|
variable capacitor question
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 |
|
****, I can't get it to link right. Here's the part number: 28-179
|
Brian wrote: 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 That link doesn't seem to take me to the kit. But I'm guessing you might need a larger 365 pf (?) air variable to build a decent loop. Can't recall where ya get em these days, but they are still available. Someone will be along momentarily to help you! |
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 |
I'm guessing 10-365pf would cover the majority of the broadcast band.
Brian |
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. |
Oh, if you mean the capacitance of the one in the kit, I really don't know.
I found the manual online, but didn't see any details for the capacitor. Brian |
"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 I see a loopstick in the kit and they usually use a 365-pF cap to tune one of those. Bear in mind the cap is probably a chintzy little thing with a tiny knob. Check oselectronics.com (#BC14400, $10.95. p 96 of online catalog) and tubesandmore.com (Antique Radio Parts, #C-V365, $11.95 and probably the same cap). For what the kit costs I think you could get a better cap, if that's all you need. "PM" |
"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 http://www.midnightscience.com/catalog5.html There's a nice one right at the top of the page.. |
Yeah, I was just looking at that page a few minutes ago, thanks. Brian |
BRIAN,
RadioShack AM/FM Radio Kit http://tinyurl.com/2ycrp RadioShack Catalog #: 28-179 Follow step-by-step directions to make an AM/FM radio that plays through the included speaker. - Change from AM to FM with a Switch - Great for Children Age 8 to Adults - Requires 4 "AA" Batteries ~ RHF .. .. = = = "Brian" wrote in message = = = thlink.net... http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=28-179 .. |
"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 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?? |
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 |
= = = "Brian" wrote in message
= = = hlink.net... I'm guessing 10-365pf would cover the majority of the broadcast band. Brian BRIAN, Loop Inductance of about 240 uH with a Capacitance Range of 10-365 pF provides Full Coverage of the AM/MW Broadcast Band: 530 kHz to 1710 kHz. AM Loop Antenna Calculator - by Bruce Carter (Version 4, 5-19-2003) http://www.mindspring.com/~loop_ante...loop_calc4.htm NOTES: * Enter Edge Length (Inches) is the Distance of One Side of a Square Loop Antenna. * Enter Loop Width (Inches) is the Distance between the First and Last Winding. (Depth) Check-Out the "Loop Antenna Information Forum" eGroup on YAHOO! LOOPS= http://groups.yahoo.com/group/loopantennas/ jm2cw ~ RHF .. |
= = = "Brian" wrote in message
= = = thlink.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.asp?catalog%5Fname=CTLG&product%5Fid=28-179 Brian - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - BRIAN, Since this is an AM & FM Radio 'Kit' it most likely uses a "Miniature Poly-film Variable Tuning Capacitor" for AM & FM Bands CAPS= http://www.oselectronics.com/ose_p98.htm Part Number: BC-88 Ideal Variable Tuning Capacitor for miniature circuitry and use as exact-duplicate replacement in current transistor receivers. Tunes AM Band from 540Khz to 1600Khz and FM band from 88Mhz to 108Mhz. Rotates through a full 180° Maximum Capacity: AM/MW Antenna Section AM 21-152PF, Oscillator Section AM, 10-74PF. FM Antenna Section FM 23-44PF, Oscillator Section FM, 14-23PF Trimmer Capacity: variable to over 12PF. Trimmer adjustment on rear of case. Completely enclosed to clear polyethylene plastic case to protect plates. Includes calibrated dial, screw, and knob. Small size, 3/4" Square x 1/2" Deep. BETTER ALTERNATIVE: AM/MW "Only" Tuning Capacitor: (Good for AM/MW Loop Antennas) For your Loop Antenna 'Project' you may wish to consider a Capacitor that is specifically designed to work within the Band Range of the AM/MW Broadcast band. "Miniature 1 Gang Poly-Film Variable Tuning Capacitor For Broadcast Band" CAPS= http://www.oselectronics.com/ose_p98.htm Part Number: BC-280 Tunes AM band from 540 kHz to 1600 kHz. Ideal Variable Tuning Capacitor for miniature circuitry and use as exact-duplicate replacement in current transistor receivers. Works great in crystal radio sets. (Loop Antennas) - Rotates through a full 180° - Maximum Capacity: Single section tunes from 10-280pf. - Completely enclosed in a clear polyethylene plastic case to protect plates. - Includes Calibrated Dial, Screw, and Knob. Small size, 3/4" Square x 1/2" Deep. REQUIRES: Loop Antenna Inductance of about 350 uH when using a Variable Capacitor Range of 10 to 280 pf. "AM Loop Antenna Calculator" - by Bruce Carter (Version 4, 5-19-2003) CAL= http://www.mindspring.com/~loop_ante...loop_calc4.htm Check-Out the "Loop Antenna Information Forum" eGroup on YAHOO! LOOPS= http://groups.yahoo.com/group/loopantennas/ jm2cw ~ RHF .. |
