June 2012 Archives

Meter Multipliers

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I bought a Simpson 1329 AC voltmeter off of eBay, which had been modified by an outside company. I haven't figured out the details of the modification yet. The meter's face reads from 0 - 1500, FS=500uA and to use an external 3 megohm multiplier.

In this context, a multiplier means a resistor in series with a meter. In current-reading meters, a shunt resistor is used in parallel. Volts = series. Amps = parallel.

The meter didn't come with a multiplier or multiplier resistor. As I don't have a need to measure voltages at 1.5kV, dividing by ten gives me a useful range: 0 - 150VAC.

Now, a FS=1mA meter movement has a resistance of about 50 ohms, so this meter, having a 500uA meter movement has a resistance of 100 ohms. In order to drop most of this 150V, we need to put a 300kohm resistor in series with the meter.

500 uA = 0.000500 A = 0.5 mA

1500V / 500uA = 3,000,000 ohms
150V / 500 uA = 300,000 ohms
15V / 500 uA = 30,000 ohm
1.5V / 500 uA = 3,000 ohms
0.15V / 500 uA = 300 ohms
0.015V / 500 uA = 30 ohms
0.0015V / 500 uA = 3 ohms

Now you know some basics on making voltmeter dividers.

Another caveat: if the meter is a rectified meter like Simpson's Rectifier series, voltage near the bottom of the scale may be inaccurate unless at least 0.4 - 0.7 V appears across the terminals of the meter. This is because the diodes have to be "switched" on, or forward biased.

Slot-Fed Corner Reflector

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Yes, it can be done.  Beamwidth is about 84 degrees from the -3dB points.

Slot-Fed Corner Reflector.png
This is interesting because when using a spacing of 1/10th (0.1) wavelength between the reflector elements, when you bridge across the final reflector elements, you have 0.75 wavelength -- which is exactly how long an Alford Slot antenna is, when designed for 0.5 wavelengths.

Kraus & Jasik, as well as other literature (Carr) suggest that the spacing between wires -- the openings between the wires -- is not less than 1/10th of a wavelength. This rule is repeated all over antenna literature when reflector elements are involved.

The Alford slot is a shorted slot antenna which radiates in the opposite polarization as the slot. A vertical slot cut 0.75m long into a longer pipe will resonate at a wavelength of one meter, as it appears to be equivalent to a half-wave dipole (f=300MHz, lambda = 1m).


Slot-Fed Corner Reflector-1.pngSlot-Fed Corner Reflector-2.png



Slot-Fed Corner Reflector-3.png
Slot-Fed Corner Reflector-4.png
Slot-Fed Corner Reflector-5.png
Slot-Fed Corner Reflector-6.png
Slot-Fed Corner Reflector-7.png
In short... it just works, as a random assemblage of related parts.


Shively Modeling

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http://www.comsistel.com/HAM%20documents/23097.pdf

Yes, it's true. Shively builds models and tries things out at higher frequencies. The above PDF shows that for a model at 91.1MHz, Shively actually built and tested the inter-bay spacing at 409.95 MHz.

Great idea, and only a few megahertz from the ham band... oh, to mix work and play....

Building Antennas from Scratch

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Midnight madness has me, and I am trying to find the answer to an idea.

So my thought is to make a four-sided corner reflector array, with each side of the 90-degree corner being an independent array.

At two meters.

So this creates some interesting problems:

First, the antenna itself will be a half-wave dipole, which means the reflector should be larger than it. So now we're looking at reflecting surfaces more than one meter tall, and one meter out from the center, which limits the options.

Second, the reflector itself has to be considered. Metal window screen? Hardware cloth? Chicken (hex) wire? Chicken wire comes in 48" widths; at 25 ft piece is $20. Then you have to get into attachment: how? Fold it over a straight-edge and clamp it? The objective is to be able to assemble and disassemble without breaking parts or needing a lot of work. Obviously, pieces that nest are great, and pieces that roll out are better.

If screen material is to be used, is it cheaper to buy a pair of screen doors and mount them at a hinge to each other? The doors are $20 each, but have the limit that they do not fold in half, limiting transportation options.

Finally, there's reflective mylar sheets, but since they do not admit air, they will act as airfoils and present windload issues. Then one has to figure out how to keep them in tension so they are reasonably flat. Fiberglass sticks? PVC is nice, but EMT is stronger. Flexibility in the core (center) shaft isn't really desired.


