Rectangular Antenna Cavity
Rectangular Antenna Cavity
(OP)
I need an expression for the fundamental frequency of an antenna constructed in the form of a simple box with one open end.
I'm also interested in any references that would be particularly helpful in design of this type of antenna in the 2.45 GHz ISM band.
I'm also interested in any references that would be particularly helpful in design of this type of antenna in the 2.45 GHz ISM band.





RE: Rectangular Antenna Cavity
'Pringle Can' antennas are similar (except they're a can versus a box). I think that they're for 2.4 GHz band.
Standby for further (probably better) info from others.
RE: Rectangular Antenna Cavity
Maybe describe the antenna a bit more.
kch
RE: Rectangular Antenna Cavity
SWR is better than 3:1 from about 2.3 to 2.5 GHz, as measured on my VNA. There is a fairly sharp increase in the SWR beyond these limits. Field tests with real radios have been good, so I think that as an antenna this device works pretty well.
Now, I'd like to alter the geometry of the cavity, and/or substitute a different dielectric, so that I can incorporate the antenna into another device. I'd like to gain a better understanding of just how this thing operates so that I can make the necessary changes while maintaining good performance.
Thanks for the help.
Walt
RE: Rectangular Antenna Cavity
Typically changing dielectric from Teflon (2.0 ish) to say a dielectric of 3.0, the frequency gets lower by sqrt(2/3) factor.
this requirement is for a narrow bandwidth antenna 8%, perfect for a "thick patch" antenna. If this is a home brew antenna, making a thick patch would cost under $10 for connector and $2 for other materials (two pieces of metal and a spacer and some glue, or some standoffs.
I'd expect 1.3:1 VSWR with an optimized 0.6 inch thick x L"x w" antenna, (L=3" min, w=2" min), gain range from 0 dBi to max. of about +7 dBi if you allow the L and W to increase to lower the backlobe. Need maybe 10"x5" size to lower the backlobe.
kchiggins
www.toyon.com
RE: Rectangular Antenna Cavity
RE: Rectangular Antenna Cavity
Tune the frequency by changing the distance between the probe and the back wall. That seems fairly simple, although I'm not clear on the meaning of statement that the "electrical distance = physical distance x sqrt of dielectric". If "physical distance" free space wavelength, and "dielectric" is the dielectric constant, then the "electrical distance" is greater than the wavelength in free space, which seems backwards to me.
Side walls need to me at least 1/2 wavelength apart, but what about the height of the cavity? Is there any rule for minimum height? Is there a reason to keep the height larger rather than smaller.
What is the affect of changing the probe length?
VSWR is about 1.5:1 in the center of the band, which is quite satisfactory.
I do like the idea of the patch antenna, in that construction would likely be simpler. However, I'm limited in the available frontal area.
Your insight has been very helpful thus far. I really appreciate the help. It's been hard for me to find straight forward answers in my paultry library of undergraduate electromagnetics texts. I'm sure there must be good coverage on this sort of thing if I knew where to look.
RE: Rectangular Antenna Cavity
No rule for minimum height as a waveguide, typically they are around half the width. Too tall is bad, energy bounces up and down, usually under half wavelength at highest frequency.
The height can be quite short, but larger antennas have a better VSWR match to air.
Look up quarter wave patch antennas. I made a 433 Mhz antenna in size 4"x2" and your antenna would be 0.75 inches length. Half wave patches are recommended with the 1.5 inch length using FR4 dielectric, or 3 inches long with air dielectric. It's very compact and simple to build.
kch
RE: Rectangular Antenna Cavity
I'll check into patch antenna designs.
By the way, I gave some incorrect specifications for the probe in the existing design in an earlier post. It is actually 1.5 centimeters long, and .85 inches from the back wall of the cavity.
Walt