Category Archives: Amateur Radio

Are ‘ham’ operators going the way of 8-tracks and VCRs?

Jeff, KE9V pointed at the following article via Twitter:

Are ‘ham’ operators going the way of 8-tracks and VCRs?

The author actually meanders a bit (into the why we call it “ham radio”, for some odd reason, but his main point seems to be this:

As National Public Radio found last March, amateur radio is an experience that can’t be duplicated surfing the Internet. That’s why when NPR attended a ham radio convention in St. Louis, a reporter found teens carrying on the hobby. As one 15-year-old said, Facebook and texting are fun, but it can’t match making friends with a $200 radio for which you don’t have to pay a monthly fee.

I groaned a bit inside, because this is complete balderdash.

Okay, it’s not complete balderdash. I am not one to tell teens (or anyone else) what is fun and what is not fun: they are capable of figuring that out for themselves. I’ve no doubt that the teens they interviewed at a ham radio convention thought ham radio was fun, otherwise, they probably wouldn’t have been there. But I also suspect that if you took away a teen’s mobile phone and replaced it with a $200 HT, you’d find that even those attending the conference would probably be more than a little upset.

The reason is obvious: a mobile phone and an HT aren’t interchangeable. They don’t do the same thing.

Most notably, an HT is a ham radio, and radios like that are almost exclusively for communicating with other hams. If you want to communicate with someone who isn’t a ham, a ham radio isn’t really all that valuable. At best, it requires a clumsy relay. In practice, it just isn’t done. You’d just fire up your phone and call them. Or text them. Or email them. Or Twitter. Or Facebook. Or post something to a blog. You’d send them a picture. Or a link to a YouTube video. If you really want to communicate with someone, I’d submit that the Internet provides a much richer environment than amateur radio.

Okay, but let’s say you do want to talk to hams. Isn’t ham radio the most obvious way to communicate with other hams?

Well, I’d submit the answer is mostly no. If your goal is to communicate, all of the ways that I mentioned above are still excellent, reliable, high bandwidth means of communication. Even if you toss in the requirement that the communication be free, the wide availability of WiFi in many areas (from coffee shops to libraries) makes conventional Internet a very attractive means of communication.

If you accept this, then it does seem that amateur radio is going to go the way of the Dodo.

But I don’t believe it will, because ham radio does provide some things that the conventional Internet doesn’t.

First of all, it relies somewhat less on infrastructure, so it provides a backup in times of emergency. This capability is widely trumpeted as the (primary?) virtue of amateur radio, and as justification for our spectrum allotment. Our pool of self-trained emergency communicators can provide some valuable assistance to local communities in case of emergency as a kind of failsafe.

Secondly, even in non-emergency cases, ham radio can provide some capabilities which would be difficult to reproduce using more conventional wired or wireless technology. Repeaters can provide broad coverage in areas not well served by cell towers. HT’s and small HF rigs can enable people hiking and camping in remote areas to communicate.

But primarily, ham radio is a kind of sport: an active pastime. A form of recreation. We do it because we like to do it. It provides us an interesting opportunity to achieve mastery. Mastery makes us feel better. The activity provides a means to connect with other humans and relate to them about our shared interests.

The invention of the motorboat didn’t end surfing or swimming. The invention of canned tuna fish didn’t end fishing. The invention of cars didn’t end walking or running. Depending on the emergency, skills like walking, fishing or swimming may save your life, but that’s still not why people do them: they do them because it’s fun.

As long as ham radio continues to be fun, ham radio will continue.

ARISSat-1 and the ISS over California

I got a tweet from twisst, the ISS pass prediction robot yesterday indicating that I’d have a good pass around 8:25PM. While I am fighting off a cold, the weather was beautiful and nice, and so I ran some path predictions to see what the path looked like, and also checked on ARISSat-1’s path to see how it was doing. I hadn’t recorded ARISSat-1 since it’s first launch weeks ago, and hoped that in spite of it’s rapidly increasing battery problems, that it would still be in sunlight, and would therefore still have a strong signal. ARRISSat-1 would lead the ISS by about 23 minutes, rising around 8:02 or so, but since I have a tall horizon to the north where it would rise, I wouldn’t expect to pick up a good signal until it cleared the hills, about 8:06 or so.

