Loop Enthusiasts,
It's ironic that a came back to work during my vacation to gain access to
this reflector to report a TX loop problem involving Q and find this Q =
500/D equation being discussed.
First here is some hard data for my 50ft X 50ft .62" OD Cu pipe loop 6ft
above ground at the lowest point that indicates this equation is way off.
Loop Perimeter = 200ft
Rac total at 185k (dry soil) = 0.38 ohms
Derived Rac for conductor based on skin effect = 0.16 ohms
Assumed ground loss (dry soil) = 0.22 ohms
Inductance = 95uh
Derived Q at 185k (dry soil) = 290
Loop current at 1W (dry soil) = 1.6A
Now here's the problem that I need some help on: I built a test loop with an
80ft perimeter from 0.88"OD Cu pipe that can be raised to 25ft (lowest leg)
in order to derive a ground loss equation. The calculated Rac for the loop
alone, based on skin effect, is 0.05 ohms. The measured Rdc for the loop is
0.015 ohms and the inductance is ~30uh. At 1 foot off the ground the loop
Rac measures 0.15 ohms. At any distance greater than 3ft the Rac measures no
less than 0.13 ohms!!
Thinking that the problem was with the Rac meter (home brewed), I built a
test coil comprised of 2mm Litz wound on a 4" form that has a 0.045 ohm Rdc.
The coil measures 0.060 ohm on the Rac meter which is close to what I
estimated. Therefore, I believe the problem is not in the measurement
technique.
So the question is: Why can't I get an Rac reading for a loop based on
applied voltage and resulting current that is close to the skin effect
resistance of the loop conductor?? Here's a list of possibilities:
1. The 16 tin-lead solder joints could have a high Rac at 185k in spite of
the Rdc = 0.015 ohms. I suppose I could invest in 80 continuous feet of .75"
copper tubing @ ~$100, or braze the joints. I can easily build a loop of the
same dimensions from 2mm Litz and see if the results are similar.
2. The surroundings could have an effect. Will move farther from the house
(currently 20ft) and note the effect.
3. There could be some theoretical issues I have not taken into account
having nothing to with a loop antenna but relating to coil Q, Rac, proximity
effect, etc.
Other thoughts: There could be 185k E/H radiation (aside from the magnetic
field) that is reflected off the lossy ground and back to the loop as an
additional Rac loss. The distance to the ground is so small relative to a
wavelength that a loop elevation change from 3ft to 25ft would see no Rac
change.
Thoughts PLEASE.
Bill A
_____________________________________________________________________
This e-mail has been scanned for viruses by the WorldCom Internet Managed
Scanning Service - powered by MessageLabs. For further information visit
http://www.worldcom.com
| 