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Whats the capacitance Brian. This is the real Brian Hill by the way
-- 73 and good DXing. Brian ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A lot of radios and 100' of rusty wire! Zumbrota, Southern MN Brian's Radio Universe http://webpages.charter.net/brianehill/ "Brian" wrote in message hlink.net... Anyone know if the variable cap, that I'm assuming is included in this kit, will suffice for the bc band loop I want to build? http://www.radioshack.com/product.as...uct%5Fid=28-17 9 Brian |
#2
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I'm guessing 10-365pf would cover the majority of the broadcast band.
Brian |
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Brian wrote: I'm guessing 10-365pf would cover the majority of the broadcast band. Yep, that would be the one. Maybe Mouser Electronics still has 'em. I know someone does. |
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"Brian" wrote in message hlink.net...
I'm guessing 10-365pf would cover the majority of the broadcast band. Brian I bet it takes double that. He will need a "dual" 365pf cap to cover the whole band with one cap, and no switching. With the dual cap, you solder the two gangs together, and end up with 730 pf. But the min value will be larger with the dual cap, and will reduce the upper range a bit. IE: two 10-365pf caps, will give a 20 pf min, instead of 10. This shouldn't keep you from tuning the whole BC band, but if you rig a way to switch to only one gang, you can increase your upper range even farther. My 44 inch per side loop uses a triple 365pf cap.It also has five smaller value gangs "maybe 25-50 pf each??" ,as well for eight gangs total. With that cap, and a switch which I mount of the side of the cap, I am covering from 450 kc to 2300 kc in two ranges. My 16 inch round loop uses a plain dual 365pf cap. No extra gangs. It covers from 500-2000kc with no switching. I really have my doubts a single 365pf cap will cover the whole BC band. To cover the low end, you will need more turns to tune with the small value cap. This in turn will reduce the upper range due to the extra turns in themselves, and also the extra stray capacitance you will see from the extra windings. If you tune for 540 kc at the low end, I doubt you will be able to tune 1600. I'm taking a wild stab, and guessing your upper range might be 1000 kc or so ?? MK |
#5
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"Mark Keith" wrote in message om... "Brian" wrote in message hlink.net... I'm guessing 10-365pf would cover the majority of the broadcast band. Brian I bet it takes double that. He will need a "dual" 365pf cap to cover the whole band with one cap, and no switching. Why would this be the case, when a standard AM radio covers the entire broadcast band with a single 10-365pf variable?? |
#6
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"Brenda Ann Dyer" wrote in message ...
"Mark Keith" wrote in message om... "Brian" wrote in message hlink.net... I'm guessing 10-365pf would cover the majority of the broadcast band. Brian I bet it takes double that. He will need a "dual" 365pf cap to cover the whole band with one cap, and no switching. Why would this be the case, when a standard AM radio covers the entire broadcast band with a single 10-365pf variable?? Q of the circuit? Not really sure. I don't build AM radios. But in judging from my loops, I'd be surprised to see one tune the whole band with a single 365pf cap. I'm fairly sure neither of mine would. If I dumped one of the gangs of my dual 365 cap on my 16 inch loop, it WOULD NOT cover the whole BC band. When you build a loop, you build it around the cap. IE: adjust the number of turns to give the desired range with the cap at hand. If you can get a loop to cover the whole band with a single gang cap, more power to you. But I'd have to see it to believe it. It ain't happening here. MK |
#7
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"Mark Keith" wrote in message m... Q of the circuit? Not really sure. I don't build AM radios. But in judging from my loops, I'd be surprised to see one tune the whole band with a single 365pf cap. I'm fairly sure neither of mine would. If I dumped one of the gangs of my dual 365 cap on my 16 inch loop, it WOULD NOT cover the whole BC band. When you build a loop, you build it around the cap. IE: adjust the number of turns to give the desired range with the cap at hand. If you can get a loop to cover the whole band with a single gang cap, more power to you. But I'd have to see it to believe it. It ain't happening here. MK You're exactly right when you say you build the loop to work with the variable cap. I made a loop with an am broadcast oscillator cap, rather than the more common antenna cap. The oscillator caps usually max out at something like 250 pf. I needed more turns on the loop antenna than usual, something like 14 turns on a form about 2 feet across. Limiting the distributed capacitance with the