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.

The Practical UHF/VHF Discone Antenna

I hope this will be an evolving topic and that this will be a part of my final antenna array.

In the short term, I want to pick up better signals in the VHF range with my scanner. Primarily, I’m trying to receive aircraft frequencies. These are around 108-130 MHz, just above FM broadcast radio and just below the bands for amateur and public service use. I’d like to be able to pick up public service broadcasts too (police, fire, EMS, etc.) and FM broadcast radio would be nice. If I have to pick between the two, FM broadcast will probably be more interesting over the long term but public service, amateur and open-use (FRS/GMRS) are much more interesting in the short term. This means that right now I want a good VHF receiving antenna.

Over the longer term, in addition to the short-term goals, I’d like to be able to receive and transmit on amateur channels in the VHF (2-meter) and UHF (70cm) range. Getting better reception in the public service band may be useful, especially if I end up volunteering with the local fire department. I’m not going to be doing any transmission on those channels or FRS/GMRS channels using this antenna; it’s much easier (and possibly against regulation) to use a pre-packaged radio and antenna in those bands. The same pretty much holds true for aircraft frequencies: if I have a need to transmit on those frequencies it’ll be in an airplane that has a radio and antenna just for that. My desire in the aircraft frequencies is local for an airport about 50 miles away so getting better reception than that isn’t really important to me. I have no interest in transmitting on FM broadcast channels — if I want to do very low-power transmission there it’ll be for a different project entirely — but having an antenna that can pick up decent broadcast radio from 120 to 350 miles away would be a fantastic add-on in my geographic location.

Transmission Power

Although not in the short-term goals, I would like to keep an eye out for being able to eventually transmit in amateur VHF and UHF bands. The maximum ERP without any evaluation according to FCC Part 97 looks like it’s 50W in VHF and 70W in UHF. Intuitively, that may be a good deal more power than I want to transmit on given the cost and complexity of equipment and cost of power.

On the other hand, I don’t want to build an antenna that I can’t easily get 10W of ERP out of on 70cm.

Concerns & Goals

Cheap is the first concern. I’d like to transcycle parts from things I have around the compound. That’s a good amount of material although I’m more likely to buy certain kinds of stock — like machined metal — than make it myself, especially if it’s cost effective or provides relatively little gain.

This is one antenna that’s going to be used for several purposes so splitting it properly is a concern. I’d like to avoid switches, which is probably going to present an interesting electronics problem. Having a jack that I can plug into a broadcast radio receiver along with separate jacks for a scanner and an amateur rig, for simultaneous use, is a target.

Physical location is another big concern. I’ve got a very clear, high-strength post that I can mount anything on. It’s about 30 feet up on what’s probably my highest ground. That’s also the largest possibility of a colossal eyesore if it’s not done right, which I wouldn’t be keen on. Building something shorter and on lower ground is possible but there are some 40-foot (and taller) trees that I don’t want to cut down. There’s much less eyesore concern for shorter antennas, which may end up directly competing against the RF constraints.

Projects

At the outset, I’ve got several things I need to figure out. They’re all somewhat correlated but I’ve broken them down into specific problems I’d like to tackle.

I’m going to need to build the antenna itself so I’ve got the discone antenna design project. This is everything between the ether and a jack for the feeder line that runs to my shack. It includes the antenna itself and the feed line down the mast.

Mounting the antenna in a weatherproof and safe way leads to the antenna site selection project. I expect that this one will be the longest-running of all of these projects because it’s the most exposed to the elements. It’s also very important for any other antennas I’m going to have because they all have to play nice together.

Feed line can be expensive and I don’t want to spend more than I have to. This gives rise to the cheap feeder project. I could easily go buy a lot of LM400 but that would cost too much. Ideally, I’d like to get closer to the performance of LM400 with something like RG11. Obviously, I’m bucking engineering here so I’m going to have to figure out a lot to optimize what I can get cheaply and figure out where the sweet spot is in how much needs to be spent for the results I want.

Since there’s going to be rework to get things closer to ideal, I need an antenna system benchmarking project. In short, this is something I can plug into the jack on the wall instead of a receiver or transceiver and get reasonable figures of system performance from the jack to ether. Figuring out what I need to measure is the first step, then I’ll need to be able to measure it so I can record how rework changes things.

I want to split one antenna for multiple uses to I have a feeder splitting project. There are many ways I may be able to tackle this problem and some are more desirable than others.