January 2014 Archives

LSI MegaRaid Quickstart on Linux

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Install some variant of MegaCli from RPM.

cd /opt/MegaRAID/MegaCli
./MegaCli64 -AdpAllInfo  -aAll
./MegaCli64 -LDInfo -LAll -aAll
./MegaCli64 -PDList -aAll

Cross-correlate as necessary.

See also: http://hwraid.le-vert.net/wiki/LSIMegaRAIDSAS

What to do when you can't get a repeater pair

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Today I became aware of an additional FM repeater pair available to ham radio operators, but it comes with a caveat of not using a standard offset. Hams, unless heavily encouraged, tend to avoid repeaters with odd spits. In Alabama, we have had several in the last twenty years, three of them in Huntsville:

Name / TX / RX
W4CFI 147.415 146.415 or 146.416
MSFCARC 147.460 146.460
KC4HRX 147.505 146.505

The 147.415 repeater is on the air, but has a slightly less or slightly more offset due to interference on the 1MHz "split". 147.505 is on the air and operational, part of a three repeater linked system covering most of the Huntsville area. MSFCARC's VHF repeater was taken out of service years ago.

In places where 70cm FM pairs are in short supply, a FM repeater can (but not necessarily may) be installed in the TV segment of the 70cm band (420-450MHz) if there is not an existing repeater present -- or even if there is. The repeater may be designed to require CTCSS to prevent "interference" from the co-channel users, but CTCSS may not be required if other methods of isolation are used, including physical separation between repeaters. Since ATV normally uses horizontal polarization and FM repeaters commonly use vertical polarization, this provides additional isolation, depending on the antennas selected and antenna mounting locations, as well as the access method required to make the ATV repeater repeat.

CATV Audio carriers in the 70cm band:
Ch 57 425.75
Ch 58 431.75
Ch 59 437.75
Ch 60 443.75
Ch 61 449.75

Each CATV audio carrier is 6MHz from the previous or next one. If user is transmitting at channel 60 to a repeater input and receiving a repeater output at Channel 57, the split is +18MHz from the user perspective. Many of the cheap Chinese-made radios (like the $30 Baofeng UV-5R) will easily handle such a split; some commercial radios may be able to as well. For a ham rig, it may require more than a few minutes with CHIRP (free / open-source radio programming software) and/or the radio's manual to program a channel with a split of 18MHz.

An 18MHz split FM repeater should be implementable without an interdigital filter, possibly without cavity filters. A pair of sharp bandpass cavities would be appropriate for the FM repeater, but a 6MHz ATV repeater will require the interdigital and vestigial sideband filters by nature.  If the ATV repeater was implemented as a 6MHz-wide linear transponder (like HARC/TVATV's was or is), the transponder may be used to repeat data using a Xagyl or Doodle Labs fractional OFDM/802.11g Atheros cards (built-in 420MHz transverter) in the 5MHz wide mode at 1-6Mbit/s or so.  A FM repeater could still be installed by using cross-polarization, but both repeaters would require hardline, and the maximum cross-polarization isolation would likely be 20dB, possibly approaching 27 or 30dB or more depending on the antenna(s) and implementation.

Interference between repeaters would largely be a function of transmitter power and the FM capture effect. As long as one carrier at 12dB or greater than the other can be maintained, only one signal should be apparent to the user. Cross polarization would help here, as there is about -3dB of attenuation with linear polarizations versus circular polarizations, and often up to 20dB or more for orthogonal modes. If vertical and horizontal separation is used -- perhaps even shielding around the antenna with a non-corroding metal, such as aluminum or stainless steel, depending on the different antenna types in use -- then 20dB or more so should be certain.

This implementation idea may be of more use for a data mode vs a data mode over the air. That is to say, a digital TV mode such as 8PSK, ATSC, OFDM, QAM, or other complex modulation (spread over 6MHz) comprising the signal in one polarization, and a FM-based FSK, GMSK, PSK, QAM or OFDM signal in the other polarization. Energy over noise temperature or Eb/N0 will provide the best estimates from the modelling side as to required signal-to-noise ratios for each modulation type.  Of course, FM vs FM interference is has a predictable result, as we've heard on two meters when two users simultaneously transmit to the a repeater.

Another plus of this concept is that one can put those "Federal Split" (403-435MHz) FM radios to use for one side of the repeater, and the more familiar UHF business band (440- or 450- 470MHz) radios for the other. The radio, by design, will be naturally less sensitive at -/+10MHz outside of the operating or design limits, and the transmitter harmonic filter will also attenuate energy that could cause desense to the receiver. For certain radios, this may be enough isolation to allow external filter-less implementation -- provided the RX filter doesn't melt down and is properly phased (one half-wave) from the combination point to minimize VSWR issues to the transmitter.

The military guidance from a publicly available Army communications manual says to use different polarizations, as much physical separation as possible in both vertical and horizontal space between antennas, and not less than 10MHz separation between adjacent radio frequencies. But then again, the military does very different things with it's radio systems, including running secure voice circuits over AM, SSB, or FM radio signals, and the modern radios support that capability with a single cable that runs between radios.

Strange, and unorthodox, this capability to implement such a repeater has been available for several years now and seldom if ever has it been taken advantage of. Thank you for reading, and thanks in advance for any feedback or comments.

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