Dave Richards AA7EE

June 18, 2012

Photography and Air Spaced Variable Capacitors

Several people have commented to me that they enjoy the pictures I take for this blog, and Mike VK1oo recently asked if I could write a post detailing how I take them. I don’t want to turn this into a photography tutorial and acknowledge that there are other (and perhaps better) ways to take pictures for a blog, but this is how I take mine.

Firstly, I use a DSLR. There is a noticeable difference in the image quality that comes from compact digital cameras with their smaller image sensors, and that of the images that emerge from a larger-sensor camera like a DSLR (Digital Single-Lens Reflex).  The number of megapixels is not that important, especially if you’re taking pictures mainly for the internet.  The common standard for images in print publications is 300 dpi (dots per inch), while you need considerably less than that to produce a great looking image on a computer monitor. Perhaps more important than the number of pixels on your camera’s sensor is the quality of those pixels. With the larger sensors of DSLR’s you can make the pixels larger, which makes for lower noise in the final images.  However, all these discussions aside, there is one factor that I think makes more difference than anything else in the perceived higher image quality of DSLR’s and that is the control over DOF (depth of field) that they give you. Put in layman’s terms, a DSLR makes it much easier to exercise some control over what parts of the picture are in, and what parts are out, of focus.  You can even control how out of focus they are. With a compact camera, almost everything is in focus.  This is good when you want everything to be in focus, but terribly frustrating when you don’t. With a DSLR, if you want very selective focus, you just open your lens aperture up to something nice and big, like f2.8, or even bigger if your lens is capable of it and you’re brave.

Why would depth of field make a difference, especially to pictures of radio and electronic parts? It doesn’t if your only interest is purely seeing how things look and how they go together, as in kit assembly instructions, for example. However, when we look at things in real life, we don’t look at everything – we concentrate on the things in our field of vision that interest us. The things that don’t interest us are not necessarily out of focus, they’re just not there (to our brains anyway).  The selective focus of a camera with a larger sensor can approximate this by throwing the things that are not central to the main purpose of the picture, out of focus. Occasionally, I have employed this technique in a rather extreme fashion. One or 2 of the pictures of my K2 build were like that, but I think the technique is more effective when it’s subtle. As an example, I’d like to offer up this photo of an air-spaced variable capacitor that turned up in the mail this week:

The background is moderately out of focus. If you look closely, you’ll see that the vanes of the capacitor are also very slightly out of focus. The front part of the capacitor is in focus, and so is the rule. Because the capacitor vanes are only slightly less sharp, there is no loss of detail, but this slightly limited depth of field helps to make the picture look natural and appealing (in my opinion). The fact that that the subject is a Millen 50pF air-spaced variable capacitor with ceramic end-plates and what looks like silver-plating is an important factor in the appeal too!

Another issue that is very, very important when you’re attempting to take good pictures is lighting.  When I say a “good” picture, I mean one that achieves what you initially set out to do. I don’t know much about how to control lighting for photography indoors.  If you don’t have flash, studio strobe lighting, reflectors, snoots, diffusion panels and all the other lighting gear that many serious photographers have, there is a quick and easy fix that will help a lot – go outside and take your pictures either out in the open if it’s overcast, or in the shade if it’s sunny. What you need is bright diffuse light. Direct light creates a lot of contrast and you don’t want that, because cameras can’t handle it. Photographers spend a lot of time controlling the contrast in their photographs, so our best defense is to look for lighting conditions that don’t create that contrast in the first place. I love bright cloudy days because it means that I can take pictures for my blog pretty much anywhere outside.  If the sun is out, I gravitate to a place in the shade that is still fairly bright.

Even when you’re photographing on an overcast day, the light does still cast shadows – they’re just not as distinct.  There may be certain parts of your project than need a bit more light so the viewer can see the details (especially the innards) so try and orient it so that a little more light gets into and illuminates the parts where you want viewers to be able to see details.

