Re: NEC-LIST: Fw: Lambda/4 monopoles lamdba/4 apart

As Chuck already noted, you need to use current excitation. You didn't
 provide specific information about the pattern you did get, so I'm not
sure how "good" your good simulation was, or how "bad" the bad one was.
 Since the antenna is included in EZNec as an example file
("4square.ez"), I ran it and noted that it predicts a pair of nulls at
+/-135 degrees from the max forward direction, with a small "bump" at
180 degrees. The nulls are better than -40dB relative to the forward
direction (at the right frequency) and the "bump" is about -27dB.
 That's at the file's elevation angle of 23 degrees, and things change
noticably as you change the elevation angle...at very low elevation
angles, you get a null at 180 degrees. Presumably the excitations
(phases+magnitudes) could be adjusted to get a "perfect" null at 180
degrees. This EZNec example simply uses current sources for
excitation, at the middle of the lowest segment.

I've looked at using such an antenna for direction finding, by looking
at the phase of the received signals in each element, and can vouch for
the strong effect of the mutual impedances among the elements, which is
important to account for when looking at phase information.

Cheers,
Tom
K7ITM

Phil Ede wrote:

>Hi
>
>Some time ago I tried to model some of the old CCIR condifurations for MF Broadcast
>arrays, and found I could not get the right radiation pattersn for Lamda/4 monopoles
>
>I have now modelled a simple pair of Lambda/4 monopoles spaced lambda/4, and fed 90
> degrees apart in HFSS using perfect ground, and get a good cardiod pattern with a small
>rear null.
>
>I have modelled an almost identical confifration in NEC, and no matter how I tweak the
> detail (frequency, number of segments, number of wires in each monopole, wire
> diameter, EK command etc) I get a much worse pattern than using HFSS..
>
>Can anyone tell me where I might be going wrong using NEC ?
>
>CM Lambda/4 monopoles lambda/4 apart
>CE
>GS 0 0 1
>GW 1 3 53.390 -0.000 0.000 53.390 -0.000 2.000 0.7500
>GW 2 19 53.390 -0.000 2.000 53.390 -0.000 100.000 0.7500
>GW 3 3 -53.390 -0.000 0.000 -53.390 -0.000 2.000 0.7500
>GW 4 10 -53.390 -0.000 2.000 -53.390 -0.000 100.000 0.7500
>GE 1
>EK 0
>GN 1
>FR 0 1 0 0 0.77 1
>EX 0 1 2 01 1.00 0.00
>EX 0 3 2 01 0.00 1.00
>RP 0 46 181 1001 0.000000 -180.000000 2.000000 2.000000
>RP 1 25 73 1000 0.0 -180 1.0 5 1000
>PT -1
>EN
>
>Phil Ede
>Brighton, UK
>
>
>

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