W3SZ 10/30/17 LF Blog – Comparison of Two Short Inverted Ls

There was not much new overnight on 630m. I had 250 simultaneous decodes on both antennas bringing the number of simultaneous decodes since I put up the new antenna on Saturday 10/28 to 604.

There were no new stations copied.

There were 8 stations simultaneously copied on both antennas, so I could calculate the difference in signal strength between the two antennas on those stations’ signals simultaneously received on the two antennas. I wanted to gather this data to see if it provided a possible explanation for why the higher, slightly longer inverted L is the poorer performer.

The data is shown below. The y axis is the difference in signal strength for WSJT signals simultaneously received by the two antennas. It is shown as superior 125-degree-axis signal strength minus inferior 35-degree-axis signal strength. The x axis is the azimuth from W3SZ to each station providing a simultaneously received signal. This is of course a discrete and not a continuous variable, which is why the x axis is scaled as it is.


Note that the horizontal axis, although it shows azimuth values in an ascending monotonic fashion, is not linear or smoothly distributed. Neverthless, it looks like there is greater superiority (by about 5 dB) of the 125-degree-axis antenna over the 35-degree-axis antenna at an azimuth of roughly 25 to 65 degrees as compared to other azimuths. Because there are no datapoints between roughly 65 degrees and roughly 205 degrees, the exact azimuth of this “peak” cannot be determined. However, there does not appear to be ANY direction in which the 125-degree-axis antenna is not superior to the 35-degree-axis antenna.

Why is the 125-degree-axis antenna superior in all directions assessed even though it is shorter and lower?

Both antennas are imperfect, with the feedline not running perfectly vertically from the ground to the horizontal element in either case. In this respect, the inferior 35-degree-axis antenna is more nearly perfect. Also, the 35-degree-axis antenna is very close to being perfectly horizontal, while the 125-degree-axis antenna has a slight upward tilt looking down its axis away from the feedpoint. The 125-degree-axis antenna also has a slightly longer feedline running along the ground to the LNA. The 35-degree-axis antenna’s feedline comes right down from the feedpoint, enters the small antenna control hut, and attaches to the LNA without ever reaching the ground. Both LNAs are grounded to a ground rod right beneath them.

Inverted L antennas are complicated beasts, with their properties very much dependent on their length relative to the wavelength at which they are used, and on the length of the feedline (vertical segment) vs the length of the upper horizontal segment. They are actually fairly omnidirectional when properly dimensioned, and several sources have indicated that they are the best “simple” antenna that one can erect.

There is an excellent review of Inverted L antennas by none other than L.B. Cebik here. Then click on “Inverted-L – Cebik.pdf. The website on which this file is located is protected and you can’t get to the file directly, but going to the directory and then clicking on the file name as just suggested will get you the pdf file.

I suspect that it is a combination of site-specific and installation-specific factors that cause the difference in antenna performance that I am seeing.

I do have star-configuration grounds around each tower, and a large conductor runs from the tower farthest from my shack to the shack, running relatively parallel to the inferior 35-degree-axis antenna and roughly perpendicular to the 125-degree-axis antenna. Perhaps that is playing a role. Or perhaps the fact that the inferior antenna is attached (via insulating cord) to relatively tall towers that extend far above it at each end of its length degrades its performance.

I will have a chance to gather more data and will report back any further useful information that I get in this regard.