Tuesday, March 29, 2005

Ten-Tec Orion S-meter & Hamlib

We have been making some progress with the Ten-Tec Orion backend software in the "Hamlib" package, an open-source project to provide a device-independent programming interface for amateur radio equipment in the Linux/POSIX world.

Here's a tid-bit you won't find everywhere: the Orion S-meter calibration curve.
#define TT565_STR_CAL { 15,  {
{ 10, -45 }, /* S 1.5 min meter indication */
{ 13, -42 },
{ 18, -36 },
{ 22, -30 },
{ 27, -24 },
{ 30, -18 },
{ 34, -12 },
{ 38, -6 },
{ 43, 0 }, /* S9 */
{ 48, 10 },
{ 55, 20 },
{ 62, 30 },
{ 70, 40 },
{ 78, 48 }, /* severe dsp quantization error */
{ 101, 65 }, /* at high end of scale */
} }
The first column is the Orion's response to the "?S" command in computer units. The second column is indicated decibels (dB) as read on the actual S-meter.

(Literary event: I just defined "S meter" in the Wikipedia! See my article.)

The Orion has a couple of peculiarities when measuring power. The minimum reading (on my unit) is about S 1.5 on all bands when looking at a dummy load. What does that mean? If S9 = 50 µV, then S1.5 is 6 dB x 7.5 S-units or 46 dB down in power = 2.5 x 10-5. The voltage ratio is the square root of power, or about 5 x 10-3. So the equivalent RF voltage level is 50 x 5 x 10-3 = 0.25 µV. The Orion's sensitivity spec (main receiver) is "< 0.18 µV" says that the receiver's internal noise will be about this level, which is consistent with the meter reading. In other words, Ten-tec seems to have this right.

(On the other hand, the tried and true RST system says that S1 means "Faint signals barely perceptible". That's an argument for using 5 dB per S-unit, as some have defined it, at least with our current generation of receivers.)

The other feature of note on the S-meter is the behavior at very high signal levels. As you can see from the calibration table above, the raw S-meter units are spread out roughly 8 units per 10 dB. However, the meter reading is highly quantized. You only see values of S9+48 and S9+65, nothing in between. This indicates that the software computation of received power has a quantization problem. Of course, it's hard to complain too much, because such power levels are almost never seen in real life. (If they are, better switch in some attenuation!)

Monday, March 21, 2005

Toward a Provisional Philosophy of Grounding

About grounding. There's so much information and advice out there that is sometimes contradictory. Partly this is what it means to be a radio amateur. Some folks have some credentials, but it's hard to know how to interpret what you read.

My background is in Physics, Radioastronomy, EE, and IT. So I have some textbook knowledge of electricity & magnetism and some practical experience with radio telescope systems. I would not claim to know much about lightning protection, but I know enough to be skeptical.

The basic problem is that few if any hams are able to test their grounding systems. Some people survive some dramatic "events", and their equipment sometimes does and does not. But in most cases there's no statistical significance. The commercial tower people probably know a lot, but they don't try to integrate RF systems into their residences. A few hams in high-risk locations survive multiple strikes every year. They're the ones to listen to.

There are some general principals that can guide us. Mainly, you want to provide a low impedance path to ground from "up high" that will not pass through you, your house, or your equipment. In a residential environment, there are always compromises. The ideal ham shack would be a windowless metal building on its own pad with a carefully laid out grounding grid and protection on every wire coming in and out. (Like a cell phone hut.) Failing that, it is probably a good idea to have as many ground rods out there covering as much territory as possible. If you don't "bond" your ham ground to your AC ground (and other lightning protection grounds, if any), you run the risk that there will be high potential differences between them during an event. What would happen if your AC plugs (all 3 wires) suddenly were at 20 kV relative to your equipment?

My local SPG system is an attempt to establish a locally bonded system that will forestall large potentials from occuring between anything near my ham gear. I will try to connect it to the AC entrance ground and a new ground rod system, difficult as that is on a granite ledge.

I don't have confidence (in a statistical sense) that this will work against the likely threats. It was fun to build. At least, it will improve the RF and AC safety grounding by a bit.

Read below to see the project at my QTH.

Ham blogging

After working with web sites for quite a while, I am just easing into the blogging world. WA1LOU had an ARRL Web Article (June, 2004), but it took a while to bubble up to my keyboard.

I did a search recently and turned up a number of interesting sites. See my links at the right. There's a lot of room for growth here!

From an email to Dan, KB6NU:
... Interesting what you come up with on http://www.globeofblogs.com/ when you look at their "amateur radio" category. I did register there, and now you can find me among the SWLs, podcasters, and what-not...

