Fun With Broadband Noise

Sit down, and let me tell you a tale of woe. As I mentioned in my last post, I got my random wire antenna up in the air and it seemed to tune reasonably well with my AT-100Pro. However (isn't there always a "however"?), the problem that I immediately noticed is that there was broadband noise at better than S9 literally on every band from 160 to 10 meters. The kind of noise that overloads the AGC and completely desensitizes the receiver. After coming from a QTH that was immersed in noise (but not even as bad as this), I was just about ready to kick my radio off of the bench and call it "game over". Once rationality once again got hold of me, I realized that I needed to cowboy up and figure out the problem.

The neighborhood has all of the utilities buried, so the chance of a nearby arcing transformer wasn't too likely. I hooked up the IC-718 to my 5 Ah gel-cell and hit the house's main breaker. Sure enough, the bands got plenty quiet (well, most of them at least). A bunch of running back and forth from the breaker box to the shack, and I was able to narrow the problem to either the shack itself or the living room. Using my FT-817, I was able to trace down the worst bit of noise to the outlet all of the radio stuff was on, along with the cable modem and router. I know, it's probably not the best place to put a cable modem and router, but it's the only place it can go for now.

I had to buy a new wall wart for the cable modem last week, so my mind immediately went to it. It's a Radio Shack switch mode supply, so it was immediately suspect. Unplugging the DC connector from the modem did provide a substantial decrease in noise. Fortunately for me, I did a lot of work organizing my components before moving, so I went right to my "toroids" tote and grabbed a FT114-43 core. Wrapping the wall wart cord around this core as a common-mode choke did a lot to improve the noise, but it still wasn't at an acceptable level to me. Shutting down the breaker confirmed that there was still some improvements that I could make to the situation. The receiver wasn't being completely overloaded anymore, but the noise still showed as over S9.

Now I was reduced to plugging and unplugging a different combinations of cords until I found the other major contributor to the problem: the shack PC. Just plugging the cord in from the PC to the Power Squid (handy product, BTW) caused a large jump in the noise floor, without even turning on the computer. I thought I was going to have to go to Fry's to pick up a new power supply or order a handful of FT240-43 cores, but then I remembered that I had a spare PC waiting to be refurbished for Jennifer's mom. The spare was dragged out of the garage and plugged in to the Power Squid with the same cord that I used on the shack PC. In that moment I found pure bliss: no increase in the noise floor at all. At this point, it was a simple matter of swapping the power supplies, since I know that Jennifer's mom wouldn't be bothered by the noisier power supply.

Now I'm happy to report that I have a very functional amateur radio station! Random wire antennas are inherently noisy compared to balanced antennas like dipoles, but the noise that's now present is orders of magnitude less than previously. Good ol' 75 meter SSB is still a bit noisy for comfortable copy, but CW and the other digimodes seem to get through OK. The rest of the bands are doing much better...probably about as good as I'm going to get here in the middle of the city with this type of antenna. It seems a bit ridiculous for an Amateur Extra ham to get so excited about something so basic, but when you've lived in RF Hell like I have for so many years, this is some thrilling stuff!

Episode IV (A New Antenna)

Now that we are pretty well situated in our new house, I decided it was time to give some serious consideration to what I was going to do in the HF antenna department. Given my reading of the CC&Rs, a full-on 135-foot dipole just wasn't going to be in the cards, no matter how much I wanted it. But, we must all walk before we can run, so I decided that I would just be happy with something that was up higher than 10 feet in the air and would snag me a few QSOs without too much grief.

Just before we moved, I put together a fairly nice version of the Cobra Jr. (Jr). using 450 Ω window line with 14 AWG stranded wire threaded in the middle of the insulation to provide a 36-foot three-conductor linear loaded dipole. I got a small piece of Lexan to use for the end and center insulators, which also worked quite nicely. The plan was to deploy this antenna in the attic, but as the saying goes, no plan survives contact with the enemy. When I first poked my head up in the attic, I realized that I did not have the stomach to do the dirty work required to hang the antenna. The height of the attic at the apex is only about 4 feet and the "floor" is covered in about 4 inches of that old-style spray-in insulation. I'd have to get a jumpsuit and respirator in order to crawl around up there, all the while trying very hard not to let my large body fall through the ceiling. Umm, thanks but no thanks. I guess the Cobra just going to have to wait for another opportunity.