It turned out to be a really good pass: I got three different SSTV images, and some really clear audio telemetry. The first SSTV image and the last were pretty marginal, but the middle one was really clear (sad, since it was the least interesting). When I first recorded ARISSat-1 shortly after launch, I had periodic fades which I hypothesized as tumbling of the satellite: those appear to be entirely gone. I’ll have to try again to see if I can get a better and more interesting SSTV image.

After ARISSAT-1 set, I waited until 8:25 to see if the ISS would come up. I tuned into 145.825 (the ISS packet radio frequency) and waited with my iPhone camera ready. By then, it was surprisingly dark, so my camera recorded mostly just blackness, but toward the end of the clip, you can see a faint dot in the recording (and very little else). Not too exciting, but I left the audio from the radio playing in the background, so you can hear the digital packet signals being echoed through the ISS. The ISS was predicted to peak at magnitude -3.1, which made it brighter than any star in the sky, and it was very easy to see.

Here’s the resulting video. WARNING, spoiler: I forgot to edit out the “secret word” in this recording. Blame it on the cold medicine I’m on.

I’ll probably try to record another pass soon. Stay tuned.

ARISSAT-1’s battery appears to be failing quicker than expected…

I didn’t get a chance to record any more ARISSAT-1 data this weekend, but I did catch up on some reading. Apparently, it’s batteries are giving out quicker than expected: the voltage is dropping low enough to cause a reset when the satellite goes into eclipse each orbit. If you were thinking of grabbing some telemetry/SSTV from the satellite, or even attempting some QSOS, perhaps sooner would be better than later.

You can read up on the Power System of the satellite here, which includes a link to this AMSAT Journal article which gave many details. Apparently the silver-zinc batteries which were used aboard the satellite (largely for safety, a dead short of these batteries does not cause a fire) have many features which were seemingly not that well understood. These batteries are typically used in a deep-discharge situation, and only recharged five times. Aboard ARISSAT-1, they are recharged in a shallower cycle, more times. There also seems to be some issues regarding temperature performance. I skimmed the article, but will reread it and rethink it some more soon.

Arduino PWM Laser Transmitter

A couple of weeks ago, I programmed an Arduino to take digitized sound stored in its rom, and send it out via PWM of an LED. A couple days after that, I used the same code to send voice using a small 5mw laser module. Ever since then, I thought it would be good to use the Arduino’s analog to digital converters to sample the sound from a small microphone, and then use that as the signal to send over PWM. Here’s my first test:



There is lots of noise remaining in the signal. Some further experimentation showed that the voltage regulation wasn’t very good: I did tap 5v from the Arduino, but the load must have been near its limit, because the overall voltage levels were varying by nearly 100 millivolts, which is only slightly less than the signal amplitude. I also have many leads which are too long, poor layout, and unoptimized values for AC blocking caps. But it does seem to work. I’ll be revising this over the next couple of weeks.

Addendum: I did measure the current through the op amp preamp, and the laser diode module. The op amp circuit only needs to drive the Arduino input, which is very high impedance, so it draws less than four milliamps. The laser diode averages 20ma, but has a peak power of twice that.

AMSAT SSTV gallery

I haven’t had the chance to do any ARISSAT-1 SSTV lately, but I thought I’d snoop over to their gallery to have a peek. Some good pictures are being received, but it appears that some earlier pictures are simply being removed: two pictures that I submitted to them earlier are no longer available. Their comment:

We may not be able to display every image because of quality or duplication but it is important for you to submit them for engineering analysis purposes. The rest will be archived onto a subsidiary page.

