Things change over time. It might be redundant to say so, since change, by definition, requires time in which to occur, therefore mentioning that "change happens over time" is like saying "given enough time things won't stay the same." You could just say "things don't stay the same."
Having made that point I have examples of things that have changed, and, true to form, they changed over time.
For months on end I had a recurrent zapping and buzzing sound on the x-band (the upper AM frequencies) coming from a defective street lamp that kept trying to turn on but was evidently short on viagra. But it kept trying and the zapping interfered with my weak AM signal. After searching for and finding the lamp I planned to pull over and write down the pole number so I could call the electric company, but they fixed it, bless their hearts, families, pets, and work ethic.
But another disturbance lurked in the background, that 2nd harmonic of AM 850 appearing at 1700 kHz as a buzz-blaster, made all-the-worse by the iBAH chainsawing at 1690 and 1710. This went on for a long time. It even started leaking into my signal at 1680 kHz forcing me to move to 1640.
But again, things don't stay the same, although they happen until they stop, which is a factor of passing time. This week 1700 is clean, and the noise is cured. At first I doubted this so I checked and found 850 is in operation, they still have iBAH, but their 2nd harmonic is finally supressed. What I think is that their engineer fixed it. I also think he had to wait for a part, custom made, which took awhile to come in. I could just call over there and ask about it, but I'd rather use my imagination to explain it away.
In my third paragraph I used the expression "months on end", and I have no idea what that actually means, but it's an expression we've all heard. What is "on end"?
Finally, since people tend to look for new posts but very quickly get bored reading them, I want to know how close two transmitting antennas can be spaced so as not to interfere with each other. For example, if a transmitter/antenna at 1640 kHz is one-foot away from another transmitter/antenna at 1680 kHz each one will be back-modulated by the other and neither signal will sound right. But how far apart do they need to be? 5-feet? 10-feet? 20-feet? 30-feet? 32-feet? What?
Some things never change, yet time goes by.
The only responsible answer I can think of for the antenna spacing situation is "it depends" which would prompt the question "upon what?" which is answered "on many things". This interaction could be calculated but, as is many times the case in such analyses, the parameters needed are not well known and need to be guessed.
The best approach therefore is to try it and make measurements. Since what is sought is "interference" some measure of this needs to be established. The most likely result of mixing will be intermodulation distortion or IMD. This can be found using a spectrum analyzer or a calibrated receiver. The mixing, if non-linear, will produce signals on several frequencies. Take, for this example, two frequencies A and B. Non-linear mixing will produce signals on frequencies A + B, A - B, B - A, 2A + B, A + 2B, 2A - B, A - 2b and so on. In general the mix frequencies will be at +/- NA +/- MB where N and M are integers. Practically N and M being 1 and 2 will be sufficient to predict where the mixing will be found.
For your query about 1640 and 1680 look first at (1640 + 1680) = 3320. If nothing is found then chances are there is no significant mixing. If something is found then a check on (2*1640 + 1680) and (1640 + 2*1680) can be made. The relative signal strengths will give an estimate of how severe the mixing is and whether this is going to be a problem.
Another check is to listen on one frequency (unmodulated) while the other is modulated. There should be no modulation heard on the first frequency.
A couple of things are working to reduce mixing: If the two antennas are resonant then off resonance signals will be attenuated and the low radiated powers will further reduce the probability of mixing problems. Be aware that there will probably be more noticeable mixing in the receiver if near the transmitters than in the transmitters themselves so measurements should be made at a distance.
Mixing can also occur in other nearby devices if non-linearities are present such as wall warts and other electronic equipment.
It will be very helpful if you will post the results if you do this experiment.
Neil
The mind isn't exactly an attic, but up in the mind we have a tremendous clutter of "previous-experience", and I just tripped over one.
My present two transmitters have been set to close frequencies and do not interfere with each other in any way, as they are 35-feet apart.
Knowing that Part 15 transmitters placed 35-feet apart operate properly is a starting point.
Now, per Neil's suggestion to move forward with tests, we need to begin testing transmitter/antennas at closer distances, let's try 20-feet apart, and luckily there is a spectrum analyzer.
This may take a couple of years.
Just a suggestion, perhaps do your next test with the antennas 1 foot apart. If there is no interference then this saves you the trouble of testing at 20, 15, 10, and so on.
Neil
Per Neil's suggestion, the next antennae proximity testing will be done at a separation of 1-foot.
Or, upon further thought, I am thinking of another way of viewing the objective... and that is to actually have an objective.
The reason for determining minimum spacing for AM Part 15 transmission antennas is to make possible the operation of up to six transmitters on as many frequencies on as little real estate as possible with no mutual interferences.
At 1-foot apart, they might be too close together to allow approach for maintenance. So, because a typical doorway is 3-feet wide perhaps that ought to be the starting distance for proximity.
But how would that look, having six 3-meter verticals in a cluster with 3-feet between each pair? Maybe there's an ideal aesthetic spacing which would best appeal for appearances.
