Antenna Extention Cord

Cool, your first transmitter - if you're anything like the rest of us, you should prepare to become quite addicted to receiving a radio signal that you have created - it's a lot of fun!

The transmitter you're working on will cover a few dozen feet, perhaps a bit more if you add a loading coil to the antenna - search this site and you'll find lots of info on a loading coil. No extension cord is needed as the AM signal will go as far as it will go without the need to locate the antenna away from the transmitter

You'll very likely find that, like boats, the first one is just the starting point 🙂 The design you are using will definitely get you on the air - start with the original design (9 volt battery and original transformer orientation) to see what that's like and then experiment away.

You will also probably find that your signal will show up all over the dial because the transmitter design is a bit prone to harmonics and that the audio quality is a bit narrow but, it's enough to get you hooked!

I appreciate the advice very much.

And I understand the boat comparison perfectly.

I have am 11 foot fiberglass with a 2 horsepower Evinrude, and am currently restoring a 16 foot bluefin from the hull up.

First of all welcome aboard. Don't be afraid to ask questions and to share your experiments with us.

It appears that the transmitter is a crystal oscillator, probably in a metal can. If so and if it is designed for a digital circuit then the output will be a square wave. As SCWIS mentioned a square wave is rich in harmonics but they will not appear in the AM broadcast band. They will appear at 2.0, 3.0, 4.0, and so on Megahertz. The odd harmonics (3,5,7) will be strong and the even weaker or non existent. Because of the low power you will be running the harmonics probably won't cause interference to other radio services but it is advisable to know about them.

If you use a loading coil (search the forum for this) and tune the antenna system the harmonics will be suppressed enough to not be a concern.

You will also find that the audio for the modulation won't be as loud as other stations because the crystal oscillator will stop oscillating before the output reaches 0 volts. This will cause audio distortion and interference in the AM band and you should tune around the band to hear it.

This information is not meant to discourage you but rather to give you some ideas about what may happen. At these power levels there is no harm in experimenting.

Neil

"Cool, your first transmitter - if you're anything like the rest of us, you should prepare to become quite addicted to receiving a radio signal that you have created - it's a lot of fun!"

Heh-heh ...So true! Now if I can just get everyone in my little town addicted as well .... 😉 ... Maybe then I can enough support to afford to set up a digital remote broadcasting unit.

Back to question ... The problem with an antenna cable is that it radiates. In most Part 15 systems (except those where an ATU is a separate unit that mounts with the antenna), the 3-meter rule applies to the whole thing, transmission line included. IOW, in this case there is no such thing as an antenna extension cord because it is by its very nature part of the antenna.

Since the oscillator operates at 1 MHz, the range will be low simply because of the low frequency. Your second transmitter should be made to operate near the top of the AM broadcast band. I don't know how well the oscillator will modulate. The high Q of the crystal will resist producing much modulation. No matter what, you will get a strong signal in your own home.

It's not too early to get familiar with the regulations, since the regulations will restrict your range much more than any technical considerations. Get a copy of Section 15.219 rules from the Government Printing Office website, which is linked from fcc.gov. To meet 15.219(a), check the DC current fron your 9 V source to be sure that the input current to the oscillator module is no more than about 11mA, so that the input power to the oscillator is no more than 100 mW. To meet 15.219(b), be sure that the total length of your antenna, ground lead, and "antenna extension cord" is no more than 3 meters. To meet 15.219(c), tune the antenna to resonance. This will be the hardest thing to do, since it is the hardest to understand. In addition to reducing out-of-band transmissions (as has already been mentioned in this thread), a resonant antenna will give the maximum range.

I really do appreciate all the advice. Both the physical and legal.

Although, does anyone know where i can find either a crystal oscillator around 1600 kHz or a schematic of that can do the same?

I hope this isn't another stupid question, but if i use a 1 mHz resonator after the oscillator and before the antenna, will it help to keep it on the frequency of 1 mHz?

I assume you mean something such as a ceramic resonator. The answer to your question is no.

The frequency of the oscillator is set by the crystal in the oscillator so anything other than a mixer added on down stream toward the antenna won't change this.

One thing the resonator will do if set to 1 MHz is suppress harmonics and splatter sidebands but it would need a bandwidth twice as wide as the highest audio frequency. It would also need to be properly impedance matched in input and output to operate within spec. and would cause some power loss in your signal.

Maybe I over answered your question but I hope you find it useful.

Neil

Try a SEARCH on this site for "crystal oscillator". There was a thread a while ago by someone where the trials and tribulations of this was discussed.

Also, a web search on the subject will yield plenty of do it yourself articles about crystal oscillators.

Neil

The answer was exactly what I needed to know. Thank you and i really appreciate the fact that everyone here is at least giving me advice to work with, rather than just a plain yes or no.

I'm only 17 and am trying to wrap my head around a lot of info.

Thanks all!

I began "playing" with electronics when I was a few years younger than you. I read everything I could about the subject and it was a great hobby. Part 15 AM was one of my first projects aided by a Knight Kit broadcaster from Allied Radio. That was a great learning experience and probably similar to yours at present.

My enthusiasm for this lead to college studies and a long career as an electrical engineer. Some warn that if you make your hobby a profession you will spoil your hobby. This was not the case for me. Regardless of your career pursuits electronics is a great hobby and many non technical folks enjoy it.

Neil