FM radio antenna

FM does it with frequency!
Bonaro
Posts: 13
Joined: Sat Dec 10, 2016 1:00 am

Re: FM radio antenna

Post by Bonaro » Wed Dec 14, 2016 1:55 am

Thank you Mike. I have responded inline below in bold

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 :D

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!
It helps a lot, thanks!

dbuckley
Posts: 91
Joined: Mon Jun 16, 2014 4:18 pm
Location: North Canterbury, New Zealand

Re: FM radio antenna

Post by dbuckley » Tue Dec 27, 2016 5:40 am

Bonaro wrote:There is a huge amount of energy being thrown off these lines. When it's cold and wet outside I can stand under them and hold a 4' fluorescent tube over my head and it will light bright enough to read by.
The tube lights up because you are standing in the dielectric of a high voltage capacitor!

There is an electric field between the wires above you and the ground you stand on. This being an AC system, that capacitance can be thought of as impedance, and a typical value at 60Hz might be 200K per mile of cable. A 750KV line is 750KV line to line, so that's about 440KV line to ground; Ohms law tells us that is about 2.2A per mile of current "wasted" in capacitance losses, or about a megawatt per mile. These losses are why one cant run high voltage overhead AC lines for an infinite distance, eventually the capacitance losses mean there is no power left at the end of the line, which is when one has to abandon AC transmission, and use high voltage DC.

Bonaro
Posts: 13
Joined: Sat Dec 10, 2016 1:00 am

Re: FM radio antenna

Post by Bonaro » Tue Dec 27, 2016 2:53 pm

dbuckley wrote:
Bonaro wrote:There is a huge amount of energy being thrown off these lines. When it's cold and wet outside I can stand under them and hold a 4' fluorescent tube over my head and it will light bright enough to read by.
The tube lights up because you are standing in the dielectric of a high voltage capacitor!

There is an electric field between the wires above you and the ground you stand on. This being an AC system, that capacitance can be thought of as impedance, and a typical value at 60Hz might be 200K per mile of cable. A 750KV line is 750KV line to line, so that's about 440KV line to ground; Ohms law tells us that is about 2.2A per mile of current "wasted" in capacitance losses, or about a megawatt per mile. These losses are why one cant run high voltage overhead AC lines for an infinite distance, eventually the capacitance losses mean there is no power left at the end of the line, which is when one has to abandon AC transmission, and use high voltage DC.

Yep. I strung a clothes line looking "antenna" under the powerlines once. I was triying to catch a bit of this stray power for some practical use. It was interesting but pointless. I gave up :shock:
https://youtu.be/g5p7MglnqZk

Bonaro
Posts: 13
Joined: Sat Dec 10, 2016 1:00 am

Re: FM radio antenna

Post by Bonaro » Thu Dec 29, 2016 12:59 am

I had decided to build a Yagi, tuned to the frequencies I normally listen to. I have worked out director size and spacing. Driven element type and dimensions, step up ratio calculations, impedance, step up ratio....on and on. I have actually learned a lot. I also learned there was a lot more to learn. Every time I figured out one component there was another that needed to be considered. Pretty interesting stuff. Then I considered the advice I read here and from others and looked into just buying an antenna. I ordered a ready made Yagi HD antenna from eBay. It covers VHF (FM) and most other bands. It's motorized with directional controls and was only $20 including shipping.

I figured out that no matter how much I learn or how careful I build this antenna, the only gauge for success was my ear..."I think it sounds better".
I appreciate all the advice you all have shared. I will continue to study this and probably have more questions in the future. :wink:

dbuckley
Posts: 91
Joined: Mon Jun 16, 2014 4:18 pm
Location: North Canterbury, New Zealand

Re: FM radio antenna

Post by dbuckley » Sun Jan 01, 2017 5:21 am

Bonaro wrote:Yep. I strung a clothes line looking "antenna" under the powerlines once. I was triying to catch a bit of this stray power for some practical use. It was interesting but pointless. I gave up
That's, err, rather more energy that I'd imagined would be reasonable to capture so easily. Ouch!

Pity you've taken it down; I'd have suggested connecting that to the secondary of an old microwave transformer, and see how much power could be drawn from it. I suspect there may be a few tens of watts there.

