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Friday, August 1, 2008

The J Pole Antenna

The J Pole Antenna is basically a half wave vertical with a quarter wave balanced line matching stub at the feed point.
J Poles are commonly built for the 2 metre band by Amateur Radio Operators due to the convenient size and ease of construction.Normally fed with coaxial cable but really could be fed with any impedance feed line, balance or unbalance with some adjustment to the point of feed on the stub.

The Matching Stub
Near to the bottom of the J Pole the iconic J section is actually a quarter wave balanced closed stub.Quarter wave stubs are quite common in antenna construction for a number of reasons.One reason is that a closed stub that is a quarter wave long has a circuit length of a half wave.
This means that currents on the two conductors at the end of the stub will be 180 degrees out of phase to each other. This alone forms the basis of coaxial 1/1 baluns.
Another property of quarter wave stub is that the closed end normally has a low impedance point at the short circuit end and a high impedance at the open end and any impedance between the two can be found some where between by tapping off.
Yet another quality of closed quarter wave stubs is that they tend to balance unbalanced currents.
Once one is aware of what these stubs can do it is no surprise that they are used to feed an end fed half wave to form the J pole.
Tuning J Poles
J Poles can be tuned to any kind of feed line at any impedance. Its just a matter of finding a matching impedance feed point on the quarter wave stub. It has been suggested for operation with coaxial cable to connect the centre conductor to the side of the stub without the radiator attached. Careful tuning can achieve a near perfect match and there are three points at which tuning can take place. Temporary connection of the feed line conductors to either side of the matching section moving them generally together up or down to provide the best match. Some independent movement may yield further improvements. Also adjusting the length of the open end of the quarter wave matching section can improve tuning.
One thing to watch is near by metallic objects. They can detune a J Pole quite a bit. Also In some cases the attachment point at the bottom may need to be insulated as some detuning can take place with a conductive masting. Its a bit unusual but it has happened.

I have heard of and seen a few different construction techniques applied to J Poles. I have heard of them being constructed from balanced feed line, copper tubing, Aluminium tubing and Printed Circuit Board at UHF.
There doesn't seem to be too many hard and fast rules just that the quarter wave stub be a quarter wave at the operating frequency and the radiator be a full half wave long from the end of the stub.
Some say that the open side of the stub needs to be of a smaller diameter tubing but this is not necessary. The spacing of the conductors in the stub is not overly critical either but a good rule of thumb would be around four times the diameter of the tubing that it is constructed from.
One notable experience I had was with creating a chock from the coaxial feed line near the antenna feed point. I found that the SWR was adversely affected by the introduction of a choke of more than a few turns.
Once tuning is complete the feed line can either be screwed or clamped to the feed point that gave best results and some water proofing is recommended around all connections and joints.

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