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Build a 17 Meter Reduced Size Coaxial Moxon
Retangle. If you have tried the Moxon Generator program to design a Moxon Rectangle, then you are familiar with the Moxon antenna. For those of you who are not familiar with the Moxon, it resembles two letter "U"s with the open ends almost touching but is squared off at the bends. One director and one reflector, each with tails pointing at each other. This antenna can achieve a respectable gain of about 3 to 4dbd and good front to back ratio and is usually made from wire or small diameter tubing. It is usually designed and operated on the higher HF bands due to the physical sizes of HF antennas on the lower bands. A standard Moxon rectangle designed for 17 meters would be about 19 3/4 feet wide. Using these plans provided by N0KHQ, the same Moxon retangle is reduced to about 11 feet wide. Man that's a big reduction. Unfortunately the MoxGen program was written only for normal wire sizes. In order for the program to generate values for different types of coax cable, the author would have to investigate different coax types. It could be done, but would require a lot of work. Here is a trick I have learned about using coax for antenna elements utilizing an antenna analyzer. If you have an MFJ 259B or better, plug a coax tee into the top, terminate one side of the tee with a 50 ohm resistor, cut a piece of RG-58/U VF .66 using the standard formula 234/freqmhz for one side of a dipole. You will find after following the instructions below that it is too long and will have to be trimmed. (For example we will use 18.050). Connect the center conducter to the other side of the tee and the shield to the ground side of the tee, leave the other end of the coax open. You will notice that when the MFJ is adjusted to resonance (50 ohm,X=0 and1:1) it will be somewhere around 17.6 or 17.8. Divide that number by the desired frequency (18.050) you will probably end up with something like .90, take the length of the coax and multiply it by .90, then cut off the valued displayed, check for resonance again....repeat as required until your coax reaches resonance at 18.050. Note, I always leave mine a little long to make connections to dogbone insulators. I have built a 75m coaxial inverted "V" using the same method. Works great! The formula for using coax instead of wire for shorter antennas: 984 x .66 = 650/18.050 = 36' / 4 = 9' x 1.2 = 10.8'. Now start testing with the MFJ 259B or the old trial and error method with an SWR meter.
An explanation by N0KHQ/St. Louis Using an MFJ 259B or better, connect a coaxial tee to the analyzer, on one side of the tee install a 50 ohm termination resistor, the other side of the tee will be connected to the coax under test. Example: We will cut a 1/4 wave length piece of coax to resonate at 18.050. When I say resonate, I'm saying that when we are done trimming the coax, the following will be displayed on the MfJ 259 = 49 to 52 ohms, 1:1/0 swr and X=0 or real close to it. Cut a piece of RG-58/U PE VF.66 10' long. At one end of the coax trim back a couple of inches of the PVC jacket, separate the shield from the center conductor, this end will feed into one side of the coaxial (center to center and shield to ground) on the MFJ. At the other end of the coax, remove a couple of inches of the PVC jacket slide the shield back, strip 1" of the PE from the center conductor, short the shield to the center conductor. Turn the tune knob on the MFJ to 18.050, you will notice that the piece of coax is not resonate (too long). Turn the tune knob slowly down in frequency until the display reads 49/52ohm, 1:1 SWR and X=0, you should end up somewhere around 16.200. Use this formula to determine how much of the 10' piece of coax you will have to trim off. This procedure is covered on page 20 of your Mfj 259b operator's manual. In this example: 16.200/18.050 = .897. Take 10' x .897 = 8.970' this is the length that your 10' piece of coax should be trimmed to resonate at 18.050. Check the frequency again with the MFJ, repeat trimming as necessary. When you have resonated your piece of coax, only the center conductor of the driven elements (remove shield) will be connected at the feed point, the other end will remain shorted. How about that, an electrical 1/4 wave length with over a 30 percent reduction in size verses using AWG. You will need 2 of these for the driven element. The reflector element will be .05 percent longer (lower in frequency), X will = 0 at 17.150 (both ends of coax are open under test). When installing the reflector element short the center to the shield on both ends. The element spacing between the driven elements and the reflector element can remain at 7'. When fabricated and tuned at 9 feet (almost 1/4 wavelength at 17m) the coaxial antenna produced the following readings: (using the Mfj 259b), (some trimming or lengthening of the driven elements may be required.
The power handling capabilities of this antenna is approximately 500 watts (continuous), this is due to the limitations of RG -58/U PE coax. In my opinion, the mark of a good antenna system is how well it receives signals. Testing of this antenna at this QTH was done over a period of a few weeks. The receive of the coaxial antenna was compared to the receive of my All Band Sterba Curtain up at 50'. Because the Coaxial Antenna is only up at 9 feet, the take-off angle is pretty high, as a result the receive is a couple of "S" units down from the Sterba. The Coaxial Antenna (weight 7 lbs.) should be raised to an operating height of 34 feet to 40 feet for optimal performance. A take-off angle of about 18 deg. To 15 deg. can be expected. Copyright N0KHQ 10/03 Click here for a large drawing! NOW JOHN CONTINUES: Its a lot of fun, there is no HF antenna that cannot be built with a 30 to 40 percent reduction in size. Its a very, very light weight antenna. With this kind of reduction in weight......now we can start stacking them............mmmmm......mm. What a DX antenna that would be.....I can see the gray line already. Talk to you later. Thanks for keeping the Sterba on your site. Hams are building those things lke crazy and are totally blown away at the performance! 73 John N0KHQ St. Louis |
