[UK-CONTEST] [CDXC] delta loops

[Colin Wilson]

Hi all
Many thanks to all who have come back to me on the Delta loop questions, the information was excellent, I shall run one for sure on 40m I think I can just about manage it on the roof!  I Look forward to working you all from Grenada.

73

Colin G3VCQ/J38CW
www.sheffield-live.co.uk


  ----- Original Message ----- 
  From: Mike Harris 
  To: CDXC at yahoogroups.com 
  Sent: Sunday, February 14, 2010 2:26 PM
  Subject: Re: [CDXC] delta loops


    
  G'day,

  Cebik - W4RNL disagrees! He has determined that the optimum gain 
  from a delta loop is obtained when the ratio of base length to 
  height is roughly determined by:

  R = log (100F) which gives a 40M base/height ratio of 2.85 at 
  7.050MHz

  The equilateral loop is the lowest gain option followed by right 
  angle apex.

  "the MFJ antenna analyser says it is R = 49, X = 8 at resonance"

  Resonance or lowest SWR? not quite the same. X = 8?

  Regards,

  Mike VP8NO

  ----- Original Message ----- 
  From: "Steve Telenius-Lowe" <teleniuslowe at gmail.com>
  To: <CDXC at yahoogroups.com>
  Sent: Saturday, February 13, 2010 11:43 PM
  Subject: Re: [CDXC] delta loops

  > Gary has already made most of the points I was going to: a very 
  > squashed
  > delta loop does not work well, so you need to have the height in 
  > order for
  > it to be equilateral or close to equilateral (you can squash it a 
  > bit).
  >
  > At present I'm using a single-element wire delta loop on 17m. It 
  > definitely
  > works better than an inverted-V dipole, sloping dipole or 
  > quarter-wave
  > vertical, all of which I've tried on this band from this QTH. 
  > Because it is
  > a small antenna (compared with delta loops for, say, 40m or 
  > especially 80m)
  > I had the opposite problem from Robert - too much height! Even 
  > suspending it
  > from the top of a 40ft tower meant I did not have enough 
  > horizontal space to
  > stretch out the support ropes in order for it to be equilateral 
  > (this would
  > not be a problem on the lower bands of course). The solution was 
  > simply to
  > lower the apex and I've compromised between having an antenna too 
  > low and
  > one with a perfectly equilateral shape, so in fact it is a 
  > slightly narrowed
  > delta loop which, as I say, works about as well as I would expect 
  > any single
  > element wire antenna to work.
  >
  > I did find that altering the height and thus the shape affected 
  > the
  > feedpoint impedance quite a bit. Despite the theory that it should 
  > be about
  > 100R, in the end mine matched perfectly to 50R coax without any 
  > 75R matching
  > section - the MFJ antenna analyser says it is R = 49, X = 8 at 
  > resonance.
  > But before arriving at the final compromise of height vs shape, 
  > when it
  > first went up the SWR was way off, so I suspect the impedance was 
  > very far
  > from 50R then.
  >
  > I agree with Gary that it needs to be longer than a wavelength: 
  > this caught
  > me out - I cut mine accurately to the formula 300/18.1 = 16.57m 
  > and found I
  > had to add quite a bit more wire to get it into the band!
  >
  > GL,
  > 73,
  > Steve, 9M6DXX



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73

Colin G3VCQ/J38CW
www.sheffield-live.co.uk