KX1 #1403 Antenna Project
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I have been following the antenna threads since I started building the KX1.
These experiments are based on that thread, the ARRL Handbook 2004, previous research into commercial verticals and my own experience. The goals are to find the best antenna for the KX1 under various operating conitions. 1. Smallest, lightest, easiest to set up 4 band backpacking antenna. 2. Smallest, lightest, easiest to set up for 1, 2, or 3 bands for backpacking. 3. Most efficent antenna for 4 bands but still backpackable. 4. Most efficent 1, 2, or 3 band antennas but still backpackable. 5. Learn Antenna Theory and Practice. Formulas, Software, Instruments Starting Conditions. DUMMY LOAD BAT FREQUENCY POWER SWR 12.4 3.5005 3.0 1.0 12.4 3.5600 3.5 1.0 12.4 7.0005 2.9 1.0 12.4 7.0580 2.9 1.0 12.4 10.1005 4.2 1.0 12.4 10.1495 4.2 1.1 12.4 14.0005 2.5 1.0 12.4 14.05800 2.4 1.0 38' TRAPPED , FLAT TOP DIPOLE, CUT FOR 7.040 PLUS/MINUS,40' HIGH. FEEDPOINT IS NEAR THE MAST FOR MY CUSHCRAFT MA5B. I REPLACE SOME OF THE GUYS FOR THE MAST WITH ROPE TO REDUCE AS MUCH INTERACTION AS POSSIBLE. WHEN I TAKE THE MAST DOWN AT THE END OF SUMMER FOR MAINTENANCE I PLAN TO REPLACE ALL GUYS WITH KEVLAR. BAT FREQUENCY POWER SWR 12.4 7.0005 4.2 1.2 12.4 7.0125 4.0 1.1 12.4 7.0255 3.5 1.0 12.4 7.0400 3.3 1.0 12.4 7.0580 3.0 1.0 FIRST BACKPACKING ANTENNA. #534 WIREMAN TEFLON COATED ANTENNA WIRE. 29'6" RADIATOR. 2-29'.6" RADIALS. THIS WAS BASED ON THE RECOMMENDATIONS OF ELECRAFT IN THE KAT1 MANUAL BUT WITH 2 LONGER RADIALS. I INTENDED TO TRY BOTH ONE AND TWO RADIALS IN THE FIELD BUT IT STARTED TO SPRINKLE SO I QUIT BEFORE I COULD DO THE ONE RADIAL TEST. THE LENGTH OF THE RADIALS WAS BASED ON THE ARRL HANDBOOK RECOMMENDATION OF QUARTER WAVELENGTH RADIALS. 29' WAS CLOSE TO THE 1/4 WAVE LENGTH FOR 80 METERS. FUTURE TEST WILL INCLUDE MULTIPLE RADIALS FOR EACH BAND. I TOOK THE GEAR TO THE TOP OF STRAWBERRY HILL (AN ISLAND IN STOW LAKE HERE IN SAN FRANCISCO) AND HUNG THE ANTENNA AS A 45 DEGREE SLOPER FACING THE NORTH. BAT FREQUENCY POWER SWR 12.4 3.5600 ? 9.9 12.4 7.0400 3.5 1.1 12.4 10.025 4.3 1.1 12.4 14.000 2.9 1.3 12.4 14.020 2.9 1.3 12.4 14.040 2.9 1.2 12.4 14.058 2.9 MY FIRST FIELD QSO WITH KX1. 449 FROM WA!!! As this note took longer to write than I expected I'll leave my guesses as to what the figures mean till later. Now it's time for food and then some air time! _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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The whole concept of radials is to improve the conductivity path for return
current to the base of the vertical antenna, when the antenna is not a complete half wave dipole, (and therefore, balanced and a complete radiator unto itself). You are replacing always lossy (to some degree), earth or rock, with copper conductors. The vertical of whatever length produces return currents that flow to couple into the earth, from the high tip of the antenna. There is some curvature of these return current paths, but generally speaking the tip current will traverse a path and reach ground (earth) about the same distance out from the base of the antenna, as the antenna element length or less due to the curvature of current paths. Therefore, there is probably no reason to make the radials a quarter wave long exactly, if the radiator element is not a quarter wave long. In addition, the presence of radials close to earth couples with capacitance such as to detune the electrical length of the physical radial. That quarter wave length you cut will not be electrically a quarter wave lying upon the ground. When you are backpacking, you may be upon stony ground in one place and good earth somewhere else. (Good in terms of RF character). You will have differing amounts of earth coupling. To get the full benefit of quarter wave radials, then, they would need to be elevated some distance above earth. Fortunately, with elevated radials, you get some benefit as to length and it has been found you no longer need a physical quarter wave