[Antennas] Query

[David Robbins K1TTT]

And of course you are right.  many years ago I was designing a 2 element 80m
wire beam and was told 'you have to make them loops'.  So I undertook to
determine why.  This was in fact based on a 2 element loop beam in the
original on4un low band dxing book that used a switched inductance to
reverse the beam pattern, but that doesn't really matter.  The basic design
used a boom at 150' that would hold the tops of the loops or inverted V's
and would be the feedpoints of them also.

Here is an extract of the results:

The inverted V beam model at 3500khz
              Impedance          34.6 - j 78.7 _
              SWR                 5.54
              Total Losses        0.31 dB
              Wire Loss           5.7%
              Load Loss           1.3%
              Efficiency         93.0%
      At 28.0° Elevation:
              Forward Gain        8.80 dBd
              F/B                15.68 dB
              Azimuth Beamwidth     77°

Replace v's with loops and reoptimize the wire lengths and load inductance
              Impedance         134 + j 49 _
              SWR                 3.09
              Total Losses        0.18 dB
              Wire Loss           3.7%
              Load Loss           0.4%
              Efficiency         95.9%
      At 28.0° Elevation:
              Forward Gain        8.63 dBd
              F/B                11.33 dB
              Azimuth Beamwidth     74°
              Radiation Peak        32° Elevation (8.65 dBd gain)

Not much difference there at all.

OK, so try this....  for high band quads the boom is in the middle of the
loops so the highest point is above the boom.  In my test above I put the
boom at the top of the loops so they were hanging below it.  So what if we
built the loops like a high band quad with the center of the loops at the
same height as the boom of the inverted V elements?

              Impedance         141 + j 33 _
              SWR                 3.00
              Total Losses        0.18 dB
              Wire Loss           3.7%
              Load Loss           0.4%
              Efficiency         95.9%
      At 28.0° Elevation:
              Forward Gain        9.64 dBd
              F/B                15.51 dB
              Azimuth Beamwidth     74°
              Radiation Peak        24° Elevation (9.77 dBd gain)

Aha!  There is that extra db or so that has normally been claimed for quads
over Yagi's... unfortunately in this case it would require a support about
35' higher to hold the tops of the loops up, not very practical in this
situation. 

So basically, if you have only a peak support, put up an inverted V and save
the extra wire for something else.


David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
 

> -----Original Message-----
> From: antennas-bounces at mailman.qth.net [mailto:antennas-
> bounces at mailman.qth.net] On Behalf Of Chris Boone
> Sent: Tuesday, January 01, 2008 17:27
> To: Antennas at mailman.qth.net
> Subject: [Antennas] Query
> 
> Anyone with software out there who can model the following for me???
> 
> Right now, our local club has a delta loop....top is about 70ft off
> ground,
> bottom part of maybe 20ft off ground; slopes maybe at a 30 degree angle
> down
> from vertical...overall length is about 250ft (all three sides). Fed with
> RG8 to tuner in the clubhouse (about 100ft or so). I feel it would be
> better
> as a inverted V instead of a closed loop. I feel the currents so close to
> the ground on the bottom section are hurting more than helping but I dont
> have a copy of any decent antenna software to prove it. BTW we are wanting
> to run this antenna on 160-10...right now 160 is a lost cause. The tuner
> wont get anywhere close...Even 80m is touchy and doesnt radiate worth a
> fip
> (of course the MFJ tuner may be responsible)...
> If someone could model this in both configs and send me the results off
> list, I would GREATLY appreciate it.
> 
> If you need more info, contact me direct off list.
> 
> Thanks!
> Chris
> WB5ITT
> <my call> at <my call>.net will get you to me if my email is not in the
> header of this message!
> 
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