[TowerTalk] Log Periodic Choice

[Gene Smar]

Mike:

     The way a log-period works is the element lengths are such that a couple are near-resonant on the highest frequency of interest, e.g., 29 MHz and a couple are resonant on the lowest, say 14 MHz.  The elements in between are resonant on the frequencies in between.  The feedline (an open-wire, transposed (crossed) transmission line) also plays into the design and phasing of consecutive elements.

     Therefore, only a couple of elements are actually radiating at a time; which ones depends on the frequency.  That's why, for a given boom length, an LP produces lower gain than does the equivalent boom length Yagi.

     However, as you have probably found out, LPs exhibit wider bandwidth by virtue of their element lengths.  The SWR varies a bit over the entire range; it's typically not the smooth parabola-like curve we see with Yagis.  However, a well-designed LP will provide a good match to 50 Ohm coax from the lowest to highest freqs, sometimes more than several decades (see Create's 5130-1N LP for V/UHF, good from 50 - 1300 MHz, or so they claim.)

     Another item you need to be aware of:  all elements of an LP are above ground.  Therefore, they are not suitable for top-loading a tower for use on the lowbands.  They also represent a different lightning risk than a grounded-element design.  FWIW.

     BTW:  Most outdoor TV antennas you used to see on chimneys and rooftops are LPs.  The design was broad enough to cover the wide spread of TV freqs from 54 - 216 MHz VHF and 470 - 698 MHz UHF, although there usually were/are two sets of elements for the two bands.


73 de
Gene Smar  AD3F


From: Mike <noddy1211 at sbcglobal.net>
Date: 2008/09/11 Thu PM 08:58:25 EDT
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Log Periodic Choice

Clint, this maybe a silly question, but how many elements on each band.  I
looked at the website, but I could not find.  Certainly a good price though.

Mike

-----Original Message-----


The T-8 from Tennadyne.  18 Foot boom for $754 - Hard to beat.

Clint - W5CPT - Moderator Tennadyne Yahoo Group -


  ----- Original Message ----- 
  From: Michael Ryan 
  Cc: towertalk at contesting.com 
  Sent: Thursday, September 11, 2008 7:30 PM
  Subject: [TowerTalk] Log Periodic Choice


  Let the games begin...
  I have a good friend with somewhat limited rotational space ( does that
make
  sense? ...small yard ).  He has a yard that does not allow for long boom
  yagis, stacks, etc.  He would like to have a log periodic, something along
  the size and boom length of the Force 12 XR-5, Teledyne, etc.  Anyone have
  some suggestions for a mulit-band ( w/WARC ) band beams with a boom length
  under 20ft?  THANKS!  - Mike


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