I know what. I'll stick six bamboo poles into the ground and study various spacings from a distant vantage. Can't do it today, though, because of the rain.
As long as it's raining out I spent some dry indoor time looking at the upper harmonics of the AMT3000 using a spectrum analyzer.
The transmitter is in modified mode, per instructions from SS Tran, using a triangular self-built loading coil driving a Wintenna and clamped to the house I-beam to ground.
Most of the harmonics didn't show up at all and a few odd ones had very small showings, never less than 50dB below the main carrier.
For novelty, I also tuned the harmonics on a shortwave radio and they are really down in the mud.
Don't underestimate the quality of the AMT3000 Part 15 Transmitter.
I'm not as good at my one language as I'd like to be, but playing with words is fun anyway.
Neil has stories about word games too, and I just noticed one...
"Tornado sirens went off."
For some reason that means the same thing as "Tornado sirens went on."
You could even say, "Tornado sirens went off for 5-minutes then shut off."
The sirens worked because the tornado got scared away.
Stand up more.
My two SSTran transmitters are located 35-feet apart. Up until now I have considered this distance safe to avoid interference between the two. Now I'm not sure.
The other day I used the TECSUN PL-310 radio, with it's signal strength and signal-to-noise LED read-out to plot the numbers in a grid-pattern over my entire yard, showing the strong-to-weak areas for the AMT3000 at 1550 kHz.
Today I decided to do the same thing for the AMT5000 at 1670 kHz, and soon ran into a snag. On the silent carrier I was hearing occasional popping sounds and the signal level abruptly lowered or came up.
When I did the readings for 1550, the 1670 transmitter was turned off. But today I took the 1670 readings while the 1550 transmitter was also on.
Abandoning the yard readings, I went indoors and considered the possibility that 1550 and 1680 are messing with each other, but there are other possibilites.
On came the spectrum analyzer and I watched the 1670 signal over a period of time, and indeed, at intervals, the signal level jumps slightly down, than comes back up.
It could be an intermittent connection in the antenna or ground system.
When I return to this exploration, the 1550 transmitter will be turned off, to eliminate the possibility of co-interference.
Another possible source of unwanted RF interference might be 850 kHz 5 kW, less than a mile away. It has caused many problems in the past and might be the problem now.
These situations make part 15 even more fun than it is.
At midnight while 850 kHz was signed off and my AM3000 at 1550 was turned off, I powered up the AM5000 at 1670 and watched/listened. The carrier remained clean and constant.
Next I powered up the AMT3000 at 1550 to find out if it would interfere with the 1670 signal, and after 15-minutes of watching, both carriers were smooth and quiet.
This morning about 7 AM I got ready to hear 850 kHz sign on with its 5 kW less than a mile away. KOA in Denver was riding in very loudly and I got a dose of morning news in that city, until at about 7:20 the local 850 switched on and began talking about things God and his friends did in a far off place a long time ago.
Hmm. My two carriers were fine. The strange behavior of yesterday wasn't happening.
At 2 PM I took the TECSUN PL-310 around the yard and got a grid of field strength readings, everything went well.
The mystery of yesterday's popping and crackling on the 1670 transmitter remains unsolved.
A few years ago, I corresponded with a gentleman who had two transmitters mounted on short poles above a mobile home. The antennas were spaced about 15 ft. apart. I seem to remember the frequency difference was about 50 kHz. He was experiencing what seemed like crosstalk between the two. He was feeding different audio streams to the transmitters. A low level of audio from one source was noticeable in the signal from the other transmitter and vice versa. After some experimentation, the problem was found and fixed. Both transmitters were sharing the same ground wire down to a single ground rod/radial system. When he ran separate ground wires, the problem disappeared. With a single ground wire, the ground-side RF current for both transmitters was flowing in one wire causing some sort of cross modulation of the RF signals.
As I indicated in my most recent post, today there were no problems between the two transmitters, but I did have an unusual experience that will be slightly difficult to describe.
With a hand held portable Grundig FR-200 tuned to 1670 (no audio) I entered the middle room between the two transmitters and when the Grundig was near a C.Crane Radio Plus which was tuned to 1550 (with audio), the audio from 1550 started being heard over on the Grundig tuned to 1670.
By holding the ferrite rods of the two radios exactly criss-crossed, the audio from 1550 came out of the Grundig at 1670 at very full level.
It seemed like I was getting a transfer between the radios, as I was about 20-feet or more away from either transmitter.
The grounding of the two transmitters is separate, but one radial from the AMT5000 runs parallel to the basement I-beam to which the AMT3000 ground is clamped.
Sounds like you're picking up the local oscillator of the other radio. If you tap on the radio playing the station you'll probably hear it in the other radio.
Indeed the C.Crane Radio's local oscillator was outting the 1550 audio on top of the silent carrier at 1670 on the Grundig, held at close right-angle.
With Part 15 radio we are reaching around in invisible space and detecting results!
It's ghostly!
Carl: You don't remember me asking you to check that?