Re: FM radio antenna
Posted: Wed Dec 14, 2016 1:55 am
Thank you Mike. I have responded inline below in bold
It helps a lot, thanks!Shane wrote:It seems you are looking for a long way around what should be a short cut. This is typical behavior for me. If there is a harder way, I have usually found it
Yagis are typically made of lightweight metal. I'm not aware of any reason why the driven element would need to be copper. Some of the designs I have seen use 1/2" copper tube to form the director.
The meters shown would not help you by themselves. You would have to make one of them the indicator in a field strength meter circuit which is taking this discussion far beyond scope to accomplish your desired end. I expected this would be true
Easy way: buy a commercially made Yagi specifically designed to receive the FM broadcast band and some "twin-lead" feedline (which is what we call the pair of wires you connect between the antenna and the radio), put the Yagi on a pole, point it toward the transmitters you are interested in hearing, connect antenna to radio and enjoy. (I may have left out a few steps but I think you would be able to fill in the blanks.)Please refer to first response. I found a brand new yagi on eBay for $30 but where is the fun in that? I actually enjoy the challenge of building stuff and I have a lot of materials on hand. Besides, nothing good is easy 2 Weeks ago the only thing I knew about antennas was that they work better when you wrap them in tinfoil...now look at me
Instructive way: Find an old copy of the Radio Amateurs' Handbook. Any year past 1956 should do. Somewhere in there will be fairly complete instructions for building the type of antenna you are looking for (a Yagi - capitalized because it's the last name of the guy who invented it) along with parts lists and theory (how it works including formulas for calculating the lengths of the elements). Excellent suggestion. Currently I am using this site
http://www.k7mem.com/Electronic_Noteboo ... troduction
A Yagi can have as few as 3 elements (even just two) and as many as needed with the majority of them being directors ahead of one driven element and one reflector. In general, the driven element would be cut to be resonant (a good match to feedline and receiver to keep it simple) at 98 MHz. The reflector would be a little longer; the directors each one a bit shorter than the last with the shortest one at the front of the array. Usually, only the driven element is not electrically connected to the boom, although there are a number of ways to use what's called a matching section so you can skip that formality and making it possible to use coaxial feedline instead. The stations I plan to tune to are 99.9 MHz and 102.5 MHz so I was going to trim the antenna to 101 MHz as a good median
Using wood is rather quaint. You will actually find some of those construction practices if you go further back into the Handbook annals. Lightweight metals, particularly aluminum, have pretty much made using wood unnecessary. LOL, that was a polite way to describe it. Yeah, I know, but the reasoning is because I planned to use 14 ga. copper wire for the director and it wont support its own weight...needed something non-conductive to attach it to. I have a lot of those aluminum arrows, I cause use them.
Two more things if this antenna is going to be outside: neighbors and lightning.
Neighbors have this awful habit of not liking ugly things in other people's back yards (or front yards for that matter). We call this the NIMBY effect, for "Not In MY Back Yard!" although it should be "not in YOUR back yard next to mine!"
Now WE don't consider antennas ugly but most people do. You may even have covenants in your neighborhood you agreed to when you bought the house that prohibit or limit this kind of activity. But then hiding one in a tree is pretty easy. I live in the country no covenants. My neighbors are pretty easy going but if they were to complain I would be forced to start a conversation about his herd of chickens and ducks that quack and crow at all hours.
If you do get the antenna up in the air much at all, you should have lightning protection on the aforementioned "feedline" before it enters the house. The "lightning arrestor" part can be found at your local Radio Shack if you still have one. You will need to supply the ground connection for it. Again, the Handbook will have info for you on that detail. I hope to stuff this in the attic of the garage. If it goes on the roof it will be low. Lightening is very rare here and I think the powerline towers nearby would probably attract any strike.
One last thought. Pay careful attention to what was written above by previous posters about element diameter. Just because you have 1" pipe laying around doesn't mean it's a good choice. In fact, besides complicating calculations, using pipe that's too big may just cause the thing to collapse on itself. The "pipe" is actually 1" thin wall tempered aluminium tube, 24" long with a short restriction on one end so they will connect together to make any length. I would use this for the beam. The 3/8" Aluminum arrows would be for the elements
Hope this helps and good luck!