Maybe Lower Power and a Lousy Antenna is Better

AM radio reception in my house is plagued with interference, buzzing from light dimmers and hum from who knows what.  Even my two solid state high efficiency transmitters, one of which is ground mounted outside and capable of 1 mile car radio range, cannot overcome the noise inside the house, but perhaps it is a mistake to think that a strong AM signal is going to overcome electrical noise in a home.

I recently noticed that a 5 kW local station also had noise on a portable receiver in my house but a 5 kW station 70 miles away did not have the usual noise on the same portable in or outside of my house.

Maybe weaker is better since it is known that AM RF can get into things such as wall warts, switching power supplies, and almost anything connected to the AC line.  A weaker signal perhaps lacks the strength to cause mixing and hence noise in devices around the house.

So, some experimenting was in order using my Part15 homebrew tube transmitter.  The transmitter operates with 67 mW final input power and uses a 3 meter wire antenna strung upward from a shelf and along the ceiling joists in the basement.  The range on a portable is about 100 feet so the combination of low final power and the antenna pretty much conspire to produce a pretty puny radiated field strength  Despite this, around the inside and the outside of my house the signal is received noise free on the portable. It also provides a noise free signal to line operated AM radios including a 1962 vintage all american five tube receiver.

It seems from this that noise problems can be avoided by using a reduced power and poor antenna for housecasting.

Anyone else have similar experience?



Carl Blare's picture

Since 2007 when I rejoined the low power life, having taken a few decades to work in pro radio after those early years with Knight Kits, many new experiences have been come and gone.

If I did something one way there would be hum, if I changed the setup the hum went away.,

The most severe form of "house hum" was a consequence of testing the newly built "Big Talker" shortwave transmitter for 13.50 MHz with the quarter-wave dipole stretched through the house just above head level. When received on any radio indoors it had a throaty hum but simply stepping outdoors with a portable radio in hand the hum did not exist. It was strictly a matter of the house responding internally.

Today the kitchen Panasonic table radio hums across the AM band when the AMT5000 is turned on because the Wi-Fi Audio Receiver connected to the transmitter has a switching power supply that injects racket into house wiring.

If I turn on the toaster oven that hum disappears.

The three CFL bulbs in the ceiling fixture over in that room throw some hum when turned on.

There's an LED bulb in the hallway the fills the long wave band with jumpy trash as viewed on the spectrum analyzer.

But after re-considering everything I cannot say that weaker broadcast signals have ever triumphed over a hum problem, and I often run the AMT5000 at minimum power (34 mW) which I am only able to enjoy because none of the hum problems showing up elsewhere happen to cross paths with the Sangean desk radio used for that purpose.

Carl Blare

Radio Joe's picture

Radio professionals,

I have been fighting buzz and hum from the RF mixing with the power line since I got on AM. I tried grounding various pipes and coax to a common point and other things like that. You get it to disappear then turn the radio a bit and you get it right back from another angle.

I got it to a point last year that I thought I had it all beat then this summer it’s all back again. Like others I often can turn on or off a light or whatever and sometimes the hum decreases a great ammount but lately none of that works.

I have grounding straps all over to vent pipes, the furnace  whatever, with instructions what to ground or change in case hum shows up, but its approaching insanity because its like fixing rust on a car, fix it here and more pops up some other place.

I tried different power levels and frequencies and lower power means less hum but also less volume and more general noise so that’s no good.

I have an outside vertical antenna and two horizontal the attic antennas and they both give me buzzing and hash to some degree. If I drive down the street the hum will be greatly reduced or disappear. I use the transmitter primarily for in the house or back yard that doesn’t do me much good only to help diagnose the problem.

I noted this before in a post some time ago but once I tried running the transmitter (TH5) off a battery and then pulled the main breaker to the house then the signal was perfectly clean, so it has to do something with the RF mixing with the electrical wiring. I think we all know this and can all agree to that.

I tried using line noise suppressors plugged into outlets and on some you plugged the noise suppress in and the noise really dropped out and others made no difference, and recently where the noise suppressors were plugged in and  I unplugged them to see what happened  and sometimes the noise got worse so no conclusion there with those.

