Hey, all, this my first post and I need some advice. I'm trying to start up a small radio club for my school and I need to be able to transmit am frequencies from an amt3000 across a high school campus while still being under part15 regulation. Any suggestions for antennas/overall advice is very welcome. Thanks in advance.
Hello Cealeous:
To give you a meaningful answer to your question it would help to know the physical environment you wish to reach with your AMT3000, including a description of the building, locations where reception will matter, and available locations for placing an outdoor antenna, i.e., rooftop, open ground.
Looking forward to hearing from you.
As was mentioned, location can be a big factor for range. Part 15 AM range can be enough to cover the HS grounds around here. I used an AM transmitter for demonstration at a college and found that with the antenna inside the building the signal could not be heard down the hall so my first suggestion is to place the transmitter and antenna outdoors.
The transmitter should be connected to an earth ground and even a simple stake driven into the earth will help. The AMT-3000 has internal loading coils and needs to be tuned according to the instructions.
As your club gains experience with this they may want to try a more advanced loading coil/antenna system such as one with ground radials and an external loading coil. It is best to experiment to find what works in a given installation.
Here is a post where I describe my AM antenna installation. The buried radials are not seen but you can see the coil and transmitter enclosure.
Let us know of your progress.
Neil
Link to modify amt3000 for base loaded antenna:
http://www.sstran.com/pages/sstran_amt3000mods.html
instructions for Base Loaded Antenna:
http://www.sstran.com/pages/sstran_buildant.html
About ten years ago, or so it seems, I started an indoor antenna project for the benefit of many who will not wish to build an outdoor antenna/transmitter. Despite much time going by, the project continues.
What we are dealing with is an AMT3000 modified according to the manual instructions and a self-built loading coil which happens to be triangular rather than circular. It is my understanding that a coil does not need to be circular, and can even be rectangular if so desired.
I name this the "Tri Coil", which is the name I'll use.
Right now I get the peak at 82-turns on the Tri Coil, but the antenna has changed properties and brought the RF output DC reading down from the optimum 13VDC to a lower 10VDC. This is because of many days of rain.
The antenna is somewhat goofy, but does the desired job very well. It is mainly a metal window frame. Yes, about a 6-foot window. I call it the Wintenna, and its advantage is that it lays the signal both indoors and outdoors without requiring the transmitter to be outside in the weather. But the weather does cause it to change values.
The next tap lower is at 72-turns, the next higher is 88-turns, both of which are way off peak.
The next step is to find if the peak point can be improved by adding or subtracting a few turns of the coil, up or down from 82.
Sure, adding or subtracting even partial turns <EM>will</EM> affect tuning.
Here's a calculator which might help: http://hamwaves.com/antennas/inductance.html
The window thing is a bit wierd, dunno how tuning affects it....i.e., does the metal frame connect all way 'round, acting at least a little like a direct-coupled loop antenna? You have to determine direction, i.e., ...What does more or fewer turns from the 72-turn mark do to the signal?
You aren't likely, 'cept by accident, do get it to resonate perfectly from taps. In one of my systems, a Sharpie pen width of length (about 1/16" or less) made a difference; above or below it caused the signal to deteriorate rapidly from resonance.
That being the case, you could adjust final length with a telescoping antenna used as a stinger, or adjust resonance with a large vernier variable air cap....or air-coil-over-air-coil movement coupling....or a small diameter air coil carefully sanded (through the coating) across its turns with a stylus for tuning....or....???
The coil inductance calculator linked above is great, and worth spending some time reading and studying. It is a virtual text book on the subject.
As to my Wintenna, it has known flaws and limitations, but I WAS able to get the AMT3000 to load properly into it back when we were having drought conditions.
Now the weather is rainy and everything is moist, taking the Wintenna just slightly out of range of being properly tuned with the loading coil.
Full tuning is indicated when the AMT3000 is adjusted for 13VDC RF output, but now I'm only getting 11.2VDC when the Tricoil is peaked.
In essence the Wintenna, being the metal window frame, is bolted to the surrounding wall, consisting of bricks and plaster. This is like bolting an antenna to the earth. The proper thing to do is insulate the antenna from the earth, or from the wall.
It would be even better to get the antenna out away from the wall, but this is an indoor/outdoor antenna, and not an outdoor-only antenna.
Also, Ken's observations about very short coil lengths having a large effect are quite interesting, and the idea of using an adjustable telescopic antenna as the trick for fine-tuning is very worthwhile.
When my home built antenna ended up resonant a bit higher frequency than I needed, I couldn't add to the coil and the radiator was already full length.
Instead, I added some short radials like a top-hat. There were six of them each about 15 inches long near the middle of the radiator. I was able to bring the resonant frequency down to the desired point.
Somewhere in PhilB's instructions for matching the AMT3000 to a loading coil and antenna combination, he explains that if the capacitance of the antenna is outside of a certain range, which certainly is the case with my Wintenna, the value of trimmer-capacitor C5 can be changed to bring the system into a matching state.
But I do not want to modify the AMT3000 by changing C5, so I found another solution by "pruning," as MRAM calls it.
I noticed that if I added a .01 capacitor between the coil and the Wintenna, the tuning was slightly improved. Trying a .1 cap it got even better. The fix came with a .068 capacitor, which sent me over the mark all the way up to 15VDC RF output!
By adjusting C5 I brought the output down to 13VDC, the optimum, and the Wintenna is resonating happily!
The capacitor that brought the Wintenna into tune was a .68 uF cap, not .068 as previously stated. It was a logical error on my part getting the numbers mixed up.
Yes, all my mistakes are logical.