Bipole green banana

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grey4trains

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Dipole green banana

Greetings,

FTposter...pretty new at this.
I 've decided to build a dipole antenna to listen to the railroad band. Receive only.
Using two aluminum rods (14.33" according to calc's.)

2 questions:
1.) Do I use 50-ohm or 75-ohm coax?
2.) Do I need a balun in between the coax and the tenna (1:1, 1:4 ???).

Probably going in the attic or at least that high. Ceiling is vaulted were the attic stops. Maybe up toward the ceiling. Can't put it outside (zoning restrictions, etc..etc..).

I'm sure this thread probably exist somewhere on the forum but any help would be greatly appreciated.

green in the bluegrass...grey4trains.
 
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KC0QNB

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you could try a standard 300:75 balun and 75ohm cable use good cable however like rg6qs.
how are you going to mount the dipole?
 

prcguy

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So Cal - Richardson, TX - Tewksbury, MA
A half wave dipole will match ok to 75 ohm cable so don't use a 75 to 300ohm balun. You should run the coax sideways away from the dipole for several few feet before running vertical to avoid interacting with the antenna pattern. If you use something larger in diameter for the elements it will probably cover the entire VHF band just fine.
prcguy
Greetings,

FTposter...pretty new at this.
I 've decided to build a dipole antenna to listen to the railroad band. Receive only.
Using two aluminum rods (14.33" according to calc's.)

2 questions:
1.) Do I use 50-ohm or 75-ohm coax?
2.) Do I need a balun in between the coax and the tenna (1:1, 1:4 ???).

Probably going in the attic or at least that high. Ceiling is vaulted were the attic stops. Maybe up toward the ceiling. Can't put it outside (zoning restrictions, etc..etc..).

I'm sure this thread probably exist somewhere on the forum but any help would be greatly appreciated.

green in the bluegrass...grey4trains.
 

kb2vxa

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Point Pleasant Beach, N.J.
First of all your calculations are off, each leg is 19". Either coax will do nicely, no balun needed, just orient it vertically and route the coax away horizontally for a few feet before routing it otherwise.

You may avoid these complications by building a ground plane out of a chassis mount SO-239 connector and 5 pieces of bare 12ga copper wire each 19" long. Since it's going in the attic I suggest making the vertical element last (after the radials are measured and cut) and put a small loop in one end before cutting it to 19" overall. Then drive a nail into the spine of the roof and hang the antenna from it with a small piece of twine.

A word to the wise about coax; always observe the minimum bending radius specified by the manufacturer to avoid conductor migration. This is VERY important in a hot attic environment where the dielectric will soften and hasten the process. There I suggest at least doubling the radius.
 

grey4trains

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Jun 6, 2008
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Thanks for the help guys. Didn't realize my calculations were off a bit. I have a preliminary antenna built that I'm using in the vaulted ceiling area of my house. Works decent...Can pick up train chatter from about 20 miles it seems. Of course, clarity is not that distinct from that distance. Gonna go with 19" legs to see what that does.

Thanks again,
M. Grey
 
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