[opendtv] Re: VHF Stations Seek Solutions for Reception Problems, by Doug Lung

  • From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Wed, 01 Jul 2009 14:25:43 -0700

Thanks Ron, good information, your search was obviously more thorough than
mine.

Unfortunately the referenced yagi antenna (
http://www.m2inc.com/products/vhf/2m/2mcp14.html)
has limited bandwidth. The elements are cut and spaced for the desired
channel frequency and circularity.

Using a Log Periodic modified Yagi provides the required bandwidth but my
thinking was that circularity is still an issue; until I found the following
abstract info on the IEEE site: (I'm not an IEEE member so can't provide the
details) and
unfortunately my searches don't find a commercial LPCP product

      Circularly polarized log-periodic dipole antenna for EMImeasurements
      This paper appears in: Electromagnetic Compatibility, IEEE
Transactions on
      Publication Date: May 1999





      Abstract
      Two types of broad-band antennas are widely used for electromagnetic
interference (EMI) measurements in the frequency range from 30 to 1000 MHz.
Log-periodic dipole antennas (LPDA) are mainly used for the range above 300
MHz and biconical antennas for the range less than 300 MHz. These two
antennas have linear polarization. However, EMI measurements can sometimes
be more conveniently made with an antenna having circular polarization and
so we propose an improved LPDA, which has circular polarization. This LPDA
has a second array of dipoles so arranged that each dipole of the second
array has a quarter-wavelength phase difference from that of the
corresponding dipole of the standard LBDA array for the given radiation
field. For this reason, we named it a cross-element LPDA. The cross-element
LPDA does not need a broadband 90° hybrid junction to produce circular
polarization. We calculated the height pattern and the frequency
characteristics of the classical site attenuation (CSA) for the
cross-element LPDA when used for both transmitting and receiving, as well as
the antenna factor. Moreover, we calculated the normalized site attenuation
(NSA) when the cross-element LPDA is used for receiving or for both
transmitting and receiving





http://www.m2inc.com/products/vhf/2m/2mcp14.html

Ron

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