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 ).. :-)

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
--

"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
.................................................. ..........

"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......

"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
.................................................. .........

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
--

"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.....

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

"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.....