extra turns is the biggest problem. I had to evenly space the windings about 1/4" apart. I was able to get coverage from a little more than 530 to 1700 kHz. Basically, the frequency range with a fixed inductance will be proportional to the square root of the ratio of the max capacitance to the minimum capacitance. The distributed capacitance adds to the maximum capacitance and the minimum capacitance of the tuning cap in the circuit. A 9:1 total capacitance ratio will give a 3:1 tuning ratio. A 16:1 total capacitance ratio will give a 4:1 tuning ratio. A 10 to 365 pf tuning capacitor will tune from 530 to 1700 kHz if the distributed capacitance can be held to below about 28 pf. An 8 to 250 pf tuning capacitor will tune the same range if the distributed capacitance can be held to below about 18 pf. This is more difficult because a 250 pf tuning cap takes more turns to resonate in the AM BC band. Wide, even turn spacing with thin wire does the job. Anyway, those are the numbers I calculate. Hope I got it right! They seem to be in the right ballpark. I won't try to make any attempt to measure the distributed capacitance of a loop antenna. I'll recommend Reg Edward's programs. These are small, ready to run programs for all sorts of radio design problems. Among them are is the RJELOOP3 programs for multiturn loop antennas. http://www.btinternet.com/~g4fgq.regp/ http://www.btinternet.com/~g4fgq.regp/page3.html#S301" Frank Dresser |
#8
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Mark Keith wrote:
"Brian" wrote in message hlink.net... I'm guessing 10-365pf would cover the majority of the broadcast band. Brian I bet it takes double that. He will need a "dual" 365pf cap to cover the whole band with one cap, Resonance in a circuit happens when inductive reactance equals capacitive reactance. They're both measured in ohms. There is NO magic capacitance for covering the broadcast band. The PROPER capacitance is the one that matches the antenna/coil over it's intended range. So, resonance occurs at: Xl = Xc That can also be said as: f = 1 / [2 * pi (sqrt LC)] http://www.electronics-tutorials.com.../resonance.htm mike |
#9
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"Mark Keith" wrote in message om... "Brian" wrote in message hlink.net... I'm guessing 10-365pf would cover the majority of the broadcast band. Brian I bet it takes double that. He will need a "dual" 365pf cap to cover the whole band with one cap, and no switching. With the dual cap, you solder the two gangs together, and end up with 730 pf. But the min value will be larger with the dual cap, and will reduce the upper range a bit. IE: two 10-365pf caps, will give a 20 pf min, instead of 10. This shouldn't keep you from tuning the whole BC band, but if you rig a way to switch to only one gang, you can increase your upper range even farther. My 44 inch per side loop uses a triple 365pf cap.It also has five smaller value gangs "maybe 25-50 pf each??" ,as well for eight gangs total. With that cap, and a switch which I mount of the side of the cap, I am covering from 450 kc to 2300 kc in two ranges. My 16 inch round loop uses a plain dual 365pf cap. No extra gangs. It covers from 500-2000kc with no switching. I really have my doubts a single 365pf cap will cover the whole BC band. To cover the low end, you will need more turns to tune with the small value cap. This in turn will reduce the upper range due to the extra turns in themselves, and also the extra stray capacitance you will see from the extra windings. If you tune for 540 kc at the low end, I doubt you will be able to tune 1600. I'm taking a wild stab, and guessing your upper range might be 1000 kc or so ?? MK Mark, Gangs in the capacitor is not the issue when trying to cover the AM broadcast band. It is the ratio of highest to lowest capacitance that is of concern. If I assume the highest frequency is 1710 kHz and the lowest is 520 kHz. The ration of highest to lowest frequency is 3.29. Square this number to get 10.82. The ratio of high to low capacitave needed is 10.82 but this must also include stray wiring capacitance. A 10-365 pf capacitor has a ratio of 36.5. More than enough to cover the band if the stray capacitance is low enough. A 10-365 pf capacitor will work if the stray capacitance is less than 28 pf. Once you have the a sufficient range of capacitance, you just need to make sure your loop has the proper inductance to match that capacitance. If you need 700 or 1000 pf to tune a loop to the AM band, then it indicates the inductance of your loop is lower and you are just using more capacitance to offset the condition. -------------------- Going back to the initial question in the thread. If the tuning capacitor supplied with the kit could cover the entire AM band with the kit's coil, then it should also cover the entire AM band with a different coil/loop. It is just a matter of getting the inductance right. craigm |
#10
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"craigm" wrote in message ...
