Josef “Jeff” Sipek

Scribbled Dummy Load Blueprints

Yesterday, I saw KM1NDY’s blog post titled Scribbled Antenna Blueprints. I wasn’t going to comment…but here I am. :)

I thought I’d setup up a similar contraption (VHF instead of HF) to see what exactly happens. I have a 1 meter long RG-8X jumper with BNC connectors, a BNC T, and a NanoVNA with a 50Ω load calibration standard.

But first, let’s analyze the situation!

Imagine you have a transmitter/signal generator and you connect it to a dummy load. Assuming ideal components, absolutely nothing would get radiated. Now, imagine inserting an open stub between the two. In other words, the T has the following connections:

  1. the generator
  2. 50Ω load
  3. frequency-dependant impedance

Let’s do trivial math! Let’s call the total load that the generator sees Ztotal and the impedance provided by the stub Zstub. The generator side of the T is connected to the other ports in parallel. Therefore:


So, when would we get a 1:1 SWR? When the generator sees a 50Ω load. When will it see 50Ω? When Zstub is very large; the extreme of which is when that side of the T is open.

If you are a ham, you may remember from when you were studying for the Amateur Extra exam that transmission line stubs can transform impedance. A 1/2 wave stub “copies” the impedance. A 1/4 wave stub “inverts” the impedance. For this “experiment” we need a high impedance. We can get that by either:

  1. open 1/2 wave stub
  2. shorted 1/4 wave stub

Since the “design” from the scribble called for an open, we’ll focus on the 1/2 wave open stub.

Now, back to the experiment. I have a 1 m long RG-8X which has a velocity factor of 0.78. So, let’s calculate the frequency for which it is a 1/2 wave—i.e., the frequency where the wavelength is 2 times the length of the coax:


This equals 116.9 MHz. So, we should expect 1:1 SWR at 117-ish MHz. (The cable is approximately 1 m long and the connectors and the T add some length, so it should be a bit under 117.)

Oh look! 1.015:1 SWR at 110.5 MHz.

(Using 1.058 m in the calculation yields 110.5 MHz. I totally believe that between the T and the connectors there is close to 6 cm of extra (electrical) length.)

But wait a minute, you might be saying, if high impedance is the same as an open, couldn’t we just remove the coax stub from the T and get the same result? Yes! Here’s what the NanoVNA shows with the coax disconnected:

The SWR is 1.095:1 at 110.5 MHz and is better than 1.2:1 across the whole 200 MHz! And look at that impedance! It’s about 50Ω across the whole sweep as well!

We can simplify the circuit even more: since we’re only using 2 ports of the T, we can take the T out and connect the 50Ω load to the NanoVNA directly. We just saved $3 from the bill of materials for this “antenna”!

(In case it isn’t obvious, the previous two paragraphs were dripping with sarcasm, as we just ended up with a dummy load connected to the generator/radio and called it an antenna.)

Will It Antenna?

How could a dummy load transmit and receive signals? Glad you asked. In the real world we don’t use ideal components. There are small mismatches between connectors, the characteristic impedance of the coax is likely not exactly 50Ω, the coax shield is not quite 100%, the transmitter’s/generator’s output isn’t exactly 50Ω, and so on.

However, I expect all these imperfections do not amount to anything that will turn this contraption into an antenna. I bet that the ham that suggested this design used an old piece of coax which had even worse characteristics than the “within manufacturing tolerances” specs you get when the coax is new. Another option is that the coax is supposed to be connected in some non-standard way. Mindy accidentally found one as she was packing up when she disconnected the shield but not the center conductor. Either way, this would make the coax not a 1/2 wave open stub, and the resulting impedance mismatch would cause the whole setup to radiate.

I’d like to thank Mindy for posting about this design. It provided me with a fun evening “project” and a reason to write another blog post.

Finally, I’ll leave you with a photo of my experimental setup.

