
FEEDBACK
A 146- and 445-MHz. J-Pole Antenna

W4ULD  11-07-2000


I received a good response from my article, "A 146- and 445-MHz. J-Pole Antenna", in the October 2000 issue of QST. The antenna worked fine for most builders. However, three builders who used the heavier  Type L copper pipe found that the antenna resonated from 143 to 145 MHz. instead of about 146 MHz. and had a very high SWR in the repeater portion of the 440 MHz. band. Other builders wanted the minimum SWR in the 440 MHz. band to be in the 440 to 450 MHz. range rather than at 436.5 +/- 1.5 MHz. for the original design.

I have constructed several antennas since writing the article. I find that those constructed from the lighter Type M pipe are fairly uniform with the 2 meter resonant frequency at 146 +/- .5 MHz. and the 440 band resonant frequency at 437.5 +/- 1.5 MHz. However, antennas constructed from Type L pipe appear to be non-uniform and tend to resonate low in frequency. One resonated at 430 MHz. causing unacceptable SWR near 450 MHz.

If an antenna constructed from Type M pipe resonates at about 146 MHz., the 440 band resonant frequency can be raised to about 441 MHz. by simply shortening the 440 band elements to 10-1/8 ins. from the specified 11-1/16 ins. The elements must be re-centered opposite the feed point. This will not change the 2 meter resonant frequency, and the SWR at the antenna should be 1.5:1 or less from 435 MHz. to 450 MHz. 

The resonant frequency of both bands for Type M construction can be increased by limited shortening (1-1/2 ins. maximum) of the upper element. A reduction of 7/8 in. will raise the 2 meter resonant frequency by 1 MHz. and the 440 band resonant frequency by 2.7 MHz. This must be done before shortening the 440 MHz. elements. The 440 band resonant frequency can be raised further by shortening the 440 band elements by about 3/16 in. per MHz.

If the resonant frequency of an antenna constructed from Type L pipe is 143 to 145 MHz., use the following procedure. Shorten the upper element to 20-1/8 ins., shorten the lower element to 38-7/8 ins., and shorten the 440 band elements to 10-1/8 in. The stub and Q-section should not be changed. If the resonant frequency is now too high, solder four 3 ins. long #12 bare copper wires to the top of the cap on the upper element. The four wires should be at right angles and horizontal to the ground. Trim each wire to 2-1/2 ins. from the center of the cap to the end of the wire. Check the resonant frequencies. They should be too low. Finally trim the wires equally to obtain the desired resonant frequency. Each reduction of 9/16 in. in the length of the wires will increase the 2 meter resonant frequency by 1 MHz. and the 440 band resonant frequency by about 2.85 MHz. One of my modified Type L antennas did not require top wires, one required 1 in. wires, and one required 1-3/4 in. wires.

I apologize for not recognizing the Type L problem earlier. Anyone desiring graphs that describe the above modifications can send me an e-mail to w4uld@mail.clis.com or a SASE to my home address of 203 Lord Granville Dr., Morehead City, NC 28557.

Andy - W4ULD

     
