Radio Scientists:
I have a question that some of you may have some experience with. I have a short horizontal antenna in my attic which I have loaded with a variable loading coil.
This antenna is feed by a RG 59 coax from the basement. The antenna tunes very well and I get very good range. The problem is the amount of hum I get on the radios in the house when listening.
The hum is also present but to a lesser degree some distance from the house, but it is still there if you turn up the volume, more than what I would like to see.
I have tried all sorts of polarizations of the long wire antenna including moving it to different parts of the attic and running it along the rafters up and down (semi vertical) and horizontal along the part where the rafters join together at the peak of the roof. I also tried running the antenna wire up and down in from the top of the roof to the ceiling joists and back up to get a vertical pattern. I was trying to get the least amount of coupling between the house wiring and the antenna
My loading coil at the coax end is grounded in the attic. I have also tried different ground routes which all connect to a copper pipe in the attic which connect to the cold water main pipe. I checked it out with an ohm meter to make sure the pipe is connected to a solid ground.
I use the standard Talking House power supply. I have also tried to isolate the hum by using a 12 volt battery and that made no difference so it’s not a power supply issue.
I have tried ground loop isolators on all sources and even made a dummy grounding plug which I used on the input of the TH5 Aux input, to make sure the hum was not originating from any attached gear.
Using the grounding plug on the input of the TH 5 AUX input made no difference the hum was still there on dead carrier.
I went up in the attic and tried all sorts of antenna variations and configurations.
Nothing really made a significant difference.
To test further I turned the transmitter on and used a 12 volt battery to power it, so I didn’t need to rely on AC power to power the transmitter and had no connection to the house wiring. I didn’t apply any audio to the transmitter because I wanted to test hum levels.
I then set up a battery powered portable radio near the circuit breaker box where I was going to turn breakers on and off. With the radio on and tuned to the TH5, I took one breaker at a time and switched off, I went through the entire panel till I got to the last breaker. As I went thought the process sometimes turning a breaker off made the hum less some time more.
The next test I did was pulled the main disconnect on the breaker box to turn everything off at once… ok finally no trace of hum in the carrier, so with all power disconnected to my house and no AC flowing in the wiring there was no hum, just a perfect carrier on the radio. I had to pull the main disconnect to get a clean carrier, with all breakers off, I still got some hum but pulling the main switch, all hum was gone.
Personally I feel since there is a lot of electrical wiring in the attic that the RF is mixing with the magnetic fields of the 110 volt power lines.
If this is true and anyone can explain to me how this happens I would appreciate it!
I think the only real answer is to this is an outside antenna mounted a distance from the house. I do have an outside antenna now on one of my AM transmitters and that is much more hum free, that transmitter I run off batteries when I use it but wanted this other transmitter for general purpose use, like I said everything is good but the darn hum.
My knees are worn out from moving around on the rafters, I tried every possible antenna configuration. The test with the breaker box confirms my beliefs that on this one there is not much of a way out but turn off all the power in my house I suppose if I want a pure signal! Bit of overkill but at least it was nice to hear that absolutely clear carrier. Any help anyone can come up with I would appreciate it!
Sad Radio Joe
My first best guess is the 60HZ from all the wiring is coupling into the transmit antenna back to your transmitter and then modulating the audio. I would most certainly get the antenna out of the attic. Looks like you have done a great job of isolating the problem.
Personally I feel since there is a lot of electrical wiring in the attic that the RF is mixing with the magnetic fields of the 110 volt power lines.
If this is true and anyone can explain to me how this happens I would appreciate it!
It sounds to me like the antenna is gathering AC hum (60hz plus harmonics) and leaking it into the modulation circuitry of your transmitter, which gets mixed in with input audio and transmitted. Im no guru on this stuff and have no suggestion on how to fix it.
From experience we know that it is possible to operate an indoor AM transmitter which sends a hum-free carrier.
But we also know that every once in awhile there is hum audible at the receiver that appears to be on the carrier.
Both of the previous answers guessed that perhaps 60 Hz is being inducted from the power lines into the antenna, which is possible.