Juniper Networks Spam

For a few months now, I’ve been getting regular mass mailing from the UK branch of Juniper Networks. Up until today, I just ignored them thinking that it must be a phishing attempt. Today, for whatever reason I looked at the headers and they look perfectly fine. It appears to be a genuine mass mailing. The thing that gets to me is that I never subscribed to this mailing. I also never talked to or did business with them. I think it is sad that even rather large and established companies resort to unsolicited mass mailings like this. In my eyes, all this kind of spam accomplishes is to market the company in negative ways. Well, done Juniper, well done.

How do I know that this is spam and I didn’t sign up for it indirectly? It’s very simple…

  1. I know how to spell my name.
  2. It’s been a while since I went to a conference. (A frequent reason to be subscribed to a mailing like this.)
  3. I do not deal with networking or network operations so I have no reason to sign up for anything even remotely related to this.

This leads me to the conclusion that for whatever reason, Juniper decided to get (buy?) an email address list of questionable origin. Tsk, tsk, tsk.

Fun fact: I clicked on the “manage subscriptions” link to unsubscribe. The web form doesn’t let me unsubscribe unless I give them more information about me—country and state. No, thanks.

Lua Compatibility

Phew! Yesterday afternoon, I decided to upgrade my laptop’s OpenIndiana from 151a9 to “Hipster”. I did it in a bit convoluted way, and hopefully I’ll write about that some other day. In the end, I ended up with a fresh install of the OS with X11 and Gnome. If you’ve ever seen my monitors, you know that I do not use Gnome — I use Notion. So, of course I had it install it. Sadly, OpenIndiana doesn’t ship it so it was up to me to compile it. After the usual fight to get a piece of software to compile on Illumos (a number of the Solaris-isms are still visible), I got it installed. A quick gdm login later, Notion threw me into a minimal environment because something was exploding.

After far too many hours of fighting it, searching online, and trying random things, I concluded that it was not Notion’s fault. Rather, it was something on the system. Eventually, I figured it out. Lua 5.2 (which is standard on Hipster) is not compatible with Lua 5.1 (which is standard on 151a9)! Specifically, a number of functions have been removed and the behavior of other functions changed. Not being a Lua expert (I just deal with it whevever I need to change my window manager’s configuration), it took longer than it should but eventually I managed to get Notion working like it should be.

So, what sort of incompatibilies did I have to work around?


loadstring got renamed to load. This is an easy to fix thing, but still a headache especially if you want to support multiple versions of Lua.


table.maxn got removed. This function returned the largest positive integer key in an associative array (aka. a table) or 0 if there aren’t any. (Lua indexes arrays starting at 1.) The developers decided that it’s so simple that those that want it can write it themselves. Here’s my version:

local function table_maxn(t)
    local mn = 0
    for k, v in pairs(t) do
        if mn < k then
            mn = k
    return mn


table.insert now checks bounds. There doesn’t appear to be any specific way to get old behavior. In my case, I was lucky. The index/positition for the insertion was one higher than table_maxn returned. So, I could replace:

table.insert(ret, pos, newscreen)


ret[pos] = newscreen

Final Thougths

I can understand wanting to deprecate old crufty interfaces, but I’m not sure that the Lua developers did it right. I really think they should have marked those interfaces as obsolete, make any use spit out a warning, and then in a couple of years remove it. I think that not doing this, will hurt Lua 5.2’s adoption.

Yes, I believe there is some sort of a compile time option for Lua to get legacy interfaces, but not everyone wants to recompile Lua because the system installed version wasn’t compiled quite the way that would make things Just Work™.

MongoDB - First Impressions

I just ssh’ed into a server that’s running MongoDB to see if it was doing anything and if there were any cool commands to get stats from it. (I’ve never used MongoDB.) I accidentally ran mongod as non-root. It died because it didn’t have the right permissions — good. However, something in the output seemed completely and utterly retarded.

warning: 32-bit servers don't have journaling enabled by default. Please use
	--journal if you want durability.

