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Coiled coax balun
Hi again!
Has anyone tried making the coiled coax balun work at UHF? Specifically, on the 70 cm band. In "Antennas for All Applications", they say the coil should be resonant at the frequency of operation. The ARRL book(s) do not cover this band. Using RG58 and a GDO, I've learned that one cannot leave long leads on one's coax coil as it will lower the resonant frequency drastically. Apparently, the long leads in conjunction with the coil forms an antenna/inductor/capacitor combination. It appears to be difficult to obtain the required resonance repeatably. Or, am I missing something (as usual)? The diameter seems to be somewhat critical. Does the diameter need to be much, much smaller than a quarter wave? What diamter would be a maximum? Thanks for any help. John (KD5YI) |
On Sun, 26 Sep 2004 15:24:40 -0500, "John Smith"
wrote: Has anyone tried making the coiled coax balun work at UHF? Specifically, on the 70 cm band. Hi John, Not when ferrites or resonant sections do the job too. 73's Richard Clark, KB7QHC |
"Richard Clark" wrote in message ... On Sun, 26 Sep 2004 15:24:40 -0500, "John Smith" wrote: Has anyone tried making the coiled coax balun work at UHF? Specifically, on the 70 cm band. Hi John, Not when ferrites or resonant sections do the job too. 73's Richard Clark, KB7QHC Hi, Richard - Please give me more information about the ferrites... How many do I need? What permeability? Who makes them capable of 450 MHz or so? Thanks. John |
"John Smith" wrote in message ... Hi again! Has anyone tried making the coiled coax balun work at UHF? Specifically, on the 70 cm band. In "Antennas for All Applications", they say the coil should be resonant at the frequency of operation. The ARRL book(s) do not cover this band. Using RG58 and a GDO, I've learned that one cannot leave long leads on one's coax coil as it will lower the resonant frequency drastically. Apparently, the long leads in conjunction with the coil forms an antenna/inductor/capacitor combination. It appears to be difficult to obtain the required resonance repeatably. Or, am I missing something (as usual)? The diameter seems to be somewhat critical. Does the diameter need to be much, much smaller than a quarter wave? What diamter would be a maximum? By the time you get to UHF it is difficult to use coils. Baluns are usually made of coax or some of the very small hard line like less than 1/4 inch in diameter. The coax is just a fraction of a wavelength. |
On Sun, 26 Sep 2004 19:38:02 -0500, "John Smith"
wrote: Please give me more information about the ferrites... Hi John, Visit: http://bytemark.com/products/content2.htm You will find background information here, but unfortunately they don't maintain the pages very well and some information is lacking. How many do I need? Depends on application. You want at least 3 times the expected load Z. Ferrite Z is bulk determined and the usual standard of bulk size is defined as a type 101 size bead. What permeability? Type 43 or 64 material. 64 being slightly better (by about 15%). Who makes them capable of 450 MHz or so? Amidon, but this is not proprietary (although the type specification may be). There are other manufacturers like Fairite. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
. . . Who makes them capable of 450 MHz or so? Amidon, but this is not proprietary (although the type specification may be). There are other manufacturers like Fairite. I don't believe that Amidon manufactures cores (except, apparently, custom ones through one if its subsidiaries), but only resells them. The last I checked, most of their ferrite cores are from Fair-Rite. The common numerical ferrite type designations of 43, 72, and so forth are Fair-Rite's. They also sell powdered iron cores from Micrometals. Roy Lewallen |