"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 |
"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 |
"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 |
"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 |
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... In my experience, a single 10-365pf cap has gotten me full MW coverage most of the time on homebrew loops, just not all of them. I have a couple of MW loops that need a switched-gang set-up like you mentioned, to get a fuller range. But, I also have a big (nearly 3 feet across) quilting-hoop-frame loop, twelve turns tightly spaced, with a single, tiny 10-365pf cap. Goes from nearly 500 to just below 1850. I didn't go through any detailed formulae when I was just experimenting with the big loop frame (I've got it put together like a Kiwa loop, it will turn in azimuth and altitude, very helpful for nulls) so I must have lucked out. But those smaller 365 caps have also been perfect for crystal sets, with full tuning range of MW, so I always figured they'd work for loop antennas in the same range (if you've matched the inductance right) and so far, nine times out of ten, they do. Linus |
"Mark Keith" wrote in message om... 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. http://www.mindspring.com/~loop_antenna/amloop2.htm |
Mark Keith wrote:
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. Sure, but being the cap is a semi-fixed limited range device, you will vary the number of turns in the loop to come up with the usable range. The loop should be built around the cap. It's the part that is unchangable within it's range. MK True to a point, but I'm sure a tap or two on that coil may be switched in or out as required. The gangs on a capacitor could be switched in and out too, in conjunction with varying the portion of coil used. mike |
"Brenda Ann Dyer" wrote in message ...
"Mark Keith" wrote in message om... 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. http://www.mindspring.com/~loop_antenna/amloop2.htm That is a standard box loop. But I do note his high end is only 1650. Also his cap is a 500 pf, not a 365 pf. Still not too bad for one cap I guess. In my case, I would have still used a dual 365, as I like mine to cover 160m also. "up to 2000 kc" With his loop, he states he is tuning the low end using only appx 365pf or so. Myself, I would have probably taken one turn or so off, and had a higher top range up to nearly 1800kc, and still cover the low end using the full 500 pf. MK |
GrtPmpkin32 wrote:
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... In my experience, a single 10-365pf cap has gotten me full MW coverage most of the time on homebrew loops, just not all of them. I have a couple of MW loops that need a switched-gang set-up like you mentioned, to get a fuller range. But, I also have a big (nearly 3 feet across) quilting-hoop-frame loop, twelve turns tightly spaced, with a single, tiny 10-365pf cap. Goes from nearly 500 to just below 1850. I didn't go through any detailed formulae when I was just experimenting with the big loop frame (I've got it put together like a Kiwa loop, it will turn in azimuth and altitude, very helpful for nulls) so I must have lucked out. But those smaller 365 caps have also been perfect for crystal sets, with full tuning range of MW, so I always figured they'd work for loop antennas in the same range (if you've matched the inductance right) and so far, nine times out of ten, they do. Linus The traditional single gang 10-365 cap' was used to tune the oscillator in a MW radio, not a front-end preselector. The oscillator usually operated at 455-Khz (I.F.) *above* the desired frequency. This would be about 995-Khz for the low end (540-Khz) of the band. That's why a variable cap' with a maximum of 365-pf is not really low enough (practical) when you want to use it as a tuning cap' for a MW loop antenna or preselector. This is because it has to tune down to the actual lower limit of the MW band (540) instead of the receiver's oscillator frequency (995) at the low end. -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