One more thing about light and this is really getting a bit esoteric, but late morning and early morning light has a special quality. The above photo of the variable capacitor was taken early in the morning. The sun had only just risen and there were some very weak diffuse rays of early morning sun lighting the capacitor. See that very slight warm coloration on the metal parts? I think it’s gorgeous but I wouldn’t go out of my way for that. I just happened to be taking photos for the blog at that time.

It’s important to really pay attention when you’re taking photographs. Make sure you get everything in the frame that you want your viewers to see, and don’t include anything that you don’t want them to see. That leaf or bit of dirt lurking behind your project – get it out of the way. It’s much easier to move these things at the time than erase them in Photoshop afterwards. Also pay close attention to the angle from which you take your photograph. Do you want the final image to show a certain detail to the exclusion of most others, or are you trying to give the blog-reader a good overall view of what your creation looks like?  If you make a habit of always asking yourself what the purpose of your picture is, then it sets the stage for the next step, which is doing what is necessary to have your picture achieve that purpose.

Many of my pictures are taken with the project at an angle and with the subject of the photograph not exactly in the center of the frame.  The reason I often orient the object at an angle is because I’m trying to give you an overall feel for what it looks like. If I just show you the front panel, you don’t get a sense of how deep it is.  The reason for not always putting the subject slap-bang in the center of the frame is more of an aesthetic one; I just think it looks nicer. If the foreground of the picture is a blurred piece of ground, and the project is set off-center, a bit further back, and is sharply in focus for example, I think that this helps to draw the viewer into the frame.  I could be wrong; this is all just opinion.

When you’re taking pictures, take your time and take lots from different angles. Don’t be afraid to lie on your tummy and get really close. This thing you’ve just built is your pride and joy and you want to show it off in the very best light to your online readers. If you’re using a DSLR, experiment with different apertures, so you can see the effects of different depths of field on the final image. I spend quite a bit of time crouched low down and lying on my stomach on the ground looking at my projects through the lens of my camera from different angles.  It’s fun.

Here are my bullet point suggestions for good home-brew blog photography:

  • If you have a DSLR (or other camera with a larger image sensor) and know how to use it, this should (IMO) be your camera-of-choice
  • If you don’t have a DSLR, use whatever camera you have
  • Take your pictures in bright diffuse lighting (unless you’re experienced with studio lighting). Outdoors on an overcast day works well. If the sun is out, look for the shade. The idea that only sunny days are good for taking pictures is a hold-over from the time of cheap film cameras with fixed (and small) apertures that, combined with the medium to slow speed films of the day, needed the bright light of a sunny day to get an image at all. Nowadays, sunny days are not your friend (unless you have the lighting tools to control the high contrast that direct sun causes);  bright overcast days are, as well as bright shade on sunny days.
  • Explore your subject – your home-brew project in this case, from different angles, both close-up and from a slightly further distance.  Shooting down from directly above might be the best way to show the layout of components on a circuit board if your main aim is show people what stages and components go where.  Pay attention to what aspects of the project you wish to display in each shot.  Take your time and enjoy looking at your creation through a lens.

Enough about photography, and back to my new preoccupation – air-spaced variable capacitors. Todd VE7BPO has a great page on his site about building VFO’s.  He has details of a very stable Vackar VFO for 40M with which he achieved a drift of the order of 5Hz/hour. That is about as good as you can get with a free-running VFO, and it’s a pretty astounding figure.  If I could get 10Hz/hour, I’d be pretty happy, and I’m thinking that the variable capacitor pictured above (which I got from an eBay seller for $12.50) might do well in this VFO. Here it is again:

I’m a novice on the finer points of variable capacitors for VFO’s but the fact that this Millen 21050 has 2 ceramic end-plates makes for a nice stable construction – more so than the ones whose frame is a single piece of metal bent in a u-shape. The vanes appear to be silver-plated (I don’t know if and why this is a good thing). I don’t know what metal the vanes are made of.  Brass is the best as it has a lower temperature coefficient than aluminum, which is a more common material for variable capacitor vanes. There are no bearings.

