Re: 8405a working and measuring resonance?

I tried to run the nec in 4nec2. It produces and error"

"ITS GM card (x,y) not supported in nec2 engine"

Which model are you using? Do you know the source of this error?

Thanks - Dan




Frank wrote: 
Dan,

Be interested to see what the exact dimensions of the coil are. Anyway, it seems we have some agreement on the 600 nH value, although my physical NEC helix models do not agree based on my estimate of your coil dimensions. I understand that EZNec uses a "Minninec" ground, which allows antenna contact with a perfect ground, but uses actual ground parameters to analyze the reflections. I am not sure about this, but would assume from the point of view of the input impedance, that the ground would be considered perfect; and therefore lossless. I also noticed I had some borderline NEC warnings when attempting to construct a coil with #10 AWG, so sometimes had to resort to a much thinner conductor.

My models showed about 17 ohms at resonance when connected to a perfect ground. The only time I observed impedances as low as 6 ohms was far from resonance when the antenna was highly capacitive. Ideally I should construct a ground screen, but for the time being will consider a perfect ground.

A free space dipole might be easier to model, but I am curious to understand why there are discrepancies in the monopole modeling.

Frank

PS, be interested in any comments on my NEC code:

CM Loaded 2 m monopole
CE
GW 1 15 0.4 0 5.6 0.4 0 1.6 0.025
GH 2 50 .32 1.6 .4 .4 .4 .4 0.025
GM 100 0 0 0 0 0 0 4 000.051
GW 3 15 0.4 0 4 0.4 0 0 0.025
GS 0 0 0.025400
GE 1
GN 1
EX 0 3 15 00 1 0
LD 5 101 1 15 5.8001E7
LD 5 102 1 50 5.8001E7
LD 5 3 1 15 5.8001E7
FR 0 41 0 0 135 2
RP 0 181 1 1000 -90 90 1.00000 1.00000
EN
"dansawyeror" <dansawyeror@xxxxxxxxxxx> wrote in message news:43D83E86.3000708@xxxxxxxxxxxxxx

Frank,

I will re-measure the coil dimensions. The recall the coil measured 600nH. That is the value I used when I modeled this antenna using EZNec. It showed resonance at about 145 MHz and 12 Ohms. (That was using an average real ground.)

If I assume the antenna measurements are correct then is it the ground that accounts for the difference between 6 Ohms and the modeled 12 Ohms?

Now I am on to model and measure a center loaded dipole.

Dan

Frank wrote:

"dansawyeror" <dansawyeror@xxxxxxxxxxx> wrote in message news:JJWdnUFF2tOojUreRVn-sw@xxxxxxxxxxxxxx


Frank,

The antenna I am trying to model is a center 'loaded vertical'. It is a 4 inch base, 5 turns at 40 percent spacing on a .8 diameter inch form and a 4 inch tip. The material is Num 10 solid copper. I adjust the frequency by stretching or compressing the coil. Currently it is resonant at about 141.7 Mhz. The 8405a shows a phase shift of 1 degree per 30 kc change in frequency.

I have used both the vertload model and the EZNEC model. Both predict an antenna R of about 5 Ohms. The 25 Ohm load shows a 12 db power difference between forward and reverse. The antenna shows a 10 db power difference between forward and reverse.

Thanks - Dan


Dan, I have modelled a 5 turn inductor, 0.8" diameter, varying in length from 0.8" to 1.6". The inductance values are 380 - 490 nH. An, approximately 9" long monopole, with a 5 turn helix appears to be resonant at about 190 MHz, with a highly reactive 6 ohm input impedance at 141 MHz. Using a lumped element simulation the required load inductance, for 141 MHz, is about 600 nH.

The only way to resolve these discrepancies is to do a standard single port network analyzer calibration and measure the actual input impedance of the antenna.

Frank 



.

Relevant Pages

  • Re: 8405a working and measuring resonance?
    ... Anyway, it seems we have some agreement on the 600 nH value, although my physical NEC helix models do not agree based on my estimate of your coil dimensions. ... I understand that EZNec uses a "Minninec" ground, which allows antenna contact with a perfect ground, but uses actual ground parameters to analyze the reflections. ... The only time I observed impedances as low as 6 ohms was far from resonance when the antenna was highly capacitive. ...
    (rec.radio.amateur.antenna)
  • Re: 8405a working and measuring resonance?
    ... Anyway, it seems we have some agreement on the 600 nH value, although my physical NEC helix models do not agree based on my estimate of your coil dimensions. ... I understand that EZNec uses a "Minninec" ground, which allows antenna contact with a perfect ground, but uses actual ground parameters to analyze the reflections. ... The only time I observed impedances as low as 6 ohms was far from resonance when the antenna was highly capacitive. ...
    (rec.radio.amateur.antenna)
  • Re: 8405a working and measuring resonance?
    ... I would like to use the windows feature to change the coil spec for instance. ... The "GH" card generates a helix with the base positioned at z = 0. ... My models showed about 17 ohms at resonance when connected to a perfect ... That is the value I used when I modeled this antenna using ...
    (rec.radio.amateur.antenna)
  • Re: 8405a working and measuring resonance?
    ... My first "GW" card positions a wire from the top of the helix with a length of 4". ... Anyway, it seems we have some agreement on the 600 nH value, although my physical NEC helix models do not agree based on my estimate of your coil dimensions. ... I understand that EZNec uses a "Minninec" ground, which allows antenna contact with a perfect ground, but uses actual ground parameters to analyze the reflections. ... The only time I observed impedances as low as 6 ohms was far from resonance when the antenna was highly capacitive. ...
    (rec.radio.amateur.antenna)
  • Re: 8405a working and measuring resonance?
    ... My first "GW" card positions a wire from the top of the helix with a length of 4". ... Anyway, it seems we have some agreement on the 600 nH value, although my physical NEC helix models do not agree based on my estimate of your coil dimensions. ... I understand that EZNec uses a "Minninec" ground, which allows antenna contact with a perfect ground, but uses actual ground parameters to analyze the reflections. ... The only time I observed impedances as low as 6 ohms was far from resonance when the antenna was highly capacitive. ...
    (rec.radio.amateur.antenna)