"starman" wrote in message ... The traditional single gang 10-365 cap' was used to tune the oscillator in a MW radio, not a front-end preselector. The oscillator usually operated at 455-Khz (I.F.) *above* the desired frequency. This would be about 995-Khz for the low end (540-Khz) of the band. That's why a variable cap' with a maximum of 365-pf is not really low enough (practical) when you want to use it as a tuning cap' for a MW loop antenna or preselector. This is because it has to tune down to the actual lower limit of the MW band (540) instead of the receiver's oscillator frequency (995) at the low end. The 10-365 pf section of the tuning cap is used to tune the antenna loop circuit (and/or the RF amplifier circuit if it's a three gang tuner). The oscillator section usually runs around 5-185 pf or so.. |
"starman" wrote in message ... The traditional single gang 10-365 cap' was used to tune the oscillator in a MW radio, not a front-end preselector. The oscillator usually operated at 455-Khz (I.F.) *above* the desired frequency. This would be about 995-Khz for the low end (540-Khz) of the band. That's why a variable cap' with a maximum of 365-pf is not really low enough (practical) when you want to use it as a tuning cap' for a MW loop antenna or preselector. This is because it has to tune down to the actual lower limit of the MW band (540) instead of the receiver's oscillator frequency (995) at the low end. I don't think so. If you have a single gang cap in an AM radio, then it is tuning an antenna coil. If you have an oscillator in a traditional radio, then you have a superhet and will see a two or three gang capacitor. The most frequent thing I've seen for the AM broadcast band is a dual gang capacitor with the oscillator section having about 75% of the capacity of the antenna/RF section. When you see a dual 365 pF cap used in a superhet, you will also see a pad cap in series with the oscillator section so that the oscillator tracks at the needed 455 kHz offset. With a 265 uH coil and a parallel capacitance of 10 pF it resonates at 3093 kHz. With a 265 uH coil and a parallel capacitance of 365 pF it resonates at 512 kHz. This is more than enough for the AM broadcast band. However in real life one gets some stray capacitance due to wiring. Adding 20 pF for stray capacitance, we get. With a 265 uH coil and a parallel capacitance of 30 pF it resonates at 1785 kHz. With a 265 uH coil and a parallel capacitance of 385 pF it resonates at 498 kHz. Still, this is more than adequate. Given the right inductance and keeping stray capacitance low, 365 pF is enough. If you need more capacitance, it probably means your stray capacitance is very high and you reduced the number of turns in the loop (inductance) to offset that problem and then added more variable capacitance to cover the low end of the band. craigm |
"Brenda Ann Dyer" wrote in message
The 10-365 pf section of the tuning cap is used to tune the antenna loop circuit (and/or the RF amplifier circuit if it's a three gang tuner). The oscillator section usually runs around 5-185 pf or so.. The older analog tuning stereo's are a great source of multi-gang caps. The one I'm using now, came from a big monster kenwood reciever from the 70's. Three 365pf stages, and 5 more small value stages. "maybe 50-100 pf??" Not sure... Great cap for any loop, if you want a wide tuning range. Some stereo's use two 365 pf gangs, and three smaller gangs. Those are good too, and will provide for sure all band coverage, and then some. I used one of those on another loop I built recently. Now, if I ever see an ancient stereo with analog tuning, toasted or not, I grab it. Just the cap is worth the hassle of dragging it home. There is at least one source on web for a good variety of caps that are good for loops. One place in particular has quite a few types/values, etc. You can do a search for variable capacitors, and it should come up. MK |
CM,
For 'common' Box Loop Antennas in the 16" to 48" size range: The Common Mistake. Many people use Hook-Up Wire (Insulated and Stranded) when building Loop Antennas with very close spacing of 1/8" or less between the windings. The result is usually a Loop Antenna that will NOT Tune the 'full' AM/MW Band 540 kHz to 1700 kHz with a single 365uf Variable Tuning Capacitor. The Better Idea. They should have simply used "Magnet Wire" (enameled single solid) with a 1/4", 3/8" or 1/2" Spacing between the Windings. This usually "Results" in a Loop Antenna that will Tune the 'full' AM/MW Band 540 kHz to 1700 kHz with a single 365uf Variable Tuning Capacitor. IMHO: Using LITZ Wire with the "InDoor" Loop Antennas in this size range and with the 'wider' Spacing can produce a Higher "Q" and is worth the extra money. FWIW: For Loop Antennas that use a one to two turn "Coupling Coil" which is about 75% to 80% of the size of the Main Tuning Loop Antenna and place inside of the Main Tuning Loop