915 mHz antenna design question
xanatos
Posts: 1,120
Regarding the picture of my very first 1/4 wave 915 mHz antenna... there seem to be two schools of thought on the "actual" 1/4 wavelength dimension.
One is the standard Wavelength = 300/f where f is in mHz; the other school takes into account that the propagation speed is slower in copper, and so takes that into account and makes the length about 5% shorter.
The second question deals with the four little ground-plane radiators - should those radiators' outer tips be 1/4 wavelength from the center of the main vertical radiator or 1/4 wavelength from their starting point on the metal plate from which they originate. I'm guessing the former - 1/4 wavelength from the center of the vertical.
Thanks for the feedback. I'm just making a few options here for getting a 915mHz signal into a receiver that's stuck in what effectively amounts to a Faraday cage!
Dave
One is the standard Wavelength = 300/f where f is in mHz; the other school takes into account that the propagation speed is slower in copper, and so takes that into account and makes the length about 5% shorter.
The second question deals with the four little ground-plane radiators - should those radiators' outer tips be 1/4 wavelength from the center of the main vertical radiator or 1/4 wavelength from their starting point on the metal plate from which they originate. I'm guessing the former - 1/4 wavelength from the center of the vertical.
Thanks for the feedback. I'm just making a few options here for getting a 915mHz signal into a receiver that's stuck in what effectively amounts to a Faraday cage!
Dave
1000 x 750 - 142K
Comments
Correct. Think of the radials as a cheap substitute for a 1/2-wavelength diameter disk, and you get the idea.
-Phil
I believe the technically correct answer is "yes", and the diameter of the elements matter too. Practically, it will be tough to be that precise and there will be other variables that affect it's performance more so.
I'm surprised none of the Hams here (who really know what they're talking about*) haven't already answered this.
It might make a difference of about 5% in the length, but other factors might also come into play a little, too, such as the diameter of your copper rods, for example. If you want to tinker with designs, you might have a look at the eznec antenna design software. It's easy to use, and the freebie download might be able to handle the design you're working with. If I can make it work, anyone can.
http://www.eznec.com/
*EDIT: Looks like W9GFO answered right before I did. Of course, I wasn't implying he doesn't know what he's talking about. :-)