Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Coil function in vertical antennas?  (Read 1591 times)
KC2NLT
Member

Posts: 39




Ignore
« on: June 08, 2008, 10:51:56 AM »

I see various UHF antennas for the 800 and 900 MHz bands, for both cellular and ham radio use, that have coils of different lengths, some have more loops, some have less, others have the coil half way up the antenna, others have it close to the base. What function does the coil serve and how do the different coil characteristics affect the antenna's performance?
Logged
WB6BYU
Member

Posts: 13482




Ignore
« Reply #1 on: June 08, 2008, 11:18:24 AM »

Typically the coil is self-resonant, or nearly so, and acts to keep the
proper phase in two adjacent portions of an antenna.

For example, imagine a 3/4 wave vertical (which would have a reasonable
match to 50 ohm coax.)  The bottom 1/4 wave has the current flowing
in one direction and the upper 1/2 has the current flowing in the other
direction.  AC is like that:  the current flows in opposite directions in
adjacent half wave sections of a linear antenna.

The problem with this is that, when seen from the receiver, the radiation
from the lower section is out of phase with that from the upper section
and will tend to cancel.  You end up with less radiation on the horizon
and more at high angles (due to the difference in distance that the RF
travels in those directions.)  If you could get the two sections of the
antenna to have the same phase you'd have better useful gain.

One solution is to use a 1/4 wave shorted stub between the two sections
to give a 180 degree phase shift.  This parallel resonant - it looks like
a parallel tuned circuit.  For mobile use it is more practical to use a
parallel-tuned circuit where the coil is wide spaced (made from the same
material as the antenna itself) and the self-capacitance of the coil is
used to establish the resonant frequency.  For any given frequency there
are a number of possible ways to configure a self-resonant coil, hence
the variety of commercial designs.

There are other ways the coil can be used, but the overall effect is
similar.
Logged
KC2NLT
Member

Posts: 39




Ignore
« Reply #2 on: June 09, 2008, 04:02:45 PM »

I must say, that is a very thorough theoretical explanation. I couldn't have asked for a more detailed response.

Essentially, one design isn't necessarily better than another. There are just different ways of reaching the same result.

Got it. Thank you.




Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!