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91
I was reading this article about AGM batteries and noticed the graph relating to cycles and depth of discharge.

http://offgridham.com/2018/03/agm-batteries/?utm_source=amateur-radio-weekly&utm_medium=email&utm_campaign=newsletter

Now what I've always been told is that if a deep cycle battery is discharged bellow 50% you will significantly reduce it's life, ie reduce the number of available cycles. Most plans for how much battery capacity you need figures on discharging down to 50%. So if you calculate that you will need 100ah per day of storage you should buy at least 200ah worth of batteries. This will keep your batteries above that magic 50% number and significantly prolong the life of your batter.

Indeed, the graph does so a sharp change in direction right at that magic 50% mark. But after looking at it for a few minutes, it dawned on me that what the graph is showing is exactly opposite the above premise. To me at least, the graph seems to be saying that the first 50% of your battery is where the number of cycles takes the biggest hit and the second 50% has less impact on the number of cycles available to you.

So I crunched a few quick numbers taken from the graph. If I take a 100ah battery and drain it 50%, I get 50ah out of it per cycle and 700 cycles. 50ah x 700 cycles yields 35,000ah over the life of the battery.
If I only sip 30% off the top I get 30ah per cycle and 1600 cycles. 30 x 1600 is 48,000ah out of the battery during it's life. That's a whopping 52% increase in power harvested over the life of the battery.

On the other hand, if I discharge the battery to where there is only 10% left in it at the end of the day (I'm discharging it by 90%) it still gives me 400 cycles, not that many fewer than only taking 50%. In fact, 90ah x 400 cycles gives me 36,000ah over the life of the battery. That's more than I get when I discharge it down to only 50%.

So the graph is definitely showing that the first 50% of the power taken out of the battery takes the biggest tole on the battery's life. The shallower line means smaller changes in discharge result in larger changes in number of cycles and the steeper line bellow 50% means larger changes in DOD result in smaller changes in number of cycles. If you are going to pull it down to 50%, you are not losing anything to keep on pulling it down to 70%, 80% or even 90%. I wondered if this graph was wrong so I looked for more graphs online and they all say roughly the same thing.

Am I missing something or have I been misled all these years?
92
Antennas / Re: Successful Wire Antennas.
« Last post by Sparks on August 04, 2018, 10:19:01 AM »
Strange, there are 237 pages in that PDF but when I download it I can only look at 145 of them

The PDF is actually only 32 pages long. It starts with the TOC and then moves to Chapter 3 Dipoles, at page 42. Then there are excerpts of other chapters, too. So the PDF is only a sample of the book.

Read more about the book in this review: http://qrznow.com/successful-wire-antennas/

They link to the same sample PDF, except this one has copyright watermarks on every page:
http://www.rsgbshop.org/acatalog/PDF/Successful_Wire%20Ant_sample.pdf

I found the book on Amazon:
https://www.amazon.com/Successful-Wire-Antennas-Ian-Poole/dp/1905086776/
93
Antennas / Re: The 49:1 Half-Wave End-Fed Transformer Campain is ON!
« Last post by gil on August 04, 2018, 05:48:27 AM »
Many of these people have no idea what they're talking about. These transformers do not have high losses. A 240 size core is 90% efficient and a 140 core around 70%. For all practical applications, you'll never hear the difference. What matters is the resonant half wave wire.

Gil.
94
Digital Modes / Re: FT8 digital comms mode
« Last post by vwflyer on August 03, 2018, 08:58:31 PM »
Just a quick anecdote about Olivia. Yesterday, my brother and I were trying to have a QSO on 20 meters over 1500 miles using Olivia, 50 watts and low wire antennas. It was tough copy but we were getting between 50 and 90 percent depending on fades. Shortly after the QSO my brother went outside and saw that one support rope for his G5RV had broken and half of the antenna was laying on the ground. Later he commented that he thought that it was odd that the tuner settings were different than usual.
95
Antennas / Re: Successful Wire Antennas.
« Last post by gil on August 03, 2018, 12:17:05 PM »
Darn. It seems the file is a preview, albeit a large one...

Gil.
96
Antennas / Re: Successful Wire Antennas.
« Last post by caulktel on August 03, 2018, 10:19:38 AM »
Strange, there are 237 pages in that PDF but when I download it I can only look at 145 of them. I really wanted to the section on EFHW's.

Joel
N6ALT
97
Tactical Corner / Re: Sending Encrypted Messages in Morse Code.
« Last post by gil on August 03, 2018, 06:04:23 AM »
Thank you.

Gil.

Sent from my SM-G928F using Tapatalk

98
Tactical Corner / Re: Sending Encrypted Messages in Morse Code.
« Last post by lpwaterhouse on August 03, 2018, 06:02:48 AM »
You're thinking Dual_EC_DRBG, which has nothing to do with AES. NSA only reviewed and approved the AES candidate algorithms, they did not have a direct hand in them like with DES (where they secretly made it more secure) or Dual_EC_DRBG (deliberately sabotaged as part of "Bullrun").

As for 128 vs. 256 the one concern is that 256 seems to be slightly weaker (though FAR from broken) against related key attacks, but given its much greater strength overall I'm not worried by that, both options are sensible, I go with 256 if speed is not a major concern.