Bonaro
Posts: 13
Joined: Sat Dec 10, 2016 1:00 am

Re: FM radio antenna

Post by Bonaro » Mon Jan 02, 2017 2:08 am

dbuckley wrote:
Bonaro wrote:Yep. I strung a clothes line looking "antenna" under the powerlines once. I was triying to catch a bit of this stray power for some practical use. It was interesting but pointless. I gave up
That's, err, rather more energy that I'd imagined would be reasonable to capture so easily. Ouch!

Pity you've taken it down; I'd have suggested connecting that to the secondary of an old microwave transformer, and see how much power could be drawn from it. I suspect there may be a few tens of watts there.
On a cold wet day I could pull an arc over an inch long off those wires. I put my meter on it and read over 1000 volts before the meter fried. I managed to measure amps and got almost .5 amp. I tried to run a wire over to the garage (+/- 100') so I could mess with it without standing in the rain. After 100 feet the arc was only about 1/4". It doesn't behave like normal electricity. I thought I may be able to conduct it over a distance using coax but never got that far. One cool thing is the flourescent lights in the garage never completely turn off. :shock:

dbuckley
Posts: 91
Joined: Mon Jun 16, 2014 4:18 pm
Location: North Canterbury, New Zealand

Re: FM radio antenna

Post by dbuckley » Mon Jan 02, 2017 2:56 pm

Bonaro wrote:It doesn't behave like normal electricity.
Well, yes and no. It's ordinary electricity, but it's from a high impedance source. As every RF engineer on this forum will tell you (which excludes me!), the secret to getting the power out of the transmitter to the listeners is impedance matching. And its the same here; you have a high impedance source, which you need to convert to a low impedance source, at a much lower voltage and and higher current. You need an impedance matching transformer. Quite what form that transformer will take depends on knowing the source impedance of the capacitive supply, and the voltage, and its power output. All of which can be figured out using Ohms Law.
Bonaro wrote:I tried to run a wire over to the garage (+/- 100') so I could mess with it without standing in the rain. After 100 feet the arc was only about 1/4".
The same magic that gave you the supply, capacitive coupling to the transmission wires, is leaking it to ground over your 100 foot run. Which is why you need to transform it by the pole.

Or you could just put a series row of fluorescent tubes up both the pole, and have permanent yard lighting!

Bonaro
Posts: 13
Joined: Sat Dec 10, 2016 1:00 am

Re: FM radio antenna

Post by Bonaro » Thu Jan 05, 2017 12:50 am

Agreed...Finding the right transformer is the trick because the strength of the supply is extremely dynamic and changes constantly with proximity, humidity, temperature and load. After I transformed it to a lower voltage I would need a regulator circuit to make it stable. I believe I could manage enough controlled power, rectified to DC, to create a near constant supply of hydrogen/oxygen via electrolysis to heat my garage. I have the hydrogen generator part figured out, pretty simple tech to make Brown's gas. Storage is an issue and random arcs of electricity would not play well with explosive gasses...but what the hell, RIght? 8)
Plus none of this technical stuff addresses the fact that this is illegal :roll:

https://youtu.be/cXhZvyGtMrk

dbuckley
Posts: 91
Joined: Mon Jun 16, 2014 4:18 pm
Location: North Canterbury, New Zealand

Re: FM radio antenna

Post by dbuckley » Mon Jan 09, 2017 5:37 pm

Bonaro wrote:...addresses the fact that this is illegal
I've been wondering about that. What one is doing here is capturing electricity that is already wasted; it is never going to reach it's destination. It's not like putting a loop of wire under the lines and making a one turn transformer; that is definitely stealing as that is electricity that would have reached it's destination.

Bonaro
Posts: 13
Joined: Sat Dec 10, 2016 1:00 am

Re: FM radio antenna

Post by Bonaro » Tue Jan 10, 2017 2:45 am

dbuckley wrote:
Bonaro wrote:...addresses the fact that this is illegal
I've been wondering about that. What one is doing here is capturing electricity that is already wasted; it is never going to reach it's destination. It's not like putting a loop of wire under the lines and making a one turn transformer; that is definitely stealing as that is electricity that would have reached it's destination.
Well, a one turn xfmr will pull power from the line but so will just a clothesline or single strand fence line....to a limited degree. I think anything you do to capture and redirect the energy coming off these lines will increase the energy coming off these lines... and represent an additional loss the power co. Plus, they probably dont want you messin around and would find a way to get you...(me)
I could explain that I was simply cleaning up their messy power spill and then returning it to earth (after I heat my hot tub) but I dont think they would buy it.

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