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I'm building a Double Cross Dipole antenna for 437 MHz to be used with a SDR receiver for CW cubesat. The idea is to follow this "classic" paper from Gerald Martes, KD6JDJ. I tuned every single dipole (8 mm aluminium bar + ferrite + coax cable) using a nanoVNA with - i think - good result(eg. here the return loss of first dipole)

enter image description here

I use a coax cable to measure the SWR like this

enter image description here

and the coax length is taken from end of the shield (as in this images :))

enter image description here but when I connect all the 4 dipole ... craziest thing happened ...and it seems to me that every dipole change the proper resonant frequency to a different one.

enter image description here

Is the problem in the cabling between all the dipole ? Or (as I suppose) I followed a wrong apporoach tuning separately every dipole and the proper way is to tune all the arms after connect them together?

Can someone give me some hints on how to proceed?

Tks a lot

Carlo, IU2KQB

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    $\begingroup$ Welcome to hamSE, Carlo. According to the article, the behavior should not depend strongly on the tuning of individual elements; this might mean that the best approach is to make all four elements the same length. Is the SWR measurement made at the input to the coax "harness?" How did you ensure that the longer coax is $\frac{\lambda}{4}$ longer than the shorter coax? $\endgroup$ – Brian K1LI Jun 16 at 14:58
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    $\begingroup$ Tks a lot for your warm welcome! Add some clarification in my post! $\endgroup$ – Carlo IU2KQB Jun 16 at 16:50
  • $\begingroup$ um, at 437 MHz, I don't think you're doing yourself a favor adding these huge metal contacts to the system - these will have very distinct RF properties by themselves. Do things get better when you not use them and directly e.g. solder your dipole branches to the coax conductors? $\endgroup$ – Marcus Müller Jun 16 at 18:14
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    $\begingroup$ by the way, what kind of coax is this? $\endgroup$ – Marcus Müller Jun 16 at 18:15
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    $\begingroup$ @CarloIU2KQB Perhaps you should contact the author. The article shows his email address. My NEC-2 simulation produces a very different pattern, with a clear "null" at the zenith, so I have no confidence in the SWR plot. $\endgroup$ – Brian K1LI Jun 16 at 21:48
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I am sure the phasing and proper connection of the coax to the individual dipoles will get you a pretty good hemispherical CP pattern. It is easy to get confused when connecting the coax to the dipole.

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  • $\begingroup$ Welcome to the site, Jerry! Do you recall if it's necessary to tune only the whole array rather than each dipole separately? $\endgroup$ – natevw - AF7TB Jul 15 at 16:44
  • $\begingroup$ I would have some difficulty convincing anyone that the element diameter, or even its length is not critical. But, anyone familiar with computer modeling can do the evaluation. The DCA is NOT an antenna which requires precision (at 137 MHz. $\endgroup$ – Jerry Martes Jul 17 at 6:07
  • $\begingroup$ Hello Jerry, I have moved your callsign and email to your profile. $\endgroup$ – Mike Waters Aug 3 at 23:36

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