I am just getting into radio, so bear with me. This is a very simple question about antennas. I have an AM 1 Mhz transmitter that I built. The antenna I am using is just a random wire antenna end fed. The wire is 18 gauge speaker wire that I have cut to what I thought was the right length for 1/4 wave operation (246 feet). Another antenna length site I looked at told me it should be 234 feet, so I am a bit confused. My problem is that it seems like the antenna has way too much reflectance because when I turn on my transmitter anywhere above half power, it starts to have feedback. The amperage draw of the transmitter begins to bounce up and down and then it stops working. I'm afraid it's going to damage it. I need a way to make the antenna less reflective (I think that's the problem). I have read about loading coils, but I thought they are only necessary for tuning an antenna that is not the right length. I always thought if the antenna is the right length it isn't necessary to have a loading coil. How close to the right length does the antenna have to be? Is a few feet ok or do a few inches make a big difference. Any help?
Thanks.
Unfortunately, Synapse 3, Part 15 of the FCC Rules, the main theme of this website, does not allow full lebgth antennas like you're talking about.
What kind of transmitter do you have?
Do you have an FCC license?
We will guide you some more once you reply.
Just curious..what is the power of your transmitter? To be getting the problems you have you don't usually get with Part 15 power. It probably is damaged already.
Yes you are right about loading coils...they are used to make a 10 foot length of wire work like a wire that is the right wavelength...when tuned right it changes the electrical length even though the physical length stays the same. With AM the ideal physical length is so long it's just not practical as compared to FM where full wave at 98MHZ is 120" so that's why a loading coil. But this is still legal because the antenna is still 3 meters(10ft) long. Also a ground is needed for the AM transmitter to work right. You don't mention if you have the transmitter grounded. Maybe that could be causing a problem.
I have an AM transmitter that I built based on a 1 MHz crystal oscillator. I have a variable power supply with amp meter that I have it hooked up to so I can make sure I don't draw too much power. I also have an SWR meter so I don't go over 100mw. I am not going to be doing any 24/7 type broadcasting, I am just doing this for fun as a hobby.
I was not aware of the entenna length rules though, so I understand if you can't help me. I just don't want to spend the time building a super elaborate antenna. What would the legal bounds be on a simple wire? Is it allowed at all?
Oh yeah, I forgot to mention that it is grounded. It's just another wire that I have attached to a metal rod that I put in the ground.
The legal length of the antenna is 3 meters. Can't see your set-up so it's hard to know what the problem is. If you have a variable power supply maybe (don't know for sure) you are giving transmitter to much..try not exceding the voltage past the point where it works ok. Is your 1/4 wave length antenna above ground horizontal in space? A 10 foot antenna with a loading coil is a lot simpler than trying to string a 250 or 500ft length of wire. You must have a lot of property to be able to do this.
Mark
1 Mhz would be 1000 Kilohertz or AM wouldn't it?
First, an end fed wire that is 1/4 wavelength long is not really a "random wire". It is resonant at 1 MHz. If your transmitter is designed to drive a "50 ohm load" and has a low pass filter, it should drive the wire with no problems. If your transmitter has an output tuning network consisting of a coil and trimmer capacitor, it's designed to drive a 3 meter antenna, and you will need to remove the coil and trimmer and replace it with a low pass filter. I was going to ask about whether you have a ground, but you answered that in a later post.
Although your 1/4 wavelength antenna far exceeds the Part 15 3 meter limit, I would love to hear what kind of range someone would get with such an antenna!
I agree with Phil. 1/4 wave antenna should be just fine if you are using a transmitter that's looking for 50 ohms. Also, once you know the length of the wire it's no longer a "random wire" antenna.