On Fri, 11 Jun 2004 23:03:16 -0000, Larry W4CSC
wrote:

Gary Schafer wrote in
:


Sorry Jack but you are wrong. It has nothing to do with microwave
frequencies. A wave guide beyond cutoff is the mode that the tube is
operating in and it simply tells you that the frequency is too low for
the given size tube to propagate through. The energy inside the tube
gets shorted out. Many 2-30 mhz signal generators use that type
attenuator.

And, if a Navy sailor has used them, the 50 ohm 1/8W resistors are cooked
from having transmitters keyed into the attenuators, too, negating any
possibility of CALIBRATION....Been there, fixed them for years for a
living...(c; Put your ohmmeter from the center pin of the output cable to
the shield and see if it measures 50 ohms....quick test.


It has everything to do with it. Skin effect is ever present in all
conductors at ALL frequencies. Note my reference to 60 hz power
transmission where it is also important.

Skin effect musta been why RG-8A melted when I keyed those twin 4-1000A
home brew linears I used to build into them...hee hee. I got accused of
hooking them up to the AC line to blow them at my ham club meeting. No,
wait, I think that was "dielectric heating" at 6KW....sorry. RG-17A/U
didn't melt.

You were right the first time. Dielectric loss is not a factor below
100 mhz. Only lack of large enough conductor surface area causes
heating / loss.



That is a contradiction to your point. You say that current flows
entirely through the walls of copper tubing and then say that is why
it is used in AM broadcast components. If that were true then they
would not use copper tubing but instead they would use solid copper
rod for better conduction.

The reason copper tubing is used is that there is no current of any
significance past a certain depth and to use solid rod would be a
waste of copper.

Hogwash. They use copper tubing because it's cheap at the local air
conditioner supply house and because, if the station is above 5KW, copper
tubing COOLS itself better because it has a bigger radiating surface than
copper wire of the same cross section. Skin effect is immeasurable at 550-
1600 Khz.....or 20 Mhz, actually. Skin effect starts rearing its head up
in the VHF to UHF range where my 2 meter kilowatt used 2" copper plumbing
tubes and Ts for a plate tank for the 4CX250Bs in push pull.


Nooo ooo, not you too Larry. Did it occur to you that copper tubing
with the same cross section as copper wire has much greater SURFACE
AREA? That would help with cooling and also, believe it or not reduce
skin effect so it didn't get as hot in the first place.

Ever heard of litz wire? I am sure you have. It is often used in small
coils to reduce skin effect losses. And guess what, it is most
effective below frequencies of 1 mhz. So if skin effect was not a
factor at those low frequencies what would be the need for litz wire?

Got any old 10khz or 50 khz coils laying around? I bet you will find
some litz wire in them. The old command set receivers with the 85 khz
IF's are wound with litz wire. Wanna guess why.


As we eventually got around to research rather than blindly arguing
positions of opinion, then the participants hopefully learned
something. I've learned that applying the math from formulas for skin
effect in conductors of known ohmic value and used with a known
frequency can determine the wall thickness of a conductor which has
full cross sectional current on it. Guess what? The original poster's
question about using copper tubing remains answered. A 1" copper tube
has more surface area and carries just as much low power RF on it's
entire cross section as a 1" wide piece of copper strap that is nearly
the same gage.



Skin effect is the reason coax cable works as it does. None of the RF
on the inside of the cable appears on the outside of the cable. Other
than leakage between strands of the shield of the cable. Those wire
strands on coax cable are pretty thin. Much thinner than your copper
pipe. Hard line has no leakage.

Geez, all this time I was told it worked in TEM mode, with the H field
around the center conductor perpendicular to the E field from center
conductor to shield, with the RF flowing up the dielectric, like RF fields
will. I never heard of skin effect at, say, 20 Khz, where coax also works
just fine, properly terminated of course. I'm gonna call WWVB and warn
'em!

Oh I am sure they already know about it!
Next time you are playing with your big amp tuning your mobile
antenna, try swapping out your solid wire loading coil with the same
size copper tubing for a coil and see if either one gets any hotter.
If skin effect doesn't exist then your solid copper coil should be
much cooler of the two.

Regards
Gary


Lots of RF appears on the outside of cheap coax with chinzy braid, which is
why we double shield RG-6 on cable systems and use aluminum hardline to
keep the FCC from kicking our asses on the Aircraft Band near the airport.

Regards
Larry