Ham Radio 2000 #2

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Text File | 1996-03-26 | 16KB | 346 lines

NOTE: This is the 'readme' file for the DOS version of the Rhombic Utility. See note at end of this file for an addendum on the Windows version. USING RHOM112.EXE, RHOM.EXE, and STUB.EXE ----------------------------------------- RHOM112.EXE is for ega/vga only, as it has some minor graphics. This program was written in C and compiled using Borland C++ 3.1. RHOM.EXE is essentially the same program, but with no graphics. It has been tested on a 286 with Hercules graphics (green mono monitor) but should run in a text-only (MDA, no Hercules) mode and in CGA mode. The source code for RHOM.EXE was written in Basic and compiled on the MS QuickBasic 4.5 compiler. This version (v 1.12) of the program is entirely based on the material con- tained in the 1984 edition of the ARRL Antenna Book. Future versions may include other design criteria from other sources; QST and other journals from the 1930s had much material on this most useful beam antenna. The program outputs dimensions and other data for the rhombic you want. At the end, this info is expressed in terms of x,y,z coordinates (in meters) for entering into an antenna analysis program like Elnec. The metric system has been declared "legal" in the U.S. since about 1870, so maybe it's time to throw away this inches and feet business. Get a tape measure that measures in meters. We're almost in the 21st century! Analysis with Elnec of the so-called 'vertical rhombic' (there are published articles advocating use of rhombics rotated 90 degrees, to the vertical, for vertical polarization) shows that it is useless. The pattern is worse than a dipole, even if you place the antenna 100 meters above the ground. I don't know if the authors of these articles ever actually used these antennas in a situation that would determine how effective they really are. I recently found the "vertical half-rhombic" in a U.S. Army antenna manual, however; if anyone has practical experience to share, please do so, it would be much appreciated. A vertical half-rhombic would be a very useful antenna to have, and it would be easy to set up because it would be essentially a long wire in an inverted-V plus a ground counterpoise. The rhombics for two meters I am building now have no termination resistor. On HF frequencies, it's no problem to build up a termination resistor for 800 ohms, say out of a bunch of 2-watt 800 ohm resistors in series/parallel. When you try that for 144 MHz, you find that considerable reactance has crept into the resistor. It is no longer purely resistive. You might try shopping around for a monolithic non-reactive 800 ohm resistor, or you can do what I did: don't use a termination resistor at all, and leave the ends open and separated by several centimeters. This makes the antenna bi-directional, with 2 or 3 dB greater gain in the forward direction as compared with "off the back", according to Elnec. Adding an 800-ohm non-reactive resistor of the proper wattage suppresses most of the back lobe, but doesn't increase the forward lobe. So why use a resistor? The rhombic for two meters can be fed as follows: 50-ohm coax --> halfwave coaxial balun --> quarterwave matching --> ant. stub A diagram on paper should be accompanying this disk showing how to do this. A program on this disk, STUB.EXE, may help you design the halfwave and quarter- wave sections. The halfwave is made of RG8-X (mini-8) which has a velocity factor of .75, and the quarterwave stub is made of 450-ohm ladder line which has a velocity factor of .95. Especially at vhf freqs, it would be very advisable to check the halfwave section of coax, before soldering it together, with a noise bridge to see if it is indeed resonant. The actual velocity factor of your coax may vary from what it is supposed to be. An error of a centimeter in the length of the halfwave section could have some adverse impact on how well the vhf rhombic radiates: extra lobes, skewed pattern, etc. So, use a noise bridge (if you can find one for 144 MHz) or a dip meter or some other instrument to test your halfwave section of coax before you solder it into U-shape. This procedure is especially important if you don't know for sure what the velocity factor of your coax is. Beware; RG-8 type has two different velocity factors depending on whether the center conductor is stranded or solid. Also, I would not advise