There is no hint of where this subsidiary page can be found. Honestly, AMSAT: if you are going to act as a centralized repository for SSTV images, why not make all received images available? If they are of interest to the principals, they are probably of interest to everyone else too.

Still, surf over and have a peek. But if you find a picture you like, archive it on your local system.

Welcome to the AMSAT SSTV gallery.

I before E except after Hellschreiber « SWHarden.com

Scott Harden had a very cool idea: sending Hellschreiber, an old fax-like teleprinter code invented by Rudolf Hell, using just an Atmel ATMega48 and a canned oscillator. The idea is pretty simple: the canned oscillator will produce a square wave with the same amplitude as the input voltage. So, you simply power the oscillator with the PWM output (presumably with some low pass filtering) of the AVR, and you can produce a simple AM modulated signal. There are some issues with this, but the basic idea is pretty cool, and I have most of the parts I need to make this happen at home. I’ll try it out and let you know how it works out.

>>> I before E except after Hellschreiber « SWHarden.com.

Addendum: I experimented with this basic idea using my LED/Laser PWM code/circuit that I built on my Arduino. I’m uploading the youtube video now, but I realize that now that I didn’t do what Scott did, and what he did was somewhat more likely to work better. If you watch my latest YouTube video (uploading now) you can anticipate that I’ll be apologizing for my stupidity. My experiment did work, after a fashion, but his idea is somewhat better. Stay tuned.

Addendum2: Here’s the video that I shot tonight. It’s still horribly wrong, but in the interest of full disclosure, I thought I’d toss it out there. I need to work at understanding the filters and the like, maybe add a buffer, and consider the biasing. What can I say, my eyes were dialated, my brain foggy. The aliased copies I hear at the end are probably not going to be present if I actually did what Scott did. Stay tuned, I’ll redo this after some thought.


Morning pass of ARISSAT-1

Success! I managed to record the pass of ARISSAT-1 from my front yard this morning.

I woke up around 7:00AM, and tried quickly to get ready. I pulled on some clothes, then I dug out my handy Arrow antenna (a hand held dual band Yagi antenna for 2m and 70cm) and my tiny Yaesu VX-3 that I had remembered to charge, a patch cable, my MacBook, and headed to the front yard. Yikes! The sprinklers were on. I turned them off, then dug a camping chair out of the back of my SUV so I had a dry place to sit. I also got a tripod with my little Kodak HD video recorder mounted on it, and aimed it roughly at the chair, thinking I’d shoot some video.

Here’s the prediction that my home grown library did for the pass:

RADIOSCAF-B will be visible from grid CM87ux starting in 00:13:35 at 14:28:04
  14:28:04  +0.1° 312.2° ?  49.1°N 144.6°W AOS 381.52
  14:29:00  +3.9° 313.7° ?  47.8°N 139.7°W     382.01
  14:30:00  +9.2° 316.1° ?  46.1°N 134.7°W     382.55
  14:31:00 +17.0° 320.3° ?  44.3°N 130.1°W     383.11
  14:32:00 +30.9° 330.2° ?  42.3°N 125.8°W     383.68
  14:33:00 +55.2°  10.5° ?  40.0°N 121.7°W     384.27
  14:33:14 +58.2°  32.9° ?  39.5°N 120.9°W MAX 384.41
  14:34:00 +43.6°  90.6° ?  37.7°N 118.0°W     384.86
  14:35:00 +23.4° 110.2° ?  35.2°N 114.5°W     385.46
  14:36:00 +13.0° 116.6° ?  32.6°N 111.2°W     386.07
  14:37:00  +6.5° 119.8° ?  29.9°N 108.2°W     386.67
  14:38:00  +1.9° 121.7° ?  27.2°N 105.3°W     387.27
  14:38:28  +0.1° 122.3° ?  25.9°N 104.0°W LOS 387.55

The satellite was supposed to start to the north, then pass to the east, reaching maximum altitude at about 14:33. I figured I wouldn’t be able to hear the satellite at all until it was above 20 degrees or so, maybe at 14:31UTC, because I have a rather high horizon to the north. But at 14:29 I tentatively turned on my radio, tuned to 145.950 and aimed it roughly north, and it was clear that there was some signal coming in. I started Audacity recording. After a minute or so, the signal was very strong, full quieting between deep fades that seemed to occur at around six second intervals.

