Matching Antenna & Feed Line Impedance

As part of the Radio Project, I’m attempting to build my own antennas and feeders out of either transcycled or inexpensive parts. I’ve already got some parts on order from McMaster as fill-ins for missing bits for the discone antenna. I’m not in the position to make an investment in high-price feeder right now. Quad-shielded RG6 is easily available in bulk at (relatively) low prices.

RG6 is 75Ω. Just about everything out of the I-don’t-know-what-RF-is market (TVs are in that market; radio gear is out of it) runs in 50Ω. I’ve got some decent 50Ω RF patch cables from the bulkhead (through the exterior wall) but getting from the antenna to the building is going to require some length of good cabling.

The first application for the discone antenna is the scanner. It’s receive-only and I bet it’s pretty well built. More than likely, it would work okay with a 75Ω feeder. At some point though, I’d like to use the same antenna to transmit on both the 70cm (UHF) and 2-meter (VHF) bands. I have no idea what the eventual impedance of my antenna is going to be either.

I ran across an article on homebrew impedance matching transformers (PDF, 222KB) on W8JI’s website. It’s a little above my head right now but I think I’m starting to get some ideas.

For starters, I think I need impedance matching circuitry. I’m pretty sure I need two circuits, one between the antenna and the feed line and the other between the feed line and my gear. The feed line is pretty short and I’m not expecting a long run from the bulkhead to my gear so those are probably not major factors. I don’t have my antenna parts yet so I’m only considering the feed line impedance right now.

I looked at some connectors and they seem to be rated for 50Ω in all the sizes I want: Type N for most of my radio gear and BNC for the scanner. I’m probably going to have to connect the feed line through a matcher outside for a 50Ω bulkhead and whatever other stuff I need to do on the shack end.

There are really easy thing you can buy to do this all for you. Mini Circuits seems to have exactly the right thing except it’s about $3 per piece with a 20-piece minimum order. And it only does 1W. And it’s surface mount.

I’m probably going to have to build my own balun (matching transformer). I may have to build two of them. It looks like I can get fairly good results from the right number of turns of the right kind of wire on the right core. I don’t have any practical experience in selecting any of those correctly (yet).

The turns ratio is the first thing. The ratio of the number of turns is the square root of the ratio of impedances. In my case, that’s the square root of two over three, which is some nice irrational number (thanks, both two and three, for that). I think I can probably get away with something close, like 5:4.

I’m going to guess that the wire is probably less important. From what I remember, there are three things to pay the most attention to in a transformer winding: inductance from the winding, DC resistance and parasitic capacitance between turns. The inductance is easy. It’s the thing I want. More on the dielectric in a minute. The DC resistance can probably be solved by using thicker wire like #18 or #20 instead of #24 or #26 that people usually use for transformers. Thicker wire usually comes with thicker insulation and from what I remember capacitance is inversely proportional to the distance between two conductors. Capacitance is also dependent on the size of the conductors but that’s increasing way slower than the separation due to the insulation so I think I can safely ignore that for now.

W8JI talks about heat being the largest factor for selection of a core, both in terms of how it affects saturation and in terms of the longevity of the device. I think that using larger wire I’ll need a larger core which should dissipate heat better. I have no idea at this point on how to select the right core. I’m pretty sure I want something torroidal because I think that’ll produce less stray RF (interference) and I’ve got a lot of other wireless gear that may not work as well with lots of interference. Plus I think that it’s just generally bad anyway and it’s my job to reduce it.

That leaves me with core selection, which includes getting the right dielectric. More research on this is to come.

Incidentally, I’ve realized that I’m not going to get through this without having an antenna analyzer. I looked at some of that stuff a few days ago and the ones off the shelf are expensive. It’s critical test gear which my gut says I should probably buy, but one that covers HF through UHF looks like it’s around $1,000. That doesn’t even cover lower frequencies like MF and LF (or even ULF) that I’d like to one day tinker in a little. It’s also way out of my price range.

I’m going to need a dummy load too. I did a little research into that too. That seems like it’s really easy and rewarding to build.

Thanks also to Bruce Conti for this article (PDF, 913KB) that explains some things about how to figure out values for baluns as well as how to test them. The notion to go to Mini Circuits to even look at that option came from there too.

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