using RG-58 as it may be too lossy. The point on the quarterwave stub where the balanced legs of the balun are attached, in my experiments turned out to be about halfway up the stub (not at all near the bottom, where you would attach the feed to a J-pole). You will need a 2-meter SWR meter to find the point of attachment giving you an SWR of 1.5 or less. I had to solder the legs to the stub, test, unsolder and resolder at a different attachment point, etc. Possibly alligator clips could be used -- if they don't slip on the ladder line. You'll have to remove the insulation from the ladder line at each tentative attachment point. The quarterwave stub is shorted at the base, not open. Other ways of matching the feedpoint impedance are possible. The articles I collected with the help of people out there in packetland suggested going from your radio to a two-meter antenna tuner (a tuner with balanced output, caution! the MFJ tuner may have unbalanced output, so you'd need a halfwave balun there, but then you're tuning the balun, not the rhombic, so you would need a tuner having balanced output ... I have an article on that if you need it), then feeding the rhombic with ladderline. Use of toroidal baluns is discouraged, as they are said to be very lossy at 144 MHz, but it's possible there are ferrite compositions and windings that will work at vhf. Another method that could work is replacing the quarterwave stub with an impedance-transforming length of 'tapered feeders'. Don't try eliminating the halfwave balun by feeding the quarterwave stub direct with your 50-ohm coax feedline. The stub could match 50 ohms to 800 ohms, but then the rhombic is not seeing a balanced feed; your radiation pattern will be skewed like crazy. As a side note: coax to a 1/4 wave stub to a LONGWIRE could work quite well. I have one design for a 2-meter long wire (not a beam antenna) which could be fed quite well with coax and a stub, like a J-pole is fed, but the radiation pattern is not too hot. HF Rhombics ----------- For HF rhombics, halfwave baluns get to be very long. I have not yet put up a rhombic for 10 or 15 meters; if I did I would feed it with ladder line coming from the balanced output of an antenna tuner. No halfwave balun, and I would try it with no quarterwave stub. If you use halfwave and quarterwave balun/ stub, you are stuck with one frequency. If you use a tuner and ladderline to feed the rhombic direct, you may lose a little power in feedline losses but you can use the rhombic for, say, both 15 and 10 meters. Experimenting is the key here. One model on Elnec for 10 meters showed very good results on 15 meters as well, with the antenna up only 30 feet above ground. Another idea for HF: make or buy a 16-to-1 toroidal balun. The coax goes to the balun at the feedpoint, and the balanced output of the balun goes to the two sides of the rhombic there. These type baluns tend to be expensive. Other notes ----------- A note on how to get the width of the rhombic correct: measure a piece of non-stretching cord of the proper length, tie it to the midpoints of each side of the antenna, and when you put it up in the air, spread the sides till the line is taut. After your antenna is up, it wouldn't hurt to do some far-field measurements with another radio: unless you have a beacon, this will require two people. One person sits at the radio/rhombic and transmits, while the second person monitors a couple hundred meters away, or a couple km away, in line with the beam of the antenna, checking the S-meter on another radio. You can do this test on FM at 144.200; the mode doesn't matter. If the remote-field person is using an HT with an S-meter, it would be best to use a J-pole mounted on a short stick, held high overhead, or else detuning by your body may occur. The point of all this: the remote-field person should take measurements on either side of the beam direction, and right on the beam direction, to verify that the main lobe is indeed in the beam direction desired. When you do this, be sure that all your measurement locations are at the same elevation relative to the rhombic, i.e. all on a horizontal line. Otherwise the results won't be meaningful. As of the date of this text file, I admit I have not actually performed this test on my rhombics. Shame shame. It might be even better if the far-field person uses a horizontal dipole on a stick held up overhead. That way the polarization is the same as the rhombic. You