Here’s the .mp3 file. It’s fine for listening to, and may (or may not) be adequate for SSTV decoding. If you really want to try SSTV, mail me a request and I’ll try to arrange a way to get you the 33M uncompressed audio file. I beeped out the “secret word” that appears in the recording, so as not to spoil it for others.

ARISSAT-1 Recording, Aug 5, 2011, 14:30UTC, 3.3M MP3 File

After I was done, I fired up MMSSTV on my windows box and did some SSTV decodes, here are the images as they appeared in the recording.

Not too bad. It’s pretty clear from the recording and from the pictures that the satellite is tumbling: periodic deep fades appear to happen about every six seconds or so. Still, the imagery is a pretty neat bonus. As the satellite’s spin slows, we should be able to easily get good SSTV imagery, even with equipment as primitive as mine.

According to the ARISSAT-1 website, the satellite was 20 seconds ahead of the ISS, and maybe a kilometer lower. It’s projected lifetime is short: maybe just sixty days, so don’t delay. Tune it in, collect your certificates, and have some fun.

Antenna? Who needs an antenna?

Drew, KO4MA, didn’t let a little thing like the missing antenna on ARISSAT-1 keep him down. He aimed his antennas at the satellite, and recorded the following:



What’s goin on here? Drew has a pretty good antenna setup, which includes an M2 CP42 for the uplink. That provides about 16.8db of gain. Since the ARISSAT-1 receiver was supposed to be reasonably sensitive to be workable with portable equipment, that gain is enough to put a reasonable signal into the transponder. I don’t think I am going to be working the bird with my setup, but i suspect that there are quite a few amateurs with similar setups that could still work ARISSAT-1 even with its lack of antenna.

Very cool.

Addendum: I was rereading my posts from yesterday, when the frustration of deploying ARISSAT-1 with a missing antenna was fresh in my mind. And I do frankly admit to a fair amount of frustration with AMSAT. But I was reading some even more negative traffic on the amsat-bb list, and that put it in a bit of perspective for me. ARISSAT-1 wasn’t a waste, nor is it useless, nor was it a bad thing for AMSAT to invest in. In terms of its primary, largely educational mission, I suspect it will be quite successful. Several times during the NASA briefing yesterday, they mentioned that this was the first of a series of educational birds to be launched from the ISS. These launch opportunities are incredibly valuable to the amateur community, and we should make sure that our frustration with problems doesn’t boil over into lashing out at our allies who are helping us achieve our goals in amateur radio and space.

ARISSAT-1 Update…

Okay folks, I’m sorry, I promised that I’d try to stay up late enough to record some of the first passes of ARISSAT-1, but physical need for sleep outpaced my natural enthusiasm and curiosity, and the first good pass found me sound asleep. Through some perverse quirk of fate, all the best passes of the ISS are occurring in the early morning hours. Running my own homebrew satellite prediction library, I find that my best opportunity for a pass where I am awake and the satellite is illuninated, and therefore in high power mode, will likely be tomorrow starting around 7:28AM PST. I’ll try to get myself all setup for that, and will hopefully have some audio and/or video tomorrow.

In the meantime, it has been heard.

VK2BRB recorded this YouTube video:



SW1OBT heard the voice beacon, and the start of the SSTV transmission:



ARISSAT-1 is off to a bumpy start…

First, the good news: ARISSAT-1 is floating free in space. I urge hams to listen for its 2m downlink on 145.950 to see if you can hear it. There is also a CW beacon on 145.919 and a special BPSK-1000 telemetry downlink on 145.920.

