Making a notch filter

One small RF project that I am working on; a 770 KHz notch filter. I always figure if I am having this problem, then others may be having it too. This is a relatively simple idea, a resonant LC circuit (AKA a tank circuit) tuned to the carrier frequency. It should have a bandwidth of +/- 15 KHz of the design frequency. Another requirement; use the parts I have available. Finally, the environment in which this is to be used is a high-noise room; with lots of computers, LED lights, etc therefore it needs to have excellent RF shielding.

Something like this would work well for anyone that lives around an AM transmitter site and is having problems with receiver sensitivity or transmitter intermodulation.

The basic design looks like this:

Parallel LC tank circuit

Time for a trip to the local storage facility known as “The Barn.” In my backyard, there is a small agricultural structure that is used for storage of just about everything. In The Barn, I found several parts salvaged from an old Energy Onyx Pulsar AM transmitter. As such, they are more than capable of receiver operation and could likely handle a fair amount of RF power in the transmit mode.

CDM F2B 0.01 uF capacitor with back of N connector inputs

Finding a type F2B 0.01 uF capacitor, rated at 2000 volts and 11 amps, the value of the inductor was calculated. For the inductor, a 20 uH coil with taps will work great. For receive-only applications, much smaller-sized components can be chosen. Also, there are many bandstop filters with multiple poles. Those are great, but I like the simplicity of the parallel resonant LC circuit.

20 uH inductor salvaged from Energy Onyx transmitter

The N connectors were salvaged from I don’t know where and the enclosure used to house a power supply for a Radio Systems console.

N connectors for input and output.

For shielding, I sanded the paint off of the enclosure where the lid is attached and tacked some brass screen down with gorilla glue. This will make a good RF contact surface. The outer of the N connectors are bonded to a piece of copper ground strap which also has a grounding lug on it.

Enclosure lid with brass screen to make contact

I used the Libra VNA to tune it up:

S12 shows return loss, S21 shows Phase

The scan shows it is -31 dB on the carrier frequency. It is -17 dB on 760 KHz and -20 dB on 780 KHz. This is good, because I may still want to listen to the station on the remote receiver. According to the smith chart, it is actually resonant on 771.5 KHz, but that is close enough for this application. I think the resonance went up slightly when I put the cover on after the tune-up.

There are several tank circuit calculators online. It is best to have more capacitance and less inductance to keep the Q of the circuit low and suppress the sidebands as well as the carrier.

Vagabond Able; The story of the USCGC Courier

The VOA and shortwave broadcasting story is interesting from an engineering perspective. Over several decades, the US government spend many millions of dollars building out transmitter sites both overseas and in the continental US to transmit information behind the iron curtain. The Coast Guard Cutter Courier WAGR-410 was a converted cargo ship fitted out with an RCA 150 KW Medium Wave transmitter on 1259 kHz and two Collins 207B1 35KW shortwave transmitters in the cargo hold.

President Truman standing in front of an RCA BT-150, 150 KW AM transmitter on board USCGC Courier, 1952.

The ship had various wire aerials, including the Medium Wave antenna supported by a barrage balloon.

From September of 1952 until 1964, The Courier was anchored primarily off of Rhodes, Greece broadcasting programs into the Soviet satellite states in eastern Europe and the Soviet Union itself. This arrangement made it difficult to jam, although expensive to support and maintain.

Coast Guard guys, operating a Collins 207B1 Shortwave Transmitter

It was, however, just one of the methods of broadcasting used.

Through the 1950s and 1960s, VOA, Radio Free Europe, and Radio Liberty built extensive sites in Bilbus Germany, RARET in Portugal, Tangiers, Tinang in the Philippines, Morocco, Udon Thailand, Greenville Site A and B in North Carolina, Dixon and Delano California, Bethany Ohio, etc. Many of these sites supported multiple curtain arrays, rhombic antennas used to receive programming, generators, living facilities, etc. In other words, no expense was spared.

In Rhodes, Greece; eventually the transmitting equipment from the CGC Courier was transferred ashore as a VOA relay site. The Courier then returned home.

Transmitter control operator position, USCGC Courier

The point of the story; a lot of time, effort, and money went into broadcasting information to the Soviet Union and its satellite states over the period of four decades. From what I am told (by people who lived through that time period), the payoff really occurred in 1986, when the Chernobyl nuclear power plant accident happened. State-run Soviet radio and television were broadcasting Swan Lake, while RFE/RL had useful information including where the worst of the radiation was spreading, the wind direction, and how to protect humans from radiological hazards. For many average Soviet citizens, this was the tipping point and the collapse of the Soviet Union was inevitable from that point forward.

After the fall of the Soviet Union, many of these sites were shut down and the facilities were torn down because the cold war was over. Shortwave broadcasting was seen as expensive and unnecessary. Unfortunately, many warning signs were missed along the way and the present occupant in the Kremlin has wild dreams of a New Russian Empire. To revive shortwave in the 2020s would require another monumental and sustained effort starting with getting good receivers into the right hands. Shortwave broadcasts could become an alternative information source, especially if the Russia-Ukraine war becomes a protracted draw. However, it is not simply a matter of turning a few transmitters on and scheduling some Russian language programming.