works better {Tunes Sharper} and has a Higher "Q". For more about Loop Antennas Check-Out the YAHHO! eGroup: "Loop Antenna Information Forum" http://groups.yahoo.com/group/loopantennas/ iane ~ RHF .. .. = = = "craigm" wrote in message = = = ... "starman" wrote in message ... The traditional single gang 10-365 cap' was used to tune the oscillator in a MW radio, not a front-end preselector. The oscillator usually operated at 455-Khz (I.F.) *above* the desired frequency. This would be about 995-Khz for the low end (540-Khz) of the band. That's why a variable cap' with a maximum of 365-pf is not really low enough (practical) when you want to use it as a tuning cap' for a MW loop antenna or preselector. This is because it has to tune down to the actual lower limit of the MW band (540) instead of the receiver's oscillator frequency (995) at the low end. I don't think so. If you have a single gang cap in an AM radio, then it is tuning an antenna coil. If you have an oscillator in a traditional radio, then you have a superhet and will see a two or three gang capacitor. The most frequent thing I've seen for the AM broadcast band is a dual gang capacitor with the oscillator section having about 75% of the capacity of the antenna/RF section. When you see a dual 365 pF cap used in a superhet, you will also see a pad cap in series with the oscillator section so that the oscillator tracks at the needed 455 kHz offset. With a 265 uH coil and a parallel capacitance of 10 pF it resonates at 3093 kHz. With a 265 uH coil and a parallel capacitance of 365 pF it resonates at 512 kHz. This is more than enough for the AM broadcast band. However in real life one gets some stray capacitance due to wiring. Adding 20 pF for stray capacitance, we get. With a 265 uH coil and a parallel capacitance of 30 pF it resonates at 1785 kHz. With a 265 uH coil and a parallel capacitance of 385 pF it resonates at 498 kHz. Still, this is more than adequate. Given the right inductance and keeping stray capacitance low, 365 pF is enough. If you need more capacitance, it probably means your stray capacitance is very high and you reduced the number of turns in the loop (inductance) to offset that problem and then added more variable capacitance to cover the low end of the band. craigm .. |
"craigm" wrote in message ...
"starman" wrote in message If you need more capacitance, it probably means your stray capacitance is very high and you reduced the number of turns in the loop (inductance) to offset that problem and then added more variable capacitance to cover the low end of the band. In my case, it's because I want a real wide range. My stray capacitance is low, but I do use only 5 turns for a 44 inch per side loop. I can tune up to 2300 kc using a single low value gang. I want AM-BC, but I also need the 160m band up to 2000 kc. Thats why I design mine the way I do. But I would always buy the biggest caps I could get, even if I didn't need it. The price difference isn't that much. The extra gangs could come in handy on a different loop...IE: one web site has about 10 various types of caps...One they sell is multi-gang, and will add to 1500 pf total...Thats the one I would buy, if I was going to buy one from that page. Might as well get your moneys worth, even if you don't need it all...:/ Use a switch, and you can use it for LW down to a certain freq in addition to MW, but still have a low value for the high end. I have some fixed caps to clip in to drop down into longwave on mine. "to about 175kc". So all total counting those, I cover from 175 to 2300 kc. Myself, I would never buy a single gang, if I could get a dual for not too much more. I like free ones out of old stereo's the best...:) If I didn't need 160m, I would use more turns, and design for LW/AM-BC, instead of AM-BC/160m.. It would be slightly more sensitive down low, using more turns. I may build another monster loop just for LW eventually...MK |
craigm wrote:
"starman" wrote in message ... The traditional single gang 10-365 cap' was used to tune the oscillator in a MW radio, not a front-end preselector. The oscillator usually operated at 455-Khz (I.F.) *above* the desired frequency. This would be about 995-Khz for the low end (540-Khz) of the band. That's why a variable cap' with a maximum of 365-pf is not really low enough (practical) when you want to use it as a tuning cap' for a MW loop antenna or preselector. This is because it has to tune down to the actual lower limit of the MW band (540) instead of the receiver's oscillator frequency (995) at the low end. I don't think so. If you have a single gang cap in an AM radio, then it is tuning an antenna coil. If you have an oscillator in a