There are many reports on this forum of noise problems associated with AM reception and indeed, here in my home there are areas where AM reception is not possible due to interference. The worst offenders here are light dimmers, some of which are still on when off (they just dim so no light appears but don't disconnect and they pound away 120 times a second producing hash). The signals are then carried via the power wiring throughout the house. If it wasn't against the wishes of my spouse all of these would be removed.
In the really old days when there wasn't a broadcast station on every block people who wanted AM reception had to rely on stations which were often remote. Many clear channel 50 kW stations were built to serve especially the rural areas of the country. Successful reception of distant stations required at least two things: a sensitive receiver and an outdoor antenna. Radios produced in the 1930s and 1940 often were superheterodyne types with tuned radio frequency amplifiers ahead of the mixer which gave high sensitivity. These were often fitted with a balanced two wire screw connector for attaching an external antenna. If one wanted to use a single wire aerial then one of the terminals could be attached to the chassis but this didn't work very well for supressing indoor electrical noise.
As an experiment I decided to get back to basics and connect my 40 meter dipole which is 70 feet behind my house and is strung from tree to tree and connected to a 50 ohm coax line which terminates in my man cave. I have a BC-1004-C receiver which has the differential two wire antenna connection and a two stage TRF amplifier. I have used it with an indoor loop antenna with some success but decided to connect it to the dipole. Doing some Gargoyle web searching revealed that the input Z of this radio antenna connection is 115 ohms and that a good match is needed for minimum noise so I needed to adapt and match a 50 ohm single ended coax to the differential 115 ohm input. To do so I made a transformer on a T50-2 core with a 40 turn primary and a 60 turn secondary which serves to convert the single ended source to a differential output and transform the Z up to 115 ohms. Now I was ready to listen and I think the proper technical description of the results is HOLY COW, AM the way it should be!
During early evening I am able to hear stations from all over the eastern US, St. Louis, Chicago, Detroit, Philadelphia, Dallas, DC, and some from Canada. These are strong with no interference. Even 5 kW stations from adjacent states are easy to receive.
My outdoor mounted AM transmitter is heard with absolutely no noise, no hum, and the fidelity is great.
If only our intended audience would return to the days of yesteryear....
Neil
Here here! The good old days.
Don't think you can get our intended audience to do this as TV is more of a priority then radio,or many wouldn't have the facilities or the radio to do this although a GE super has the RF stages and external antenna connections. So does the Eton field radio which is made now. Oh yes Ccrane has one too don't they?
As for a younger audience, you have to explain AM radio to them.
Mark
Very good, Neil. Additionally...
Inside my house when using either my Tecsun PL-880 or Sangean PRD-15 each with their built-in loopstick antenna -- from early evening through early morning I can also listen to the signals of many AM broadcast stations from all over the eastern 2/3rds of the US and Canada (depending on their licensed facilities, and skywave propagation conditions at the time).
Neither of those receivers was very expensive, nor required an outside antenna to achieve that performance.
Even my < $50 Tecsun PL-310 with its internal loopstick does well in this, but not quite as well as the other two.
What if we advertised the best Radios to receive our AM stations when we put up signs for our stations? One way to increase our listenable range.
Next how to educate the public?
Neil,
It is funny that you wrote about this as I have been battling with noise in my house for a several days now and still have not narrowed down what it is yet.
As some of you know, I mess around with cb radio and other license free bands as well, but i have had the most horrendous noise on all 40 channels AM and SSB both.
Of course both modes are affected because each AM channel produces two sidebands, other modes do as well but I won't get into that right now.
This noise occaisionally goes away and I can once again hear stations that are further than 1/4 mile away from my antenna with ease. I don't think the problem is coming from inside my home, i think it's something new that a neighbor may have bought or received since Christmas because, that is when the noise started.
It sounds FM in nature, like taking one of those FRS walkie talkies and turning the squelch off where all you hear is the white noise. And on the S-meter the scale goes from 0 to 30 dB, the noise is about 6 dB on AM and practically undetectable on SSB but, if someone talks in either Upper Sideband or Lower Sideband, that noise comes in with their signal and all i can do is strain to listen or turn the radio off.
I will post a short video clip of the noise received on my local cb channel 27.205 Mhz at the bottom as well as a video of a Canadian Time Standards station CHU 3330 Khz I received during a power outage. The difference is easy to see...ermm hear.
