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Re: Direct Sequence Spread spectrum modulation
On Wednesday, November 4, 2020, 08:53:56 AM CST, James Amos <jimamos@...> wrote: [snip] > Several mentions were made to summing the different multipath rays for better detection. This >does not work on a single antenna, as the singles get summed at the input! [snip] This will be more clear with some numerical experiments, but those will take a while to find time. The general model of multipath arrivals at a single receiver in Z transform notation of sorts: D(z) = S*A0*s(z) +S*A1*s(a*z) + S*A2*s(b*z) + ... with S signal, A0... being path strength and s delay with exponents a,b... over path. If S=M*PRN where * indicates convolution, then if one one crosscorrelates D(z) with PRN, one then gets the bit for that transmission frame prior to multipath filtering. Without correlation with the PRN the signal sounds like low level white noise. Because the bit frame is long, precise resolution and tracking of multiple arrivals is easy. So the travel time sequence for the arrivals is simple to determine. If you know the PRN pair used by a station, you can listen for that signal at very low computational cost. There is a very big difference in what is possible if you allow enough time per bit. By using GPS synchronization, multipath at 10-30 seconds per bit is a bit different. You can exploit the additional signal by phase aligning and summing multiple arrivals. This can even be used to separate arbitrary signals on the same frequency if you have enough compute available. My goal is reliable peer to peer radio mail at a few kB/day. Have Fun! Reg |
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Re: Direct Sequence Spread spectrum modulation
DSSS remains the low rate modes for 802.11 chip sets at 1 and 2 Mbps operation.? ? It might be good to review the issues around using DSSS in 802.11, as this is very well understood in terms of packet recovery, protocol and other issues.?
As mentioned below, multipath remains the primary challenge for DSSS demodulation.? Early Wifi DSSS radios used various architectures to mitigate multipath, the primary one being the rake receiver.? ?Several mentions were made to summing the different multipath rays for better detection.? This does not work on a single antenna, as the singles get summed at the input!? ?OFDM MIMO radios (802.11n, ac, and ax) can use signals from different receive chains and combine them into a single packet. a technique called Maximum Ratio Combining (MRC).? Going back to DSSS modulation, early Wifi systems sometimes used diversity antennas to select the best path with the lowest amount of multipath and or flat fade.? ?The best / easiest algorithm determined the transmit antenna with the highest packet success rate (or even last success packet) and used that antenna.? ?The use of diversity antennas can still be seen with many IOT radio modules.? The use of multiple receive or transmit antennas is being enabled by todays SDR technology even at HF.? ? Some of the available SDR boards support 2x2 transceivers.? ?I was looking at a SDR board that showed up on ebay yesterday that had to receiver inputs and A/D converters.? ? Signal processing could be done in either the FPGA, or in the backend computer, allowing for a lot of experiments in signal processing and signal combining techniques.? ? If you had all the time in the world (Wifi is time bound by it's protocol) to process a packet, you could attempt to demodulate it at various time slots until you found a time slot that correctly decoded the packet (this is usually determined by a valid CRC-32).? ? Something similar could also be done if you had two diversity signals.? ?Decode them individually to see if one decodes properly, and add them together (OK - cross correlate) to see if that helps!? You might also experiment with Forward Error Correction, another technique used by OFDM. 73's Jim N8CAH |
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Re: Direct Sequence Spread spectrum modulation
Primary focus is the HF ham bands. However, I've also studied the part 15 rules rather closely. My goal is reliable, long distance, peer to peer communications of short messages at very low power levels in a manner that does not interfere with other users except as a increase in QRN at high utilization. It will take me a while to get back up to speed with Octave. As I figure out how to get data in and out of Octave so I can plot it with gnuplot I'll have some interesting displays which will make clear how simple this is. The hardest part is the lexicon. Of course, if you want the proof, you'll have to go to the nearest university math department. Though I can recommend some books. Have Fun! Reg On Tuesday, November 3, 2020, 03:25:29 PM CST, Steve Hendrix <stevehx@...> wrote: I want to hear more as your work progresses. I too have beenthinking about something like this. What band do you plan to use? Iassume you're in the ham bands. Steve Hendrix |
