|
Magloop capacitor padding?
Hi!....
I`ve just built a magloop 5`square and i find the variable cap only tunes down to 4.45mhz.. can i pad the cap out with a coaxial ur213 stub to reach 3.5mhz ???.... Lee.......G6ZSG..... (The 1metre loop works fine - 15 to 7mhz ).. :-) |
Magloop capacitor padding?
On Wed, 02 Aug 2006 22:09:43 GMT, "Lee"
wrote: Hi!.... I`ve just built a magloop 5`square and i find the variable cap only tunes down to 4.45mhz.. can i pad the cap out with a coaxial ur213 stub to reach 3.5mhz ???.... Lee.......G6ZSG..... (The 1metre loop works fine - 15 to 7mhz ).. :-) This is a riddle, right? If you are after a few pF, then the losses in the stub will be very low, but at lower reactances, the resistance or loss element becomes more significant. So you have hinted a bit at this loop.... lets do some guessing: An inductor of 1.5m square and 20mm conductor diameter has an inductance around 5uH, thats about 140 ohms at 4.45MHz... so it looks like your tuning capacitor is 250pF. You probably need around 400pF at 3.5MHz, so lets add another 200pF in shunt from an RG213 o/c stub. You need an admittance of ????+j0.0044 S. Using the line loss calculator at http://www.vk1od.net/tl/tllce.php , it looks like 1.95m of RG213 terminated with 1e8+j0 will have an input admittance of 0.000004+j0.004401. So this stub is dropping around 4e-6S of loss conductance across the loop, or 250kOhms in shunt if you like. This shunt resistance is somewhat lower than you would expect with a good quality capacitor. You can estimate the effect by plugging the 250KOhms resistance into your design model and examining the change in efficiency. Owen -- |
Magloop capacitor padding?
"Lee" wrote in message . uk... Hi!.... I`ve just built a magloop 5`square and i find the variable cap only tunes down to 4.45mhz.. can i pad the cap out with a coaxial ur213 stub to reach 3.5mhz ???.... Lee.......G6ZSG..... (The 1metre loop works fine - 15 to 7mhz ).. :-) ===================================== Lee, You will need at least 70 percent more capacitance. But where you'll find that much, high voltage, high Q capacitance is anybody's guess. You would do better to fit a larger variable capacitor designed for the job. Or make a larger loop. A lumped capacitor made from coax cable is low Q anyway because of the small diameter of the inner conductor. Compare coax inner conductor diameter with loop conductor diameter. ---- Download program MAGLOOP4 from website below to give you some ideas. ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
Magloop capacitor padding?
"Owen Duffy" wrote in message ... On Wed, 02 Aug 2006 22:09:43 GMT, "Lee" wrote: Hi!.... I`ve just built a magloop 5`square and i find the variable cap only tunes down to 4.45mhz.. can i pad the cap out with a coaxial ur213 stub to reach 3.5mhz ???.... Lee.......G6ZSG..... (The 1metre loop works fine - 15 to 7mhz ).. :-) This is a riddle, right? If you are after a few pF, then the losses in the stub will be very low, but at lower reactances, the resistance or loss element becomes more significant. So you have hinted a bit at this loop.... lets do some guessing: An inductor of 1.5m square and 20mm conductor diameter has an inductance around 5uH, thats about 140 ohms at 4.45MHz... so it looks like your tuning capacitor is 250pF. You probably need around 400pF at 3.5MHz, so lets add another 200pF in shunt from an RG213 o/c stub. You need an admittance of ????+j0.0044 S. Using the line loss calculator at http://www.vk1od.net/tl/tllce.php , it looks like 1.95m of RG213 terminated with 1e8+j0 will have an input admittance of 0.000004+j0.004401. So this stub is dropping around 4e-6S of loss conductance across the loop, or 250kOhms in shunt if you like. This shunt resistance is somewhat lower than you would expect with a good quality capacitor. You can estimate the effect by plugging the 250KOhms resistance into your design model and examining the change in efficiency. Owen -- Heh!....too technical for me i`m afraid Owen....Heh heh..... :-) I`m using a tuned RG213 stub about 2meters long as a temporary measure for 80, till i can sort the cap problem....... i just need a simple answer is all....... Thanks..... Lee.........G6ZSG...... |
Magloop capacitor padding?