So what is the answer? How do broadcast stations get around this problem? I suppose that’s one reason they place the transmitter out in the boonies some place to get away from that, however I know of one 5 KW station in this area the antennas are about 500 feet from the studio and always have been since the station came on the air long ago.

I do know that if I drive under heavy power lines listening to AM stations, I sometimes get the same hum or hash noise so how can we kill it off once and for all?

No problems on FM with this noise.  Thought I had it all fixed but now, I can’t sit in my CHAIR and listen to my TUNES when transmitter is in my own house and antenna few feet away in the attic... Doesn’t make sense. I do know for a while there was very little hum and didn’t change anything since, but the hum is back again.

Radio Joe

Rich's picture

This post won't help anyone in fixing this hum problem, but it may give an understanding as to why it occurs physically close to a Part 15 AM setup.

The graphic below shows the strength of the magnetic field in mA/m (vertical axis) with respect to a range of 5 meters to 30 meters of horizontal distance from the Part 15 AM antenna (horizontal axis).  The Part 15 AM system modeled (in NEC4.2) is approximately that of the outdoor installation of radio8z, as recollected.

In the near field of a radiating antenna, the electric fields and the magnetic fields of a radio wave are not related to each other by the ~377 Ω impedance of free space.  Both fields change exponentially in the near field, rather than a 1/r rate as in the far field.  Those near fields can be much greater than the far fields.

Consumer-level AM broadcast receivers (both a-c and battery operated) normally use ferrite-loaded loop antennas that respond to the magnetic field.  Auto radio antennas normally respond to the electric field, and ignore the magnetic field.

This graphic shows that the magnetic field at 5 meters is over 34X as strong as it is in the far field, which begins somewhere around 15 meters from the transmit antenna.

It is difficult to provide sufficient shielding of those high magnetic fields so as not to react with other electric/electronic systems.

Carl Blare's picture

The subject of indoor antennas is of particular interest because so many part 15 stations will always operate from indoor antennas.

For my station, KDX Worldround Radio, I will end up with both because I am in mad pursuite of setting up a maze of transmitters and backup transmitters, with both indoor and outdoor antennas.

Sticking with indoors for this thread, AM and FM have diffent issues.

Indoor AM antennas, as you've mentioned, are subject to the hum problem.

With indoor FM it's the multi-path problem caused by human or pet traffic in the same airspace used by the RF energy.

Usually the manufacturers OEM (Original Equipment Manufactured) antennas are used to cleave to the certification. But others of us delve into experimental antennas while always being sensitive about the FCC FM rule part 15.239.

What's wrong with the OEM antennas? They are inherrently mismatched and subject to high standing-wave-ratios (SWR) which contributes to multipath.

A closely matched antenna improves the SWR but requires rolling back the RF power to contain the field strength, all legal.

But proximity to walls, furniture and living beings sets up reflections that will result in unpredictable multipathing by something as ordinary as opening the refrigerator.

FM antennas are either DIY (Do-It-Yourself) from wire or metal coat hangers or manufactured.

Manufactured FM antennas are usually intended for outdoor use and are hard to accomodate indoors.

Enter the Decade GP32, an actual indoor FM antenna from the same people that make Decade transmitters.

Decade GP32 Indoor FM Antenna

I have ordered one of these and will file a report.

Carl Blare

Radio Joe's picture

So how far away from my house do I need to set up a vertical antenna so the magnatic field dosent get into my house electrical  system?

Radio Joe

Rich's picture

That depends on the configuration of the antenna system and other local conditions.

In the case posted in Reply 4 the far field begins about 15 meters from the antenna, and that might be considered as the minimum distance for those conditions. YMMV.

Carl Blare's picture

The Decade GP32 Indoor FM Antenna arrived a day ahead of the delivery schedule, and I became stalled by being unable to locate a UHF connector to BNC or F-connector adapter, which I've now spent half a day searching for.

Doing things slowly comes easily so we'll order the necessary parts to connect the antenna to the EDM transmitter while you wait.

The item is neatly built and rated for 75 Watts, which may come in handy if the FCC raises the allowable power for part 15 FM.

So far as I know it is the only antenna in the world labeled as an indoor transmitting antenna.