length. Thus, if you pick an antenna length for its being easy to backpack, and at least 60 per cent of the full resonant antenna length, you get a shorter load to carry, but also you benefit from the current return path being no taller height than your antenna, such that radials can be less than quarter wave and still provide a good current return to the base of the antenna. SWR can be "good" as being near one to one, and you can still have a lossy vertical or other antenna. Don't get too wrapped up with seeking 1:1. A good quarter wave vertical is not 50 ohms, ever. And thus, if the vertical is quarter wave resonant, you will never have 1:1 SWR. But, that is OK. The goal is maximum field strength. The character of the surrounding terrain 2 to 5 waves out from the antenna will have more effect on reflections that could give you reflection gain, and help your antenna work well in terms of skip. In every back packer's kit should be a simple field strength meter, which can consist of a miniature meter movement, a diode and a RF bypass capacitor. This detector and a short whip can be used at some standard distance from your antenna over flat terrain to establish a "normal" field reading. Then, when you set up in a camp, you can quickly check if the antenna is radiating as well as it did when you first tested it under controlled conditions. Note the meter reading or mark the scale where your antenna tuned up best on your standard "antenna range", and you will be able to ensure you are getting out if that mark is reached by the field strength during backpacking operations. -Stuart K5KVH _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: KX1 #1403 Antenna Project
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What Stuart says about radials is quite true, but there is another important
reason for a tuned radial (or other ground connection) other than to improve the efficiency of an end-fed wire. Consider the end-fed dipole; it's probably the most extreme example of what I'm talking about. Fed at the end, a dipole (1/2 wave radiator) presents a very high impedance to the source. Hence, the RF voltage at the feed point will be very high. Without a ground on the ATU and rig, the whole rig will tend to "float" at a high RF voltage. At QRP you may not notice any RF "bites" or "burns" (although you certainly can get burns at less than 5 watts under the right conditions) but other problems abound. RF gets into the controllers and other logic in the rig causing erratic operation, and just touching the rig may detune the whole system, because of your body capacity to ground. An electrically 1/4 wave long "radial" connected to the rig ground can prevent these problems since a 1/4 wave wire has an impedance of only 35 ohms. That means that the rig case and ground will see an impedance of only 35 ohms, maximum, to ground even though the feed point at the "hot" side of the antenna connection may be looking into thousands of ohms. The Elecraft ATUs, like most tuners, can't handle the very high impedance of an end-fed dipole. Still, they can match to a sufficiently-high impedance that RF on the rig can easily become a problem, as many have reported here on the reflector. Usually a 1/4 wave long wire, elevated a few feet off the ground (over 6 feet if it's where people might walk into it) is a good investment. Another approach, as Stuart observed, is a wire on the ground that will capacitively couple to it and help hold the RF voltage on the rig down. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Good points Ron.
I usually call those RF control radials, an RF counterpoise, as sometimes you can use one cut for each band of concern and run it around the baseboard of a shack. I like to think of a radial as an outside wire, and counterpoise as an insulated inside the shack RF control aid. But, the word counterpoise seems not to have as good a general meaning as radial has come to mean to hams. Stuart K5KVH _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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