"Reg Edwards" wrote in message ... "Lee" wrote in message . uk... Hi!.... I`ve just built a magloop 5`square and i find the variable cap only tunes down to 4.45mhz.. can i pad the cap out with a coaxial ur213 stub to reach 3.5mhz ???.... Lee.......G6ZSG..... (The 1metre loop works fine - 15 to 7mhz ).. Sorry, should read 15 to 40 meters..... :-/ ===================================== Lee, You will need at least 70 percent more capacitance. But where you'll find that much, high voltage, high Q capacitance is anybody's guess. You would do better to fit a larger variable capacitor designed for the job. Or make a larger loop. My thoughts also on both likely answers Reg but the cap is a foot long 4 inch sq 12/14kv 26 rotary vanes, now 28 as i`ve added (squeezed) two more rotaty vanes and four stators into the frame (came in kit form), also designed the loop using loopcalc program which gives 407pf at 3.5megs...my cap is supposed to be 500pf !!...... A lumped capacitor made from coax cable is low Q anyway because of the small diameter of the inner conductor. Compare coax inner conductor diameter with loop conductor diameter. ---- Download program MAGLOOP4 from website below to give you some ideas. Got it, good prog...... Thanks Reg... Lee........G6ZSG...... .................................................. ......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. ......... |
Magloop capacitor padding?
On Thu, 03 Aug 2006 01:30:45 GMT, "Lee"
wrote: Heh!....too technical for me i`m afraid Owen....Heh heh..... :-) I`m using a tuned RG213 stub about 2meters long as a temporary measure for 80, till i can sort the cap problem....... i just need a simple answer is all....... Well, my guess from you data is you will need about 2m of RG213, and you will find that it "works", but it doesn't work as well as a low loss capacitor. Owen -- |
Magloop capacitor padding?
"Owen Duffy" wrote in message ... On Thu, 03 Aug 2006 01:30:45 GMT, "Lee" wrote: Heh!....too technical for me i`m afraid Owen....Heh heh..... :-) I`m using a tuned RG213 stub about 2meters long as a temporary measure for 80, till i can sort the cap problem....... i just need a simple answer is all....... Well, my guess from you data is you will need about 2m of RG213, and you will find that it "works", but it doesn't work as well as a low loss capacitor. True, but it works......altho` it`s very narrow band, would much prefer a variable cap` or even a vacuum cap would be better still....can`t afford one tho`......ho-hum...... Thanks Owen.... Lee.......G6ZSG..... |
Magloop capacitor padding?
Lee,
You can greatly increase the Q of a long length of coax cable by cutting it into 7 equal short lengths. Then put six lengths round one length and connect all seven in parallel. In theory, this increases Q by 7*7 = 49 times for the same capacitance. And it occupies less space. But when outdoors it is more difficult to seal against the weather. ---- Reg, G4FGQ |
Magloop capacitor padding?
"Lee" wrote in message . uk... Hi!.... I`ve just built a magloop 5`square and i find the variable cap only tunes down to 4.45mhz.. can i pad the cap out with a coaxial ur213 stub to reach 3.5mhz ???.... Lee.......G6ZSG..... (The 1metre loop works fine - 15 to 7mhz ).. :-) Sorted!!!!...... I rebuilt my variable capacitor and found a bad link in a bank of stator vanes ...effectively, only half the stators were coupled ...... i can now cover 80 metres.....yippee!!!.... Thanks for everyones help and advice.....find me on 3.697megs....... ;-) or, more likely, 3.733megs, QRV SSTV, analogue or digital.... Regards..... Lee......G6ZSG..... |
| All times are GMT +1. The time now is 02:56 AM. |
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
RadioBanter.com