A ton of software

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http://tonnesoftware.com/

John Tonne has put together a lot of programs and posted them there for free. Among them is a program to make faces for analog meters, which has entirely too many options.

http://tonnesoftware.com/meter2.html

Considering what he offers, the $35 package isn't a bad idea, but you have more options than you know what to do with.

John also has a great treatise on AM demodulators and linearity. If you're researching basic radio theory, it's worth a read:

http://tonnesoftware.com/appnotes/demodulator/diodedemod.html

The basic concepts of AM and FM modulation are of interest to anyone doing software-defined radio (SDR).

Parasitic Antenna Elements

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http://faculty.nps.edu/jenn/pubs/veredAPS.pdf

I just ran across this, and I was struck by the concept of a parasitic ring element for an antenna. I've seen dipoles before, but not a ring. This warrants research, of course.... It's a neat idea for driving a dish without much interference/blockage.

http://ham-antennas.blogspot.com/2011/02/2m-70cm-open-sleeve-vertical-dipole.html

This is an interesting concept as well. The standard 2m dipole has a pair of parasitic elements added that allow the antenna to be used at 70cm. I have to wonder how far one can stretch this concept; for instance, adding elements for 915MHz and/or 1270MHz. One can probably add parasitic elements to a 10m vertical and cover 6m, or 2m elements to a 6m antenna. The question is: Do you have the real estate?

This link explains the theory a little better:

http://9amtalk.net/new2/sites/default/files/DualBand2.pdf

Adding Environment Variables Automatically On Login

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This website:
http://forums.fedoraforum.org/showthread.php?t=742

Documents this solution to auto-adding proxy or environment information to your user's profiles or shells. This works for RedHat Enterprise Linux 6.2.

create the following files in /etc/profile.d, and then this will work in *any* shell for *any* user of the system

#proxy.sh
export http_proxy=http://host.com:port/
export ftp_proxy=http://host.com:port/
export no_proxy=.domain.com
export HTTP_PROXY=http://host.com:port/
export FTP_PROXY=http://host.com:port/

#proxy.csh
setenv http_proxy http://host.com:port/
setenv ftp_proxy http://host.com:port/
setenv no_proxy .domain.com
setenv HTTP_PROXY http://host.com:port/
setenv FTP_PROXY http://host.com:port/

The files don't have to be set executable. When the next person logs in, those environment variables are automatically added to the user's environment, so the proxy is auto-configured for them.

Stupid PERC6 Tricks

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Ghetto-cloning

I've discovered, quite by accident, a reliable way to clone one system to another using only the SAS hard drive controller, the PERC6 (LSI Logic).

When a pair of drives are installed in a system, the PERC6 will 'claim' the blank drives and put some data on them for management. This data relates to the drive, drive position in the enclosure, and the drive's unique descriptors like the WWN. If these drives appear in another RAID controller's configuration, that RAID controller will consider those drives as a Foreign Configuration, or Foreign Config for short. By default, the PERC6 will not do anything with these drives.

Certain circumstances will cause the PERC6 to automatically start a rebuild if a drive is in a degraded state. Experience has shown that the RAID controller 'does the right thing' when a bad drive is removed and is replaced with a blank drive. Another way is if an array is in a degraded state and a designated or global hot-spare becomes available. However, if a drive contains data on it, or a previous configuration, the PERC6 will not automatically import that configuration, nor will it overwrite the data. In this way, the PERC6 may recover without rebooting and going into the RAID BIOS. However, for situations which warrant it, such as advanced intervention, the user is forced to enter BIOS to manipulate the settings.

In this example, I will be using three servers, Blade1, Blade2, and Blade3. These may be abbreviated "BL1," "BL2," and "BL3." For hard drive one, I will use "HD0" and for hard drive two I will use "HD1". Blade1's first hard drive is BL1HD0 and Blade 3's second hard drive is BL3HD1.

First, the desired image is created using Blade1, with both BL1HD0 and BL1HD1 loaded and configured as a mirrored RAID set. One the image been built and the drives are synchronized, the server is powered down.

At this point, all hard drives are removed and labeled for current location. Blade 2 is booted with BL1HD0 in hard drive slot one. Blade 3 is concurrently started with BL1HD1 in slot two. The remaining slots are kept empty.

