Weak Signal Propagation Reporter

Slightly off-topic, but includes radio.

The antennas are the most interesting aspect of Radio Frequency Engineering to me. The transfer of power in the form of voltage and current to the magnetosphere and back again is where the rubber meets the road. Any opportunity to experiment with the art of antenna design and fabrication is welcome.

This is for the Amateur Radio community. With the upswing of Solar Cycle 25, predicted to peak in July of 2025, I decided it would be fun to get back on the air with some type of HF setup.

My past experience with HF radio and peak solar cycles is that wild fluctuations can occur creating band openings at unusually high frequencies or no propagation at all. The geek in me finds this very interesting. HF Propagation is a complex matter. Long-distance communication can be carried out with very low power levels provided the ionosphere is bouncing signals back to the earth instead of absorbing them.

Weak Signal Propagation Reporter (WSPR) is an HF beacon system, where stations transmit a digital signal containing your call sign and Maidenhead Gird locator for several seconds. The challenge is to have an efficient antenna and use as little power as possible. In this case about 200 mW (0.2 watts) or 23 dBm. The modulation type is MFSK and the bandwidth is 6 Hz. According to Wikipedia, which is mostly accurate about things like this; WSPR uses a transmission protocol called MEPT_JT. That sends messages composed of:

  • 28 bits for callsign, 15 bits for locator, 7 bits for power level, total: 50 bits.
  • Forward error correction (FEC): non-recursive convolutional code with constraint length K = 32, rate r = 1⁄2.
  • Number of binary channel symbols: nsym = (50 + K − 1) × 2 = 162.
  • Keying Rate is 12000 ⁄ 8192 = 1.4648 baud.
  • Modulation is continuous phase 4 FSK, with 1.4648 Hz tone separation.
  • Occupied bandwidth is about 6 Hz.
  • Synchronization is via a 162-bit pseudo-random sync vector.
  • Each channel symbol conveys one sync bit (LSB) and one data bit (MSB).
  • Duration of transmission is 162 × 8192 ⁄ 12000 = 110.6 s.
  • Transmissions nominally start one second into an even UTC minute: e.g., at hh:00:01, hh:02:01, etc.
  • Minimum S/N for reception is around –34 dB on the WSJT scale (2500 Hz reference bandwidth).

Distant stations report reception to a database. Several good websites display reception in a map or table format.

WSPR report

This map shows a good path to coastal Maine on 40 meters. The received signal-to-noise ratio is -2 dB at a distance of 423 KM.

80 Meter End Fed Half Wave antenna supported by trees

My antenna is an End Fed Half Wave (EFHW) cut to 3.568 MHz which can be used on any harmonically related frequency (7, 10, 14, 18, 21, 24, and 28 MHz). To accomplish this, a 49:1 Unun (Unbalanced feed to unbalanced feed) transformer is used to transform the 2,400-ohm impedance of the wire to the 50-ohm impedance required by the transmitter. The antenna works best against a ground system that is not less than 0.05 wavelength or 18 electrical degrees on its lowest frequency. That works out to about 4.2 meters (14 feet). A little bit longer is a little bit better. Six 20-foot long 14 gauge bare copper ground radials are attached to an 8-foot ground rod.

Diecast aluminum box containing 49:1 Unun

The Unun is two FT240-52 (not an affiliate link) cores with 14 gauge enamel wire consisting of 2 turns on the primary and 14 turns on the secondary. The antenna is 40 meters (132 feet) of 10 gauge hard-drawn stranded copper wire. This should be good for about 800 watts CW/SSB on HF if I want to use it in that capacity.

Unun transformer
Unun wire tied to a DIN rail with 100 pF 5 KV capacitor

There are several guides on how to make the unun available via Google search. There is some debate on whether a 64:1 transformer should be used. Most indicate a 49:1 is the best match. The diecast aluminum (not an affiliate link) enclosure is a nice feature. It cost $33.00 on Amazon.

I used the network analyzer to trim up the antenna a bit. I made a few measurements, the first was just the wire with no ground connected. The next was the wire and ground system after trimming the length for resonance on 3.5 MHz.

The transmission line is LMR-400 with N connectors. I loath PL-259s and use N connectors whenever possible.

I did a series of broadband SWR sweeps. The first was just the wire prior to trimming.

First sweep, frequencies are a little low, SWR is a little high

The next was with a ground rod and six ground radials, #14 bare copper wire twenty feet long.

EFHW trimmed up and looks good on everything except 60 Meters (10 MHz)

This demonstrates the effect of a good ground system. It is worth the effort (and it is an effort) to put in some buried ground radials with this type of antenna. I think above-ground radials would work too.

Here is a screenshot of the little Zachtek desktop WSPR beacon transmitter I bought. This is a great addition to the toolbox and works well for testing the radiation efficiency of an HF antenna. It has a GPS antenna input for timing and location reference. The frequency bands are selectable if you are testing a mono-band antenna. It will work into a fairly poor load, so I suggest sweeping the antenna first with an analyzer.

Zachtek configuration web interface
WSPR beacon, 0.2 watts

This shows that my signal is getting out. So far, the furthest distance is 17,030 km with an SNR of -10 (Australia, VK5ARG). That is quite amazing when you think about it. I am letting this run overnight to see how the propagation changes. Overall, this was a good recreational project and now I have a known working HF antenna.

How long should a transmitter last?

This Broadcast Electronics FM3.5A is 40 years old. There was a small problem that took the station off the air for a couple of hours this morning. The high voltage shorting solenoid fell apart, causing the 40 amp breaker in the service panel to trip.