But the reverse is also true. The AM carrier may be loading into the power wiring and re-radiating with a mixture of 60 Hz.
The total length of all the house wiring might be a perfect wave-length or fraction of a wavelength of your operating frequency, in which case it would be "matching" to your frequency-energy.
BUT there are other possibilities too, and from THINGS YOU DIDN'T MENTION I will come back later on with some more ideas.
Try changing the length of the coax between the antenna and Xmtr. Add a jumper to it or shorten it a bit if you have extra..
How many places is your system run to ground?
You say it's grounded in the attic. But that's high above earth ground and all the pipes and wiring between the attic and ground is lifting that ground point by perhaps 100 to 200 ohms.
If it's also grounded down at the bottom end, there might be floating voltages of different potential between the two ground points, contributing "ground loop hum."
The audio eqipment is attached to the transmitter, thus yet another ground path which, being so far removed from the others, and especially if you have some equipment plugged into one branch of the AC and other equipment to a different branch of the AC, that could be a problem.
Connect all associated equipment to a single branch of the AC power.
Try a dead-carrier test with nothing connected to the audio input and the audio control turned down on the transmitter.
Can't offer much more than the excellent answers already but when you used a battery I was surprised that there was still hum! I can give you some insite to the problem as the battery supply didn't work. By the way, I have the Spitfire AM transmitter and the hum wasn't that bad but was still there.
The A/C you are surrounded by in your house is radiating the 60 hz frequency, as EMR and from other sources as sound waves too. This is picked up by your 10 ft antenna , transmitter, and input cables. Turn power off to your house....problem solved!
AM transmitters and receivers are based on amplitude modulation.....same as all electrical noise and EMR hum from the A/C all around you.
FM transmitters don't have this problem because they are all based on frequency modulation where noise isn't and the audio is separate fron the EMR interference.With a proper power supply there's no hum.
Other than going without electricity the only thing to try(sounds rediculous) but get a metal box and put the transmitter, audio source, inside to try to shield the whole thing! or a regular box lined with foil. or put the transmitter outside away from the house.
Mark
After fixing audio ground loops and transmitter power supply filtering/regulation problems, the remaining (and most common) hum problem is caused by the maze of AC wiring surrounding the indoor antenna. More specifically, it's caused by devices connected to the AC wiring. Outdoor antennas don't suffer from this problem.
Indoor antenna hum caused by AC wiring can usually be identified as follows. Place a radio within a couple feet of the antenna: no hum. Take the radio to other rooms: hum. Take the radio outside to a distance where you can just receive a decent signal: no hum. I'll come back to the reason for this behavior later.
To start with, consider that the "reactive near field" at the upper end of the AM band extends to about 90 ft (.159 wavelength). This simply means that any wires within 90 ft are reactively coupled (capacitive and inductive) to the antenna. So, they become part of the composite antenna. The total radiation is from both the antenna and the AC wiring. Also, there is radiation from the ground path to the transmitter through the AC wiring ground wire connected to the transmitter which is a separate, but equally important phenomenon.
Consider a house that has all the circuit breakers on, but nothing hard-wired or plugged in to any outlets. The composite antenna consisting of the transmitter antenna, the transmitter ground path through the wiring, and the nearby wiring yield a constant, stable RF impedance and there is a constant signal level with no hum on receivers everywhere in the house. Now flip on a light switch for an incandescent light. Depending on where you are in the house with your radio, you may notice a change in the signal strength caused by a change in the RF impedance of the AC wiring due to the connection of the incandescent light. There still won't be any hum.
Now consider switching on an appliance or electronic device that has an RF impedance that varies at the 60 Hz AC cycle rate. The result will be just as if you flipped the incandescent light switch on and off 60 times a second. You will receive a signal that gets louder and quieter at a 60 Hz rate. In other words: hum.
Most appliances and electronic devices won't present a varying RF impedance to the AC line because they have proper RF filtering at their inputs. The filtering makes the RF impedance constant. But, out of the myriad of things you can buy to connect to your AC lines, there are many that aren't properly filtered. The FCC is supposed to regulate this, but many imported devices seem to constantly sneak under the FCC radar.
