T FM Antenna? - Page 1
Forum section: electronics / general

author: vlad777
author_rank: Frequent Contributor
timestamp: December 31, 2013, 12:28:39 am
content: Hi,I know nothing about antennas but I want to make a T FM antenna for my stereo.Should I go With option a) b) or c) and WHY?My stereo says 75ohm coax characteristic impedance,so how should I go about termination to prevent reflections?What should be the formula for L, I think it should involve75 ohms and 98MHz?Many thanks.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 31, 2013, 12:53:06 am
content: Just use a TV antenna.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 31, 2013, 12:56:26 am
content: (a)will look like 300 Ohms at the feedpoint (where the coax is connected)The classic form of this uses 300 Ohm ribbon for both the loop & the feeder,with a 300 Ohm to 75 Ohmbalun at the receiver end.A simpler version for coax feed is to ditch the loop & just use single wires connected to the centre conductor & shield of your coax.(each of these wires will beL/2,with the total length of both wires combined beingL.Lin metres will be 150/98 =1.58m.---this is a "halfwave dipole".There are a few niceties which you could use if you are fussy,but which shouldn't matter much with what you are using it for.They are:(1) "End effect"--this normally means you should makeLabout 5% shorter than the value calculated above.& (2) I/2 wave dipoles are balanced devices,& coax isn't,so in theory you should have a 75 Ohm: 75 Ohm balun at the feedpoint.(b)&(c)are not appropriate for your application.

author: vlad777
author_rank: Frequent Contributor
timestamp: December 31, 2013, 12:57:38 am
content: Quote from: NiHaoMike on December 31, 2013, 12:53:06 amThere is a variety of those too, besides it would be less fun and learned (but maybe more effective?)

author: vlad777
author_rank: Frequent Contributor
timestamp: December 31, 2013, 01:07:00 am
content: @ vk6zgoThank you, lets call your suggestion a1).Can I make it longer I have about 10 meters or more at disposal?

author: tsmith35
author_rank: Frequent Contributor
timestamp: December 31, 2013, 01:14:03 am
content: Helpful?FM Broadcast Receive AntennaOrThe informal antenna survey.

author: vlad777
author_rank: Frequent Contributor
timestamp: December 31, 2013, 01:29:58 am
content: Yes helpful thanks.BTW I am far from religious but "Jesus-Mounting" sounds wrong.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 31, 2013, 01:42:34 am
content: Quote from: vlad777 on December 31, 2013, 01:07:00 amNo,there is no advantage in that,as it is a tuned antenna.

author: vlad777
author_rank: Frequent Contributor
timestamp: December 31, 2013, 01:57:26 am
content: Cant I tune it to integer times halfwave? (just a dumb question)Look what I found googling "tuned antenna"http://www.qsl.net/va3iul/Antenna/Wire%20Antennas%20for%20Ham%20Radio/Wire_antennas_for_ham_radio.htm

author: vk6zgo
author_rank: Super Contributor
timestamp: December 31, 2013, 03:18:02 am
content: Yes,but it is doubtful you will get a lot of advantage,as the directivity changes from at right angles to thedirection the antenna runs, to much more complex patterns,so max signal pickup may not be where the FM station is.Also,even multiples of a 1/2 wavelength will present very high impedanc4es at the feedpoint,so you are stuck with odd multiples.

author: tsmith35
author_rank: Frequent Contributor
timestamp: December 31, 2013, 04:44:24 pm
content: Quote from: vlad777 on December 31, 2013, 01:29:58 amAgreed.

author: hagster
author_rank: Frequent Contributor
timestamp: December 31, 2013, 05:31:05 pm
content: This is technically called a folded dipole.this is a good site to read morehttp://www.antenna-theory.com/antennas/foldeddipole.phpPersonally I'd go for a sleave dipole as it's easier to construct and has an impedence closer to your 75ohm.

author: kc9qvl
author_rank: Regular Contributor
timestamp: January 01, 2014, 07:15:01 am
content: Use the 300 ohm to build a slim jim.http://www.m0ukd.com/Calculators/Slim_Jim/