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Re: Direct Sequence Spread spectrum modulation
¿ªÔÆÌåÓýHi Reg ¨C ? Yes ¨C also the data rate will be of great consequence when you consider the effects you mention, as well as the accuracy of the timing source. ?For real high data rates even a good TCXO may work fine. For low data rates, GPS is almost needed for sure. 73 ¨C Mike ? Mike B. Feher, N4FS 89 Arnold Blvd. Howell NJ 07731 848-245-9115 ? From: [email protected] <[email protected]> On Behalf Of Reginald Beardsley via groups.io
Sent: Tuesday, November 3, 2020 4:51 PM To: [email protected] Subject: Re: [qex] Direct Sequence Spread spectrum modulation ? Mike, On Tuesday, November 3, 2020, 02:28:17 PM CST, Mike Feher <n4fs@...> wrote: ? ? It seems they would not only have to have the same seed, or key, but also the same timing. 73 - Mike |
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Re: Direct Sequence Spread spectrum modulation
Mike, Yes, GPS time is key. You've also got to determine the propagation and multipath delays so they can be summed properly. You *can* synchronize by using a special PRN. With a 0.5 ppm TCXO you could stay close enough to determine the phase and frequency difference, but lots more trouble. It appears I was so blinded by the need for GPS that I failed to notice the lack of Rx. But my RSP2 should take care of those chores with the 2nd RFzero providing the clock. But more work too :-( Have Fun! Reg On Tuesday, November 3, 2020, 02:28:17 PM CST, Mike Feher <n4fs@...> wrote: It seems they would not only have to have the same seed, or key, but also the same timing. 73 - Mike Mike B. Feher, N4FS 89 Arnold Blvd. Howell NJ 07731 848-245-9115 -----Original Message----- From: [email protected] <[email protected]> On Behalf Of Craig Johnson, AA0ZZ Sent: Tuesday, November 3, 2020 3:03 PM To: [email protected] Subject: Re: [qex] Direct Sequence Spread spectrum modulation Reg, Fascinating!? I've wanted to play with this for many years also. Quick question that I've never understood about PRN's.? Does everyone (ham community) use the same formula for generating PRNs such that if a station's call is used as a seed then both stations would have the same sequences for encoding and decoding? 73, -Craig, AA0ZZ -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Reginald Beardsley via groups.io Sent: Tuesday, November 03, 2020 1:34 PM To: [email protected] Subject: [qex] Direct Sequence Spread spectrum modulation This is a simplified outline of a digital mode I have wanted to test for almost 40 years.? It's a direct application of the basic theory of probability and random processes.? I later learned it had been done in the 1950's.? I'll skip the math for now. Generate a pair of Pseudo Random Number (PRN) sequences using the station call sign forward and backwards as seeds. Clock the PRNs using a GPSDO time base and fixed set of frame epochs.? Spread the signal out over most of the band allocation and use a set of binary window lengths so each step in frame length is 3 dB of power gain. One of the two station PRNs is used for MARK and the other for SPACE.? So at the receiver the signal is digitized and crosscorrelated with the 2 PRNs. The one with the largest peak energy at the expected arrival times is the symbol.? This simplifies additively including multipath as useful signal.? This also permits easily listening to other stations on a selective basis. CQ is done with the same PRN generator system using "CQ" and "QC". Once contact is established shift to the communicating stations unique PRNs. For net operation,? use the net name as the seed value when joining or leaving the net.? This allows stations in the net to track what stations to listen for. Identification is by sending the station call using a standard PRN pair (e.g. "FCC"). The effective signal power at the receiver is the time-bandwidth-power product.? So the power level and data rate can be adjusted as conditions permit.? Mathematically, the system? should get as much power gain relative to the noise level as desired? at the price of reducing the data rate.? There should not be significant mutual interference despite stations occupying the same BW.? For a 30 second/bit data rate one should get around 55 dB of gain. Technically there is nothing new in? the concept outlined above.? It simply was too expensive and complex to implement in the past.? ? The economics have changed.? A Raspberry Pi 400 and an RFzero? should be able to implement this with ease and drive a 4K display. Have Fun! Reg |
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Re: Direct Sequence Spread spectrum modulation
¿ªÔÆÌåÓýReg, Thanks.?? I won¡¯t even pretend to say that I understand how to do this but it¡¯s something I¡¯d like to dive into someday. -Craig ? From: [email protected] [mailto:[email protected]] On Behalf Of Reginald Beardsley via groups.io