Carl Blare

radio8z's picture

I doubt that there is a good way to calculate this distance but my outdoor transmitter delivering 85 mW into a base coil loaded 3 meter antenna with buried ground radials is installed 45 feet from the house and it produces hum on receivers inside the house but not on portable or mobile radios away from the house.  The other confounding variable is that this transmitter is fed with audio/power via a buried cable which originates in the house and which conducts RF into the house.

As mentioned in Post #1, my low power tube unit with a wire antenna inside the house produces a signal strong enough to overcome light dimmer hash and with the light dimmers on or off there is no noticable hum or noise on the received signal.  This is not the case for the other two transmitters I use which produce more power ouput and use coil loaded antennas.  Hum is noticed even with the dimmers off.

First, make sure that the hum is not originating in the transmitter or audio system.  It appears that you have done so which verifies that the noise is coming from somewhere else.  Try operating without a transmit antenna or with a very short one and note if the noise is less.  If all you want to do is house and yardcasting then maybe the "sweet spot" for signal strength providing hum free operation could be found and used.  I found this by accident with my tube transmitter.



Radio Joe's picture

Well, best just to wait for anohter day because it all varies with variables, day to day- if that makes sense.

It has someting to do with electrical radiated power, and loads and connections on the AC lines in and around and out side the house to the pole and beyond. Going to listen to my 8 tracks again, no poroblem with those humming - only way i got rid of the hum during testing is run the transmitter on battry, pull main breaker to cut all power to house, then carrier nice and clean, you get connected to AC mains and you got problems again.

Radio Joe

Carl Blare's picture

Radio Joe Knows

Radio Joe said:  "...out side the house to the pole and beyond."

Carrier current broadcasting also encounters the ever changing AC lines in aggravating ways.

As you may realize, your neighbors are usually fed by the same power transformer as you, and everytime they turn on a washing machine or other heavy-duty appliance the impedance of the AC line abruptly changes, always upsetting the careful matching we may have done with our Transmitter Coupling Unit.

Many colleges abandoned AM carrier current not because it became old fashioned, but because of all the AC line issues.

Isolating our regular transmitters from disturbances on the electric wires might be possible, but for now we only know that it's a problem.

Carl Blare

Carl Blare's picture

The Decade GP32 Indoor FM Antenna, now being written about in 3 threads at 2 forum sites, works like a good dream. I am absolutely pleased with it and will never again look for another FM indoor antenna.

But there may be a bump in the road.

The antenna is so efficient, even though it's rated at 0 dB Gain, that I anticipate being above 15.239 field level with the EDM power minimized.

The instructions for tuning the dipole present a 1/4 wave dipole antenna, and if necessary it might solve the field strength problem by re-tuning to 1/8 wave.

Why not?

On Neil's original premis.. "lousy antennas", I am wondering if a badly tuned antenna might accidentally go resonant at a harmonic of the fundamental and generate a violation of 15.209?

Why not?

Carl Blare

wdcx's picture

With the antenna properly tuned I would have to think your signal level would be in violation of the rules. Dipoles have gain. There is no such thing as a 0 db dipole.

Druid Hills Radio AM-1610- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC. We do not censor free speech and hide public information.

Rich's picture

... Dipoles have gain. There is no such thing as a 0 db dipole.

The term "gain" is meaningless unless the reference for it is accurately stated.

A 1/2-wave, center-fed dipole has a maximum free-space gain of 0 dB with reference to a 1/2-wave, center-fed dipole (obviously), but the correct way to state that gain is 0 dBd, not 0 db.  Such a dipole is sometimes used as a reference for the gains stated for other types of antennas.

The maximum gain of a 1/2-wave center-fed dipole in free space is 1.64X greater than that of an isotropic source.  An isotropic source is a theoretical concept for an antenna that radiates equally in all directions in a spherical volume of space, and is the common reference for stating the gain of an antenna.

The value of 1.64 converted to decibels is 2.15.  So a 1/2-wave, center-fed dipole has a gain of 2.15 dBi, that is, it radiates 1.64 times as much power in the directions of its maximum field as an isotropic radiator would.

wdcx's picture

That didn't take long.

Druid Hills Radio AM-1610- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC. We do not censor free speech and hide public information.