Building a Yagi for FM

FM antenna discussion
Post Reply
Fmscanner
Posts: 2
Joined: 14 Oct 2020 18:39

Building a Yagi for FM

Post by Fmscanner »

Hi guys,

I am building my own Yagi antenna for FM, have cut the elements to the sizing etc but I am just wondering something.

Have seen a guide where it says I need to make a balun, to do this I can use a run of coax. I need to make this one into a U shape and connect all the shields together, then connect all center conductors together but place one on each side of the dipole.

My question is, where do I then put the shield? and what if I don't use a balun? Because my setup right now is an open dipole and I didn't use a balun there, just connected the sheild and center to separate sides of the dipole and it seems to work fine!

One last question, when mounting the Yagi vertically, am I right in thinking the end of the boom attaches to the pole, as opposed to attaching it in the middle?

Regards,

Matt.

daveB
Posts: 1686
Joined: 07 Oct 2014 16:49

Re: Building a Yagi for FM

Post by daveB »

Hi Matt,

With your new aerial - is it a folded dipole, or is it still an open dipole? From how you describe the guide the balun is for a folded dipole, but the connections you describe don't sound right
just connected the sheild and center to separate sides of the dipole and it seems to work fine!
"seems to" is the operative phrase. Check out this web page - lots of info including an explanation of why a balun is needed.

http://ham-radio.com/k6sti/balun.htm

Well down the page it describes a 300 ohm half-wave coaxial dipole - which sounds like what you are describing. if you "google" 'half wave coax balun diagram' you'll find some clear diagrams. The shields are only connected to each other and must be insulated from the dipole connections.

If your new aerial is an open dipole you need a different type of balun - also described in the above web page.

And finally - yes you don't stick a metal pole up the middle of a vertical dipole (unless you just want to listen to local radio) as it messes up the directivity. The support for a vertical yagi ought to be 300 mm / 1 foot or more behind the reflector - any closer and it messes the f/b ratio. I've modelled this on the 4nec2 program when building a Moxon vertical aerial.

All the best on your construction project.

David
Location: Sittingbourne, Kent JO01ji
SDRPlay, AirSpyMini, RTL dongles, ST311, Yamaha TX930. 5 ele yagi 20ft agl - 70 deg. FM5 - 80 deg, 5.1 Yagi 16ft agl - 120 degFM5 -180 deg. Vert 3-ele Moxon - 80 deg. Shielded magnetic loop 0.1-200 MHz - 10 ft agl

User avatar
Julian Hardstone
Posts: 571
Joined: 17 Sep 2014 20:49
Location: Stockport
Contact:

Re: Building a Yagi for FM

Post by Julian Hardstone »

Good advice Dave. From practical experience and several threads on the old Skywaves group I can suggest that a small vertical Yagi will retain most of its rather modest directivity on a metal mast if mounted at the top of the mast. There is an optimum position but half way between dipole and 1st director is good enough

Fmscanner
Posts: 2
Joined: 14 Oct 2020 18:39

Re: Building a Yagi for FM

Post by Fmscanner »

Thanks for the replies, yes it's a folded dipole.

Ok so I connect the shields together and then just leave them be. Because someone told me I should connect the coax shields to the boom of the antenna or to the pole itself.

Sounds like I'm better off mounting it vertically then, or perhaps I can mount it at a slant, so it's still vertical but the elements are away from the pole.

Post Reply

Return to “Antennas”