Test Results - Round 2

Well, after receiving several emails stating that my test procedure may have been flawed, I decided to rethink some things. The biggest concern that people had was the fact that my dipole only measured an axial ratio of 3dB. All have said that I should have measured 10-20dB. After some explanation and head-scratching, it began to make sense to me. So I decided to run the tests again with the suggestions in place and to perform some additional testing.

Dipole

The first thing I did was get rid of the bench vise I had supporting the dipole and move the RF generator to the floor. This would minimize any reflections from these two items and hopefully give me a little more accurate results. Remember, I made this dipole based on calculation only and it has never seen a network analyzer. That said, I ran the tests and low and behold, I measured almost 14dB of axial ratio!! Problem solved as far as I know.

Patch

Along with the concern for the dipole axial ratio, several people wrote to me stating that my patch is not as bad as I think it is. Charles Suckling, G3WDG in particular has been very helpful and I pass along his findings below.

Construction Notes

It has been determined that the nylon spacers between the patch element and the reflector originally shown in the drawings, degrade the performance of the antenna. It is suggested that the spacers are not installed.

The dimensions of the patch element are critical. Try to keep all dimensions within 0.5mm during construction.

The shape and size of the reflector does not seem to effect the performance of the antenna. However, I've been told that the larger the reflector, the better the side lobe reduction. This has not been verified through testing.

The addition of a "cup" reflector has had some mixed results. Mostly, it appears to reduce over-illumination of the dish and increase the signal to noise ratio. I use a 1" (2.54cm) tall cup reflector on my 1M offset dish. The size of the cup is best determined by trial and error (I haven't figured out how to calculate the optimum size yet). The size of the cup reflector will be determined by the type of dish you have and its characteristics. Others have used this antenna without a cup reflector with good results.


Charles Suckling - G3WDG

Charlie wrote me stating that he made a patch to my specifications, and connected it to his network analyzer to run his own tests. His setup included the use of a WA5JVB Log Periodic antenna that he used as his receiving antenna. He told me that he had much better results using it over a dipole possibly because of the better front-to-back ratio and side lobe reduction to reduce reflections in his home laboratory. He sent me two plots from his network analyzer; they are labeled "Bend State 1" and "Bend State 2". What he means by this is he used an alligator (or crocodile if you prefer) clip that held a small piece of wood applying pressure on the patch element. This provided him a means of "tuning" by changing the angle and spacing of the patch element to the reflector. Also note the D=12" on each plot. This is the distance in inches between the patch and the WA5JVB antenna.

Both plots are included below. In "Bend State 1" he mentioned to me that the patch element was NOT parallel to the reflector. In "Bend State 2" the patch element was very close to being parallel to the reflector. Both plots show the S11 parameter (input reflection coefficient, lower is better) and the S21 parameter (forward transmission gain). There are multiple S21 plots showing that Charlie rotated the patch in 45 increments. Notice how they converge very close to the marker frequency of 2401MHz.

Click on any thumbnail below to open a new browser showing the full-size picture.


G3WDG Patch
Here is Charlie's version of the patch antenna and the wooden "tuning rod" held on by the clip mentioned above.

Patch Setup
Here is Charlie's setup showing the WA5JVB Log Periodic as his receiving antenna.

Bend State 1
Here is the network analyzer plot of Bend State 1. Notice the double dip in the S11 plot possibly indicating two resonant frequencies.

Bend State 2
Here is the network analyzer plot of Bend State 2. This is a much nicer S11 plot and shows the importance of getting the patch parallel to the reflector.




Just for fun, I merged my simulation plots over Charlie's plots to see how close the S11 simulation matches reality. I realize this is not a real good indication because of the many variables involved but, I did it for the sake of curiosity and present the results here.


Bend State 1
I found it interesting that the simulation shows a double dip in just about the same place as Charlie's measurement did.

Bend State 2
This is where the simulation started out matching up fairly well with Charlie's measurement but quickly diverges.





As another experiment, I rotated the patch in 15 increments while taking signal strength measurements on the spectrum analyzer. I thought this test would give me another way of looking at the axial ratio. The results are, in my opinion, a little disappointing. The difference between minimum and maximum signal strength was 9.66 dBm. This occurred 90 apart at 120 and 210. I rotated the patch around the connector axis which is NOT at the center of the patch. I don't know if this played a part in the results. When I perform this test again on the next version of the patch, I will rotate it around the center point of the patch to see if it makes a difference. The chart is below, click on it to open a new browser window to view the full size.


Patch Rotation
vs
Signal Strength
Excel chart showing patch rotation vs signal strength.

Signal Strength
vs
Frequency
This is another experiment I did for the sake of curiosity. It is a chart showing received signal strength vs frequency. I added a trend line to smooth out the gaps in the measurements. Maximum received signal strength is around 2.35 GHz.




Conclusions

After running some more tests, the patch does appear to be circular polarized. Again, the design can definitely be improved and I am working on that as time permits. Overall, good performance has been obtained with this antenna from others that have constructed their own versions.


21MAR2002