Upon booting, the RAID BIOS notices a large change in configuration and offers to the user to Import the Foreign Configuration or enter the setup utility. One should enter the setup utility.

From the setup utility, a new "tab" appears in the menu at the top: "Foreign Config". Operations may normally be navigated by using "F2" to produce a menu of options and Alt+N for switching between the pages (tabs).

Navigating to the VD (Virtual Drive) tab, one may notice that, at the controller level, there is no configuration, or a missing configuration. Hitting "F2" at this point will produce a menu with a number of options, one of which is "Foreign Config", with the sub-options of "Import" or "Clear".  Take note of this: disaster is a keystroke away.

By selecting Import Foreign Config, the original drive configuration will be removed from the controller memory and a new one supplanted. Immediately, the system should show that two hard drives are configured, one is present, and one is missing.

At this point, one of the original hard drives should be reinserted into the empty slot. BL2HD1 and BL3HD0 are inserted into the respective vacancies left from earlier activity. "F5," or "Refresh" is applied after the drives have spun up.

The BIOS shows no change to the existing configuration. Switching to the "PD" or Physical Drive tab (Alt+N), note that one drive is Online and the other is Ready. Flip over to the VD page, make sure the cursor is on the Controller-level.
Hit "F2", and select "Clear Foreign Configuration".  Switch back over to the PD page and highlight the drive which is Ready. Press F2, and select "Make Global Hotspare". Select "Yes" to give priority to the enclosure. This will make the array rebuild faster. Page over to VD again and notice that the array is now rebuilding and a progress meter is present.  Rebuilding will continue into the recently installed original drive has been mirrored.

Once the mirroring process has finished, the "borrowed" drive should be pulled from the chassis. The borrowed drive will be marked as "bad" by the RAID controller, which will shift to the remaining drive to as the 'good' drive.

Now the original drives should be re-inserted into their respective systems and slots. BL1HD0 and BL1HD1 are re-installed, and BL2HD0 is installed. BL3HD1 is installed in Blade 3, hard drive slot two. The process is repeated, removing the foreign config, making the 'new' drive a Global Hot Spare, and rebuilding completed.

Blade 1 usually experiences some issues because it's drives have been in a foreign server, so one of the drives may come up as foreign even after the foreign config has been imported. At this point, the 'bad' or 'missing' drive should be manually made a Global Hot Spare. The PERC6 will start rebuilding that drive.

Once rebuilding has started, the systems may be booted, as the mirroring operation will continue until it completes without majorly impacting operations. 

Recovering From a Lost Configuration

It was discovered that a server which has a PERC6 had no configuration information present in it. Talking with the admin, it was discovered that the Import Foreign Config had already been performed. Since the server is remotely located, it is known that nothing changed about this system. The BIOS showed two hard drives in the "Ready" state and no virtual drive configured. The "Action" options were "Clear", "Create", and "Manage Persistent Cache". "Import" was greyed out.

Realizing there was no option at this point except to rebuild the machine, the "Create" option is exercised to attempt to recreate the virtual drive, which is not present in the configuration. Everything else being the same, as long as the drive geometry is the same, the system should boot. However, the BIOS would not let the administrator create a new virtual drive.

After some consternation and head scratching, this "Manage Persistent Cache" option was explored. In that option, the controller showed that it still had data in the cache from the previously defined virtual drive. Further, the verbiage explained that if data is present in the cache, 'certain operations' will not be performed.

I made the call to wipe out the cache. It was done. From here, the administrator recreated the Virtual Drive using the defaults. This process presented the least amount of change and the best possibility of not damaging the drive. The RAID controller now displayed that a virtual drive existed, and that two hard drives existed in that configuration. Further, it reported that Drive 00 (Enclosure 0, Slot 0) was failed, and that Drive 01 was fine. I told them to boot the system, and it reached the login screen; back to normal, just one hard drive down.

I hope your RAID hijinks aren't nearly as fun. I earned my fee; the trip it saved and the triage time spent on the server cost more than my hourly rate.