"Ralph Mowery" wrote in message nk.net... "John Smith" wrote in message ... By the time you get to UHF it is difficult to use coils. Baluns are usually made of coax or some of the very small hard line like less than 1/4 inch in diameter. The coax is just a fraction of a wavelength. Hi, Ralph - Yes, I can see that now. A little over two turns of RG58, tightly wound, on a .75 inch diameter is about right to be resonant around 440 MHz. However, the resonant frequency moves with compression and expansion of the coil (not unexpectedly) making it difficult to settle on the dimensions. The problem here is that I must have no leads on the coil while experimenting because the leads affect the frequency. Later, when I create the coil at the base of the antenna, I will not be able to determine the coil's exact resonant frequency and adjust it for my frequency of operation. I think I'll give up on this idea. Thanks for your comments. John |
"Richard Clark" wrote in message ... On Sun, 26 Sep 2004 19:38:02 -0500, "John Smith" wrote: Please give me more information about the ferrites... Hi John, Visit: http://bytemark.com/products/content2.htm You will find background information here, but unfortunately they don't maintain the pages very well and some information is lacking. How many do I need? Depends on application. You want at least 3 times the expected load Z. Ferrite Z is bulk determined and the usual standard of bulk size is defined as a type 101 size bead. Thanks for the info, Richard. Is there a way to add toroids to RG58 and measure the resulting effectiveness? Maybe measure the RF on the shield with an RF Voltmeter? John |
On Mon, 27 Sep 2004 09:36:21 -0500, "John Smith"
wrote: Is there a way to add toroids to RG58 and measure the resulting effectiveness? Maybe measure the RF on the shield with an RF Voltmeter? Hi John, Apply a steady, known power to a resistor (25 - 75 Ohms) and measure the voltage (more for sanity's sake). Add one core to the lead (make leads as short as possible, but long enough to allow ONE core to be added. Measure the drop across BOTH the resistor AND the core. Measure the drop across EACH, the resistor and the core. Across the core means at the wire passing through the center, as if the core were a leaded component with leads emerging from either side (it will NOT act like a short! - ain't science wunnerful?). Standard ratiometrics will reveal the core's Z. You need only measure one core at a time, the addition of cores is additive just like series resistors. Unlike chained resistors, successive passes of the same wire through the core raises the Z by the square of the turns. A "turn" is defined as each passage of a conductor through the center (do not confuse with loops which may give rise to the appearance of a "turn" that in fact does not have a lead through the center; or that you lose a turn by counting only loops). 73's Richard Clark, KB7QHC |
"Richard Clark" wrote in message ... On Mon, 27 Sep 2004 09:36:21 -0500, "John Smith" wrote: Is there a way to add toroids to RG58 and measure the resulting effectiveness? Maybe measure the RF on the shield with an RF Voltmeter? Hi John, Apply a steady, known power to a resistor (25 - 75 Ohms) and measure the voltage (more for sanity's sake). Add one core to the lead (make leads as short as possible, but long enough to allow ONE core to be added. Measure the drop across BOTH the resistor AND the core. Measure the drop across EACH, the resistor and the core. Across the core means at the wire passing through the center, as if the core were a leaded component with leads emerging from either side (it will NOT act like a short! - ain't science wunnerful?). Standard ratiometrics will reveal the core's Z. You need only measure one core at a time, the addition of cores is additive just like series resistors. Unlike chained resistors, successive passes of the same wire through the core raises the Z by the square of the turns. A "turn" is defined as each passage of a conductor through the center (do not confuse with loops which may give rise to the appearance of a "turn" that in fact does not have a lead through the center; or that you lose a turn by counting only loops). 73's Richard Clark, KB7QHC Thanks, Richard. I'll give this a shot. Might not get to it until the weekend, though. 73, John |
On Mon, 27 Sep 2004 11:11:16 -0500, "John Smith"
wrote: Thanks, Richard. I'll give this a shot. Might not get to it until the weekend, though. Hi John, Good. Let us know of your results whenever you finish. This feed back encourages others who might not be participating in this thread, but who are interested nonetheless. 73's Richard Clark, KB7QHC |
I've made coiled-coax choke baluns at 440MHz. Seems to work fine.
