Lesson 11

Interference Outside the Radio

Signals harmful to your radio reception might not be coming from your radio gear. They may be sneaking out of other electrical signals in your home and neighborhood.

Personal computers are a particular risk for unwanted noise. A PC may introduce the appearance of unstable modulated or unmodulated signals at specific frequencies. Many PC peripherals like a mouse use shielded cables. These have a layer of conductor on the outside of the cable that might radiate or receive interference. That’s due to common-mode currents on the shield and conductors. Inexpensive mic cables seem to be very susceptible to this. Why? Because common-mode current flows equally on all conductors of an unshielded multi-conductor cable.

Do you live within a few miles of a local AM transmitter? AM band signals can combine to generate spurious signals in the MF or HF bands. It happens when nearby corroded metal joints are mixing and re-radiating the broadcast signals.   

Corroded metal which causes re-radiation and mixing

Power lines can also generate AC noise.  

A loud roaring or buzzing AC line interference that comes and goes at intervals can be caused by:

  • Arcing contacts in a thermostatically controlled device
  • A defective doorbell or doorbell transformer inside a nearby residence or
  • A malfunctioning illuminated advertising display

All of these choices are correct when asked about buzzing AC interference.

Fighting Noise Suppression and Interference

Time to start reviewing ways to fight interference. These tools help fight the noise! They have names like attenuators, filters and noise blankers. 

An attenuator can be used to reduce receiver overload on the lower frequency HF bands. This has little or no impact on signal-to-noise ratio because atmospheric noise is generally greater than internally generated noise even after attenuation.

Here’s an option to reduce or eliminate intermodulation interference in a repeater. Install a properly terminated circulator at the output of the repeater’s transmitter. A circulator prevents power from passing backwards from the antenna. That backwards flow causes interference.

What are options to improve issues we discussed with HF and VHF superhet radios? One way to reduce possible receiver desensitization is to insert attenuation before the first RF stage. You can also add a narrow-band roofing filter to the first IF. It improves blocking dynamic range by attenuating strong signals near the receive frequency.

Another tool available to manage incoming signals is a preselector. It’s used in a receiver to increase rejection of signals outside the desired band.   

Alternator on engine

Have you heard a whine on your mobile radio’s signal? That’s usually electromagnetic interference from an alternator. You can remove that type of impulse noise by using a noise blanker. There is a potential bad side effect of using a noise blanker. Strong signals may be distorted and appear to cause spurious emissions. Think about a more permanent fix. Consider reducing the conducted noise by your car’s battery charging system. Do that by installing ferrite chokes on the charging system leads. 

Other RF interference from AC motors can be suppressed. One way is by installing a brute-force AC-line filter in series with the motor leads. This source of this noise might be from a home air conditioner unit. 

Is there just a carrier peak right in the middle of a signal you are trying to receive? An automatic notch filter (ANF) is a tool in some radios to remove interfering carriers. CW operators might not find an ANF as useful. An ANF can cause the removal of the CW signal as well as the interfering carrier. On the other hand, an ANF can be very helpful in cleaning up SSB signals.

Some high end radios and many SDR’s have a a tool called a DSP. This digital signal processing filter, or digital noise reduction can impact:

  • Broadband white noise
  • Ignition noise and
  • Power line noise

All of these choices are correct when asked about DSP processing. 

Here’s a problem I’ve had. One of my home network routers was causing interference. I saw it as a series of carriers at regular intervals across a wide frequency range. The problem turned out to be related to switch-mode power supplies. Sometimes they are inside equipment, other times they are “wall warts.”

Equipment is expensive. What are options for protecting your equipment from power line issues and lightning. A single point ground system bonds equipment together at one spot. That includes RF and home electrical. One purpose of this is to ensure all lightning protectors activate at the same time. You would also put your station’s AC surge protector on the single point ground panel. 

Let’s wrap up with a discussion about the performance of your antenna network. Link margin is a sum of total gains and losses in your antenna network. Your antenna analyzer and other tools would measure some of these factors. You’ll need to calculate a couple for the exam. As we said, this is a sum, so just add and subtract to get your totals.  

You’re given this problem to calculate. What is the link margin in a system with a transmit power level of 100 W (+40 dBm), a system antenna gain of 10 dBi, a cable loss of 3 dB, a path loss of 136 dB, a receiver minimum discernable signal of -103 dBm, and a required signal-to-noise ratio of 6 dB?

Let’s pick out the right figures. First the positives, we have 40 dB of power and 10 dB of antenna gain, so that gives us +50 dB to use. We’ll ignore the m and i for this calculation.

40 dB + 10 dB = 50 dB into the antenna network.   

The minimum discernable signal is also a positive for our link budget, so we’ll add that in as well.  Flip the sign and add 103 dB to our positives or gains.  

50 dB + 103 dB = 153 dB gains

Now let’s subtract out the losses. We have a 3dB cable loss. The path loss  is 136 dB. We need to account for the 6 dB required SNR so that goes into our negative list as well.

3 dB + 136 dB + 6 dB = 145 dB losses.

Now, just one more calculation. 153 dB gains minus 145 dB losses leaves us with our answer. Our link margin is good at +8db. 

There is one more calculation to cover. We’re using the same formula of adding gains, and subtracting losses. So, we can handle this easier question about received signal level.

What is the received signal level with a transmit power of 100 W (+40 dBm), a transmit antenna gain of 6 dBi, a receive antenna gain of 3 dBi, and a path loss of 100 dB?

Our gains are 40 dB, 6 dB and 3 dB. Our path loss is 100 dB. 

Add those up and 40 + 6 + 3 is 49.  Subtract 100 in losses to get our answer of -51 dBm. While this question doesn’t specifically ask for link margin, notice it’s calculated the same way.

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