Balun Basics
Displaying 1-7 of 7
| Mar 2nd 2012, 05:05 | |
W4WCSTotal Topics: 0 Total Posts: 0 |
I got my General a while back. Finally getting into HF! Bought a FT-857D and researching antennas and tuners. My sitiation is such that I need to use my attic for my antenna farm:) I am considering several wire dipoles and possibly a loop tied to an automatic tuner. MFJ-993B or a remote tuner. Oh and COAX for the moment. I can figure everything out except BALUNS.........!!!!!!!! I see the purpose, prevent rf from feeding back down the shield, balance between the coax and dipole. BUT........why a 1:1 or 4:1 or current????? I have no idea what I need and why???? There has to be a good tutorial somewhere? I just cant find it.. HELPPPP and thanks in advance. Bill |
| Mar 2nd 2012, 11:53 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
It is like choosing the best suit when nothing meets your needs. It comes down to choosing the best compromise for the situation. Coaxial choke baluns offer the highest performance--but on only one amateur band. But, for well matched antennas, like triband Yagis, they perform adequately on adjacent bands. Ferrite baluns work on more bands, but they typically offer just 1000 to 2000 ohms of choking impedance, not the 20,000 ohms or more one would want for feeding a center fed dipole on on bands. Because their impedance is low, they can be a lossy circuit. This is evidenced in the shack via SWR drift as the balun heats up. To compound the problem, ferrites are lousy at getting rid of heat, so a big toroid may only be able to handle 10 watts, worst case--even though they can handle the legal limit in ideal conditions. The calculation of when they are lossy is complicated by the transmission line, which acts as an impedance transformer when not terminated in its characteristic impedance. As for current or voltage, a current balun provides equal currents at the output, while a voltage balun provides equal voltages at the output. Ideally, the voltages and currents would be equal at the output of a balun, but when you provide unequal impedances to ground, the laws of circuit theory say that something has to give--you can't have both. Typically, you choose the impedance transformation that provides the greatest system efficiency--but this is sort like deciding on tax deductions--you really need to run the numbers. Some tax breaks don't count if you make too much money. On the other hand, you may not get any benefit unless you make enough money to meet certain thresholds. A 4:1 coaxial balun is often used on 2M because it is unusually practical and efficient--so folks use it even though a 200 ohm output impedance isn't exactly what we want. But, such a choice makes little sense on 40 meters, the long length of coax makes it heavy impractical. Zack Lau W1VT ARRL Senior Lab Engineer |
| Mar 2nd 2012, 14:01 | |
W4WCSTotal Topics: 0 Total Posts: 0 |
Thanks for that info Zack. I am trying to digest it. I was thinking about a multi band center fed wire dipole. What would you recomend in that case with coax feed? Thanks Bill |
| Mar 2nd 2012, 14:56 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
Hi Bill, First, I'd consider this attic vertical antenna designed by Kai Siwak, who has over 30 patents http://www.arrl.org/limited-space-and-indoor-antennas An All-Band Attic Antenna Kai Siwak, KE4PT, describes how to make an effective inverted L with an Icom AH-4 autotuner. QST October 2007 pp. 33-37. You might also look at the other options on our limited space antenna web page. The best two options for attic antennas are to install an automatic antenna tuner at the feedpoint, or to cut and try until you get system that works acceptably. Your choice of operating mode is a big factor--if you want to operate voice, you can't really afford to throw away signal, unless you are rare DX, in which case folks will point big antennas your way to make up the difference. CW and PSK31 operators can typically get away with antenna compromises and still have fun, provided they have reasonable expectations--like getting a DX contest pin for working 100 stations, rather than winning the section. Thus, a phone operator may need a resonant half wave antenna for each band--while a PSK31 may do quite well with a non-resonant antenna that just loses a few dB on all bands--not ideal--but good enough to work. Due to the harmonic relationship of ham bands, an antenna that is resonant on 80/40/20 meters won't work well on higher harmonics when fed with lossy coax--hence the popularity of non-resonant lengths for multiband work. But, harmonic resonances are a somewhat iffy concept in limited space situations--you might try putting up the longest antenna and see where the resonances are--and then shorten it for optimum performance on all the bands you want. Zack Lau W1VT ARRL Senior Lab Engineer |
| Mar 3rd 2012, 06:38 | |
W4WCSTotal Topics: 0 Total Posts: 0 |
Thanks Zack I did not know there was that much antenna info on the website. Shame on me for bad Searching! Great advice! Very helpful! Bill |
| Mar 4th 2012, 20:25 | |
WD0GOFTotal Topics: 0 Total Posts: 0 |
There are two sites that deal well with RF baluns and chokes. http://www.hamuniverse.com/balun.html http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12643 A lot of good info that does not get lost in Calculus |
| Apr 13th 2012, 17:55 | |
KD7KCPTotal Topics: 0 Total Posts: 0 |
These three books helped me immeasurably when I was in the same situation. Stealth Amateur Radio By NT0Z Lew McCoy On Antennas By W1ICP Building And Using Baluns And Ununs By W2FMI The antennas to be used in the station I am building now, are based on these books and the 1955 ARRL Antenna book. Good Hunting KD7KCP |
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