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#1
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On 3 abr, 00:42, "Richard Fry" wrote:
Followup -- the link below compares the relative current distribution, directivity and radiation efficiency of a helical and a linear radiator system when the helical radiator described in my earlier post is operating at the frequency of its first self-resonance, and the linear monopole height is set for its first self-resonance at that same frequency. It is interesting to note that linear form has better performance than the helical form. http://i62.photobucket.com/albums/h8...inear_1st_Reso... Hello Richard Fry, Why is this so interesting, as it is what I expect (and I think you expect this also)? The current*(physical length) product is more, so given same feed current it produces more field (hence more radiated power). This results in higher input impedance, hence reducing the 10 ohms ground loss. The small change in shape of pattern is just due to the less isotropic array pattern of the 0.25 lambda radiator (w.r.t. to the array pattern of the 3 m radiator). If it is not time consuming, I would like to see what happens when you extend the helix until it gets its second (half wave) high impedance resonance (current maximum in the middle). I expect some gain increase due to small change in antenna pattern and reduced ground loss. ¡Very informative thread! 73, Wim, PA3DJS, www.tetech.nl. |
#2
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On Apr 2, 7:33*pm, Wimpie wrote:
I would like to see what happens when you extend the helix until it gets its second (half wave) high impedance resonance (current maximum in the middle). Also at its third (3/4WL) low impedance resonance where there are two current maximum points on the helix. -- 73, Cecil, w5dxp.com |
#3
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"Wimpie" wrote
If it is not time consuming, I would like to see what happens when you extend the helix until it gets its second (half wave) high impedance resonance (current maximum in the middle). I expect some gain increase due to small change in antenna pattern and reduced ground loss. The NEC software that I used for the results shown earlier doesn't allow enough segments to double the length of that helix. So I reconstructed the model in one that does -- link to screen clip from it below: http://i62.photobucket.com/albums/h8...onopole_6m.gif |
#4
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On 3 abr, 21:11, "Richard Fry" wrote:
"Wimpie" wrote If it is not time consuming, I would like to see what happens when you extend the helix until it gets its second (half wave) high impedance resonance (current maximum in the middle). I expect some gain increase due to small change in antenna pattern and reduced ground loss. The NEC software that I used for the results shown earlier doesn't allow enough segments to double the length of that helix. *So I reconstructed the model in one that does -- link to screen clip from it below: http://i62.photobucket.com/albums/h8...onopole_6m.gif Hello Richard Fry, Thanks for taking the time to do the simulation. It seems from your graph that the electrically half wave (180 degr) helical has slightly more gain than the physically and electrically quarter wave (90 degr) radiator (5.61 dBi for the helical versus 5.14 dBi for the quarter wave whip). Given the about 4 kOhms for 1/(Re(Y)), the loss in the 10 Ohms ground resistance is negligible (so at can be significantly higher before presenting significant loss). 900Vp at the feed point (100W input) will not give problems with matching. With kind regards, Wim PA3DJS www.tetech.nl without abc, PM will reach me very likely |
#5
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On Apr 3, 3:42*pm, Wimpie wrote:
It seems from your graph that the electrically half wave (180 degr) helical has slightly more gain than the physically and electrically quarter wave (90 degr) radiator (5.61 dBi for the helical versus 5.14 dBi for the quarter wave whip). A full-length half-wave monopole has gain over a full-length 1/4WL monopole because the current maximum point on the half-wave is elevated by 1/4WL. The same concept has to be a technical fact for the helical even though muted by the smaller physical distance between the current maximum points. -- 73, Cecil, w5dxp.com |
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