BE FM3.5A defective shorting solenoid

These types of failures will become more frequent as the transmitter ages. Things like air switches, blower motors, tuning and loading mechanical assemblies, circuit breaker fatigue, plate rectifiers, screen and plate bypass capacitors, exciter and controller fans, etc. The list of potential failure points can get quite long. The fact is, nothing lasts forever.

Manufacturers nameplate

There is no backup transmitter for this site and there is no easy way to get a temporary unit on line, if needed. This is not the oldest main transmitter that we service with no backup. That honor goes to a CCA DS-3000 built in 1970.

The question is; how long should old tube transmitters be kept in service? Also; how long should we (an independent service company) agree to maintain them? The temporary solution for the above failure was to remove the broken shorting bar and turn the transmitter back on.

Broken shorting bar removed

That creates a safety issue for anyone who may need to work on the transmitter before the replacement arrives. It also creates a potential liability issue for my company.

I put a big label on the back door indicating that anyone doing service needs to discharge the power supply capacitor with the grounding stick (which they should be doing anyway). But I will feel better when the shorting solenoid is working again.

I Audited the RF Noise in my House

The largest problem facing analog AM broadcasting (and digital Medium Frequency and High Frequency broadcasting) is RF Noise.

Like most people, I have many modern conveniences that make my life easier than previous generations; electric lights, central heat and air conditioning, appliances like vacuum cleaners, microwave ovens, and whatnot. I enjoy the wireless internet, have an LED TV, use LED light bulbs, and get free electricity from my photovoltaic solar system. These devices can contribute to the high levels of RF noise found in most buildings. RF Noise which is the bain of AM broadcasting. Digital modulation schemes use variations in amplitude to transmit data bits. They are not immune to RF noise, they simply mask it better until they don’t.

I thought it would be interesting to isolate the various noise generators that may be present.

To make measurements, I used the Siglent SVA-1032X spectrum analyzer. This unit has a noise floor of -140 dB. My methodology is to turn everything off except the Device Under Test. Set the spectrum analyzer up for a wide band sweep, then narrow the bandwidth on any detected noise. Turn the DUT off to make sure that the noise goes away. Turn the DUT back on to make sure that the noise comes back.

The first thing I noticed; there is more noise during the daylight hours than at night. This is interesting. I thought it might be coming from my solar system, which uses individual inverters for each panel (so-called microinverters). These are wired to 240 VAC but have an internet gateway device that is in the house and communicates with the inverters using a power line data scheme. It turns out this was a minor contributor below the AM broadcast band.

By process of elimination, here are things that were not contributing to RF noise on Medium Frequency (AM band):

  • Cable Modem (Motorola MB7420 DOCSIS 3.0)
  • Router/WiFi gateway* (Netgear R6700v2)
  • GB Ethernet Switch (Netgear TLSG116E)
  • Dell Desktop PC’s (three models)
  • Dell Laptop PC (two models)
  • Android phones (two models)*
  • Phillips 4K LED large-screen TV (5PFL5604/F7)
  • LG LED computer monitor (24MK430H-B)
  • Refrigerator (Frigidaire FFTR1835VSD)
  • Stove (GE BP63D W1WH)
  • LG washing machine (WM3400CW)
  • LG clothes dryer (DLEX4501)
  • Bosch dishwasher (SGV68U53UC)
  • Dehumidifier (GE APEL70LTL1)
  • LED light bulbs (Sylvania 9W Ultra LED)
  • Generic incandescent light bulb
  • Furnace (fancy controller)
  • Furnace burner motor**

*These are intentional RF emitters

**The furnace burner motor made a small broadband RF signal on startup, likely the igniter which uses an electric arc. Once the unit was running, there was no further RF emissions noted.

Medium Frequency baseline noise level

The yellow line is the peak hold, the magenta line is the 100 sweep average and the cyan line is the minimum peak hold. I live out in the sticks; there are no streetlights, no stoplights for miles, the nearest cellphone site is four miles away, and houses are spaced far apart.

First, I measured the noise with everything turned off. I then turned things on one by one, noting any changes in the spectrum. For the list noted above, this is the way it looked.

These are a few things contributing to RF noise levels on the MF band.

We have cheap Chinese grow lights to start seedlings for our vegetable garden. We were using these during the daytime hours to augment the low sunlight in early spring. I initially thought this was coming from the solar system. The interference was making a massive noise hump between 750 and 957 KHz. The brand of growlight is BestVA B-1000 LED which was purchased from Amazon.

RF noise from Grow Light

Next, somewhat surprisingly, the LG computer monitor on my desk was creating a pretty decent rise from 1120 KHz to 1700 KHz. I have three LG computer monitors, this is the newest only this one creates any RF noise.

LG 240P500 LED monitor

Then, pretty much every florescent lamp (compact or full-length tube) created a broadband noise increase across the entire MF band and well into HF.

Florescent lighting

The vacuum cleaner makes a little bit of broadband RF noise when near the receiver. However, you cannot hear the radio when the vacuum is running, so that does not seem to matter.

None of these are surprising. However, I was more surprised that many other electronic devices are not contributing to RF noise in my house.

A little bit about data over power line or power line communications. Searching for power line data can be a bit tricky. First, there is this large voltage 60 Hz (plus harmonics) waveform to deal with. Secondly, there are many different protocols and many different frequencies. I narrowed down my solar system by listening to my Kenwood R-2000 below 300 KHz. Some noise went away when I completely disconnected the inverters. I don’t know the exact frequency, the protocol, the modulation type, etc. But there is something.

Data Over Power line is popular with home automation systems, it can be used to extend Ethernet LAN, and some power companies are using it to control substation equipment, smart power meters, and/or to function as an ISP for their customers. I have heard some HF users complain about BBPL, but I have not experienced it for myself.