Now, the bad news. It appears that the UHF antenna was damaged or broken off at some time. The satellite was supposed to operate a 70cm->2m linear transponder, but without this antenna, it is unknown what the status of that capability will be (politely put, it’s almost certainly not going to work).

Okay, now the commentary:

It seemed rather obvious that the handling and deployment of the satellite was ill-conceived and poorly handled. You might imagine that some pre-EVA check of the satellite could have determined that the satellite was missing its antenna. You’d also imagine that the relative fragility of the solar cells (mentioned by one of the cosmonauts well into the EVA, after the satellite had been spun around and “contacted” all sorts of objects on the space shuttle) would have been part of a pre-mission briefing. Watching the video, it seemed that these cosmonauts were using all the care that a bunch of high school kids you hired to move your couch might have had. In the end, it appeared to me that they deployed the satellite simply because they couldn’t be bothered to stow it again as much as to achieve the goal of launching this amateur payload.

And lack of communication is one of the single most frustrating things I find about AMSAT. Surf on over to their website at arissat1.org and look for an update. It’s been two hours since deployment, and here is the only update in information that you can see:

Really? That’s all you have to say about the deployment? The culmination of four years of planning, building and testing, and it’s finally in space, and you can’t even file a report that contains any actual information?

I was curious about what the configuration of these antennas actually was. After finding a couple of broken links to the February 2010 design review, I found this page:

Design Review of ARISSAT-1

Lou McFadin, W5DID did a presentation on the antenna systems. It appears that the antennas were designed to be detachable and use a flange system similar to other antennas deployed on the ISS. There is a lot of detail missing from the presentation, but it seems to me that the obvious failure mode is simply forgetting to install the antennas rather than damage to them. The fact that the antenna situation was only detected once outside the ISS seemed very odd indeed. It should also be noted that one of the design goals was to be able to endure “rough handling”, which means that if the antenna were damaged, then one of its primary design goals was not adequately met.

Frustrating!

Oh well. I’ll be listening for telemetry on its first good pass, which appears to be a 38 degree pass after 1:00AM this morning. I’ll let you know what I find out. I’m monitoring a bunch of blogs to see if anyone is picking up telemetry, expect updates as addendums to this post later in the day.

A nice Flickr set of images from the NASA feed of the deployment

Addendum: It lives! Masa, JN1GKZ reported on the ARISSat-1 website that he heard the FM, CW and BPSK signals on 2m during its pass over Tokyo. It can run in “high power” mode when illuminated, the late night passes I will see tonight will be in eclipse: we’ll see how low power mode works.

On the Ham Nation podcast, or the perils of perpetual promotion.

Ham Nation is a relatively new weekly podcast that is brought to us through the power of the TWIT network, Leo Laporte’s mighty podcast empire. Fellow blogger KE9V had some comments on it, some of which I agree with, and some of which I do not.

Of Dits and Bits | KE9V’s Ham Radio Blog.

First, the positives. Hosts Bob Heil and Gordon West are practically legends in the world of ham radio. Both are charming gentlemen, deeply concerned with promoting ham radio.

And in a strange sort of way, that’s what I find to be most disconcerting about the show. It tries really, really hard to convince you that ham radio is exciting and relevant. I do firmly believe it’s a great hobby. Or more precisely, it’s a bunch of great hobbies, all united under the need to get a license to operate. Some aspects of the hobby I like better than others. I can appreciate that other people might like aspects of the hobby that I do not. I can also appreciate that some people might want to promote the hobby.

But I can’t help but think that we overstep our promotion of ham radio sometimes. In trying to understand why I feel that way, I see two primary causes.

