A new twist for the HF J-pole antenna
Feel like tinkering? AD1B describes a nifty little antenna project that also makes use of your computer.
I recently became interested in the possibility of building a compact high frequency antenna for portable workand did some reading on the subject. It seems that the price for most physically small antennas is a narrow band width and exotic coil construction. My thoughts then went towards the possibility of a J-pole antenna, a simple design that is anything but compact when used in the normal fashion, as I was to discover after I did some computations.
There has been a lot of interest in the J-pole antenna for VHF work, particularly on the six and two meter bands, but much less information on HF applications. The "J" is, however, a popular and simple vertical antenna that has been described in a large number of articles that emphasize that the J-pole features ease of construction, low cost of materials and the convenience of direct feed by coax. Another nice feature is that there is no need for a ground system or for a group of radials.
The ARRL Antenna Book describes the J-pole as a half-wave vertical radiator fed by a quarter-wave stub matching section (see fig. 1). It also indicates that the J-pole has been popular in mobile applications, although it is not as widely used as the quarter-wave whip.
Fig. 1 - The basic J-pole antenna. At (A) it is fed with coax and at (B) twinlead or openwire.
My interests are in development of a simple 20-meter antenna for fixed and perhaps mobile work, one that does not rely on coils, traps or other electrical methods of size reduction. I wanted the derivative of a full sized antenna using some type of compacting approach for the problem of achieving a reasonable size.
How about a Helical J-pole antenna, I thought; it may have been tried, but I have never found a reference to that particular configuration. To test the concept, I used the cardboard tube from a paper towel roll and would a two meter J-pole antenna around it. The VHF antenna worked fine and seemed to offer a reasonable match to my Yaesu Memorizor. The next step was to try the concept on a larger scale. Since cardboard was impractical for the larger size of the high frequency antenna, I settled on 1.5 inch PVC tubing for a supporting mechanism.
The next question was the determination of the dimensions of the antenna; many slightly different formulas have been reported in the literature. The most useful formulas were in an article by VE2CU ((QST March, 1981), which are shown in fig. 2.
½λ section = ½λ × .965
¼λ section = ¼λ × .830
Stub = .0136λ
λ = 300/MHz Meters
Fig. 2 - The VE2CU formulas for the HF J-pole antenna. Refer to fig. 1(B).
I prepared a short BASIC program for my IBM PC in order to determine the dimensions of the J-pole at various frequencies (see fig. 3 for the program listing - it is easily adaptable to a variety of computers). Settling on 14.3 MHz, I ran the program and built the antenna from 300 ohm twin lead that I purchased at Radio Shack. The process of winding the coil around the mast is tedious, but not particularly difficult. At ten inch spacings, I made a band of electrical tape to prevent the spirals from slipping. The initial spacing between turns was about one-half inch, later trials were made with one inch spacings.
10 CLS 20 REM "J-pole program" 30 A$ = "********************************************************************" 40 FOR N = 1 TO 6: PRINT A$: NEXT N 50 PRINT 60 PRINT 70 PRINT "HF J-POLE ANTENNAS" 80 PRINT 90 PRINT "COMPUTATION OF DIMENSIONS" 100 PRINT 110 PRINT "AD1B 120 PRINT 130 FOR N 1 TO 6: PRINT AS: NEXT N 140 PRINT: PRINT 150 PRINT " ENTER 'C' TO CONTUE" 160 Z$ = INKEY$:IF ZS = "" THEN 160 170 CLS:FOR N = 1 TO 6: PRINT :NEXT N 180 PRINT 190 PRINT "To determine the size of a J-POLE antenna for a selected HF band, enter the FREQUENCY in MHZ:" 210 INPUT F 220 L = (984/F) 230 X= .0136*L 240 Y= .2075*L 250 Z= .483*L 260 CLS 270 FOR N = 1 TO 4:PRINT: NEXT N 280 PRINT 290 PRINT "HIGH FREQUENCY 3-POLE:" 300 PRINT " I I" 310 PRINT " I I" 320 PRINT " I <- .5 wavelength" 330 PRINT " I I" 340 PRINT " I I" 350 PRINT " I 360 PRINT " I break <- separation between Y and Z" 370 PRINT " I I" 380 PRINT " + + coax <- center to left; braid to right" 390 PRINT " I I <- .25 wavelength " 400 PRINT " --- <- stub (distance from short to coax)" 410 PRINT 430 PRINT "Half-wavelength: ";INT(Z*10)/10 440 PRINT "Quarter-wavelength: ";INT(Y*10)/10 450 PRINT "Antenna Length: ";INT((Z+Y)*10)/10 460 PRINT "Stub Length: ";INT(X*10)/10 480 PRINT 490 PRINT "FOR ANOTHER COMPUTATION, HIT ANY KEY...." 500 Z$ = INKEY$: IF Z$ = "" THEN 500 510 IF Z$ = "C" THEN GOTO 170 520 STOP
Fig. 3- The J-pole computer program.
Well, how did it work? Not bad, I answered myself. I live near Boston, Mass. and was able to work stations in Illinois, Arizona and Tennessee with reasonable signal quality with the antenna attached to a fence at a height of six feet above the ground.
What are the pros and cons of the compact J-pole? It is small, one version was only four feet high. It is cheap enough, with a little scrounging it should be possible to build one for under $10. It does work, my experiences were quite a pleasant surprise.
As to the cons... the helical J-pole is a compromise antenna that will never perform as well as a full-size model, but perhaps this is something that will work for you in portable or mobile applications. I suspect that the helical J-pole would be easy to clamp to the rear bumper of a car, using two clamps and a metal plate. This would certainly be a very simple and readily available antenna for anyone on a limited budget who is interested in mobile operations.
HIGH FREQUENCY J-POLE: I I" I I" I <- .5 wavelength" I I" I I" I I break <- separation between Y and Z" I I" + + coax <- center to left; braid to right" I I <- .25 wavelength " --- <- stub (distance from short to coax)" Half-wavelength: 33.2 Quarter--wavelength: 14.2 Antenna Length: 47.5 Stub Length: .9 FUR ANOTHER COMPUTATION, HIT ANY FEY.... Break in 510 OK
Fig. 4 - A sample run of the computer program.
AD1B, Thomas M. Hart.