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Glastar Dipole comm antennae behind doors

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nuckolls.bob(at)aeroelect
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Posted: Mon Oct 28, 2019 7:52 am Post subject: Glastar Dipole comm antennae behind doors

 	  Quote: 	
		  Iâ€™ve got dipoles in the tail, one close behind bulkhead A, and one in the vertical tail. This was what was being done in TWTT back in 2006.



 I measured the fraction of reflected energy (measured in terms of whatâ€™s called â€œSWRâ€) and the impedance of both antennas a few years ago. The ideal impedance is 50 ohms (to match the coax). The ideal SWR is unity. For an antenna in a free-field (like on a pole), one would hope for an SWR less than 1.5 over the full VHF aviation band. For less than ideal circumstances, < 2.0 or even 3.0 might be considered tolerable. 	


   Actually, if one can get 3:1 or less over the

   entire spectrum of interest, then performance

   for aviation purposes will be adequate . . .

   ASSUMING that locality conditions don't add

   factors un-related to SWR.



 [img]cid:7.1.0.9.0.20191028104902.06ca0978(at)aeroelectric.com.0[/img] 



  	  Quote: 	
		  As you can see in the attached graphs, impedance is all over the map, and rarely near 50 ohms. The SWR seems to indicate that the front antenna is barely tolerable up to 127 MHz, above which it gets pretty bad. Conversely, the tail antenna is pretty crappy until 116 MHz, then tolerable right up to 135 MHz. 	


  The aft antenna (vertical fin?) does show a smooth

  curve with pronounced dip at 130MHz. This suggest

  the antenna is doing a reasonable job of accepting

  energy from the feedline over the range of interest.

  Further, it's under 3:1 SWR . . . I'd call it

  good. 



  The front antenna plot suggests complete lack

  of resonance at any point in the explored spectrum.

  I'm assuming this antenna is wrapped around the

  inside surface of the composite shell with a

  predominantly vertical polarization.



  If the SWR scan had been taken lower frequency,

  I suspect that a pronounced dip in SWR would

  be discovered . . . the trend of the plot as

  it approaches 113Mhz suggests this.



  It makes sense. Close proximity of either/both

  CONDUCTIVE or DIELECTRIC features tend to

  lower the resonant frequency of a radiator.

  Pasting the antenna to the inside surface of

  the fuselage will lower it's resonant frequency

  measurably . . . proximity to steel structure

  inside the fuselage will lower it more . . .

  perhaps MUCH more.



  Suggest the ends of the forward antenna be

  shortened a bit and new scan taken . . . start

  at some much lower frequency to see if there's



  a real resonance artifact down there.I suspect that the interaction with conductive structure, such as the cage in front and the tail spar in the rear have more to do with the anomalies than the modest curvature. Not only that, the coax is connected to both antennas directly. Since the coax is an unbalanced conductor, and a dipole antenna is balanced, there is supposed to be something called a balun between them. I don't know if it was ignorance, cheapness, or weight-consciousness behind the absence of a balun, but no doubt thatâ€™s also part of the problem.

   

  A BALUN can improve on energy transfer between

  feedline and antenna but the thing needs to be

  RESONANT in the range of interest first.



  I'm fiddling with an dual-band (121.5/406) ELT

  antenna intended for installation on the inside

  surface of an RF friendly composite fuselage. Looking

  at the value of some BALUN options. Discoveries

  would applied to the VHF comm antenna being

  discussed here. 



  Get the SWR down and 'centered up' first.



 

   Bob . . .

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LouNathanson

Joined: 31 Oct 2019
Posts: 2

Posted: Thu Oct 31, 2019 11:45 am Post subject: Re: Glastar Dipole comm antennae behind doors

Thanks, Bob!  It had never occurred to me to step back and see the obvious:  that that front antenna was simply peaking well below the av band.  I will experiment with trimming it



I will also look forward to your balun.  



Thanks,

-Lou

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