Monday, April 20, 2015

It should have not been BPL that worried us! (And using a separate receive antenna at HF)

About 10-15 years ago - more or less - the ham magazines and press were abuzz with concerns about the impending doom that was "BPL" - Broadband Over Powerlines.

Ostensibly, there was good reason to be concerned.  These devices utilized much of the same frequency spectrum as HF and low VHF communications and were bound to cause some degree of interference.

It did not help that, despite optimistic predictions by advocates of the technology, power lines were not very good RF transmission lines in the sense that they were lossy and DID radiate rather badly - not to mention being rather noisy in their own right!  These were all factors that contributed to emissions from BPL systems radiating much more broadly than initially "predicted" by its proponents and the subsequent need to use far more RF power to convey data with the integrity necessary to present to the consumer what appeared, on the surface, to be a viable product.

Interestingly, despite the attempts to assuage the amateur community that BPL would not cause significant interference, some independent "field readings" were taken indicating that the radiated signal strengths were far higher than expected and that the utilized power levels were far higher than predicted - apparently in an effort to overcome the inherent shortcomings of using the power utility's conductors as a conduit for data.  It got to the point that, allegedly, some attributes of the laws of physics themselves were conveniently "revised" to favor the BPL proponents without regard to potential interference issues - a fact touched upon in the video linked below:

The video above:
"BPL Unredacted":  ARRL's Dave Sumner reviews
redacted FCC info on BPL interference.
Total Run time:  10:59
(From the ARRL Forum, 2009 Dayton Hamvention)

Not too unpredictably, BPL as a technology has all but died:  It was just too "slow", even compared to the competing technologies at the time, and it had the inconvenience of not being "wireless" in any sense of the word!  Inevitably, even the "non-wireless" technologies such as DSL and "Cable Internet" quickly trumped the available capacity of the proposed BPL systems with their more practical "scalability" - that is, the ability to practically construct a data network in which many people can simultaneously use bandwidth by being able to sub-divide the resources to the "local" area.

Nevertheless, many hams attempting to use HF have reported an ever-rising tide of background noise on HF - not from BPL or even the plethora of "wireless" electronic devices that are designed to emit radio signals, but from devices that the uninitiated may not even consider as a potential RF radiator.

I am talking about power supplies and the devices that contain them.

True, there are a few devices such as Plasma TVs that are known to cause all sorts of "racket" but these, too, are a dying technology - literally:  Very few of these TVs are sold these days and even fewer are repaired as they quickly die of old age and failing electrolytic capacitors!

The type of power supply to which I'm referring is a switching power supply.  For the very same reason that BPL tended to radiate more more widely than "predicted" (it was not much of a prediction, actually:  As the video states, the FCC's own engineers expected problems from the beginning!) a badly-designed (or failing) switching power supply can radiate its "crud" quite effectively.

These devices are ubiquitous these days, being lighter, cheaper, and more efficient than their older, iron and copper counterparts.  These devices are essentially an oscillator powered by the AC mains voltage that feeds a transform that is made small, a feat possible by the high frequency (typically 30-60 kHz) of the oscillator, with the output of the transformer rectified back to DC again with some feedback used to regulate the voltage.  These devices may take the form of the small, external "wall wart" or they may be built into the appliance or product - but if it has been built since the early 2000's and it doesn't feel as though it has a big chunk of iron somewhere in it, it probably has a switching supply!

Being that these devices are oscillators at their core it is no surprise that some of this energy can "escape" and appear on HF frequencies in the form of harmonics.  While it is well within practicality to design these devices such that they would radiate a negligible amount of energy in the HF - or even VHF - frequency range, it may not surprise you to know that many of these devices actually do radiate significant amounts of energy.

Any electronic device sold in the U.S should be subject to certain levels of certification and one of these certifications is related to FCC part "15" which dictates limits of how much stray energy a given device may emit at various frequencies.  Now, it should be made clear that this does NOT mean that a device that complies with these rules DOES NOT emit stray signals at all, but rather it means only that they comply with a certain limit.  In other words, if a certain device meets these rules, it can still radiate - and even cause interference.

This brings us to another portion of FCC part 15 that says, in a nutshell, that if the device causes interference, it is incumbent on the user of that device to take care of the interference - even if it means that they must stop using that device.