"Mark Keith" wrote in message om... "Brian" wrote in message hlink.net... I'm guessing 10-365pf would cover the majority of the broadcast band. Brian I bet it takes double that. He will need a "dual" 365pf cap to cover the whole band with one cap, and no switching. With the dual cap, you solder the two gangs together, and end up with 730 pf. But the min value will be larger with the dual cap, and will reduce the upper range a bit. IE: two 10-365pf caps, will give a 20 pf min, instead of 10. This shouldn't keep you from tuning the whole BC band, but if you rig a way to switch to only one gang, you can increase your upper range even farther. My 44 inch per side loop uses a triple 365pf cap.It also has five smaller value gangs "maybe 25-50 pf each??" ,as well for eight gangs total. With that cap, and a switch which I mount of the side of the cap, I am covering from 450 kc to 2300 kc in two ranges. My 16 inch round loop uses a plain dual 365pf cap. No extra gangs. It covers from 500-2000kc with no switching. I really have my doubts a single 365pf cap will cover the whole BC band. To cover the low end, you will need more turns to tune with the small value cap. This in turn will reduce the upper range due to the extra turns in themselves, and also the extra stray capacitance you will see from the extra windings. If you tune for 540 kc at the low end, I doubt you will be able to tune 1600. I'm taking a wild stab, and guessing your upper range might be 1000 kc or so ?? MK Mark, Gangs in the capacitor is not the issue when trying to cover the AM broadcast band. It is the ratio of highest to lowest capacitance that is of concern. Of course. I've already noted that. If I assume the highest frequency is 1710 kHz and the lowest is 520 kHz. The ration of highest to lowest frequency is 3.29. Square this number to get 10.82. The ratio of high to low capacitave needed is 10.82 but this must also include stray wiring capacitance. A 10-365 pf capacitor has a ratio of 36.5. More than enough to cover the band if the stray capacitance is low enough. If you say so. It would depend on the spacing of the wires to a large degree. I'm just saying most all the loops I've built so far need a wider range cap. If they are getting by with a single 10-365 cap, I'd like to see the loop, the size, winding spacing , etc..I bet it will be quite different than the ones I build. Most of my loops are the standard solenoid type loops. The spacing varies, but on my 44 inch per side loop, the ratio is pretty wide, maybe 5 to 1. It's smaller on my small loop, as it uses thick wire, that is closer together as far as the ratio. A 10-365 pf capacitor will work if the stray capacitance is less than 28 pf. The stray capacitance of my large loop is appx 9 pf. "assuming rjloop3.exe is fairly accurate as a calculator. It seems to be." . There is no way in heck a 10-365 pf cap would cover the whole BC band on that particular loop. I can tune up to about 2300 kc on the upper end if I use a single low value gang. "I have a switch". According to my calculations, if I used only a single 365pf, my lower limit would be appx 810 kc. I calculate a dual 365pf to drop down to about 580 kc. I calculate needing 1220 pf at 450 kc. And this is pretty close to what I'm using. As you can see, yea, I'm using a multi-gang cap, which is switchable to allow a low value for the high end, but my loop also covers a wider range than just the BC band. 450-2300 kc in two ranges. My 16 inch round loop uses a dual 365 pf cap, which I assume is maybe 20-30 min-730 high value. With that particular loop, it covers from 500 to 2000 kc. I built a loop a while back for another poster of this group. It was a diamond loop on a 30 inch frame. It used a multi-gang cap with a dual 365 pf, and a bit extra in three other small gangs. That loop covered 540 to 1830 kc. Once you have the a sufficient range of capacitance, you just need to make sure your loop has the proper inductance to match that capacitance. Of course. If you need 700 or 1000 pf to tune a loop to the AM band, then it indicates the inductance of your loop is lower and you are just using more capacitance to offset the condition. Maybe so, but all the loops I've built need that range, and I always build the loop around the cap at hand. I haven't looked at the loop he's considering, but it must be quite a bit different than the box solenoid type loops I've been building. Maybe a smaller pancake type loop? I'd have to look at it. -------------------- Going back to the initial question in the thread. If the tuning capacitor supplied with the kit could cover the entire AM band with the kit's coil, then it should also cover the entire AM band with a different coil/loop. It is just a matter of getting the inductance right. It's quite possible I guess, if the loop specs fits that low a range to allow whole band coverage. None of mine do though. BTW, by whole band coverage, I am including the new upper range also to 1700 or whatever it is... All mine go higher than that. My large loop is a diamond, 5 turns, 44 inches per side. The PVC cross support is 5 ft across. It's here in the room and rotates. It's a kick butt loop. Very sensitive, and very balanced. I'm not saying a loop can't be made to cover the whole band with a single 10-365. I'm just saying that for the average "box" type loops many will try to build, I don't see it happening. I guess just general box/diamond loop info from what I see here...:/ MK |
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