The graph above is a comparison of signal levels for 630m WSPR signals simultaneously received at my station between December 10 and December 12, 2017 on the 500 foot BOG “aimed” at 250 degrees azimuth and the 200 foot BOG aimed at 40 degrees. You can see that nearly 10,000 signals were simultaneously received.
Each black data point represents (signal strength in WSJT dB units received on the 250 degree BOG minus signal strength in WSJT dB units received on the 40 degree BOG). Mean and standard deviation for all data points for each callsign are shown in red. The horizontal green line shows 0 dB, so points above this line had stronger signals on the 250 degree (500 foot) BOG, and points below this line had stronger signals on the 40 degree (200 foot) BOG.
You can see that the vast majority of the signals were received more strongly on the 250 degree, 500 foot BOG and that there does not appear to be a significant directionality to the difference between received signal strengths for the two antennas.
This is not surprising. Although Beverage antennas are directional, their directionality depends on their wavelength, and significant directionality begins at approximately 0.25 wavelength. As length increases, so does directionality. At 630 meters (474 kHz), my 500 foot BOG has a length of 0.24 wavelength ignoring velocity factor. If one assumes a velocity factor of 0.5, the 500-foot BOG would have length of 0.48 wavelengths. But the 200 foot BOG would have length of less than 0.1 wavelength without considering the velocity factor, and even considering a velocity factor of 0.5, the length would be only 0.2 wavelengths. So one would not expect to find significant directionality when comparing these two antennas. Obviously, at 136 kHz the directionality of these antennas would be even less, as the wavelength at that frequency is approximately 2200 meters.
You can see that on average, the 500 foot BOG is on the order of 5 dB better than the 200 foot BOG. In a prior post I showed that this shorter 200 foot BOG was 5-15 dB better than my 40 foot inverted L antenna. And in a post prior to that one, I showed that the 40-foot inverted L was 5-10 dB better than the first inverted L that I had installed, which I ran between my two towers. So, starting with a relative reference signal level of 0 dB, I improved the relative signal to 5-10 dB with the second inverted L. The 200 foot BOG improved the relative signal to 10-25 dB. And going to the 500 foot BOG improved that to 15-30 dB. So a signal that with my first antenna would have been -30 dB would be -15 to 0 dB with the 500 foot BOG. Quite an improvement!!
What is happening on the graph above at 242.5 Degrees with the WA4SZE signal? Well, WA4SZE has a notoriously terrible signal and has WSPR signals on at least 4 separate frequencies, each with a different signal strength. A gross test of your receiver’s capability is how many spurious WA4SZE signals you can receive at once on the 630 meter band. I did not take the time to separate out the individual signals from WA4SZE for this analysis, and so sometimes the WA4SZE spurii with weaker signals on the 500 foot BOG were compared with the WA4SZE signals with greater signal strength, thus producing the strange graphic result above. This graphic anomaly could be corrected by evaluating each frequency component of the terrible WA4SZE signal separately, but it is not worth taking the time to do that. Who knows, maybe someday WA4SZE will even clean up his signal!
My BOGs are lying directly on the ground. At one end there is a transformer to match the antenna impedance to the coaxial line, and at the other end each Beverage is grounded to a 4 foot ground rod through a 220 ohm resistor. The transformers were taken from a 160 meter installation.
In addition to monitoring the WSPR signals as noted above, I also monitored JT9 traffic on 630m for two nights, using the 40 degree, 200 foot BOG. With this I copied 23 separate stations transmitting JT9, and 707 JT9 signals. Below is a set of graphs showing the signal strengths for each of the 23 JT9 stations copied vs time:
Many of these stations are also represented in the WSPR data given above and also given in prior blog posts.
I also compared the relative signal strengths when using these antennas on 136 kHz. This dataset was also acquired over two nights from 12-10 through 12-12, but this dataset only has 947 datapoints divided among 3 stations, one of which is in Maryland and two of which are in Arizona. You can see that the 250 degree, 500 foot BOG is about 5-6 dB better than the shorter antenna:
Below is an image of what happened when I connected a new “wall wart” to one of the SDRs so that I could independently power it up and down. The vertical lines on the waterfalls are birdies that are not caused by the wall wart. All of the diagonal lines represent spurious signals generated by the wall wart that gradually drift upwards in frequency. The device is a ZOZO Multivoltage switching AC power adapter called the “ZOZO 12W Multi Voltage Charger” that I purchased at Amazon. You might want to avoid it. Here is the image:
Buyer Beware!
Finally, below is an image showing my bandscope for 136 kHz receive on the 200 foot BOG on top, and for the 500 foot BOG on the bottom. Note the horizontal noise bands on the 200 foot BOG tracing. The 200 foot BOG roughly parallels (and in the horizontal dimension is within 5 feet of) the main underground 220 VAC service coming into my shack for roughly 2/3 of its (the BOG’s) length. The mains supply actually crosses under the BOG as the mains supply approaches the shack. In contrast, the 500 foot bog is at its closest 150 feet from the power line, and extends to 650 feet away from the power line. On the spectrum display, the noise that produces the horizontal bands on the waterfall is seen of course as the noise floor rising above the “quiet” baseline and then dropping back down to the baseline between noise events.
73,
Roger
W3SZ