Monthly Archives: May 2015

Retro log: TV DX from the great Thanksgiving 1986 tropo episode

This just came up on a message board, so I though I would post a few frames from what apparently is considered one of the best tropospheric propagation episodes on record. I was in high school at the time in rural Iowa, and in addition to a log-periodic VHF antenna that we needed to get network stations out of Des Moines, I talked my Mom into buying me a UHF yagi antenna. This was nominally to get 17-KDSM out of Des Moines, but I also used it for DXing. Anyway, over the Thanksgiving holiday in 1986 atmospheric conditions were such that persistent long-range reception of TV signals (and other frequencies) occurred in the central and eastern US. I picked up a lot of UHF stations from Indiana and Ohio; UHF was more common in those areas than in most of the country. Here are a few samples; remember that these are single frames, so the actual perceived quality of the video was better; maybe someday I’ll do some frame averaging and put up more pictures, or post some videos to YouTube. Distances are approximate, I haven’t tried to re-calculate them from my road atlas measurements back in 1986.

(PS – These are frames from a VHS-to-DVD conversion I did back in 2004 when I threw out most of my VHS collection.)

14-WFIE Evansville IN, 340 miles (with some co-channel interference)frame12729_WFIE14

23-WAKC Akron OH, 580 milesframe18153_WAKC23

28-WTTE Columbus OH, 510 milesframe18485_WTTE28

36-WUPW Toledo OH, 500 milesframe16583_WUPW36

40-WHMB Indianapolis IN, 350 milesframe11009_WHMB40

45-WRGT Dayton OH, 460 miles<frame20553_WRGT45

Gear: A filtering option for strong stations on upper AM broadcast band

I will be writing more about broadband antennas for AM BCB in the future, but one of the issues I have is the dual problem of using an inexpensive low dynamic range RTL-SDR dongle as a receiver and having two local stations right next to each other on the dial.  Granted, this is a much easier problem to deal with than in a big city where there might be many strong stations spread across the dial.  At least if one’s worst problem stations are at either the low end or high end of the dial, filtering becomes a practical option.

I live within 2 miles of the transmitters for 1 kilowatt stations on 1450 and 1490 kHz.  While I’m lucky in that the station on 1490 only broadcasts from 5am to midnight even though it is licensed for 24-hour operation, with the stations so close to each other, the frequencies can interact and create intermodulation products at 40 kHz intervals below 1450 and above 1490. (In fact, 40 kHz is the closest allowed separation for stations serving the same community; why our only two AM stations are this close on the dial is beyond me.)

Anyway, when using an untuned loop antenna for broadband coverage, the signal tends to be quite weak, requiring significant amplification to get decent signals at all broadcast frequencies and to pick up trans-Pacific or trans-Atlantic signals if so desired.  But, strong local signals can overload with amplification and cause problems at other frequencies.  I find that I can’t otherwise amplify a broadband loop such as a Conti Superloop due to the strong 1450 and 1490 signals. A more expensive SDR with a higher bit rate A/D converter would probably help with the dynamic range, but this seemed like a good opportunity to experiment with filtering.

It’s possible to build one’s own filters out of discrete components, but the frequency precision required for something like this makes the project quite challenging. Thus, I went with off-the-shelf filters from Mini-Circuits. They seem to have the best options at lower frequencies like this. I narrowed it down to two filters, the LPF-B0R7+ and the LPF-B0R8+. These are both surface mount filters as opposed to “plug-and-play” filters. Unfortunately, they have to be purchased in bulk, at least 10 units. Thus, the cost is about the same ($80-90) as a cased filter with connectors. (Note that the plus sign at the end of the part number just means that it satisfies the international RoHS guidelines for hazardous substances.)

The following plot uses the data available on the Mini-Circuits site, showing from top to bottom the response of a single LPF-B0R8 filter, two LPF-B0R8 filters in series, and a single LPF-B0R7:
You can see that while the LPF-B0R7 kills 1450 and 1490 kHz, it also wipes out too many lower frequencies. A single LPF-B0R8 is actually pretty decent, but I wanted to make sure I really diminished 1450/1490 and had a sharper rolloff. Plus, having to buy 10 units, I had plenty to spare, so I build a double LPF-B0R8 filter. When you multiple numbers together, the logarithms add, so the loss depicted on the graph for the double filter is exactly twice the value for a single filter.

The data sheet for the filter indicates which of the solder pads on the bottom of the filter are input, output, and ground, so doubling up is just a matter of making sure that the output for one filter is connected to the input of the second filter. I put the filters on perfboard, soldering from below to get enough solder through the plated hole up to the ground pads to attach the filter to the board. One has to be a little careful not to overheat the component while soldering. I ran 20-gauge connector wire from the “hot” lead of an SMA connector to the input of the first filter, another wire from the output of the first filter to the input of the second filter, and a third wire from the output of the second filter to the “hot” lead of the output SMA connector. I then ran “ground” wires from two of the ground posts of the input SMA connector to the corresponding posts of the output connector, making sure to make a solder connection through the perfboard to the ground pads of each filter along the way. The following pictures show the result from the top and from the bottom:


I found a box on Amazon that was perfect for this particular 2×8 cm board, resulting in the following finished product:

It turns out that the filter works pretty much exactly as expected. Here is a screenshot from the gqrx SDR software of AM BCB from 540 through 1490 kHz:LPF-B0R8_example
You can see the general signal dropoff at high frequencies along with the dropoff in noise level. The stations at 1450 and 1490 kHz are still fairly strong, but without the filter they would by far be the strongest signals in the band and at this gain level would overload the RTL-SDR dongle. The signals beyond that are basically buried in the noise, which is collateral damage in this case. However, that’s a small part of the band and there are very few stations beyond 1600 kHz in any case. I can use a tuned loop in this area to sample individual frequencies.