If you have shut off all circuit breakers except for the one that powers your transmitter and still hear hum somewhere indoors, then look for any devices that are powered by that same breaker and disconnect them. Just turning an electronic device off won't guarantee it is really completely off. Unplug it.
Now, going back to what I said at the top, why is there no hum within a couple feet of the antenna and very far away from the antenna, but there is hum in between? It has to do with the ratio of the signal received directly from the antenna and the signal received from the AC wiring. At very close range, you're receiving almost all of the signal from the antenna. At medium range you are receiving the antenna/AC wiring composite signal. At maximum distance you are receiving much less of the AC wiring portion due to the extreme inefficiency of the AC wiring portion of the composite antenna. If you walk around the house with a portable radio, you will likely find some areas where the hum is maximum. Those are the areas where you are receiving most of your signal from the AC wiring.
Phil's explanation is consistent with what I have observed with my station. The transmitter and antenna are outside approx. 45 feet from the house. It is powered and audio fed via a buried cable originating at my "studio".
What I observe is that inside the house (using a portable receiver) there is hum on the signal and also when near the house outside. However, as I move away from the house and power lines the signal is clean. This is also heard using my car radio. Near the house or in the garage there is hum. At the street no hum.
Neil
Adding to and supporting the comments of Phil and Neil, such hum effects can even affect reception of licensed AM broadcast stations, though the measured hum levels they actually transmit are 50 dB or more below program modulation.
Such hum effects can be present across small geographic areas at some receiver locations close to overhead a-c power lines, but miles from the transmit antenna. This occurs because the direct signal from the transmit antenna is reduced/nulled at the receive antenna by reflections of the direct signal from nearby re-radiators such as the metal poles used for street lights, the ground wires leading down the wooden poles used to support a-c distribution wires, etc.
The strongest signal present at a receiver in those locations then can be re-radiation from those nearby a-c power lines, and subject to whatever AM noise they add to the original signal.
From the several posts on why hum occurs with indoor antennas it might seem like a hopeless situation, but it is possible to have an indoor AM transmitting system with virtually no hum. Both of my AM transmitters fall into this category.
My antennas are "in-phase" with outer building walls because they consist mainly of metal window frames extended at the top and bottom with wire sufficient to total 3-meters vertical length.
I believe this "in-phase" factor aids with radiation both indoors and out because the signal suffers zero multi-pathing from reflections from the inner or outer wall boundaries.
Yet another cause of disruption for both AM & FM indoor antennas is reflections off of wall, floor and ceiling surfaces. This is minimized by making the antenna integral with one of the boundaries (surfaces).
The PZM (pressure-zone-microphone) operates on the same principle.
All good advice in the help here on the subject. I went up last night and tried to cover all the wires i could with tinfoil and ground the foil that dindt do anything but use us two boxes of tin foil so got rid of that stuff up there. On the ohter hand today the hum is much less but still there but its at resonable levels, has to do with even whose who knows what is on in the house or even the neighbors I dont know. I do know that after I set up the antenna at first it wasnt so bad thought i hit paydirt but then yesterday it was bad so figured i had to do someting about it.
I did do the thing with the breaker box, and also first went around turning things off at different out lets to see what might be causing the hum. In one room I unpluged a turned off comptuer and it made a big difference but in other rooms the hum was still there with the comptuer unpluged so you figure. Thats when I decided to try the breaker box.
One thing for sure it had a nice quiet carrier with no power going into the house. Not a practical fix though.
Some times I think its the goverment making the hum show up on my radios I think its from the DRONES that may be watching me I dont know. You NEVER know anymore.
I know some Drones.
I wonder how the AM broadcast stations get it hum free? might be somting to do with the cost of the equipment im sure there not runnin talking house and ten foot piece of wire in the attic some place.
I did work at an AM that had a hum every few seconds. Traced it to a bad choke for the tower light.