Generally expect to use pretty small line, to be able to make a small enough coil. The self-resonance is nice but not critical. You're just looking for a moderately high impedance. I have a program that does a good job of estimating the self-resonant frequencies of coils, but it's not on this machine so I can't check for you...I think one of Reg Edward's programs also estimates self resonance for coils. If you can get an impedance that's a few times the load impedance, you should be OK. In feeding an antenna, it may be as important to use a couple such chokes, spaced about a quarter wave apart, to keep the antenna from coupling strongly to a possibly resonant section of feedline. Cheers, Tom "John Smith" wrote in message ... Hi again! Has anyone tried making the coiled coax balun work at UHF? Specifically, on the 70 cm band. In "Antennas for All Applications", they say the coil should be resonant at the frequency of operation. The ARRL book(s) do not cover this band. Using RG58 and a GDO, I've learned that one cannot leave long leads on one's coax coil as it will lower the resonant frequency drastically. Apparently, the long leads in conjunction with the coil forms an antenna/inductor/capacitor combination. It appears to be difficult to obtain the required resonance repeatably. Or, am I missing something (as usual)? The diameter seems to be somewhat critical. Does the diameter need to be much, much smaller than a quarter wave? What diamter would be a maximum? Thanks for any help. John (KD5YI) |
"Tom Bruhns" wrote in message m... I've made coiled-coax choke baluns at 440MHz. Seems to work fine. Generally expect to use pretty small line, to be able to make a small enough coil. The self-resonance is nice but not critical. You're just looking for a moderately high impedance. I have a program that does a good job of estimating the self-resonant frequencies of coils, but it's not on this machine so I can't check for you...I think one of Reg Edward's programs also estimates self resonance for coils. If you can get an impedance that's a few times the load impedance, you should be OK. In feeding an antenna, it may be as important to use a couple such chokes, spaced about a quarter wave apart, to keep the antenna from coupling strongly to a possibly resonant section of feedline. Cheers, Tom Thanks for that info, Tom. If resonance is not necessary, then I need only one turn of .75 diameter to make sure resonance is higher than my operating frequency. That would make the coil with parasitic capacitance look inductive. 73, John |
"John Smith" wrote in message ...
... Thanks for that info, Tom. If resonance is not necessary, then I need only one turn of .75 diameter to make sure resonance is higher than my operating frequency. That would make the coil with parasitic capacitance look inductive. ??? A single turn 3/4" diameter won't give you much reactance at 440. Looks to me like if you used about three turns with that ID with a total length around 1.5 inches, you'd be a lot better off. That'll be about 300 ohms inductive reactance, shunted by enough parasitic capacitance to come close to resonance (thus a higher net reactance). Cheers, Tom |
"Tom Bruhns" wrote in message m... "John Smith" wrote in message ... ... Thanks for that info, Tom. If resonance is not necessary, then I need only one turn of .75 diameter to make sure resonance is higher than my operating frequency. That would make the coil with parasitic capacitance look inductive. ??? A single turn 3/4" diameter won't give you much reactance at 440. Looks to me like if you used about three turns with that ID with a total length around 1.5 inches, you'd be a lot better off. That'll be about 300 ohms inductive reactance, shunted by enough parasitic capacitance to come close to resonance (thus a higher net reactance). Cheers, Tom Actually, Tom, my GDO measures about 440 with two tight turns. Add a couple of inches for leads and it goes lower. So, I figured I needed something slightly higher in resonance to be sure it is on the inductive side of 440. No? John |
The objective is to get a sufficiently high value of impedance. It
doesn't matter whether the impedance is inductive, capacitive, or resistive. You'll get the highest impedance at resonance, so that's the best choice for a single band and there's no point in modifying the design to "be sure it is on the inductive side". Typically, the impedance is higher even considerably above resonance than it would be with an inductive choke of fewer turns. With HF air-core coax chokes, the impedance is very high at resonance and on the bands immediately above and below the resonant band -- and adequately high for some or most applications at frequencies above and below that, since the resonance is quite broad. Roy Lewallen, W7EL John Smith wrote: Actually, Tom, my GDO measures about 440 with two tight turns. Add a couple of inches for leads and it goes lower. So, I figured I needed something slightly higher in resonance to be sure it is on the inductive side of 440. No? John |
Roy, W7EL wrote:
"The objective is to get a sufficiently high value of impedance. It doesn`t matter whether the impedance is inductive, capacitive, or resistive." True, at a particular frequency. But, inductive impedance increases with frequency. Capacitive impedance decreases with frequency. The parallel resonant circuit may have extreme impedance, but at higher frequencies it may have a very low impedance, especially if the resonant circuit is a high-C circuit. Best regards, Richard Harrison, KB5WZI |
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