First, in our haste to bring new people into the hobby, we try to talk about ham radio in a way that the uninitiated can understand. That’s okay, but unless we are careful, we end up never talking about anything else. And frankly, I see that a lot in the first ten episodes of Ham Nation. Like KE9V, I haven’t seen that there is anything being presented in Ham Nation that anyone whose been in the hobby for more than a year will care about. In an effort to make the hobby understandable to the layman, we are stripping out everything interesting from our hobby. We are providing no inspiration to deeper achievement in ham radio. We perpetually talk about what equipment to buy, but never talk about the equipment we build. We promote simple wire antennas, without working to provide information on more complex and therefore higher performing antenna systems. And Ham Nation has this in spades. It’s not too surprising: Gordon West has made a career out of selling study aids for ham radio and getting new people into the hobby. But I can’t help but wish that there was a podcast that actually talked about ham radio in a way that at least periodically reached beyond how to solder and your first wire antenna.

Second, there is the idea that ham radio is unique among hobby in promoting virtue and intellectual development. Most hams are great people, but I believe that most people are great people. There are some really amazingly intelligent people in ham radio, but there are intelligent people in all walks of life, with all sorts of hobbies. I suspect that if your hobby was windsurfing, most of your fellow windsurfers would be good, intelligent people too. The difference is that windsurfers don’t spend a lot of time trying to convince you that windsurfers are really great people. They just accept that if you want to be a windsurfer, you’ll learn what you need to, and you’ll join a great group doing something they love. Hams want to convince you that they are a bunch of great guys, doing great stuff, but they seem to spend all their time talking about how great they are, and relatively little time showing you how great they are.

Ham Nation suffers from both of these ills.

It’s not enough to have celebrity hams. It’s not enough to tell everyone how great ham radio is and how much they will love it and how easy it is. You have to get on with doing ham radio, to demonstrate how cool you think it is, and let other people make up their own minds. When people like Kevin Rose express skepticism about the hobby, we shouldn’t tell him he is wrong: we should show him he is wrong. And we should accept that perhaps he isn’t wrong, at least for him.

It’s just a hobby after all.

On RC Filters…

Over on the #savagecircuits IRC channel on irc.afternet.org, Atdiy was trying to decipher the mysteries of a mainstay of analog circuit design: the RC filter such as the one pictured on the right (diagram cribbed from Wikipedia) It dawned on me that (newbie as I am) I didn’t really have a clear understanding of them either, so I thought it would be worth the exercise to go through the mathematics of them as well, and try to derive the simple formulas from (more or less) first principles.

First of all, it’s interesting to just try to understand what happens at DC voltages. No current flows through the capacitor at DC, so the output voltage is just the input voltage (when measured as an open circuit, more on this later). If you are a total newbie like me, you can look at the capacitor symbol, and note that one side is not connected to the other (it’s an open circuit), so no current can flow across it (at least, at DC) frequencies. But at higher frequencies, we need to consider the behavior of the capacitor, which means that we need to consider impedance.

If you don’t grok complex numbers, the road ahead will be a tiny bit bumpy. I didn’t have enough brain power to write an intro to complex arithmetic, but if it’s new to you, consider reading up on it. For now, just remember that complex numbers can be manipulated like regular numbers (adding, multiplying,dividing and the like) but have different rules.

Rather than derive anything at this point, we’ll just cheat a tiny bit and introduce a formula to find the reactance of the capacitor. (You should not feel too uneasy at this point by introducing what seems like magic at this point. The following equation just describes how capacitors work. You probably accepted that resistors could be described by a single number (expressed in ohms). Because capacitors have more complicated behavior in AC circuits, they need a bit more complicated description, but not too much).

The capacitive reactance of the capacitor is given by:

[latex]X_C = \frac{1}{2 \pi f C}[/latex]

where [latex]f[/latex] is the frequency of operation, and [latex]C[/latex] is the capacitance in farads (be careful, not microfarads). For instance, a 1 nanofarad capacitor has a reactance of about 160 Ohms at 1Mhz.

I glossed over something a bit there. The reactance we computed is measured in ohms, which makes it seem like it’s a resistance. But in reality, it’s an impedance (usually written as the symbol [latex]Z[/latex]). Impedance is a measure of the opposition of a circuit to current, but it generalizes resistance by taking into account the frequency dependent behavior of capacitors and inductors. In particular, impedance varies with frequency.

