Match & Diplex

This is a bump update from some research for the Radio Project.

I’ve gotten some more clarity on the desired end game for the antenna array. Here’s what I’m looking at:

  • I’m working on an optimized discone for reception in FM broadcast (88-108 MHz), air operations (118-137 MHz), all of VHF and UHF. By, “all,” I mean everything that’s worthwhile listening to and isn’t going to cause any issues. I’m not supposed to — and I have no interest in — snooping in any bands where I have no business. This gives me a wide receive bandwidth of (in round numbers) 80 MHz to 450 MHz. Discone antennas are supposed to have a 10:1 bandwidth so I figure I can squeeze out 80 MHz to 800 MHz and not use all of it..
  • The same discone antenna will be my VHF transmit antenna for 2-meter amateur bands. Discone antennas are advertised (colloquially, on the Internet) as having an, “about 5:1,” transmit bandwidth so with a low end in the 80 MHz range, I should be able to do 2-meter relatively easily. If I design it right, tuning it to 88 MHz instead of 80 MHz as the lowest frequency, I may be able to get it to do a decent job of transmitting on 70-cm amateur bands. Either way, using the discone as a transmitter is mostly a stop-gap bridge to get me on the air and DXing enough to figure out what more I’d want to build.
  • For HF bands, I’m going to build something bigger, along the lines of a dipole. I’m vague about that because I’m not building a 160-meter dipole. There’s got to be something that’ll get me good performance given sufficient ground planes and what not in all the HF bands I want to work in. This will be a transmit and receive antenna and it’ll probably go through several iterations.
  • Once I figure out my VHF and UHF transmit needs in more detail, I’m going to put appropriate dipoles together with the HF antenna. Other than matching, they should be able to coexist if I get the physical configuration right for phasing and what not.
  • Down the road, when I’m comfortable that I have a working and stable platform, I’m going to build some lower-frequency experimental stuff. I’m leaving an empty, “slot,” for a third antenna system that is yet to be determined. That slot may be used for other experimental things as well but having the infrastructure to pop up another antenna — feed lines, a cleared pad, etc. — will be nice. By then, I’ll have experience, I’ll probably have better tools to figure out the electrical characteristics of my experimental antennas and I may very well work towards an upgrade from the FCC.

All of this together means I’m going to have three feed lines into the shack with several wide-band antennas. Those lines will feed several radios:

  • My main ham rig. I’ve got a bunch of equipment but some of it is yet to be determined.
  • The scanner I have right now. I’m not going to build anything for this because I don’t want to mess with any regulatory stuff outside bands I’m allowed to use. I’m pretty sure a scanner out of the box won’t mess with anyone else’s reception of whatever they’re receiving.
  • Hopefully there will be an upgrade to a desktop scanner at some point. Same deal as the current scanner, just a bigger and badder version of the same thing.
  • An FM broadcast radio. The regular deal, listen to music and what not. No reason to tinker with any of that since I can probably get everything I ever wanted and more from Goodwill.
  • Whatever experimental stuff I’m going to cook up. I think there may be an SDR in my future but that’s probably a long way off. It’s nice to have the headroom.

This means that I will have five radios sharing three feed lines. I need to split lots of things. I need to diplex — maybe more — to some of the feed lines as well.

This article (PDF, 718 KB) shows how to build some simple DIY splitters. For some background, this article (PDF, 348 KB) also gives great information. I haven’t processed much of it yet but it looks like splitters are similar to impedance matchers in that they’re mostly LC circuits. In the simplest sense, they’re LC filters, which means I can tune them to the correct impedance with careful work.

Over time, I’d like to design some more isolation and possibly active splitting as well. No definite plans or even feasibility thoughts in the area yet.

This does however give me hope that I can get everything I want, engineered the way I want it, and that I can probably build most or all of it myself. I’m going to have to learn how to wind coils and transformers but I’m not sure how much radio work I can do myself without that anyway. That’ll also be really useful to know for PWM-based work, like power supplies, class D amplifiers, SDRs, etc. so learning that is going to be an important next step.