This is really cutting edge stuff, I have to say -- ham blogging. Ham
radio, for me, has morphed into 10% on-air and 60% Internet, with maybe 30% non-Internet reading and building. (I've been retired for a couple of years.)

Ham radio as a literary form! Unfortunately, [a lot] of hams can't relate to that. I recently looked at QST's guide for authors. Their advice about avoiding high falutin' language is quite charming. Or maybe I could find another adjective.

Saturday, March 19, 2005

In-shack Ground System

The grounding project is coming along. We now have all the radio gear, computers, and internet devices protected against a Single Point Ground, which is a bus plate on the wall of the shack. For now, the SPG is referenced only to a waterpipe (hydronic, at that) ground that is good at 60 Hz, but not for fast surges. The SPG also ties back to the house AC ground through a green wire connection to a power wall-plate, but this is not much protection.

Here is some construction information. Maybe it will help others who are contemplating a similar project.

The SPG plate will attach to the shack wall, right over the existing panelboard. A backing of 3/4 inch plywood will sit behind the copper panel.

Wall attachmentBack panel

The SPG panel itself, after lots of work.

SPG panel

Here are a few details. The custom cut copper sheet is 15" x 20" x .0647" Copper 110 = 16 ga. or 48 oz, supplied by OnlineMetals.com. They provide great service. You can see a number of 1/4-20 wingnut connections for miscellaneous ground strap connections. There is also a clamp for 1.5" copper strap, for eventual connection to a new external earth connection. The Polyphaser PLDO and ICE 348 devices are described in earlier posts below. The Hyperlink DSL protection is also visible, along with a number of PolyPhaser coax surge devices. Various antenna switches surround the panel, but are not part of the grounding system.

Tabletop Ground Bus

Using another copper sheet, 6" x 48" x .0647", we constructed a tabletop "ground distribution system" to provide a well-defined reference for the radio desk.

wide shot of table

detail shot of table

One connection bonds the ground bus to the metal office desk. Another connection goes a couple of feet across to the SPG plate on the wall. One-inch tinned braid is convenient for these connections.

Sunday, March 13, 2005

ICE 348 Surge Protector

The model 348 from International Communications Engineers is an 8-terminal device intended to protect rotator control wires. I am using 3 of these: one for a Yaesu G-1000DX rotator and two for a SteppIR 3-element yagi controller. Here is what we see when we open the box:

The MOV devices fire at voltages over 50 V. Bypass capacitors help reduce any RF problems. Construction is "basic", to be charitable. The MOVs are rated at 1000s of A of surge current, but it's not clear if the wiring will handle the rated surge gracefully. The top and bottom plates, not shown, are secured by 4 bolts to the rectangular extrusion seen here. There is no "reliable" connection to the mounting tabs, which are on the bottom plate, except compression of the alumninum plates. For a more reliable surge ground, I am using the screw terminals visible at either side.

Compared with the Polyphaser devices in the coax lines, I worry that the rotor/SteppIR protection with the ICE unit is fairly weak. The upside is that the Rotor and SteppIR controllers are relatively robust, and in the worst case they can be considered "sacrificial". Still, a multi-stage spark gap/inductor/MOV system (similar to the Hyperlink DSL device) would offer more security, at higher expense.

Saturday, March 12, 2005

Ground Map at AA6E

The figure shows the house plan at AA6E and how I am looking at the grounding problem. (Click for full scale view.)

Map of Proposed Grounding

The proposed stages of the grounding project, in order of increasing difficulty:

  1. Cross-connect ham radio "Single Point Ground" bulkhead to AC Service Ground at water pipe entrance using 1.5" or 3.0" copper strap.

  2. Develop a new buried ground system outside radio room. This would involve ground rods or cable buried in a trench in our shallow soil over rock ledge.

  3. Run a perimeter ground cable connecting the two lightning grounds, the AC service ground, and the ham SPG.
There is also an option 0, much inferior, which is to connect to the copper pipe hot water heating system that runs immediately adjacent to the SPG. This has a good DC connection, but obviously much higher inductance than other options.

One fairly strong argument for option 3 is that the tower is bonded to the lightning ground system (the bottom one), which could lead to problems if the ham SPG is not cross-connected.

6/16/2006 Note: Finally installed the 1.5-inch strap -- about 30 feet -- from the SPG panel to the waterpipe entrance ground (1-inch copper pipe). Before connecting the strap to the SPG, I measured a 3 mV AC differential. That's somewhat reassuring. "Soundings" in my lawn show that there is only 4 to 10 inches of soil above our rock ledge. That limits my ability to make a good new exterior grounding system.

Hyperlink HGLN-DSL surge protection

My DSL service is with SBC (formerly SNET). I seem to live at an electrically "long" distance from the Branford, CT central office. At any rate, I had quite a bit of trouble getting my modem to sync when the service was originally installed. Even now, while the service has been very stable, I am "capped" at 384 kbs download and upload.

