antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank,

Thank you. Your assumptions were correct. The only difference is 54 uH vs 75.

The model below runs on nec2. I tried a quick load it into 4nec2 without
success, it seems to be confused by the GH and GS cards. I will have to pick
this up tomorrow.

Thanks again - Dan

Frank wrote:
Frank,

I observed by playing with the relative inductor values on the vertical
segment and the radial elements that it was possible to move the relative
feedpoint. This supports tuning the antenna by either inductor.

Reg's c_poise program predicts a 75 uH loading coil.

I am excited with prospect of coil models.

Thanks - Dan


Dan, I have done some minor approximations with your coil. I took the
length and diameter to be 12", rather than 300 mm (11.8"). The coil copper
pipe diameter is, as specified, 5/16" (0.3125"). I was a little confused
with your use of the term "Pitch" as 0.5". In the sense of a screw thread
pitch is the distance between adjacent thread peaks, but I took it to mean
the actual distance between the outer walls of the pipe; in which case the
actual pitch is 0.8125". If this is the case the total pipe length is just
over 47 ft. The inductance calculates to 54.2uH, and the Q = 2990. I have
not yet run the program in NEC 4, for greater accuracy, since I would like
to get the model as close as possible in NEC 2.

If I have gotten the pitch definition wrong then the model dimensions will
violate the NEC criteria of the minimum distance between adjacent turns.

The code for this preliminary run is shown below. Some of the odd-ball
dimensions are just to approximately equalize segment lengths.

Despite some of the weirdness of 4nec2, concerning "GH" cards, you should be
able to run it.

Frank

CM Inductor Q Calculation
CE
GH 1 300 0.8125 12 6 6 6 6 0.15625
GW 2 3 0.72322 -5.95625 12 .35542 0 12 0.15625
GW 3 6 .35542 0 12 .35542 0 0 0.15625
GW 4 3 .35542 0 0 6 0 0 0.15625
GS 0 0 0.025400
GE 0
EX 0 3 3 00 1 0
FR 0 5 0 0 3.7 0.02
LD 5 1 1 312 5.7001E7
RP 0 181 1 1000 -90 90 1.00000 1.00000
EN

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank,

Thank you. Your assumptions were correct. The only difference is 54 uH vs
75.

The model below runs on nec2. I tried a quick load it into 4nec2 without
success, it seems to be confused by the GH and GS cards. I will have to
pick this up tomorrow.

Thanks again - Dan

No problem Dan, I find all this very interesting. You could change all the
dimensions to metric, and drop the GS card. I don't know why it is having
trouble with GH. If you run some of the inductance programs, such as:
http://www.captain.at/electronics/coils/, where I have used the number of
turns as 14.7, and length and diameter 12"; the inductance calculates to
44.71 uH.

Minimizing the proximity effect, with #18 AWG, NEC computes z = 3.3 + j1274.
Therefore Q = 433, and L = 60.8 uH; an even greater difference that the
inductance programs, but closer to your requirement of 75 uH. I have tried
to run the program on NEC 4.1 to see if it agrees with NEC 2, but am having
trouble with connecting the helix since the end points don't appear to be in
the correct position.

Frank