You can split the impedance into the sum of two parts, the pure resistance (usually written as [latex]R[/latex]) and the reactance (usually written as [latex]X[/latex], as we did above when we were computing the reactance of the capacitor). The total reactance of a circuit is the inductive reactance minus the capacitive reactance. If the overall reactance is negative, the circuit looks like a capacitor. If the sum is positive, it looks like an inductor. If they precisely cancel, then the reactance disappears, and the circuit presents no opposition to current at all.

But how do we handle resistance? Here’s where the complex numbers come in.

[latex]Z = \sqrt{R^2 + X^2}[/latex]

[latex]X = X_L – X_C[/latex]

The nifty bit is that you can treat impedances exactly as if they were resistances (e.g. we can use Ohm’s law) just so long as we remember that we are dealing with complex quantities. I decided to write a small Python program which could be used to compute the filter responses of RC filters:

[sourcecode lang=”python”]
#!/usr/bin/env python
#
# Here is a program that works out how RC filters
# work. Here is a diagram of the prototypical
# RC low pass filter, sampled across a resistive
# load Rload.
#
# R
# Vin -> —/\/\/\/\—-+——–+ Vout
# | |
# | /
# | \
# C —– / Rload
# —– \
# | /
# | \
# | |
# —– —–
# — —
# – –

import math
import cmath

RLoad = 1e9
R = 600 #
C = 0.0000001 # .1uF

for f in range(10, 100000, 10):
XC = complex(0., -1/(2.0*math.pi*f*C))
Z = cmath.sqrt(XC*XC)
t = RLoad * Z / (RLoad + Z)
Vout = t / (R + t)
print f, 20.0 * math.log10(cmath.sqrt(Vout*Vout.conjugate()).real)
[/sourcecode]

If you take the data that gets printed out here and graph it, you get the following:

I built the same circuit in LTSpice just to check my understanding, and the graph appears identical. If you search through the data to find the place where the response drops to -3db, you find it occurs around 2650 Hz. The traditional formula for filter bandwidth is

[latex]F = \frac{1}{2 \pi R C}[/latex]

which when you plug in the values I chose, you end up with 2653.9 (which is pretty good agreement).

Addendum: This filter works best when the input impedance is very low (compared to the 600 ohms that we chose for the input resistor [latex]R[/latex]) and where the load impedance [latex]Rload[/latex] (here purely resistive, but it could also be complex) is high. If the load were low (Rload was comparable or less than than R) then the losses would be higher. (Ignore the capacitor. If Rload is small compared to R, then the voltage, even at DC, is already divided down to a lower value, this shifts the entire curve downward).

Addendum2: I wrote this hastily. If it doesn’t make sense, or people have suggestions, don’t hesitate to add them in the comments below.

Addendum3: Atdiy asked me to run some simulations showing how the load [latex]Rload[/latex] changes the output to demonstrate loss. Here, I kept [latex]R = 100[/latex], but varied [latex]Rload[/latex] from 1M ohms, to 1Kohms, and then down to just 100ohms. You can see the entire curve moves down, with only 100 ohms of load resistance, the voltage is -6db (multiplied by 0.25) even at DC).

Addendum4: Atidy has started her series on filter design, which includes a qualitative description of how these filters work that you might find easier to understand. Check it out here.
.

Dave Richards, AA7EE constructs The WBR by N1BYT

Thanks to Bill at the SolderSmoke blog for posting a link to Dave Richards’ construction project. He made a slick version of the Wheatstone Bridge Receiver, a regenerative variant created by N1BYT and published in More QRP Power. I’ve looked at this receiver before, and found the design to be pretty interesting, but Dave goes above and beyond by showing all the things my projects lack in great abundance: like neatness, care, and planning. Check it out:

The WBR – A Simple High Performance Regen Receiver for 40M by N1BYT « Dave Richards AA7EE.