My thanks to Ed and others here for the supporting commentary regarding
mindset and open mindedness.
In his reply, Jim K9YC mentioned something that is often overlooked and
often adds confusion and challenges to a search: Multiple sources.
I'm not an Engineer so I'm allergic to the words "assume" and "assumption."
That being said, there is much truth in Jim's statements about multiple
sources. Every modern home does have multiple interference sources. Nearly
every neighborhood does have multiple power line sources.
As a utility interference investigator, I travel widely around northern
Florida. I have a well-equipped work vehicle. If I'm not obligated to
listen to something else, I'll leave the vehicle AM/FM radio tuned to 1710
AM, the IC7000 tuned to 15 MHz AM (WWV) and the IC2820 tuned to 121.500 MHz
AM mode. All three run unsquelched with the volume turned down so only a
slight hiss is heard. All 3 receivers are connected to resonant or close to
resonant vertical antennas. The myriad sources I hear are both amusing in
their musicality and shocking in number!
I use this setup to survey the area around a power line interference
complainant's premises as part of my process for preparing to meet the
complainant. I use this survey as a baseline.
Nearly every neighborhood (not every, but most) has multiple utility and
private sources. I make note of the sources I do hear. Surprisingly often
the sources I hear during this initial survey are not the sources affecting
the complainant!
For the ham who wakes up one day, walks into the shack, fires up the gear
and hears buzzes, frying, hums and other new and undesired noises, the
first step is to listen...carefully. If it seems like different interfering
sounds are being heard, even on the same frequencies, it's likely there are
multiple sources.
Listening, carefully, is the easiest way to determine how many sources are
affecting your receiver. Take notes about:
1. Signal strength
2. Highest frequency at which each source can be heard
3. A simple description of each source's character
4. Any other descriptive data
5. Brief notes about weather conditions
Keeping a log of episodes is very helpful when dealing with intermittent
sources.
It's fairly easy to make an audio recording of each source. I like to use a
pocket digital voice recorder. (Dozens of cheap, quality makes & models are
available.) When searching for the source an audio playback can be compared
to sources heard during the search.
Radar Engineers Model 200 series receivers are equipped with a display and
the ability to store the signature image of sources. Signature matching is
a popular and effective method of finding the specific source among several
that is affecting a complainant. This system is more useful than a spectrum
analyzer because it provides real identifying information about each unique
source. But few are willing to spend multiple kilobucks for this capability.
I find that tackling the strongest source first then the next strongest and
the next strongest is an effective way of dealing with multiple sources.
Patience is a requirement when prosecuting multiple sources. Sometimes,
perhaps even often, a strong source will mask a weaker source. More than a
few times I've had to face the disappointment of learning that the repair a
crew and I have just completed has now revealed a secondary source that
continues to affect a complainant's equipment though at a lower signal
strength. The primary source was so strong that the secondary source was
completely masked.
My surveys are usually able to determine if the "main breaker" test needs
to be performed. But I will agree with Dave that shutting off power to the
whole house can be a very, very revealing, if not shocking, experiment for
any ham to try.
Multiple interference sources are a frequent reality. Careful listening,
record keeping, patient careful searching and a step by step elimination
process will ultimately win the reward of an interference free receiver.
73,
Frank N Haas KB4T
Utility Interference Investigator
Florida (on the road in NC)
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