traditional radio, then you have a superhet and will see a two or three gang capacitor. The most frequent thing I've seen for the AM broadcast band is a dual gang capacitor with the oscillator section having about 75% of the capacity of the antenna/RF section. When you see a dual 365 pF cap used in a superhet, you will also see a pad cap in series with the oscillator section so that the oscillator tracks at the needed 455 kHz offset. With a 265 uH coil and a parallel capacitance of 10 pF it resonates at 3093 kHz. With a 265 uH coil and a parallel capacitance of 365 pF it resonates at 512 kHz. This is more than enough for the AM broadcast band. However in real life one gets some stray capacitance due to wiring. Adding 20 pF for stray capacitance, we get. With a 265 uH coil and a parallel capacitance of 30 pF it resonates at 1785 kHz. With a 265 uH coil and a parallel capacitance of 385 pF it resonates at 498 kHz. Still, this is more than adequate. Given the right inductance and keeping stray capacitance low, 365 pF is enough. If you need more capacitance, it probably means your stray capacitance is very high and you reduced the number of turns in the loop (inductance) to offset that problem and then added more variable capacitance to cover the low end of the band. craigm I was thinking of a superhet with a single gang cap' to tune the oscillator and no front-end preselection tuning, which would require another gang on the cap'. -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
"starman" wrote in message ... craigm wrote: "starman" wrote in message ... The traditional single gang 10-365 cap' was used to tune the oscillator in a MW radio, not a front-end preselector. The oscillator usually operated at 455-Khz (I.F.) *above* the desired frequency. This would be about 995-Khz for the low end (540-Khz) of the band. That's why a variable cap' with a maximum of 365-pf is not really low enough (practical) when you want to use it as a tuning cap' for a MW loop antenna or preselector. This is because it has to tune down to the actual lower limit of the MW band (540) instead of the receiver's oscillator frequency (995) at the low end. I don't think so. If you have a single gang cap in an AM radio, then it is tuning an antenna coil. If you have an oscillator in a traditional radio, then you have a superhet and will see a two or three gang capacitor. The most frequent thing I've seen for the AM broadcast band is a dual gang capacitor with the oscillator section having about 75% of the capacity of the antenna/RF section. When you see a dual 365 pF cap used in a superhet, you will also see a pad cap in series with the oscillator section so that the oscillator tracks at the needed 455 kHz offset. With a 265 uH coil and a parallel capacitance of 10 pF it resonates at 3093 kHz. With a 265 uH coil and a parallel capacitance of 365 pF it resonates at 512 kHz. This is more than enough for the AM broadcast band. However in real life one gets some stray capacitance due to wiring. Adding 20 pF for stray capacitance, we get. With a 265 uH coil and a parallel capacitance of 30 pF it resonates at 1785 kHz. With a 265 uH coil and a parallel capacitance of 385 pF it resonates at 498 kHz. Still, this is more than adequate. Given the right inductance and keeping stray capacitance low, 365 pF is enough. If you need more capacitance, it probably means your stray capacitance is very high and you reduced the number of turns in the loop (inductance) to offset that problem and then added more variable capacitance to cover the low end of the band. craigm I was thinking of a superhet with a single gang cap' to tune the oscillator and no front-end preselection tuning, which would require another gang on the cap'. I've never seen a superhet without an antenna tuning stage (at least not before the new garbage which is PLL tuned and has no tuned antenna stage, and therefor also has no gain). All the older superhet designs, whether tube, transistor or IC (analog and almost all portable digital tuners) has a tuned input. |
starman ) writes:
I was thinking of a superhet with a single gang cap' to tune the oscillator and no front-end preselection tuning, which would require another gang on the cap'. If they make them, they are going to be rare. If you don't have front end tuning, the receiver is going to overload on strong signals. If the IF is too low compared to the signal frequency, you also will never know which signal you are receiving is the one you want, and the image frequency that you don't want. If you've got a receiver with a low IF frequency, but front end tuning not ganged to the local oscillator tuning, you may be able to null out the image frequency, but you will have to keep adjusting both knobs. It would be easy to mistune the front end tuning, and tune in the image frequency. That's why all receivers have ganged tuning, at least after it was invented decades ago. If the IF is higher in frequency, of course one can use other techniques. Put the IF in the HF range, and the image response will be MHz away, and the front end will not need constant tuning. That lead to the separate front end tuning in the sixties, where it only needed peaking every so often. For limited range receivers, such as for only the ham bands, a suitably high IF could mean that one could use bandpass filters at the front end, ie they tuned a fixed 500KHz or so segment, and did not need tuning as you crossed the band. Or put the IF above the shortwave frequencies, and you have more leeway. There, the image frequency is the other side of the IF, so one could use a low pass filter, with a cut-off of 30MHz, though that still means the active stages before the first IF filter see a 30MHz range of frequencies, which may lead to overloading. At least some receivers, once first IFs went that high, allowed for a low pass filter and some sort of preselection, so you could choose. Michael |
Michael Black wrote:
starman ) writes: I was thinking of a superhet with a single gang cap' to tune the oscillator and no front-end preselection tuning, which would require another gang on the cap'. If they make them, they are going to be rare. If you don't have front end tuning, the receiver is going to overload on strong signals. If the IF is too low compared to the signal frequency, you also will never know which signal you are receiving is the one you want, and the image frequency that you don't want. If you've got a receiver with a low IF frequency, but front end tuning not ganged to the local oscillator tuning, you may be able to null out the image frequency, but you will have to keep adjusting both knobs. It would be easy to mistune the front end tuning, and tune in the image frequency. That's why all receivers have ganged tuning, at least after it was invented decades ago. If the IF is higher in frequency, of course one can use other techniques. Put the IF in the HF range, and the image response will be MHz away, and the front end will not need constant tuning. That lead to the separate front end tuning in the sixties, where it only needed peaking every so often. For limited range receivers, such as for only the ham bands, a suitably high IF could mean that one could use bandpass filters at the front end, ie they tuned a fixed 500KHz or so segment, and did not need tuning as you crossed the band. Or put the IF above the shortwave frequencies, and you have more leeway. There, the image frequency is the other side of the IF, so one could use a low pass filter, with a cut-off of 30MHz, though that still means the active stages before the first IF filter see a 30MHz range of frequencies, which may lead to overloading. At least some receivers, once first IFs went that high, allowed for a low pass filter and some sort of preselection, so you could choose. Michael The most common tube type AM radios (5-tubes) used a two gang tuning cap' for the front-end tuning and oscillator but I think there were some cheap models with no front-end RF amp' or preselection tuning. The antenna was connected to the mixer which had some gain to off set the lack of an RF amp' stage. -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
"starman" wrote in message ... The most common tube type AM radios (5-tubes) used a two gang tuning cap' for the front-end tuning and oscillator but I think there were some cheap models with no front-end RF amp' or preselection tuning. The antenna was connected to the mixer which had some gain to off set the lack of an RF amp' stage. Armstrong's first superhets didn't have a tuned RF stage, but those radios were designed to operate at frequencies too high for the TRFs of the WW1 era. Something like 3 mc, if I recall. There wasn't any interfering signals at the image frequency.` I'm not aware of any commercially available AM superhets without preslection and such radios would have a big problem with images 910 kc above the intended frequency. The images would include other stations in the broadcast band, the old Loran buzzsaw, hams on 160 meters, etc. Few AM band only radios had RF stages, and most of those were simple RC coupled stages for extra gain, but not selectivity. But, as far as I know, they all had a tuned loop antenna or antenna coil. Frank Dresser |
| All times are GMT +1. The time now is 11:55 AM. |
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
RadioBanter.com