Log: FM E-skip, May 14th and 17th

Successfully hearing and identifying a distant FM radio station via meteor scatter, is a very hit and miss proposition. The other main mechanism by which FM signals can travel long distances is very hit and miss in general, but when it is “in”, it can be fairly stable for many minutes. Sporadic E-layer propagation, or “E-skip”, is a condition in the upper atmosphere whereby electrons are stripped from atoms, creating an electrically charged “cloud” that can reflect radio signals back to Earth. Under normal circumstances, the E-layer is transparent to FM and TV frequencies, but as a stronger cloud develops, higher and higher frequencies are reflected, and the maximum usable frequency (MUF) can reach the FM radio band and even higher at times. (In particular, there is an amateur radio frequency band at 144 MHz, and those operators eagerly anticipate strong E-skip conditions to log distant contacts. Operators on the 50 MHz band have much more frequent openings from E-skip.) The formation of these clouds is poorly understood, but they are very seasonal, mostly occurring within 6-8 weeks of the summer solstice. Unlike meteor scatter, the clouds can be fairly large and long lasting.

FM radio stations in North America run as high as 100 kilowatts of power, and for line-of-sight coverage they mostly need that much power to penetrate into buildings. With true line of sight, even much lower power stations can be clearly received for tens of miles. Thus, with a good reflection of a signal by an E-cloud, there’s still enough energy available to produce a clear signal from hundreds of miles away. In fact, the typical reception distance for E-skip is around 1000 miles! Basically, you can hear stations that are exactly the same distance on the other side of the E-cloud as you are from the cloud, within a margin depending on the size of the cloud. Clouds drift and evolve, so one might get E-skip from a sequence of cities during an opening, but if the cloud is in the wrong direction (i.e., giving you a path into the ocean or otherwise middle of nowhere), you might not notice anything unusual.

Okay, that was pretty long-winded, you can use your favorite search engine to get more info. Apparently, this season has gotten off to a poor start, but I was able to log a few stations with some automated recordings with my RTL-SDR dongle and FM6 yagi antenna set up indoors.

March 14

I did a 3-hour recording covering 87.9-89.9 MHz from about 7-10am MST (14-17UT). My clearest frequencies in this range are 88.3, 88.5, 88.7, and 89.1. I had some apparent meteor skip during the recording, but the main “star” was 88.9 with some significant flare ups. I had a station in and out from about 8:55am (1555UT) through about 9:30am (1630UT).
I never managed to get call letters, but I got the “88.9 Shine” slogan, which is quite rare (I could only find stations in Calgary and Ohio), and an ad for the Royal Canadian Circus, which was indeed appearing in Calgary at the time. Thus:
88.9 CJSI, Calgary AB, decent at 9:18am, 100kw, slogan and ad (new; FM6 indoor, 1147mi, Es)
Here are two audio files:


Without knowing what I had, I started a new recording and went to work. This recording covered 94.5-96.5 MHz from 10:20am-3:21pm MST (1720-2221UT). My best frequencies there are 94.9, 95.3, 95.5, and 95.7. E-skip was already in progress on 95.7 at the start and this lasted for at least a half-hour. Again, no call letters, but I got just about everything else I could hope for, repeated mentions of their “Easy Rock” slogan, repeated mentions the “Kootenay Boundary” region in Canada, many ads for the town of Trail and surrounding towns, and other reference to British Columbia and Canada. Thus:
95.7 CJAT, Trail BC, good at 10:22am, 13.5kw, slogan and ads (new; HD6 indoors, 1036 mi, Es)

This recording is 4 minutes long, but illustrates just how persistent E-skip can be and how listenable the signal can be from more than 1000 miles away:
After this faded out, I was getting an over-the-horizon signal from KWKM in Saint Johns AZ at 166 miles, but that’s a lower-atmosphere effect that is fairly common; upper atmosphere effects are always several hundred miles or more.

Interestingly, while CJAT was in, I was also getting a signal on 95.3. This only lasted from around 10:30-10:40am (1730-1740UT), and I was unable to clearly identify the station. There was an ad for Walker’s Furniture and Mattress, which has several outlets in Washington and Idaho, and at some point there was a mention of Coeur d’Alene. The only station I can find on 95.3 that serves Coeur d’Alene is KPND out of Sandpoint, but I just don’t feel quite right about counting it without a station slogan or something like that.

May 17th

I made some attempts on the 15th and 16th to no avail. On the 17th, there wasn’t anything in my morning automated recording, but I did some live DXing in the evening. From 7:10-7:50pm (0210-0250UT on the 18th) I was flipping back and forth between 92.7 and 93.7. I was getting a couple of stations on 93.7, but not sure if those were E-skip or just usually weak Arizona stations. However, on 92.7 I heard (and recorded) “(something) country 92.7” at 7:36pm, and while trying to decipher that, I kept recording. Lo and behold, in my next audio recording at 7:39pm, I caught “Saint Joe’s Q Country 92.7”! Combined with checking their webcast, that is certainly:
92.7 KSJQ, Savannah MO, good at 7:39pm, 50kw, slogan (new; HD6 indoors, 1031mi, Es)
And the recording:
(Savannah is a town just north of St. Joseph MO.)

This faded out, but I was also getting a station playing the syndicated “Sunday Night Slow Jams” program, which was definitely not a country station! But, there are too many affiliates to be sure about the ID, and I didn’t hear anything specific when the station was in.