it is also mentioned that, when you use J Pole, 2 m J pole, for 70 cm, the angle of radiation is not ideal for terrestrial repeaters or terrestrial communication, but it is OK for satellite communication. A slightly different angle of radiation directed towards the satellite is there, compared to that of terrestrial repeaters. So, a single J Pole antenna on 2 m, has been successfully used, for satellite communication for the cross band repeater. You know that International Space Station and many other satellites, have V/U transponders, with uplink on VHF, and downlink on UHF. So, if single antenna works for both, that is convenient for the operator. That is meant for those who are operating using base station, from within the shack.
This diagram shows the current and voltage pattern, that is the RF current, not DC current. RF current pattern in the stub and the main segment of the antenna. You can see that, the current is peaking at the lower end and also at the quarter wave region. This portion will be a quarter wave. Peak is at lower end and also at the quarter wave segment and at the top, the current is minimum, RF current is minimum. Voltage is the otherway round. Voltage is maximum at the upper end, and for the lower one third. Here also the voltage is peaking. And, in case of the quarter wave stub, voltage is peaking at the upper end and current is peaking at the lower end. This is the current pattern in the two regions of the J Pole antenna.
This diagram illustrates the radiation pattern from a J Pole. So, as you can see, slightly higher towards the matching stub region. And a slightly higher angle would mean that it will be directed towards the sky partly, and that is how this is useful in satellite communications. You want the radiation to be slightly in a higher angle towards the satellites in the sky. Of course, a single J Pole antenna, is not the ideal one for satellite communication, because this will not have a high gain, and being omnidirectional, the whole of the radiofrequency which you radiate from the J Pole, will not reach the satellite. Only a small fraction will reach the satellite, unlike in the case of Yagi antenna, which has a high front-to-back ratio and directivity, most of the signals would reach the satellite.
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