Sent: Tuesday, November 03, 2020 2:35 PM To: [email protected] Subject: Re: [qex] Direct Sequence Spread spectrum modulation ? Craig, |
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Re: Direct Sequence Spread spectrum modulation
Craig, Yes, that's the idea. The crosscorrelation is an FFT and multiply by the PRNs of the selected senders. This makes it very computationally cheap to monitor several senders in a net and ignore everyone outside the net. After you've done the FFT once for each frame it's just a vector multiply and add. Reg On Tuesday, November 3, 2020, 02:03:25 PM CST, Craig Johnson, AA0ZZ <aa0zz@...> wrote:
Reg, Fascinating! I've wanted to play with this for many years also. Quick question that I've never understood about PRN's. Does everyone (ham community) use the same formula for generating PRNs such that if a station's call is used as a seed then both stations would have the same sequences for encoding and decoding? 73, -Craig, AA0ZZ |
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Re: Direct Sequence Spread spectrum modulation
It seems they would not only have to have the same seed, or key, but also the same timing. 73 - Mike
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Show quoted text
Mike B. Feher, N4FS 89 Arnold Blvd. Howell NJ 07731 848-245-9115 -----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Craig Johnson, AA0ZZ Sent: Tuesday, November 3, 2020 3:03 PM To: [email protected] Subject: Re: [qex] Direct Sequence Spread spectrum modulation Reg, Fascinating! I've wanted to play with this for many years also. Quick question that I've never understood about PRN's. Does everyone (ham community) use the same formula for generating PRNs such that if a station's call is used as a seed then both stations would have the same sequences for encoding and decoding? 73, -Craig, AA0ZZ -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Reginald Beardsley via groups.io Sent: Tuesday, November 03, 2020 1:34 PM To: [email protected] Subject: [qex] Direct Sequence Spread spectrum modulation This is a simplified outline of a digital mode I have wanted to test for almost 40 years. It's a direct application of the basic theory of probability and random processes. I later learned it had been done in the 1950's. I'll skip the math for now. Generate a pair of Pseudo Random Number (PRN) sequences using the station call sign forward and backwards as seeds. Clock the PRNs using a GPSDO time base and fixed set of frame epochs. Spread the signal out over most of the band allocation and use a set of binary window lengths so each step in frame length is 3 dB of power gain. One of the two station PRNs is used for MARK and the other for SPACE. So at the receiver the signal is digitized and crosscorrelated with the 2 PRNs. The one with the largest peak energy at the expected arrival times is the symbol. This simplifies additively including multipath as useful signal. This also permits easily listening to other stations on a selective basis. CQ is done with the same PRN generator system using "CQ" and "QC". Once contact is established shift to the communicating stations unique PRNs. For net operation, use the net name as the seed value when joining or leaving the net. This allows stations in the net to track what stations to listen for. Identification is by sending the station call using a standard PRN pair (e.g. "FCC"). The effective signal power at the receiver is the time-bandwidth-power product. So the power level and data rate can be adjusted as conditions permit. Mathematically, the system should get as much power gain relative to the noise level as desired at the price of reducing the data rate. There should not be significant mutual interference despite stations occupying the same BW. For a 30 second/bit data rate one should get around 55 dB of gain. Technically there is nothing new in the concept outlined above. It simply was too expensive and complex to implement in the past. The economics have changed. A Raspberry Pi 400 and an RFzero should be able to implement this with ease and drive a 4K display. Have Fun! Reg |
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Re: Direct Sequence Spread spectrum modulation
Reg,
toggle quoted message
Show quoted text