CHU was received on a Grundig G3 Shortwave radio using just the telescopic antenna, the cb is connected to a Tram No-Ground Apartment Dwellers Antenna using 100 feet of 50 ohm RG-8X coaxial cable.
I have no clue exactly what has changed since Christmas, maybe the neighbors on either side of me bought a new plasma tv or game system, i just need to track this down and try to remedy the problem somehow.
The best 2 radio's i have ever owned and would love to get back are the Radio Shack (Sangean Rebranded) DX-390 and an AM tube type alarm clock radio, both had exceptional reception.
Videos:
CHU 3330 Khz during a power outage
Noise on 27.205 Mhz AM CB Radio
Ok i looked it up out of curiosity, that tube clock radio i mentioned was RCA CLOCK RADIO MODEL 2-C-0511 about the 12 th picture down on this website.
Mine was red though and came with a box of tube radio parts, one other radio that worked out of that purchase was a Zenith G725 AM/FM radio seen here. I had trouble finding parts for it as the FM was not working, AM however shined.
Neil, I am glad to hear you can enjoy such a great radio with stellar reception.
Barry
Your discussion reminded me of a DIY AM loop antenna I had come across a long time ago (and posted it about it here somewhere years ago), It's simple and could even be attractive if spray painted, but it requires a direct connection to the radio antenna connections which manyortable radios do not have: http://www.mile204.us/PotsdamDan/RubeGoldberg/Ultimate_AM.html
Another even simpler antenna, which does not invole a direct connection, but involves long stretches of outdoor wire (like Neils) is illustrated by CCrane: http://www.ccrane.com/How-To-Make-a-Simple-Powerful-AM-Loop-Antenna-For-Free
Neil said: To do so I made a transformer on a T50-2 core with a 40 turn primary and a 60 turn secondary which serves to convert the single ended source to a differential output and transform the Z up to 115 ohms. Now I was ready to listen and I think the proper technical description of the results is HOLY COW, AM the way it should be!
Although it's not very feasable to have long wire strung out outside in my location, I am trying to comprehend your use of a homemade transformer to drastically reduce noise interference.. It's totally a subject outside my knowledge, I'm a rather non-technical part15 hobbyist, but I would be interested to understand how the use of your transformer results in both extendeding AM reception while at the same time reducing interference..
The transformer in Neils post was built and used to step up the required impedence from 50 ohms to 115 ohms as the Hammurland external antenna connection required this much to preform at it peak.
The coax running into the house from the dipole has an impedence of 50 ohms and now i am curious as to what Neil has on the dipole antenna to step up or down impedence IF the antenna ever needed to be used for Ham HF operations.
Barry
Rich,
The transformer I described performs two functions, the first to match the feedline impedance to the receiver but the second is more important in reducing the noise which is to provide a differential signal to the antenna input of the receiver.
You may already be familiar with the concept since it is often used in microphones with XLR connectors. It is known as a differential signal or a balanced signal.
The simplest way to connect a signal from a source to a destination is to use one wire to carry the signal and connect the chassis commons together which is know as a single ended system. The destination receives both the signal from the wire and any noise which is induced in the wire or the common chassis connection.
The two wire system is where the signal is delivered to the destination as the differnence in voltage between two wires which are isolated from the chassis. The receiver responds only to difference between the two wires and not to the difference between these and the chassis or circuit ground. Noise induced into the differential wires is done so in what is called common mode and the common mode signals cancel when the difference is taken at the destination. This makes the system almost insensitive to noise.
External noise tends to appear as a difference between the signal line and the circuit ground but since a differential system is set to only see the difference between the two wires and not between them and ground where noise is induced the noise is virtually eliminated.
Keywords are single ended, common mode noise, common mode rejection and differential signals.
Search for explanations on these keywords and you should find some good articles on the topics.
Neil
Balanced audio was a big lesson for me to learn at my home studio. I hooked several audio pieces to a mixer panel I'd wired up from a line of pots from an old wooden stereo. It was a metal frame with volume, bass, treble, and tuning, and I connected a phono, and tape decks to it, then out to a preamp.
I had awful hum in the system, and I kept trying things to get rid of it, testing after midnight in "special hours" like AM stations would do.
