I still have a very basic antenna system here both for Rx and Tx, and I have substantial improvements planned, but even so I have made some improvements that are interesting from a technical standpoint and have also significantly improved transmit capability here. I am still using the 500 foot BOG for receive, and my receive capability signficantly outstrips my transmit capability, so I have no plans to upgrade receive capability until it is more nearly matched by my transmit capability.
My Inverted L with 15 foot height and 60 ft horizontal element when matched with the BC-306A variometer showed an Rs (series resistance) of 200 ohms at 475 kHz. Last week I improved the ground for the transmit antenna by connecting its ground to the star ground of the tower near its vertical element. This star ground is also connected to the star ground of the second tower, and also connected to a ground ring that encircles the building housing my station. Every 8-10 feet along the entire length of this extensive ground system is an 8 foot ground rod. After I connected the transmit antenna to this extensive ground system, Rs fell to 25 ohms at 475 kHz. On the left below is the VNA graph before the improved grounding was added, and on the right is the VNA graph after the improved grounding was added. Rs is colored burnt-orange in both graphs:
You can see that the SWR before improving the ground was approximately 4:1, and after improving the ground the SWR was about 2:1. I improved the SWR further, to 1.35:1 using a Nye-Johnson MB-V-A antenna tuner.
After doing all of the above, I also added 10 70-foot radials centered around the vertical portion of the Inverted L.
The next day I went up to Hilltop to do some work, and I was surprised to find my antenna lying on the ground. I decided to remake the inverted L with #12 stranded wire, and to extend it from 60 feet to 100 feet for the horizontal segment, by angling it a bit at the end farthest from the vertical element, so that I could extend it beyond the second tower without coming close to that tower. I was hoping to get some improvement with this longer antenna, not only because it was 66% longer, but also because I had moved the one end away from the tower to which it had been attached.
At first I was very excited, because this change seemed to give me a 12-17 dB boost in my signal strength at K3MF, who is the closest-in station to me at 74 km, compared with prior measurements. Prior to my putting up the new antenna, my WSPR signal at Wayde’s was on the order of -10 dB, and immediately after I put up the new antenna the first six spots I got from Wayde ranged from +2 to +7.
But here is where it gets interesting! After I got these first very encouraging spots from Wayde, the situation changed and the next spots that I got from Wayde were on the order of -13 to -20 dB; see the graph below, where NEW Tx antenna spots are in pink and OLD Tx antenna spots are in blue. You can also see NEW vs OLD Tx antenna spots for many other stations on this graph. K3MF at 74 km, WA3U at 77 km, WB3AVN at 128 km are the closest stations to me.
You can see on the graph that WA3U also shows a dichotomy of signal levels, but in his case the NEW Tx antenna spots by him are roughly 8 dB WORSE than the OLD Tx antenna spots by him. K3MF is the station near the middle of the graph with 6 pink dots above the green zero-dB baseline, and WA3U is 6 groups (4 labeled callsigns) to the left of K3MF. If you want to enlarge the graph for easier viewing, then right click on it and click “Open image in new tab” and then move to that tab to view the enlarged image.
It turns out that the apparent 12-17 dB boost in my Tx signal that I initially saw after putting up the new antenna is a “time of day” effect. My initial spots from K3MF after I put the the new antenna were all before the band opened for the night, and all of my spots by K3MF before I put up the new antenna were after the band had opened for the night. My spots with the new antenna by WA3U were all after the band had opened for the night, whereas my spots by WA3U with the old antenna had all been before the band opened for the night.
I believe the reason for the initial “good” spots of my new Tx antenna by K3MF is that these measurements were made before the band “opened” for the night when K3MF was getting my groundwave signal but not yet getting distant skywave noise to increase the noise floor at his site. Once the band “opened” and markedly increased the noise baseline at his QTH, the signal-to-noise ratio of my signal there decreased even though my absolute signal strength likely didn’t change and this produced the subsequent “bad” spots of my Tx signal from his QTH.
Because all of my prior spots by K3MF had been at times that were after the band had opened, I didn’t notice this behavior of my signal worsening after the band opened before, because I had no point of comparison.
I believe that this behavior was more prominent at K3MF and WA3U than at the other stations that received me because they are the closest stations to me and so my groundwave signal predominates over skywave more at their stations than at the other stations.
WB3AVN is next closest at 128 km, and his spots of my Tx signal were also much better before the band opened, at -14 to -3 dB, whereas after the band opened and the noise floor rose my signal strength there was -17 to -28.
I made a plot of nighttime vs daytime signal strength and you can see the results in the graph below. The close-in stations K3MF, N3FL, W3LPL, and WB3AVN with both nighttime and daytime measurements are clustered in the center-right portion of the graph. All of them show the dichotomy in signal strengths, with the daytime signal strengths being significantly stronger than nighttime signal strengths :
You can see on the two graphs below what the noise floor looks like at my location before the band opens (Top) and after the band opens (Bottom). There is at least an 8 dB difference, with the noise floor of course higher after the band opens:
The effect is reversed for stations farther away than these close in stations, for the more distant stations cannot receive my signal at all until the band opens.
I did the calculation of antenna parameters for my new Inverted L with the 100 foot horizontal top wire using the calculator at 472kHz.org as I had done for the old antenna. You will recall from the prior blog that covered this that I was very impressed with the accuracy of the online 472 kHz calculator in terms of estimating the antenna capacitance, as the number calculated was very close to the measured value. For the new antenna with height 15 feet (2.615M) and length 100 feet (30.48M), the calculated antenna capacitance is 223 pF, and my BK Precision 875B RLC meter measures a capacitance value of 236 pF…quite impressive!:
With the new antenna and estimated ground loss of 50 ohms, 100 watts Transmit Power Output (TPO) gives me 340 milliwatts EIRP. 150 watts TPO would give me 510 milliwatts EIRP. So I will need to increase my Tx antenna efficiency by 9.8 times if I am to achieve the maximum-permitted 5 watts EIRP at these TPO levels. If I push the amplifier to its maximum of 400 watts, then I only need to increase antenna efficiency by 3.7 times.
If the ground loss could be reduced to 25 ohms, then the antenna performance would improve even more, with 511 Watts TPO required to get 5 Watts EIRP and with 100 Watts TPO giving just under 1W EIRP (978 mW) and 150 Watts TPO giving 1.47 Watts EIRP.
With the current Tx and Rx antenna setup I am currently collecting more than 1000 WSPR spots per night on other stations. The most distant station out of the 39 unique callsigns that I have received in the past 2 weeks was W7IUV in DN07dg at a distance of 3549 km.
Since I started transmitting 12 days ago I have had my transmitter spotted on WSPRnet by 57 different amateur stations and 1 SWL. The furthest spot of my transmitted signal was by W0SD in EN13gp at a distance of 1812 km. 15 stations have copied me at distances of over 1000 km, and an additional 17 stations have copied me at distances of more than 500 but less than 1000 km.
In the past 2 weeks I have completed 16 QSOS, all but one of them using digital (all JT9 except for one FT8), with the remaining QSO being CW. Most of this has been done operating remotely, as described in the last blog. I am now using my Hermes transceiver exclusively, having retired the K3 from MF and LF duty.
The WSPRNet map at the top of this page shows both stations that I have received and also the stations that have received me using WSPR. Below are maps of the JT9 stations that I have received on 630m in the past 24 hours (top map), and of the stations that have received my JT9 signals in the past 2-4 hours (bottom map):
73,
Roger
W3SZ