Fascinating! I've wanted to play with this for many years also. Quick question that I've never understood about PRN's. Does everyone (ham community) use the same formula for generating PRNs such that if a station's call is used as a seed then both stations would have the same sequences for encoding and decoding? 73, -Craig, AA0ZZ -----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Reginald Beardsley via groups.io Sent: Tuesday, November 03, 2020 1:34 PM To: [email protected] Subject: [qex] Direct Sequence Spread spectrum modulation This is a simplified outline of a digital mode I have wanted to test for almost 40 years. It's a direct application of the basic theory of probability and random processes. I later learned it had been done in the 1950's. I'll skip the math for now. Generate a pair of Pseudo Random Number (PRN) sequences using the station call sign forward and backwards as seeds. Clock the PRNs using a GPSDO time base and fixed set of frame epochs. Spread the signal out over most of the band allocation and use a set of binary window lengths so each step in frame length is 3 dB of power gain. One of the two station PRNs is used for MARK and the other for SPACE. So at the receiver the signal is digitized and crosscorrelated with the 2 PRNs. The one with the largest peak energy at the expected arrival times is the symbol. This simplifies additively including multipath as useful signal. This also permits easily listening to other stations on a selective basis. CQ is done with the same PRN generator system using "CQ" and "QC". Once contact is established shift to the communicating stations unique PRNs. For net operation, use the net name as the seed value when joining or leaving the net. This allows stations in the net to track what stations to listen for. Identification is by sending the station call using a standard PRN pair (e.g. "FCC"). The effective signal power at the receiver is the time-bandwidth-power product. So the power level and data rate can be adjusted as conditions permit. Mathematically, the system should get as much power gain relative to the noise level as desired at the price of reducing the data rate. There should not be significant mutual interference despite stations occupying the same BW. For a 30 second/bit data rate one should get around 55 dB of gain. Technically there is nothing new in the concept outlined above. It simply was too expensive and complex to implement in the past. The economics have changed. A Raspberry Pi 400 and an RFzero should be able to implement this with ease and drive a 4K display. Have Fun! Reg |
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Re: MilliWatt asynchronous antipodal communication
Hello Reg and the group
Glad you like your new toys. I am sure your endeavors will keep you busy for the winter. The RFzero are assembled by an EMS so the quality is fully pro: Indeed the MCU is not powerful enough for sophisticated tasks. Joe, HS2JFW, has a remote RPi + RFzero setup running: Bo www.rudius.net/oz2m :: www.rfzero.net |
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Direct Sequence Spread spectrum modulation
This is a simplified outline of a digital mode I have wanted to test for almost 40 years. It's a direct application of the basic theory of probability and random processes. I later learned it had been done in the 1950's. I'll skip the math for now.
Generate a pair of Pseudo Random Number (PRN) sequences using the station call sign forward and backwards as seeds. Clock the PRNs using a GPSDO time base and fixed set of frame epochs. Spread the signal out over most of the band allocation and use a set of binary window lengths so each step in frame length is 3 dB of power gain. One of the two station PRNs is used for MARK and the other for SPACE. So at the receiver the signal is digitized and crosscorrelated with the 2 PRNs. The one with the largest peak energy at the expected arrival times is the symbol. This simplifies additively including multipath as useful signal. This also permits easily listening to other stations on a selective basis. CQ is done with the same PRN generator system using "CQ" and "QC". Once contact is established shift to the communicating stations unique PRNs. For net operation, use the net name as the seed value when joining or leaving the net. This allows stations in the net to track what stations to listen for. Identification is by sending the station call using a standard PRN pair (e.g. "FCC"). The effective signal power at the receiver is the time-bandwidth-power product. So the power level and data rate can be adjusted as conditions permit. Mathematically, the system should get as much power gain relative to the noise level as desired at the price of reducing the data rate. There should not be significant mutual interference despite stations occupying the same BW. For a 30 second/bit data rate one should get around 55 dB of gain. Technically there is nothing new in the concept outlined above. It simply was too expensive and complex to implement in the past. The economics have changed. A Raspberry Pi 400 and an RFzero should be able to implement this with ease and drive a 4K display. Have Fun! Reg |
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Re: MilliWatt asynchronous antipodal communication
My RFzero boards and accessories arrived yesterday. Quality is excellent. I've not powered anything up yet as I am still collecting documentation.