This is all well and good, but if such a device belongs to an uncooperative neighbor or, perhaps worse yet, it is a device that is in your house - perhaps the "wall wart" (plug-in wall adapter) that runs something that you or someone in your house finds "essential" or is inconvenient,, expensive or impractical to replace or discard, what can be done?

Resources on identifying and fixing the problem:

Followers of this blog will have noticed that there are a number of entries devoted entirely to the quashing of "grunge" emitting from these very devices - some of which appear to be well-designed pieces of equipment.  A few such blog entries include:

In the above articles may be found some descriptions of the inner-workings of such power supplies along with how and why they may radiate.  A bit of searching on the internet will yield many hundreds of similar articles related to taming noise problems with such power supplies.

While it may not be just one power supply that causes a problem - or even "too much" of a problem, the fact that one may have a house that contains dozens of these things, each emitting a small amount of "grunge" across HF, can cause a cumulative degradation of the reception of HF.  Unlike the "old-fashioned" type of interference caused by a fluorescent lamp or noisy power line that one may recall where there is an obvious "buzz" at twice the line frequency (120 or 100 Hz), the noise from these supplies often takes the form of a "hiss" - often with a bit of AC mains-frequency modulation on it - but sometimes not.

What's worse is that if there are a lot of these devices all interleaving and summing their energy together, it may not be immediately obvious that there is an interference issue at first, particularly when one goes around the house and starts to unplug devices one-at-a-time:  If it is a multitude of devices, unless all of the devices are powered down at once by turning off the main breaker to the house, the magnitude of the problem - if the source is confined to your house - may not be readily apparent!

There is hope:

For the most part, even if you live in an urban area it is most likely that the worst offenders will be in your house, on your electrical system.  If you live in an apartment/condo/townhouse where electrical utilities are shared and you live in much closer quarters, there will be more of a challenge in reducing noise sources, but identifying and fixing (or removing) the offending devices in your possession will certainly help!

Not mentioned previously is the fact that in HF operation one can "split" antennas effectively between receive and transmit.  While it is well-known that a "small" HF antenna such as a mobile whip or a loop typically has a disadvantage when it comes to effectively radiating a signal, there is usually less of a problem when it comes to receiving signals.  An antenna such as a loop that is both directive in its ability to cast "nulls" in specific directions and it is somewhat resistant to local noise "E" field noise pickup which makes it a good candidate for receive-only while while a larger antenna may be used for transmitting.  If it is used for receive-only purposes, it need not be constructed using the same large components that might be required for an antenna that would need to handle 10's or 100's of watts, greatly simplifying its construction and lowering costs.

The caveat with the use of "split" antennas is that most radios cannot be easily configured to transmit with one antenna and receive with another, but there are some T/R (Transmit/Receive) relays available that may be keyed by the output on the radio that would be used to key an external HF power amplifier:  A bit of quality time with a search engine should locate a bit of information regarding such devices - including ones that you can build!

Sources of external TR switches - Buying or building:

One such device, commercially available, are these offerings from MFJ:

Another commercially available device is this:

I have not personally used either of the above units so I cannot personally vouch for their suitability, but they look as though they suitable for the purpose.

If you want to build one of your own, consider this article:

While designed original for an Icom radio, it can easily be used with almost any transceiver, as the article describes:  All it needs is a "keying" line from the transceiver - the very same one that would be used to key an external power amplifier.


While some of the commercially-made devices can automatically switch upon sensing transmitted RF, it is strongly recommended that one "hard wire" the T/R relay to the keying line of the transceiver - the same line that would be used to key an external RF amplifier.  This is recommended to prevent any possibility of "hot switching" the relay or sending any RF at all to the receive antenna - even for an instant!

Because an RF-sensing switch requires, by necessity, that there be at least an instant where there is RF being sent to the receive antenna, this could result in damage to a receive-only antenna that uses an internal RF amplifier or components that cannot handle transmitting power/voltages - even for an instant!

Additional resources related to the use of separate receive antennas:

A review on a receive-only loop antenna that describes how it may be used to reduce local noise sources:

Video above:
VK3YE demonstrates the efficacy
of a separate receive antenna.
The video is of rather low resolution, but you get the idea!

Comment about other wireless devices around the home:
  • There are a lot of devices that are designed to transmit, such as wireless modems, access points, etc.  By themselves, these will NOT cause HF interference because they do not operate anywhere near that spectrum, but rather in an amateur bands approximately 100 times above the HF range.  What can cause interference with HF operation is the power supply that operates these devices as they are likely of the "switching" type!


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