I had two big problems, one was the magnetic phono preamp. The RCA jacks into it were on a phenolic insulating board. Initially I had some hum, and went into the preamp, finding that the "ring" part of the jacks that's supposed to be grounded solidly to the chassis just had one skinny little wire connected to the preamp board inside, along with the left and right. So that's what they did, cheap out on the ground!
I fixed that by adding wires between the grounds and the metal case of the preamp, the output side had its ground going to the chassis after all. It didn't help a thing, the hum could have been more! I tried everything to reduce the hum, because I thought connecting to the chassis ground just had to be right.
What I didn't realize was that was being a balanced system, from the pickup coils in the cartridge, and the skinny ground wire was bringing the signal directly to the input of the preamp's ground, and you didn't want a ground anywhere else in that circuit.
The other issue was with power cords. I didn't have access to power strips then, and was using a number of extension cords, like the kind for christmas lights, 3 outlets. I didn't have enough power connections for all the equipment on that side of the room, so I ran cords across the room to another outlet, which apparently caused a ground loop and hum whenever the pot was turned up. At the time I thought it was just the long cord picking up hum, and also buzz from a TV station, and even audio from other radio stations sometimes.
Now I run all station equipment from the same outlet, or at least ones I know are connected to that same branch directly. That's easy now, with PCs, harder in older times with all the equipment sucking up the juice.
I also note that the commercial part fifteen transmitters, with the transmitter and antenna on a roof, and the power and audio going to it through a remote injector, those use transformer feed with the audio through that long cable.
Now more pro audio equipment has gotten rid of transformers in favor of differential op-amp inputs, doing the deed with ICs instead of transformers.
It's odd that when I moved
to this house last year I was prepared for
huge noise problems with regard to the
AM BCB DXing that I do.
I was even prepared to place
portable radios under buckets
(or whatever) in the backyard.
So that I could retransmit those
received signals into the house
via Bluetooth or Part 15 FM.
It turns out this house is fairly noise free
on the AM BCB. This place is a lot
better than the previous house.
There are other stories connected
with this. I'll stop here for now.
Best Wishes To All
Brooce, Part 15, Hartford CT
I found the cause of the noise on receivers in the house, that new Arrus modem.
Without even trying to find the cause the other day, i had found the solution by accident.
Or rather, the dog did.
She decided to take a short cut under my desk which did not turn out the way she expected, she got confused and in her confusion she pulled the plug to the modem/router while the CB was turned on. Ironic huh?
As the modem dropped out so did the 5 to 6 dB noise level on the Cobra 148 GTL, it was a happy accident. Now , how do i shield things so this noise doesn't happen anymore?
If it was up to me, the devices in the house would all be wired but some of the devices my family use do not have the option to use wired connections, as it stands my desktop is the only device in the house that is wired.
Any ideas?
You may have already ruled this out but since I don't see it mentioned I will mention it. I have read that many homes built in the 70s were built with aluminum wiring which can induce noise. Secondly have you tried something like this power filter?
I have one of these which also protects my equipment although it of course doesn't protect from noise coming in through the ether.
Didn't know that a router caused or added noise to AM which includes anything working on amplitude modulation like the device you mentioned....you can't stop it unless you wrap it in metal sheilding which will stop your wireless signal also....maybe try shielding everything but the antenna?. Aren't these things supposed to not cause interference per FCC rules?....Ha Ha! Try getting a power bar with good noise filtering like up to 60DB but may not make much difference.
Just the problems with AM as what this thread is about and what I have been posting for a long time. Nowadays unless the power goes off you only get good AM outside away from the house.
Mark
That will only work to filter RF noise directly on the power line to your equipment not the RF noise radiating from the a/c wiring all around you in the house. If you have an AM radio and are on batteries and then you plug it in the noise gets twice as bad....that filter will only remove the amount the noise gets worse when plugged in.
Mark
Indeed, there are many sources of interference in today's homes which would be very inconvenient if removed (dimmers, routers, computers, wall warts, etc.)
Back in the early 70s I consulted with a friend who was involved with mainframe data processing. One of his installations required some means of alerting the operator when the computer halted and the manufacturer had such a device available for rental of $60 per month. We worked out a scheme using a $5 "Flavoradio" to pick up the hash from the computer and alarm when the hash stopped. In this case the interference was an advantage.
Back to AM in the home, my report of success in post #1 is based on two tactors, first place the antenna outdoors away from the noise sources, and second transport the signal to the indoor receiver so that no noise is picked up by the transmission line system.
Neil