Hi All,

I keep noticing something which looks odd to me - I am sure it is
correct because this appears to be standard practice, but I would
appreciate it if someone could shed some light on the reasons...

Take a peek at http://web.telia.com/~u85920178/tx/500w-txt.htm (first
example I found). Specifically look at the 5 position switch on the
output matching, in position 5, the bulk of the inductor is shorted
out. This just looks plain wrong to me.

I assume that having shorted turns on an inductor like this drops the Q
drastically, or is this the whole point ?

Is the same true for roller coaster type variable inductors ?

Comments appreciated.

Thanks,

Mark.

"MarkAren" wrote in message
oups.com...
Hi All,

I keep noticing something which looks odd to me - I am sure it is
correct because this appears to be standard practice, but I would
appreciate it if someone could shed some light on the reasons...

Take a peek at http://web.telia.com/~u85920178/tx/500w-txt.htm (first
example I found). Specifically look at the 5 position switch on the
output matching, in position 5, the bulk of the inductor is shorted
out. This just looks plain wrong to me.

I assume that having shorted turns on an inductor like this drops the Q
drastically, or is this the whole point ?

Is the same true for roller coaster type variable inductors ?


It might cause some small ammount of power loss but it has been done that
way in many other amps over the years. Probably not enough loss to notice
or others would have done it differantly.

MarkAren wrote:
Hi All,

I keep noticing something which looks odd to me - I am sure it is
correct because this appears to be standard practice, but I would
appreciate it if someone could shed some light on the reasons...

Take a peek at http://web.telia.com/~u85920178/tx/500w-txt.htm (first
example I found). Specifically look at the 5 position switch on the
output matching, in position 5, the bulk of the inductor is shorted
out. This just looks plain wrong to me.

I assume that having shorted turns on an inductor like this drops the Q
drastically, or is this the whole point ?

Is the same true for roller coaster type variable inductors ?

Comments appreciated.

Thanks,

Mark.

I've pondered this question myself. I've only seen this done on
air-wound coils -- on a coil wound or a core this would be disaster. I
think the reason it's done is because if you _don't_ short the turns
you'd have all sorts of wierd resonance and/or high RF voltage effects
in the unshorted coils.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Bill Sabin W0IYH posted an enlightening comment on this topic just a
month ago in this newsgroup. Look in groups.google.com for his posting
on April 27. The topic was " Shielded inductors -- why do they work
so well?"

Roy Lewallen, W7EL

MarkAren wrote:
Hi All,

I keep noticing something which looks odd to me - I am sure it is
correct because this appears to be standard practice, but I would
appreciate it if someone could shed some light on the reasons...

Take a peek at http://web.telia.com/~u85920178/tx/500w-txt.htm (first
example I found). Specifically look at the 5 position switch on the
output matching, in position 5, the bulk of the inductor is shorted
out. This just looks plain wrong to me.

I assume that having shorted turns on an inductor like this drops the Q
drastically, or is this the whole point ?

Is the same true for roller coaster type variable inductors ?

Comments appreciated.

Thanks,

Mark.

On 26 May 2005 00:17:44 -0700, "MarkAren"
wrote:

Hi All,

I keep noticing something which looks odd to me - I am sure it is
correct because this appears to be standard practice, but I would
appreciate it if someone could shed some light on the reasons...

Take a peek at http://web.telia.com/~u85920178/tx/500w-txt.htm (first
example I found). Specifically look at the 5 position switch on the
output matching, in position 5, the bulk of the inductor is shorted
out. This just looks plain wrong to me.

I assume that having shorted turns on an inductor like this drops the Q
drastically, or is this the whole point ?

Is the same true for roller coaster type variable inductors ?


Mark,

Nominally those coils are loaded to a Q of less than 10. So the
shorted turns do not significantly affect the Q of the coil. Also the
turns are off the "end" of the inductor where the least effect is
felt.

In some cases the shorted turn can be used to tune the coil there
is a 6M amp that used a shorted turn inside the primary
inductor the make the inductance variable with out mechanicaly
altering the primary coil.

Part two of that is if t he unused sections were left unshorted
they may resonate and develope high voltages or other
undesired parasitic effects.

Roller inductors are indeed used that way as well.

Allison

It's really not all that hard to estimate what the degradation will be.
Use your favorite inductance formula or program to find the inductance
of the whole coil, and then find the inductance of the unshorted part
if it were standing alone, and the inductance of the shorted part if it
were standing alone. From those three values you can find the mutual
inductance between the coils, and the coefficient of coupling. You'll
probably find it's much lower than you had guessed. The result is that
shorting the unused portion really doesn't have that much effect on
either the inductance or the Q of the remaining portion.

For example, an inductance program I use tells me that 20 turns of
8AWG wire 2" inside diameter helix, spaced with a pitch of two wire
diameters, will give me a coil just over 4" long, 8.705uH, unloaded Q
around 860 at 10MHz. (There's some question if the unloaded Q would
actually be that high, but that's another topic.) If I use 10 turns of
that same winding geometry, I get 3.637uH and Qu about 720 at 10MHz.
The mutual inductance between the two 10-turn halves of the 20-turn
coil is then 0.5*(L(20t)-2*L(10t)) = .716uH, and the coefficient of
coupling is mutual inductance divided by sqrt(L(10t)*L(10t)), or .197.
If you leave the unused portion of the coil open, the inductance of the
10t part you're using is, of course, 3.637uH. If you short out the
unused portion, the inductance of the used part drops to about 3.496uH,
which is only about 3.9 percent. The Q drops by about twice that
percentage. But if the LOADED Q of the network is around 10, and the
unloaded Q is up around 500, then only 1/50 of the input power is
dissipated in the coil resistance, and a change of even 10 percent on
the unloaded Q will result in a change of 10 percent in the power
dissipated in the coil...so in a 1kW system, the power dissipation in
the coil might go from 20 watts to 22 watts, in this example.