A bit more meditation on the subject made me realize that perhaps a random wire wasn't such a bad idea. A new plan began to take shape in my brain. The shack is at the rear-center of the house. A large tree is in the back corner of the backyard about 30 feet from the shack window. A wire could be run from the shack up to one of the tree limbs without being visible at all from the front of the house. My immediate neighbors could see it in the backyard, but I don't think it would be very offensive. So the new antenna is 70-some feet of 26 AWG stranded teflon-coated wire run from an access hole in the exterior shack wall, routed up the wall by some plastic wire staples, and up to a high tree limb. This would be worked against earth ground, provided by a short, direct wire connection to the 8-foot ground stake that I just killed my hands driving into the ground.

The big problem was figuring out how I was going to deploy the antenna. The house is in a subdivision  with your standard mid-sized city lots. A few neighbor's houses are pretty near the tree, and I could easily see an errant lead weight flying through the tree limbs and into a nearby window. Fortunately, I remembered that somewhere I had a leather throwbag and 75 feet of slickline stored away. A bit of digging turned up these two gems, and gave me hope that I could pull this off. I fashioned an end insulator out of a small piece of the Lexan (1" x 2") that I cut out with the jigsaw. One end of the random wire was tied off to one end of the insulator. 50 feet of poly/dacron rope was tied to the other end of the insulator, then the slickline was tied to the free end of the rope. After about 10 tries, I got the throwbag over a high limb and back down to the ground. After getting the feedpoint end of the wire into the shack and attached to the tuner, the length of the antenna wire turned out to be just about perfect. Only a little bit of the wire was doubled back over the limb towards the ground.

So how's the performance? On transmit, the antenna seems to load up just fine on every band from 80 to 10 meters, with the exception of 12 meters. A bit of trimming will probably correct that. The receive side is a different story for a different post. I haven't made any QSOs with it yet, but I suspect that will be remedied by this weekend. Now excuse me while I go sacrifice a few chickens to the Antenna Gods...

Stealth Antenna Options

Since the ham shack is now mostly packed up in cardboard boxes and plastic totes, the only ham radio activity that I've been working on lately is research on antenna options for the new QTH. I'll have a fairly good-sized lot at the new QTH with tall trees on opposite corners. The problem is that there's a nice CC&R which restricts any kind of external antenna or wire drop to the house. So if I want to keep my new neighbors on my good side, I had better plan for a stealthy antenna installation to get me started.

Right now I've got my options narrowed down to two: an attic antenna or a steath longwire in the tree. There's a nice, tall oak in the back yard where I could probably get a wire up vertical about 60 feet or so, and let it hang down through the limbs. The biggest problem that I see with this plan is that problem of deploying the antenna. There's another house very close to the back side of the tree, which means that I stand a good chance of hitting it if I try to shoot a weight with a leader up to the top. And there's no way that my fat carcass is going to climb up more than 20 feet or so.

The attic antenna option is a bit more appealing for it's ease of installation, but not so much for the performance factor. I still think it's what I'm going to try for first, just because it won't attract any unwanted attention from the neighbors. The house is a typical one-story 70s ranch-style, so the antenna won't be any higher than about 15 feet I'm guessing. Not very appealing, but it should work OK for local contacts. I've been leaning towards the linear loaded dipole, or more specifically a variant called the Cobra Jr. There's not very much information about this antenna on the Interwebs, but I found a pretty good site by N4SPP. The antenna found on this page is more like a "Cobra Jr. Jr.", since it seems to be cut for a lowest frequency of 40 meters. I've decided to try to lash up a version of this antenna using 450Ω window line with another wire threaded down the middle of the insulation to give three conductors. I'll give an update when I get a chance to finish the antenna and maybe try it out temporarily here before we move.