[Note, 7/2008: I got upgraded not long after this. I now have AT&T "Elite" DSL, which is 6 Mb/s down and 768 kb/s up.]

As part of my station grounding project, I wanted to include my computer network in the "safe" area protected by a single point ground (SPG) and appropriate lightning/surge protectors. Devices for DSL circuits shared with POTS telephone ringing currents are not very easy to find. I decided on a Model HGLN-DSL-1S from HyperLink Technologies, available at www.SharperConcepts.com.

The unit appears well constructed, but when inserted in my DSL line, my modem immediately dropped sync and would not come back. There is a problem, at least with my rather weak DSL connection.

My modem, an Efficient Networks Model 5260, reports the following line parameters when the filter is not in the circuit:

Current SNR Margin 8.5 dB
Current Attenuation 66.0 dB
Current Output Power 12.0 dB

The modem also claims a maximum supportable transfer rate of 640 kbs with this SNR. This apparently is bad enough that SBC gives me a 384 kbs rate cap.

I tried a lot of things to get DSL back. The most significant was to replace my phone jack connection temporarily with 250 ft of Cat 5e cable -- using only one twisted pair of the four available. I didn't want to cut my 250 ft spool of cable, but the real run length from my telco entrance to the shack is more like 60 ft. Without the DSL filter, I now see the following modem report:

Current SNR Margin 10.5 dB
Current Attenuation 66.5 dB
Current Output Power 12.0 dB

Two dB better SNR is very good. That corresponds to a 711 kbs line limit, according to the modem. However, with the filter, the modem would still not lock up.

Time to take a look at the schematic: (Click for larger view.)

I was able to identify the components via Google. The Spark Gap SG is a type 3R-230 from Beijing Tehy. The spec says it has a trigger voltage of 230 V +/- 20% and an inter-electrode capacitance of <5 pF. The resistors are 3.3 ohms, 1 watt (?). The transient absorber is not actually as drawn. It is a composite of three devices, thus:

The transient suppressor seems to be similar to a Bourns CD214B "ER" type. The Bourns part has a breakdown voltage of 189 - 218 V and a junction capacitance of 2000 pF at 5 V standoff running down to 70 pF at 200 V. At typical phone line bias, 50 V, we should have 200 pF. The two cross-connected diodes seem to be similar to Diodes Inc. type FR1J, 600 PIV.

The short story is that the diode suppressors load my marginal DSL line too much. Unsoldering one end of the ER device allows the modem to sync up. I haven't run full tests to see if it is as reliable as without the filter, but there seems to be almost nothing left in the circuit that could plausibly degrade the signal.

Why does the surge suppressor cause so much trouble? The capacitance and resistance seem immaterial. Perhaps it is the non-linear characteristic with voltage that may add too much distortion.

So, will we have enough protection without the second stage of suppression? The modem faces a greater risk of being fried in an impulse event, but I expect that most of the "downstream" computer gear is protected well enough. One never really knows without full-scale and expensive tests.

Monday, March 07, 2005

The Polyphaser PLDO

The Polyphaser PLDO-120US15A Inline Impulse Suppressor is designed to protect equipment power circuits from large voltage surges. I am using it in my ham station in connection with a "single point ground" system to protect against lightning surges. The Polyphaser catalog does not give design details, but I wanted to know just how my equipment is going to be protected. Here is what I've found to date. (Click for more detail on photo.)

PLDO-120US15A Photo

The unit is solidly built. I traced the schematic: (Click for more detail.)

PLDO-120US15A Schematic

Most of the protection comes from MOV devices that begin to fire above 130 V. The inductors, by rough estimate, are about 1.2 µH. That converts to 15 ohms of series reactance at 1 MHz. If the second stage MOVs are in conduction, the inductor should provide a significant voltage drop. The series resonance of the two inductors and the capacitor is at about 400 kHz.

I am puzzling over the handling of the system grounds. The AC ground connection naturally goes to the PLDO case, which will bolt onto my local ground plane - where all the surge arresting gear is to be mounted. The AC ground is carried back about 30 feet to the AC service entrance. From that point to a solid earth connection (water pipe entrance) is another 40 feet or so. [I believe this is according to code. The situation is complicated by the location of the house on granite ledge, which rules out a driven ground rod.]

These ground paths go through the crawl space under much of the house. The ham shack antenna feed entrance and ground plane are on an exterior wall, with a possible earth ground only a few feet away. On the one hand, we shouldn't have the radio ground system with a separate ground from the AC, but on the other hand, that AC system "ground" is rather inductive and lengthy compared to a direct ground near the radio room.