Another contender for a VFO could be this one, which was part of a care package of parts from a very generous ham:

The ham who sent me that variable also sent me this one  (this picture featured in an earlier post on the DSB80) :

I also received this variable capacitor, reduction drive and mounting bracket from The Xtal Set Society this last week (the candy and copy of The Xtal Set society Newsletter was also in the package):

It’s great to find a supply of new reduction drives (I don’t know of too many). The air-spaced variable capacitors are also new and while they are not the very highest quality in terms of mechanical stability and Q, there are many uses I can think of for them.  Even for non-critical VFO applications I think they’d do OK – they have to be significantly better than the polyvaricons which are used in some VFO’s nowadays.  It’s a single gang 365pF with a built-in 8:1 reduction drive, which has a nice smooth action.  The actual maximum capacitance is closer to 390pF.  I’d quite like to make a broadcast band regen with this little fellow – or perhaps a direct conversion receiver for 160 or 80M. The neat thing is that the mounting bracket in the above picture is specifically designed to hold the reduction drive at the right height for their variable capacitors, making the mechanics of connecting it all together a lot easier.

Here’s one more photo of the little beauty:

There has been some progress on the DSB80 this week – more on that in a future post.

June 3, 2012

The DSB80 – A Direct Conversion DSB Transceiver for 80M By G4JST and G3WPO – First Stage Of Building

I was 19 years old in March 1983 when the UK magazine Ham Radio Today published the article “A Low Cost DSB/CW Transceiver for 80M”. Being short of cash and wanting to get on the air, I sent away for the kit and soon after, was surfing the phone portion of the UK 80M band on my new DSB rig.  I didn’t get too many QSO’s due probably, to my poor 80M dipole, although G3UMV who lived just a mile down the road heard my home-brew signals and came over to see where they were coming from.  The receiver seemed to work very well, and I spent many hours listening to the chatting between 3.6 and 3.8MHz (80M only goes as high as 3.8MHz in the UK). The whole thing was enclosed in an aluminum case and tuned with a Philmore vernier attached to a polyvaricon.  I don’t remember any drift, so it must have been stable enough for sideband, and it didn’t have any noticeable microphonics either.  As it was my first DC receiver, I didn’t even know that this type of circuit often suffered from microphonics, as this one didn’t have any to speak of.

That little rig made it with me across the Atlantic and met it’s end one day in my apartment just a block from Hollywood Blvd. In a passing wave of nostalgia for my earlier radio days, I hooked it up to 12V DC to see if it still worked. It would have, had I not connected the 12V the wrong way round, and if I’d had the foresight as a kid to provide it with reverse polarity protection. I still don’t know why I didn’t just put it aside so that at a later date I could have replaced the damaged active devices. Unfortunately, I tossed it into the dumpster of my apartment building. What a shame – and it had an SBL1-8 mixer too!

From time to time either when moving or thoroughly tidying my apartment, I come across the copy of the original article that came with the kit. Trouble is, whenever I looked specifically for it, I could never find it, and the only copy of the article I was able to find on the internet is of too low a resolution to be of much use. I’ve been wanting to recreate this rig for a while and recently, when the desire became too strong to ignore, decided that I was going to find that article even if it took a day or two of searching. It did, but I did.

Pure nostalgia wasn’t the main reason for my wanting to build this rig again. A big reason is that I have always been drawn to simple receiver topologies such as regens and direct conversion receivers, yet not all designs are created equal. I remembered this one as working well and on top of that, it used something in the circuit that you don’t see too often in DC receiver designs these days – a passive diode ring mixer package (NE602 anyone?)  I wanted to build a DSB rig that used a diode ring mixer package, so this is why I am here.

The schematics for this rig aren’t that easy to come across.  I eventually a found a low-res version of the article online after some searching but it’s not really good enough to work from.,Ham Radio Today is no longer being published, and the company that sold the kit back in the 80’s, G3WPO Communications,  went out of business a long time ago. On top of that, Tony Bailey G3WPO is no longer an active radio amateur. On this page on his website he gives a link to a reprint of an article about another of his projects, the Minisynth VFO. Judging from this, and what he says in the whole of that 3rd paragraph, I don’t think he’d mind my publishing the schematics for the DSB80 here on my blog.  That is what I am hoping as I’m pretty sure that some readers will want to see the schematics, and there doesn’t seem to be any other way to get them. I’m having a bit of trouble with the VFO (more on that later), so by showing you photos of my construction and the schematics, I’m hoping someone may be able to help me.