[initandlisten] MongoDB starting : pid=12682 port=27017 dbpath=/data/db/
	32-bit host=hkn2012
[initandlisten] ** NOTE: when using MongoDB 32 bit, you are limited to about
	2 gigabytes of data
[initandlisten] **       see
[initandlisten] **       with --journal, the limit is lower

Was it the 2 GB data limit? Nope! Was it the fact that 32-bit systems default to non-durable mode while 64-bit default to durable? Bingo! Seriously, that is retarded. Why would anyone think that it is a good idea to cripple 32-bit systems like that? Was performance so bad with journaling that to avoid gaining a reputation for being slow the developers decided to keep the data unsafe by default? I doubt it. Was it to squeeze in a couple more MB of addressable data? Probably. Good job, folks.

For what it is worth, I decided to see if the 2 GB limit was something silly (i.e., using an int in some on-disk structure). It turns out that mongod likes to mmap the data for fast access. This is reasonable. (More so than using architecture specific integral types in on-disk structures.)

I wonder what other gems are hiding in this software.

Gentoo's --as-needed insanity

I feel a bit ranty.

Recently, I got to make an ebuild file (this is Gentoo’s equivalent of RPM spec file) for PCP. The ebuild file is pretty simple to make — the only trickiness was PCP’s little strange makefile setup (the makefile runs configure which makes another makefile that the original makefile includes). This was pretty easy to work with anyway.

The issue I spent about a day and a half on was Gentoo’s idea of making better executables … the –as-needed flag. The idea behind this flag is to only link in the libraries that are actually needed (have symbols referenced) and ignore the rest. This sounds great at first, but then you run into an issue where you want to build a Perl module but instead you end up with a shared object which Perl won’t load because of unresolved references. I don’t know how exacly Perl does its magic, but –as-needed was not linking the Perl module against libpcp.

Part of the fix was to add a bunch of filters in the pkg_setup hook:

        append-flags $(no-as-needed)
        filter-flags -Wl,--as-needed
        filter-ldflags -Wl,--as-needed
        filter-ldflags --as-needed
        append-ldflags $(no-as-needed)

Additionally, I had to modify a src/cpan/PMDA/Makefile.PL to disable this insanity in the Perl-invoked builds.


I’ve resumed watching Stargate Universe. I’m more or less convinced that it’s crap. The plot is painful (well, the tiny bits of plot that are there), the characters are annoying, and there are a few “issues” I have with the production. This is going to be my short rant about one aspect of the production.

At some point, someone thought of making a device so that you can hand-hold a camera but still have it stable & steady. This invention is commonly called a Steadicam. Anyhow, I would like to know when it became artsy and cool to not use steadicams, and instead have this unsteady shot. I find this motion distracting and even un-natural. SGU isn’t the first show to do this. I vaguely remember BSG doing the same. Why are you doing this?

Of course, not doing silly things with the camera will not fix the other issues with the show - like annoying characters and lacking story lines.

So, if you are responsible for the production of SGU, use those steadicams! They’ll improve your show!

Dear Facebook

Dear Facebook,

I tried to use your @ name completion feature in Opera, and it didn’t work…at all!



During an experiment, I needed to install Fedora 12. I made a few mistakes:

  1. I went with the netinstall. Unlike Debian’s netinstall, Fedora’s is very slow.
  2. The installer was a bit sluggish under KVM, and so I accidentally clicked though the window that let me unselect Gnome. So it’s installing the whole shebang.
  3. For whatever reason, it is installing CJK fonts. I do not speak either of those languages, and therefore they are useless to me. Furthermore, I’ve been told that something in the neighborhood of 20% of Fedora users make use of CJK. That just sounds wrong. Why install a package by default that only 20% of your userbase will benefit from? Aren’t there more useful packages?


Dear Flash,

You Suck.


Josef ‘Jeff’ Sipek.

P.S. do I really have to justify this?


Dear Firefox,

You Suck.


Josef ’Jeff’ Sipek.

P.S. xulrunner-stub using 4% CPU when the window is not visible and 36% when re-rendering parts of the page is a bit too excessive.

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