Total posts : 45366
“Thanks much for your willingness to share your adventures and for the great writeups with pics. Some of the best technical advances happen when investigators share the step by step investigations during development rather than waiting until everything is perfect and polished. For example, anyone wanting to follow your lead knows the small wire is not optimum and can save $$$ and time. You also are getting feedback which we all hope is useful.
So far as feedback, it’s always a mixed bag. It’s good to read it and learn from all the assorted sites and measurements done under as close to lab conditions as possible, so long as it doesn’t stifle your own spirit of “try it and see” in the non-optimal real world.
I’m fairly sure that most of the conditions mentioned for measuring the Q of *very* high Q coils would be nullified to at least some degree in mounting it as part of a physical antenna. Particularly so with an indoor antenna where even if we discount how the parts of the rest of the antenna would damp the Q, there are likely to be objects like walls made of diverse building materials and movable parameters like people walking near to it.
My interest in the spider coil was initially that it is physically not as large as the solenoid coil. With an indoor antenna, in many cases people’s houses don’t have 3 meter high ceilings. So if a person had to make do with say 6 ft of vertical radiator (allowing a foot or two for some sort of stand for the antenna system and also a bit for the wire to ground) , taking a foot or so of that height for a solenoid coil would have a more significant impact on how much radiator is left to work with than a spider web coil that might take up an inch or so of the available space. The general concensus has seemed to be that loading coils do not generally radiate and so it seems most people don’t count them in the height with an outdoor antenna, but with an indoor antenna the ceiling simply may not be that high.
One of the first experiments (albeit a crude one) that I did with the spider web coils I built was to bring metal objects near while watching the inductance readout on my meter. I noted that non-ferrous metal (a roll of aluminum foil from the kitchen cupboard) moved the inductance down when brought near, while ferrous material (the crescent wrench seen nearby in one of the photos) moved it up. Which was as expected. BUT.. The shift in both cases was only pronounced when the object was brought near the “hole” in the center of the coil.. there it started to shift uH when a few inches away.. Bringing the metal object up to the edge of the coil didn’t shift the inductance even 1 uH until I was within a couple wire diameters. Whether the object was held in fingers or hung from a bit of twine didn’t seem to make a noticeable difference.
I haven’t wound a similar solenoid coil to compare how it “picks up” the presence of objects compared to the spider web coil, but my current logic based on past experience with the small spaced-turn air core coils used at FM frequencies is that they are very sensitive to the presence of a hand near the winds. Might be more a VHF thing, might not.. But that’s what experiments are for. If the spider web coil is better than a solenoid coil for not shifting inductance (or maybe other values, since I’m assuming the Q is being affected long before a shift in inductance of the degree I was looking for is observed) when objects are near or pass near, that might be another advantage to consider for indoor antennas.
Also I picked the spider web because it is fairly easy and inexpensive to construct as an air-core coil. With the typical solenoid coils used with the sort of outdoor antenna usually made for the SStran, I’ve wondered how much the tuning is being affected by moisture/dampness. A formless air core coil would dry quicker after rain or condensation and as such might be more consistent. I’d say the 22 gauge “coil 2” I made “might” be physically robust enough for outdoor mounting. But a heavier gauge like 18 or 16 would probably hold up pretty well outdoors. The current project is an indoor antenna experiment, but I hope that some of it is also applicable to an outdoor antenna this summer.
I learn best by tinkering. For example, all the things I’d read about “stub tuning” with a piece of coax made a lot more sense after I got a cheap capacitance/inductance/resistance meter and nipped some bits off some old rg 59. And slipping a piece of coax into a piece of brass tubing that was a fairly tight fit and tinkering with the coax’s braid and the tube hooked in parallel to the tuning cap of a cheap transistor radio made the concept of a “trombone” capacitor considerably clearer for me and was also quite entertaining. (Ok, so maybe I’m easilly entertained. LOL)
Projects that can be tinkered together have a definite place in the hobby if we want to encourage learning and more (and/or better) part15 stations. I have *some* tech background, but far less than some folks here. Many people that might look into part15 have more of an interest in dj-ing/programming than in engineering. But I’m pretty sure most of them could manage at the very least putting together a salt-box or oatmeal-box crystal radio receiver from clear instructions if that was the *only* way to get an affordable receiver to listen to radio on. That’s kind of the situation they’re in with part15 antennas. They could order an outdoor one from Carl the antenna guy maybe.. But for indoor, I didn’t find much of anything out there on the net at all. Not for sale *or* for homebrewing. Summer is a ways away yet here in the northern part of the US and for some people who may have an interest in the hobby, even when it gets here they might not have a yard/situation that allows for an outdoor antenna anyway. If they don’t have a yard or they’re in a bad neighborhood, an indoor antenna may be their only option.
Now while it does take a bit of soldering ability to homebrew or build a kit, a person can solder well enough (or teach themselves or find someone to teach them) if they want to get on the air badly enough, but still know nothing at all about antennas. “Optimal” may not be in the reach of everyone, but “better” almost always can be. Especially when compared with a simple short wire indoor antenna. A lot of people learn best a bit at a time as they go along, and even “a bit better” can be a powerful and fun motivator. If we want more people in the hobby, those are people we have to reach. And the easiest way I can think of are easy inexpensive projects they can use with their stations.
Crystal radios have been making a comeback, from all I can tell. I’ve looked at a lot of crystal radio sites and forums, especially lately.. And you know the one thing I don’t think I’ve ever seen on even one of them? I don’t think I’ve seen *anybody* telling a newbie to “not bother” with making an oatmeal box radio if they can get the parts easily. Oh, yeah, some of the sites and discussions are very tech heavy and some of the advanced DX designs could scare off any newbie. But the overall tone of that community seems encouraging to “hands on” and even very simple projects to learn solid basics if that’s what it takes to get people to learn and do. We need that for part15 if we want to keep enough people involved to keep the hobby alive and growing in good ways.
It goes without saying that we also stress and reinforce the legal requirements and rules and responsibilities, since transmitting stations (even very small ones) need to have those priorities where someone with a crystal receiver does not. We do plenty of that here, to the point where a newbie might find it redundant or tiresome. Heck, I think *all* of us find it tiresome at least once in a while. LOL But it is a necessary part encouraging *responsible* operation of a part15 transmitter/station. Better to say it too many times than to say it too few and have someone unknowingly put themselves at risk for fines and etc.
So part of my indoor antenna project will be taking into account the 3 meters total for the antenna, feedline and ground wire. May not work everywhere or for every situation, but at least *one* way it can be done. I’ll get to that as I figure out a practical way of doing it with easily available materials, though.
I think I’ve monopolized the soapbox for long enough just now, though.. LOL