Log periodic antenna to Coaxial cable... balun right?

[eccerr0r]

I acquired a (large) used rooftop log periodic broadcast reception antenna and it didn't come with wiring. I figure I could use it for parts if nothing else but first I'd like to see if this thing actually works for receiving distant stations.

From passing from amateur knowledge, these log periodic antennas need to be matched to the impedance of coaxial cable, nevermind the balanced nature to the unbalanced nature of co-ax. So I'll need to locate a balun.

So I was wondering about people who worked on rooftop terrestrial broadcast antennas...are these log periodic "dipole" typically 300Ω and I'll need to get a 4:1 balun to attach to 75Ω co-ax? Should I feed twin lead 300Ω down to the house and have the balun inside the house or is it just like transmitting antennas where it's best to have these matching networks close to the feed point?

I just so happen to have a 4:1 300Ω to 75Ω balun but it's not sealed for outdoor use, so would be best to keep it indoors or perhaps take it in when it rains...

[CapLeaker]

Re: Log periodic antenna to Coaxial cable... balun right?

Not all log periodics are 300 Ohms. Kind of really need to know what it is. Let’s say it has a 300 Ohm feed point. You could use 300 twin lead and run it inside under the roof, then use that 4:1 balun and run coax to wherever you want to.

[R_J]

Re: Log periodic antenna to Coaxial cable... balun right?

One of these should work

[eccerr0r]

Re: Log periodic antenna to Coaxial cable... balun right?

hmmm what are typical log periodic impedances?

I need to look for a large pipe or something I can use as a mast, and wonder how high I need to have it to exceed performance of indoor rabbit ears... Roof mount would be best but not for now...

[CapLeaker]

Re: Log periodic antenna to Coaxial cable... balun right?

The most typical is 300 Ohm for those TV antennas, unless there is a tuning stub or something. If there isn’t any matching mechanism, it plain 300 Ohm. Now if you use twin lead 300 ohm cable, it has less losses an works over a wider frequency range vs coax. As soon as you put that balun in there and go coax, you have more losses.
Coax advantage: 75 Ohm, unbalanced, shielded, can be laid anywhere. Disadvantage: higher losses.
Twin lead advantage: 300 ohm, balanced, works over a very wide frequency range, less losses. Disadvantage: unshielded, cannot be laid in metal pipe, or touch metal etc. It needs holders of some sort.

Years ago, I put an antenna like that in my barn roof. Look around, people are giving away old TV towers.

[eccerr0r]

Re: Log periodic antenna to Coaxial cable... balun right?

Due to the HOA and the government provision to not deny me outdoor OTA antennas, but towers are a no-no. This antenna I picked up is fairly large, so I wonder how well it performs though as a log periodic, it's not a yagi...

I never thought of running it in my roof alas I don't think this is an option. My roof access hole is like 2x2 and this antenna is like 10 feet long, 4 ft high, and 8 feet wide... have to do some serious disassembly to get it into the roof...