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

  • From: Albert Manfredi <albert.e.manfredi@xxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Wed, 1 Jul 2009 21:16:25 -0400

Dale Kelly wrote:
 
> Check this out. Helical antenna bandwidth is
> discussed, amongst other design aspects.
 
http://www.antenna-theory.com/antennas/travelling/helix.php

Good description. By the way, the equations are said to be mostly empirical. It 
looks simple enough to build. And the bandwidth is as uncritical as it is for 
yagis.
 
Sounds like you just pick the midband frequency, in UHF or even high VHF. The 
only two parameters to worry about are:
 
1. Circumference of the helix, which should be equal to mid-band wavelength, and
 
2. Distance between coils, which should be 1/4 of the wavelength.
 
At least three coils are needed to get circular polarization. The 
characteristic impedance is a bit of a mystery, estimates go from 50 to 200 
ohms, so I bet 75 ohms in a receive antenna will be okay. The more the turns, 
the greater the gain, the tighter the main lobe.
 
Also, the polarization is clockwise when viewed from the transmitter, so that's 
the direction the receive antenna helix has to be looking in toward the 
reflector plate. Outside looking in.
 
Cliff, go for it!
 
Bert
 
_________________________________________________________________
Windows Live™ SkyDrive™: Get 25 GB of free online storage.
http://windowslive.com/online/skydrive?ocid=TXT_TAGLM_WL_SD_25GB_062009 
 
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:

- Using the UNSUBSCRIBE command in your user configuration settings at 
FreeLists.org 

- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word 
unsubscribe in the subject line.

Other related posts:

  1. Home
  2. opendtv
  3. Archive
  4. 07-2009