If I HAD to pick an alternative today I'd go for ChaCha20 (A Stream Cipher, so not fully comparable), Serpent or Twofish (in descending order of preference), but for the moment I trust AES. Much of that is of course personal preference and gut-feeling, but I read the relevant papers too ;-)
99
Tactical Corner / Re: Sending Encrypted Messages in Morse Code.
« Last post by gil on August 03, 2018, 05:13:56 AM »
Great info, thank you!

I remember reading that AES might have been voluntarily compromised by the NSA, on the random number generator... Also that AES-128 is better than the 256 bit version...

Wouldn't Blowfish be more secure than AES?

Gil

Sent from my SM-G928F using Tapatalk

100
Tactical Corner / Re: Sending Encrypted Messages in Morse Code.
« Last post by lpwaterhouse on August 03, 2018, 05:04:23 AM »
Hi,

I just discovered the youtube channel and the OTP video drew me here. I'm a bit of a Cryptology nerd and want to chime in on some of the commonly glossed-over details, misconceptions, etc. that are nonetheless surprisingly vital in keeping your communication secret, especially regarding the perfect secrecy of OTPs (which is _far_ more impractical than most people assume). Gil does mention all of the problems, but I think they bear pointing out even more explicitly:

While it is true that a properly generated One-time Pad is information-theoretically perfectly secure, the devil is in the details of that "properly generated". The perfect security ONLY applies when a source of actual, physical, randomness is used, with proper precautions to eliminate biases, etc. The problem is that it is very hard to be sure your source _is_ actually random (Generally nuclear decay, Thermal (Johnson-Nyquist) noise, Avalanche Diodes or the last bit of a microsecond time difference between keystrokes are usually considered "good, as far as we know"). If instead you use any kind of Pseudo-random number generator [PRNG] (e.g. /dev/random, rand(), that formilab site, etc.) then the security guarantees drop from "perfect" to that of an ordinary StreamCipher (in fact "Adding/XORing with an algorithmically generated Key-Stream" is a definition of StreamCipher), subject to the quality of the PRNG, but always lower than "perfect". Some such PRNGs are "cryptographically secure" [CPRNG], meaning that while they aren't "perfect" we _currently_ know no way of breaking them. The majority of PRNGs however are utterly unsuitable for cryptographic purposes, they were designed for statistical properties instead.

So, if you truly want to approach the perfect security of the OTP you MUST use a hardware or "true" RNG [TRNG], and pay attention to whether the cryptographic community considers that particular design "good, as far as we know". Some possible options are http://moonbase.tictail.com/ or https://www.tindie.com/products/WaywardGeek/infinite-noise-true-random-number-generator/.

If you have no access to a TRNG (or you realize that "perfect" is the enemy of "good enough") then you are probably better off using an established cryptographic algorithm, because they a) are likely equal or better than anything you can assemble with a PRNG, even if cryptographically secure, and b) don't share the major drawback of OTPs (or rather: pre-shared key-streams), namely having to _securely_ share a "pad" with the recipient ahead of time, that is at least as long as all messages you ever intend to exchange concatenated (The only advantage of this over communicating securely directly is that it can be shared ahead of time, when circumstances can make it easier, e.g. face to face). There is a good reason even governments/spy agencies don't often use OTPs, exchanging and storing that much highly sensitive "pad" is truly difficult (And then you still have to be extremely careful in using it correctly, or you can easily make non-obvious mistakes that render you encryption surprisingly weak, read up on the "Venona project" for when that happend to the Soviet Union. For example: NEVER EVER repeat the pad! One repetition IS enough to break it, relatively easily even!)

So, what if you only need "good enough"? Meaning something that, according to current research (publicly available only, of course...) would either take longer than the expected life-time of the universe to break, or something that only needs to be secure against non-government adversaries, like your neighbors?

Your best bet is well-established and internationally vetted systems, preferably using public-key algorithms (So you don't even have to exchange the, relatively short, compared to an OTP, key securely), for example RSA with AES.

But those of course require computers and access to cryptographic software (something that many governments consider restricting from time to time...), so you want something that can either be programmed from memory or works without any computer at all, "in the field". I suggest having a look at these two options in particular:

    - CipherSaber-2 (http://ciphersaber.gurus.org/) using RC-4, an easily memorizable algorithm that most programmers can code in a few minutes, with practice. RC-4 itself is considered insecure against determined and knowledgable adversaries, though the modification in CipherSaber-_2_ is likely to alleviate that somewhat. In a SHTF (as opposed to an Orwellian) scenario I wouldn't hesitate to use it.

    - Solitaire (https://www.schneier.com/academic/solitaire/), also known as "Pontifex" (in the "Cryptonomicon" Gil mentioned) working without a computer, using an inconspicuous deck of playing cards. Not much research seems to have gone into the security of Solitaire (though its designer is a respected cryptographer), but its construction suggests it may be vulnerable to the same kind of attacks as RC-4. Still: Certainly good enough against your neighbors. Try it (the password is "GL", which is too short for real use, and I'm not using the optional keying step of setting the Jokers): "LEZVJ RUQLK BCTED"

And last, but not least: In case you want to learn some cryptanalysis and break some ciphers (including a few used by the US army in WW2), have a look at https://www.mysterytwisterc3.org/en/ It can be quite eye-opening...

Regards,
Lawrence
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