After more thought, my plan is to connect an RFzero and step attenuator and a diode noise source and attenuator to a hybrid combiner whose output is the input of an RFzero. Then start measuring achievable processing gain for different modulation systems. At ~13 dBm output, I can safely connect most instruments to it without worry. The ARM M0 just doesn't have the resources to do anything exotic, but a Raspberry Pi 400 certainly can and is an ideal host leaving all the resources of the RFzero to implement convenient control protocols. Unfortunately, I shall have little time to play as I have a short weather window in which I need to get some outside work done. But I should be able to do some numerical simulations of various protocols. Thanks, Bo! Have Fun! Reg |
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Re: MilliWatt asynchronous antipodal communication
David, Wow! Thanks a lot for that link! That is absolutely perfect for my project. And a very nice suite of other boards for filters, etc. I'm getting a pair so I can do short range tests with a dummy load for the antenna. That will strongly incentivize getting relicensed. These let me skip the RF design step and go straight to the DSP which I know much better than RF design, though I am working hard on that. Have Fun! Reg On Saturday, October 24, 2020, 06:44:38 PM CDT, David McQuate <mcquate@...> wrote: Reg, ? An easy to use and fairly inexpensive RF source is the RFzero? ? ? ? ? about $64 + shipping, designed by?Bo, OZ2M.? Frequency range?2289 Hz to 200+ MHz, with frequency resolution on the order of milliHertz.? Output power +13 dBm from 400kHz to 200MHz.? It has a built-in GPS receiver, so it can frequency-lock to GPS.? It's controlled by an arduino microcontroller, and the source code is available for several working applications, including a WSPR beacon.? (You will need to add a low-pass filter to suppress transmission of harmonics.)? ? Its modulation capabilities are limited; certainly low BW FSK is possible. David WA8YWQ |
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Re: MilliWatt asynchronous antipodal communication
Reg,
? An easy to use and fairly inexpensive RF source is the RFzero? ? ? ? ? about $64 + shipping, designed by?Bo, OZ2M.? Frequency range?2289 Hz to 200+ MHz, with frequency resolution on the order of milliHertz.? Output power +13 dBm from 400kHz to 200MHz.? It has a built-in GPS receiver, so it can frequency-lock to GPS.? It's controlled by an arduino microcontroller, and the source code is available for several working applications, including a WSPR beacon.? (You will need to add a low-pass filter to suppress transmission of harmonics.)? ? Its modulation capabilities are limited; certainly low BW FSK is possible. David WA8YWQ |
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QEX is available as a membership print option!