may have to put an attenuator in line if the received signal is too strong to give meaningful S-meter readings. Any comments or suggestions welcome. / │ │ │ │ method #1: halfwave balun │ ant and 1/4-wave stub │ │ ╔└───────────────/ ║ 1/4 wave--------------║ stub ╔║ ║ ║--------------halfwave balun ║ ║ █ █ █ █ █ █ █ █ █ █ █ █ █ coax feed █ █████████████████████████████████████████████████ / method #2: no balun, no stub; │ feed antenna with │ ladder line; in shack, │ antenna tuner with │ balanced output. │ ant │ │ ╔└───────────────/ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ladder line to ║ tuner in shack; ║ no sharp bends & ║ keep metal objects ║ about a wavelength ║ away from the line ║ ║ ║ ║ ║ Orrin C. Winton WN1Z, PO Box 89, Big Sur CA 93920 17nov94 Add'l note: 30 September 1994 ------------------------------ In my construction of 2-meter rhombics, am not yet totally convinced that the halfwave balun --> quarterwave stub --> antenna is the best possible match. I.e., it might (or might not) be too lossy. Have gotten some contradictory results. You will have to try and test, and let me know what you get. Remember tho, the rhombic has a very narrow beamwidth. The leading alternative to the above match would be: radio --> 50-ohm coax --> 2-meter antenna tuner with a BALANCED output --> 450-ohm ladder line --> to rhombic feedpoint The MFJ 2-meter tuner may OR MAY NOT be the answer: call MFJ to ask them if the output of the tuner is balanced, or is it unbalanced (coax). If the latter, you are out of luck, the tuner won't do you any good unless you can modify it inside to give you balanced output at around 200 ohms (actually 450 ohms would be ideal for 450-ohm ladder line). Putting a halfwave coaxial balun on a tuner's output will not help you either, because then you are just tuning the balun, not the feedline. Putting the balun at the feedpoint and feeding it with coax is no good either. You need a tuner that has its innards designed to give a balanced output. I have one or two homebrew designs (articles); send sase for those if you need them. On HF, this problem doesn't exist. Just use the balanced output of your HF antenna tuner, to 450-ohm ladder line, to the feedpoint. Then you have a good match AND multi-band use too, typically 10m and 15m for a "small" (200 ft) rhombic. Again, if i haven't said it elsewhere in these notes, the far end of the rhombic is left open, unconnected, with the ends several centimeters apart, with no resistor at the end. If you plug the antenna into Elnec with and without the load resistor, you'll find you're better off with no load resistor. And don't forget to use the 'taper' feature on the wires menu: the sharp angles of the rhombic require the use of 'taper'. Taper BOTH ends of each of the four wires. Note: 11/17/95 --------------- Internet address as of 11/95: orrin@redshift.com Other Internet addresses: wn1z@amsat.org Since I wrote this readme file, I have been in touch with other rhombic designers/users who have indeed used the stub/balun method of matching for two-meter rhombics. So please, if you're a cw/ssb 2m user, experiment and let's share results. Note: 11/30/95 --------------- Regarding the Windows Rhombic utility: as of this date, i have not converted the stub.exe file to a Windows version. The DOS version is included here so you can use it if you need it. Note: 12/3/95 -------------- Bug fix: fixed rhom.exe (Basic version) and C++ version as follows: Basic version and 1.10 would give an error message (division by zero error, domain error, illegal function call error) if user entered a height above ground of 0.25 wavelengths or less. The function used to calculate wave angle based on height above ground, has a limit of 90 degrees (straight up) when the height is an infinitesimally small amt greater than 0.25 wavelengths. The fix: if user enters a number below 0.26 (example: 0.1 which would not be uncommon for the low bands) then the error is trapped and the wave angle is given as "> 75 degrees." I.e. "greater than 75 degrees." Note: 3/26/96 -------------- The most current Windows (3.1) version of this utility is 2.04. Also, the older RHOM.BAS and RHOM.EXE (which was my original attempt at this rhombic program) are now bug-fixed. Someone reported it was giving a "type mismatch" error after you enter the height above ground in wavelengths. Apparently the QuickBasic 4.5 compiler didn't like some of my GWBASIC code. This bug is now fixed.