tech stuff – Engineering Radio

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.

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 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.

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.

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.

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.

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.

Rohde Schwarz Test & Measurement Fundamentals

I found a great resource for learning about test and measurement on Rohde Schwarz’s YouTube channel. Each video is about 5 to 15 minutes long and covers the basics of RF test equipment, measurement parameters, and definitions.

Rohde Schwarz Test and Measurement Fundamentals

Measuring RF systems is an important part of Broadcast Engineering. Many folks think that RF plants are going away, replaced by all IP content distribution. I disagree; Terrestrial Broadcasting will be around for a while yet. AM and FM radios are still ubiquitous in cars, homes, businesses, etc. There is no other information distribution method that is as simple and robust as over-the-air broadcasts. That is why Federal Emergency Management is still spending money on hardening broadcast facilities.

The Internet and Mobile Data in particular are susceptible to failure in emergencies. Cellular networks were almost useless due to congestion or system outages during the 9/11 attack or a natural disaster such as Hurricane Sandy.

Radio still has a role to play.

As the older Broadcast Engineers retire, there is a dearth of qualified RF specialists who can make accurate measurements on antenna systems, filters, and other transmission system components. There are very few mentoring opportunities, especially in commercial broadcasting. Gone are the days of several engineers on staff, when there was time to teach the younger people some hard-learned lessons. One of the reasons I write this blog is to pass along some of that knowledge to others so that the industry might survive.

Summer Time Atomspherics; Why is there another FM station on our frequency!!??

That is indeed a good question. There may be several explanations; a pirate, somebody’s Part 15 device, or atmospheric ducting. If the weather is good, tropospheric ducting can cause VHF (FM broadcast) and UHF (TV broadcast as well as Remote Pickup units and STLs) signals to travel far beyond their intended reception areas.

The Troposphere is the zone in the atmosphere closest to the Earth, ranging from 0 to 15 km. It is the area where most weather phenomena take place. For VHF and sometimes UHF, refraction can bend the signal back towards the surface of the Earth. Refraction at lower altitudes (called surface ducts) can cause radio signals to travel shorter distances than normal unless they are over water. Refraction at higher altitudes (elevated ducts) can cause those same signals to travel far beyond their normal range, sometimes hundreds or thousands of kilometers.

Three things affect the tropospheric refraction index (or N); water vapor, air pressure, and air density. At higher altitudes, the air is normally cooler, less dense, and dryer than air closer to ground level. However, high barometric pressure will often bring warm, dense, moist air to high altitudes. This can create a layer of warm air over a layer of cooler air known as a temperature inversion. This can create a “duct” in the upper troposphere similar to a waveguide. These signals can be very strong, sometimes overpowering a local FM signal due to the capture effect.

There is an online source that predicts atmospheric ducting, mostly used by Amateur Radio operators, but it can also be a useful troubleshooting tool: https://vhf.dxview.org/

That site produces a map like this:

This can happen any time of the year but is more common in summertime. Tropospheric ducting is not an effect of ionization from the sun. This phenomenon is known as Sporadic E, which will be covered below.

The good news is tropospheric ducts normally last a few minutes to a few hours. Sometimes they can last longer however changes in the width or length of the ducts will change the frequencies and distances that RF signals travel along that duct. In addition, if you are hearing a co-channel FM station from many hundred kilometers away, listeners of that station are now hearing your station the same way.

Another long-distance VHF propagation phenomenon is called Sporadic E layer propagation or simply Sporadic E. This happens when the Ionosphere is heavily affected by a solar storm or sunspot. Sunspots run in an 11-year cycle. We are approaching the solar maximum for Solar Cycle 25, predicted to happen in July 2025.

NOAA Space Weather Solar Cycle 25 progression

Sporadic E is much less predictable, more random, and short-lived. Solar storms can create highly ionized areas in the E layer of the Ionosphere, creating skywave conditions for VHF signals. These signals will skip in the same way that HF and MF signals do. Fortunately, these conditions usually last a few seconds or minutes at most. More on the solar cycle can be found here: https://www.swpc.noaa.gov/products/solar-cycle-progression

Once again, Amateur Radio operators are interested in this as a mode of communication. There is a Sporadic E map online at: https://www.tvcomm.co.uk/g7izu/radio-propagation-maps/north-american-sporadic-e/