View Single Post
  #27   Report Post  
Old February 20th 08, 02:02 PM posted to rec.radio.amateur.antenna
LA4RT Jon Kåre Hellan LA4RT Jon Kåre Hellan is offline
external usenet poster
 
First recorded activity by RadioBanter: Aug 2006
Posts: 16
Default Horizontal loop antenna

(Richard Harrison) writes:

Roy Lewallen wrote:
"Perhaps you build antennas for radiation resistance, but I think most
people build them to maximize radiation in some particular direction.."

You must have some radiation resistance or you have no signal, but not
all antennas are highly directional.

Page 8-10 of the 20th edition of the ARRL Antenna Book has (Eq 5):
Efficiency = radiation resistance / radiation resistance + loss
resistance

Zero radiation resistance = zero efficiency.

Best regards, Richard Hsarrison, KB5WZI


You're right that low radiation resistance makes it hard to attain
high efficiency. You want radiation resistance to be significantly higher
than loss resistance.

But ohmic losses usually aren't that bad in large dipoles and
loops. EZNEC or other modeling progarms can tell you how much you gain
by going up in wire size. For common geometries and wire sizes, the
difference will be a small fraction of a decibel.

For small ("magnetic") loops on the other hand, efficiency is very
much an issue, and you want to push the radiation resistance as high
as you can, and ohmic losses as low as you can. (Increasing the size
of the loop is the easiest way to increase radiation resistance, but
presumably there's a reason that you'er using a small loop in the
first place.

73
LA4RT Jon, Trondheim, Norway