300 ohm to 75 vs 75 ohm to 300 for homebrew antenna

[CrowsSDR]

So I'm looking to build one (or both) of the antennas on this page.

I think I understand the basic concepts, but looking for the hardware I realized that I'm getting stumped regarding the balum. I'm not an electrician, but is there a functional difference between a 300ohm to 75ohm adapter, and a 75ohm to 300ohm adapter?

The 75 to 300 looks like it makes sense in regards to the written instructions for the larger antenna, but the 300 to 75 looks like the one used in the small wire antenna.

I am most likely missing something basic here, any help would be greatly appreciated.

 

[prcguy]

i don’t think either of those antennas are worth building, they don’t appear to be designed by anyone with antenna knowledge and there are much better antenna projects to pursue.

 

[nd5y]

I think I understand the basic concepts, but looking for the hardware I realized that I'm getting stumped regarding the balum. I'm not an electrician, but is there a functional difference between a 300ohm to 75ohm adapter, and a 75ohm to 300ohm adapter?

They are both 4:1 baluns. Low impedance unbalance to high impedance balanced. Probably the same thing except the form factor, connectors, and one has screw terminals and one has spade lugs. They could vary depending on the quality of the components used and the frequency range they are good for.

I agree if you are building antennas there are better ones than the off-center fed dipole.

 

[W4AXW]

Difficult to see the point in an OCF, requiring a transformer, when a CF would work well without the need for a transformer.

 

[prcguy]

Difficult to see the point in an OCF, requiring a transformer, when a CF would work well without the need for a transformer.

The problem with the OCF in question is it’s intended to be vertically polarized and the lobes will point just about everywhere but at the horizon. It’s possible to choose lengths of wire that can provide a reasonable match to 300 ohms on several bands but it will never work well. OCFs are used all the time on HF but in horizontal mode providing both NVIS and low angle depending on the band. If you hang a 40m or 80m OCF vertical e you will have the same problem of the lowest band having a takeoff angle towards the horizon but all the other bands will point up in the sky or into the dirt.

 

[CrowsSDR]

i don’t think either of those antennas are worth building, they don’t appear to be designed by anyone with antenna knowledge and there are much better antenna projects to pursue.

This would be the first one I've built, the reason I was looking at that one was primarily because I've seen it recommended on RR, and because it seemed relatively simple.

However, I'm not married to it and will gladly consider suggestions if you have anything in mind! If you've built one antenna, you've built at least one more than I have.

They are both 4:1 baluns. Low impedance unbalance to high impedance balanced. Probably the same thing except the form factor, connectors, and one has screw terminals and one has spade lugs. They could vary depending on the quality of the components used and the frequency range they are good for.

I agree if you are building antennas there are better ones than the off-center fed dipole.

The main thing that had me confused about the hardware was that one of them was specified as "300 to 75", where the other was "75 to 300". So you are saying that functionally they serve relatively the same purpose?

 

[nd5y]

The main thing that had me confused about the hardware was that one of them was specified as "300 to 75", where the other was "75 to 300". So you are saying that functionally they serve relatively the same purpose?

The links are to scamazon, not someplace that specializes in electronics or RF parts. You can't trust the descriptions. They should be the same thing but there is no way to tell who made them or the quality of the inductors and capacitors they used or the exact frequency range they are good for.

A balun is a balun. A 4:1 balun for RF/antenna use can be 75 ohms unbalanced to 300 ohms balanced or the exact opposite depending on what you are using it for. Also 50 to 200 ohms or 100 to 400 ohms, etc.

Coaxial cable is unbalanced and parallel conductors like twin lead and ladder line is balanced.

 

[W4AXW]

The problem with the OCF in question is it’s intended to be vertically polarized and the lobes will point just about everywhere but at the horizon. It’s possible to choose lengths of wire that can provide a reasonable match to 300 ohms on several bands but it will never work well. OCFs are used all the time on HF but in horizontal mode providing both NVIS and low angle depending on the band. If you hang a 40m or 80m OCF vertical e you will have the same problem of the lowest band having a takeoff angle towards the horizon but all the other bands will point up in the sky or into the dirt.

Thanks for the lesson. Exactly the point of my question. Why use an OCF in this situation when there are much better alternatives?

So I'm looking to build one (or both) of the antennas on this page.

I think I understand the basic concepts, but looking for the hardware I realized that I'm getting stumped regarding the balum. I'm not an electrician, but is there a functional difference between a 300ohm to 75ohm adapter, and a 75ohm to 300ohm adapter?

The 75 to 300 looks like it makes sense in regards to the written instructions for the larger antenna, but the 300 to 75 looks like the one used in the small wire antenna.

I am most likely missing something basic here, any help would be greatly appreciated.

A simple, vertically polarized, Center-Fed, Half-wavelength dipole would be a good place to start. Would match up well with your RG6 without the need for a transformer. And you can learn about antennas as you go.

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