Amateur Radio and 'Served Agencies'

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I've been listening to Steven Lampereur (KB9MWR) rant on YouTube about ARES/RACES and "Emergency Communications". It's linked here: http://www.youtube.com/watch?&v=N7OtmQ11ssw

One of the thoughts that immediately comes to mind when faced with this "entity" which has a desire to communicate is to establish a clear demarcation, and setup a link from that point to another. This is questionably legal, however, because as amateurs, we are responsible for the content, and agencies, like many people, tend to know exactly one subject eight miles deep, and know other subjects less than the thickness of a sheet of paper thick. Anyway, the idea is this: An agency needs communications. Setup a demarc point, like a channel bank. Capture audio from the channel bank, convert to SIP, shoot across microwave/wireless/900MHz. Since the audio is SIP, many CODECs may be used, from the most bandwidth efficient, to the most bandwidth inefficient. Of course, the radio amateur is limited by the amount of bandwidth that may be used for such purpose; he should select the mode which provides the most bandwidth savings to send the given information. In the case of twenty-four channels of PCM audio, the bandwidth required is 1.536Mbit/s. Using OFDM or even MSK, this may be transmitted in less bandwidth than the average TV channel. However, by converting the above traffic to the GSM codec, only 384Kbit/s is required. SIP will add overhead, of course, but this should be minimal.

However, this creates an arguable point. The argument is many-fold, as is typical for this sort of thing. First, the amateur is now providing services which a commercial company may provide. Federal law prohibits this. However, it is definitely arguable that, in the time and space required, that a commercial entity cannot provide such service -- licensed microwave links require coordination and licensing, not to mention equipment. Short of having prior coordination and licensing in place, these cannot be delivered in a short period of time, into the middle of a disaster situation. Compliance with the law would require a high-priority given to the licensing activity, which would required GPS coordinates be fed to someone outside of the disaster zone, and monies paid to a frequency coordinator and to the FCC or a licensing consultant to actually be granted the license in a short amount of time.

Secondly, the amateur is still responsibile for the content of the traffic, but it has now scaled beyond what he is capable of monitoring himself. Does the traffic become "data" and he responsible for just the bit, or the actual content of those bits? Is his license in jeopardy if his "customer" fires up a VPN connection over an IP link he has set up? Where is the line drawn? Does the amateur become a common carrier? I would think not, since common carriers are not responsible for content, yet amateurs are.

There are many ways to go with this, unfortunately, we need to do the one thing we are loathe to: poke the bear (the FCC) and ask for guidance.  

GE S-950 or S-990 Control Heads

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http://kb0nly.info/s950_s990_modifications.html
http://www.mindspring.com/~dchopper/s990/

The modification listed in the above websites has a few caveats which are not explained well. In order to effectively bankswitch 128 channels, the channels are divided into eight "modes" in GE parlance, or "zones" in Motorola parlance. In Midland-speak, they are "groups". To many of us, they are "banks". To reach 128 channels, the control head switches either four banks of thirty-two channels or eight banks of sixteen channels. When the bank is changed, the radio is reprogrammed by the head with the contents of the bank selected.

My process for preparation was to burn the "new" firmware to an EPROM, insert the firmware, and to pull the EEPROM (codeplug) and wipe the codeplug.

Big no-no.

The instructions say that the head should already be programmed. What they don't say is that if you want or intend to use the full 128 channels, you must start with a codeplug that has 128 channels defined. Finding myself with a blank codeplug, I dug around to see what I could do to make a codeplug.

Sometime ago, I found a copy of the Niles programmer software. Supposedly, this was available for free download or some such, so I downloaded it and stored it away. I found that I could enter a few channels into the Niles software and it would create a codeplug image that I could program into the EEPROM using an external EPROM programmer.

So I programmed the EEPROM with a few channels, plopped it in the head and started poking around... and found that I couldn't access banks beyond two. This was, as I mentioned before, because I didn't have 128 channels programmed into it. Pull the EEPROM, jump in the car, back in front of the computer, to define 128 channels. When selecting a bank, two options are listed for VCO -- the 138 - 151 MHz version, and the 150-174 MHz version. Since this was a 150-174 MHz radio, I chose that option. Burn the EEPROM, get in the car, drive to the radio, plop the codeplug into the head... fire up the radio and... VCO unlock.

So I started dorking around with it, and paged through a few memories to make sure the information was correct (and it was), then hit the key to exit... And noticed that the VCO was now in lock.

Apparently, something about reviewing the channels, one by one, corrected a programming error in the codeplug that was in the EEPROM. So one channel at a time, all 128 channels, to review and reprogram the banks until all the channels worked.

So keep that in mind, if you run into issues with the head. Sometimes it is smarter than you give it credit for. Enjoy.

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