In summary, the assumption that the (unloaded) Q drops drastically is
incorrect for practical coils commonly used in transmitter tank
circuits. But it does suggest that you should understand what the
coupling is, and insure that the shorted part of the coil really isn't
coupled too strongly with the unshorted part: if the coil is very
short compared with its diameter, you'd get into trouble shorting
turns.

Cheers,
Tom

I keep noticing something which looks odd to me - I am sure it is
correct because this appears to be standard practice, but I would
appreciate it if someone could shed some light on the reasons...

Take a peek at http://web.telia.com/~u85920178/tx/500w-txt.htm (first
example I found). Specifically look at the 5 position switch on the
output matching, in position 5, the bulk of the inductor is shorted
out. This just looks plain wrong to me.

Hi Mark,
If you take an inductor, make a tapping and pass a hugh RF current through
the smaller end, then a voltage will be induced in the other side. If the
tapping ratio was (say) 10:1 rati,o and 300 volts was developed across the
1, then the bigger end would have 3000 volts at the other end = arcing and
"sparkalating", smoke and switch breakdown :-(

Regards

From: "K7ITM" on Thurs 26 May 2005 13:32

snip of good stuff

In summary, the assumption that the (unloaded) Q drops drastically is
incorrect for practical coils commonly used in transmitter tank
circuits. But it does suggest that you should understand what the
coupling is, and insure that the shorted part of the coil really isn't
coupled too strongly with the unshorted part: if the coil is very
short compared with its diameter, you'd get into trouble shorting
turns.

Good stuff there, Tom. The COEFFICIENT OF COUPLING isn't
considered enough by others (in my opinion) but makes all
the difference as far as losses.

Toroidal power transformers are going to be in for a hot
time in the old town if they are mounted with a non-
insulating strap. That's the "shorted turn effect" with a
toroidal form having a high coefficient of coupling due to
concentration of flux lines. The conductive mounting strap
will suddenly become very hot being a one-turn winding that
has very low resistance (always insulate such mountings).

Air-core coils having a high length to diamerter ratio will
have a lower possible coefficient of coupling...which
actually varies somewhat depending on the position of the
coupling location along the length...lowest at the ends.
The flux density is very low, spread all over as compared to
a toroid where the flux density is concentrated IN the torus.
"Pie-wound" multi-layer coils have an intermediate coefficient
of coupling, less than toroids but greater than high L
solenoidal windings.

There will be some losses due to that "shorted turn effect"
but that hasn't unduly bothered high-power transmitter
designers and users for over six decades. I got started
in HF communications in the Army in the fifties and one of
my tasks was changing the shorting link assemblies on a 15
KW Press Wireless transmitter running FSK RTTY. Two tank
coils about four feet long and about a foot in diameter using
tubing about 3/4" in diameter (memory may be short on the
exact dimensions, I didn't take notes due to having to QSY
as quickly as possible when so ordered). Those got thermally
HOT sometimes but then the final amplifier compartment was
generally thermally hot all over despite forced air cooling.
At least 20 KW of the AC electrical feed of about 45 KW
was wasted in various forms of HEAT on those critters.

Shorting bars were ALWAYS placed beginning at the bottom end
of the coil. A similar shorting system was used on the
old BC-340 power amplifiers (10 KW CW, also running FSK).
Those beasties were operating 24/7 and cranked out the RF
continuously until another QSY was required. BC-340 was
water-cooled with the water flow going through the tank
coil tubing from the final amplifier tube jacket. Was
thermally cooler than the PW-15.

For those who are interested in details and comparisons with
actual result numbers can consult with almost any old text
for ways of measuring Q, coefficient of coupling, etc.
Hours of drudgery to derive all that...as lots of EE students
can testify to. :-)

Tim Wescott wrote:
MarkAren wrote:

Hi All,

I keep noticing something which looks odd to me - I am sure it is
correct because this appears to be standard practice, but I would
appreciate it if someone could shed some light on the reasons...

Take a peek at http://web.telia.com/~u85920178/tx/500w-txt.htm (first
example I found). Specifically look at the 5 position switch on the
output matching, in position 5, the bulk of the inductor is shorted
out. This just looks plain wrong to me.

I assume that having shorted turns on an inductor like this drops the Q
drastically, or is this the whole point ?

Is the same true for roller coaster type variable inductors ?

Comments appreciated.

Thanks,

Mark.

I've pondered this question myself. I've only seen this done on
air-wound coils -- on a coil wound or a core this would be disaster. I
think the reason it's done is because if you _don't_ short the turns
you'd have all sorts of wierd resonance and/or high RF voltage effects
in the unshorted coils.

Most commerical receiver designs that used band switched coils
had a special rotary switch that shorted out all the unused coils
while they unshorted and selected the coil for the current band.
The reason was to avoid 'suckout' at the resonate frequency of the
unused coils.

Be aware that even if you do short out the unused section of the
coil in a tank circuit, if the lead lengths and the switch form
a resonate ground loop you will have arcing while you tune up
on some bands. Been there done that! (2kw amp with a pair of 4-400A's
in grounded grid. Selecting a good bandswitch is VERY IMPORTANT!!!!)