Could the Russian state propaganda machine be engaged and defeated; yes. Will it be quick and easy; no. Is it worth it; yes. The real problem is apathy. The Russians have been subjected to terrible governance since pretty much the beginning of their existence. How to overcome that apathy and the corresponding sense of helpless victimhood of the Russians themselves is the question.

Regarding Vagabond-Able, after the USCGC Courier returned to the US, it was used as a training ship until 1972, when it was decommissioned.


Disinformation and Shortwave radio

I came across an interesting article on the Engineering and Technology website:

How to defeat disinformation with short-wave radio

The takeaway is this:

“When you’re in an environment where infrastructure has been damaged, where transmission towers have been destroyed or where the power supply to the transmission equipment isn’t reliable and robust, such as some parts of Ukraine, then you end up with a fallback to older equipment, such as battery-powered radios,”

Griffiths, Sarah. “How to Defeat Disinformation with Short-Wave Radio.” RSS, The Institute of Engineering and Technology, 9 Nov. 2022,

That applies not only to war zones but also to natural disasters or other situations where widespread disruptions occur in communications or power distribution networks.

The article focuses mainly on the BBC’s efforts to get information to Ukrainians who may be listening on shortwave radios in occupation zones. That is an effective use of shortwave radio, to be sure. One problem with this idea; if there are no regularly used shortwave frequencies in the affected areas, who will have access to a shortwave radio? There may be a few receivers around in any given community, but the vast majority of people will not have access to them. The idea that a broadcast service can be neglected for years if not decades, then be quickly dusted off and put into use is simply not realistic. This applies to AM and FM radio as well.

The State of Media in Russia

I read with interest the Radio World Article: Why reviving Shortwave is a non-starter. The main premise of the article is that reviving Shortwave broadcasting to Russia is just wishful thinking. Since I am still in contact with persons in Russia, I figured I would attempt to ask them what they thought about it (without putting them at risk, of course).

The short answer; the opinion by Daniel Robinson and Keith Perron is correct. Shortwave is a non-starter in Russia. I asked a series of questions of my Russian friends and received the following answers:

Question: Do Russians own AM/FM radios?

Answer: Yes, especially those with cars. Most listening to analog AM or FM radio is done in cars and sometimes at work wearing headphones.

Question: How about Shortwave or Longwave sets?

Answer: Amateur radio operators and some hobbyists may own these radios, but not the general population.

Question: Are there any leftover shortwave sets from Soviet times/the cold war period?

Answer: Not likely, and if there are, they probably don’t work. Soviet equipment was not high quality.

Question: What about listening to the radio at home?

Answer: Not very many people do this. Most people watch TV while at home or stream movies.

Question: Where do most people get their news?

Answer: The state media TV services (Russia-1 or Channel-1). A few people may stream news from overseas, although this is getting harder and harder to do. Anyone who does this will likely use a VPN.

Question: How many people can find, download, install, and configure a VPN?

Answer: Mostly young people will do this, but it is getting harder to find VPN apps. They are getting removed from online sources and people are afraid to install them on their mobile devices in case they have interactions with the police. Approximately 10% might be using a VPN.

Question: Could Russia disconnect the internet completely from the rest of the world?

Answer: Yes, but it would be difficult. The internet in Russia was not designed to do this the same way as say North Korea or the Great Firewall of China. There are many physical connections out of the country and not everything is well documented. It is much more likely that they will block, censor and track users rather than completely disconnect.

Question: Are there any sources of outside information available in Russia right now?

Answer: For those that are searching for it, yes but likely not for much longer.

Question: If Radio Free Europe/Radio Liberty were available on AM/FM or Shortwave, would people listen to it?

Answer: No one cares. Unlike during Soviet times, no one really cares about western news or opinion. Things changed in 2014 with the first set of western sanctions. The cultural ministry (Roskomnadzor) began blocking access to internet sites they deemed detrimental. There were few independent media voices left. With the 2016 US election and anti-Russia investigations, most Russians believe that the west is turned against them. The news, as presented on Russia-1 is believed as true.

As a radio professional, I believe in the medium including HF broadcasting. The point of my Russian friends; even if the programs were directly receivable via radio (of any type), most Russian people would ignore them. It is not a technical matter of getting programming to them. It is more a perception problem, brought on by years of propaganda and negative press from the west. If shortwave had been reinstated in 2007 when the first signs of censorship began to appear in Russia, things might be different. Instead, no one seemed to notice or care.

HF broadcasting, like all radio services, can be effective when nothing else works. Much of the VOA’s HF service is targeted to Africa and Cuba, two places that do not have a high percentage of internet users. It is very expensive to build and operate. To revive HF, it will need to get smaller and less expensive. DRM (Digital Radio Mondial) shows promise in this regard, but there is a dearth of available receivers (a quick search on Amazon nets zero usable results). Instead of trying to burn holes through the ionosphere with giant 500 KW AM modulated transmitters coupled to huge antennas with +20 dB gain or more, smaller digitally modulated transmitters with simpler antennas could net the same or better results. If these systems were placed closer to the intended target audience, so much the better.

Clearly, the world has changed. It is time to re-examine (and perhaps update) some of this old technology for 21st-century use. Whatever happens, there are no short, quick, or easy answers. It took decades of time and a momentous effort to bring western ideas to the communist block. This was all undone in a few years by budget cuts.