FYI I had asked a few days ago about getting QEX as the membership print option. Much to my delight I received the following today from Becky Schoenfeld:
" Our Membership Department has advised me that they can make QEX the magazine you receive as your primary ARRL membership benefit. They will put this into effect. If you have any questions about the matter, please contact circulation@..." Have Fun! Reg |
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Re: High Q UHF coils and lines
I've got a considerable pile of WW II RF stuff Dad bought on Radio Row (Cortlandt St, torn down to build the World Trade Center) in NYC after the war. I don't recall ever seeing a silver plated coil that was tarnished, so I must presume that there was some coating applied over the silver. Switches and such are commonly tarnished. One of my winter projects is to move and inventory the pile so I'll try to remember to take some pictures. If anyone is interested in buying such stuff send me a PM. Dad bought cases of really high quality parts. I need to sell this stuff off as it exceeds anything I could ever use up. The high power RF stuff is amazing. Steatite coil forms and silver plated wire. Very bulky, but it had to be big to allow the high voltages. The tubes in the standard B-17 radios had a 1500 V plate rating!. These have become *the thing* for audiophool amplifiers and the tubes sell for $300-400 each. Dad left me 23 of those tubes :-) For anyone interested in tube stuff, I've got a rather large quantity of a tube which is basically a 6L6 with a 3 V filament in addition to a lot of other well chosen general purpose tubes. So if you want to make an Ameco AC-1, my only station transmitter, it's a bargain relative to a 6L6. Thanks for posting. I'm currently struggling with writing some comments about the QEX WSPR QRN paper in the 9-10/20 issue. I'm very intrigued by the work, but not entirely sure I know enough about the ionosphere yet. My career was spent looking down, not up. Have Fun! Reg On Thursday, October 22, 2020, 07:52:22 PM CDT, Daniel Ricardo Perez via groups.io <danyperez1@...> wrote: Hi! I understand VHF / UHF coils are silver plated, not because silver has a slightly lower resistivity than copper, but more because silver oxide is conductive, whereas copper oxide not. This statement sounds a little strange to me, because silver oxide has a quite higher resistivity than the pure metal anyway. But let's leave it there. My question is: wouldn't it be far superior to use enamel copper wire, possibly adding a perfectly airtight lacquer on top of the enamel? That way Q would be preserved along the years better than with silver plating. Obviously I must be wrong, because otherwise nobody would take pains to apply the silver. Can anybody clarify this? Kind regards Daniel Perez LW1ECP |
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High Q UHF coils and lines
Hi! I understand VHF / UHF coils are silver plated, not because silver has a slightly lower resistivity than copper, but more because silver oxide is conductive, whereas copper oxide not. This statement sounds a little strange to me, because silver oxide has a quite higher resistivity than the pure metal anyway. But let's leave it there.
My question is: wouldn't it be far superior to use enamel copper wire, possibly adding a perfectly airtight lacquer on top of the enamel? That way Q would be preserved along the years better than with silver plating. Obviously I must be wrong, because otherwise nobody would take pains to apply the silver. Can anybody clarify this? Kind regards Daniel Perez LW1ECP |
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Re: Introductions
I'm mostly a lurker on several test equipment groups, enjoy
the learning and the insights from the "old heads" who designed
the great test equipment in the early days of electronics when it was
mostly magic (ALL magic, to me!). I earned a Novice license (N8EKC)
around 1968 at the urging of my dad. Never had much success with it. When
I was in college (USAF Academy) the new thing with repeaters and phone
patch appealed to me so I found a professor who was a VE and got my
Technician. I put on the form that my permanent address was Ohio, so
please give me an '8' call sign, but my current mailing address was
Colorado. So of course they gave me WB0YJU and mailed it to
Ohio.
After my time as a USAF flight instructor and instructor-instructor, I left the USAF, got married, started a family, and went to work in a couple of software engineering jobs, continuing to market my HexDos operating system for Ohio Scientific computers on the side (I hitched to the wrong horse....) Later struck out on my own, making Hx Engineering, LLC my full-time occupation, corporate world headquarters in my basement. But that let me beat the work-from-home thing by 28 years, and I got to be much more involved with my kids than would have otherwise been possible - 3 by birth, 2 by adoption, and 36 foster babies. The only real product of my own these days is KISS-488, an IEEE-488/GPIB/HPIB to Ethernet adapter that I'm still selling on eBay and at my own . When one of my sons wanted to get his ham license as a result of earning Radio Merit Badge with me as his counselor, I went along and took the Extra class exam, so now I'm W8HXS and he's KC8UQX. Another son is just now getting interested Kids are mostly grown now, but I'm still involved with Scouting, mostly as a merit badge counselor, and I'm also a volunteer CASA/GAL advocating for foster kids in court. On the side, I'm a part time senior flight instructor for single-engine, multi-engine, and instruments, and also a sideline beekeeper with up to 30 hives producing about 1200# of honey each of the past few years. So you could say I'm busy as a one-armed paperhanger with the hives - both kinds! Steve Hendrix, PE, CFII Hx Engineering, LLC W8HXS |
Bo, OZ2M
Daniel Ricardo Perez LW1ECP
Steve J. Noll, WA6EJO
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