The plan is to get this working well as a receiver first, after which I’ll add the transmitter stages.  So to kick things off, here’s a block diagram of the receiver, a pretty standard diagram of a direct conversion receiver:

I built the VFO first of all. It seemed to work OK and be reasonably stable, with drift of less than 80Hz/hour after warm-up. I know that’s not stellar, but a bit of temperature compensation could help that.  Somewhere in between adding the buffer and adding the rest of the receiver, I noticed that the VFO was drifting a bit more and FM’ing, which makes SSB sound pretty bad. However, if I can lower the drift on the VFO and get it to stop FM’ing, I think I’ll have a nice-sounding direct conversion receiver on my hands. There are virtually no microphonics – you have to turn the volume way up and really be listening in order to hear them. For all practical purposes, microphony is just not a problem; something I like very much in a DC receiver.

I’m getting ahead of myself. Here’s the original circuit for the 80M VFO in the DSB80. I did leave out a trimcap, 1N4148 diode and associated components that were used to switch in a CW offset, as I won’t be using this rig for CW:

Important – please note that I experienced instability with the buffer transistor Q2 (it wasn’t doing a lot of buffering). I don’t understand why Q1 was coupled to Q2 with a 100 ohm resistor instead of a capacitor, but at the suggestion of K4AHO, I replaced it with a coupling capacitor (I used a 39pF NPO) and put a 100K resistor from the gate of Q2 to ground. My problems with the buffer cleared up and the receiver sounded really great. I’ll publish the schematic of the entire receiver section of this rig in a future post.

The oscillator is a Colpitt’s configuration and the 260pF variable was, in the original design, a polyvaricon.  I wanted to modify the VFO for varactor tuning (at what point did we stop calling them by the more descriptive term varicaps and start using the name that makes them sound like a prehistoric bird?) and also figured that a 78L08 regulator in place of the 8.2V zener diode could only help. This is what I came up with:

All the caps marked “poly” are polystyrene. I changed the values of the 2 x 1000pF caps to 1200pF simply because that’s what was available.

Projects always look pretty when I first start them, before I’ve had a chance to mess them up –

Here’s the VFO, although I have yet to add the varactor at this point – it will be located at the far right end of the board –

One more view just for the heck of it –

Somehow, by the time I got around to adding the mixer, AF preamp, AF amp and bandpass filter to complete the receiver, the whole thing started to look just a little bit messy. You’ll notice that I ditched the nylon mounting hardware for the VFO toroid in favor of a single nylon strap. I figured it would be one less material in contact with a frequency-determining part of the circuit that might cause drift. I didn’t clean up the board for it’s photo-op, as this is a work-in-progress that may not make it out of the emergency room –

VFO drift was steadily downwards after the initial warm-up and probably something that could be brought to within useable limits with some temperature compensation work.  There are almost no microphonics to speak of (always a good thing in a direct conversion receiver), and only a small amount of broadcast band break-through which is only coming through at the kind of high volumes that will rarely be used. This breakthrough is not coming from the DBM, but from the preamp, which is set for a gain of 1,000 (60dB). If this rig makes it to a later stage of building, I may reduce the gain of that preamp just a bit – we’ll see. The receiver sounds pretty good on 80M SSB with one big problem – the VFO FM’s when receiving signals, and that IS a problem.

The documentation that came with my kit for this rig in 1983 had something to say about  FM’ing of the VFO:

Hmmm….but I was using J310’s in this re-creation and was still getting FM’ing of the VFO.  As far as I can remember, it was not happening in the original version I had.

I decided to build the original VFO circuit with the zener diode regulation and with an air-spaced variable capacitor instead of a varactor to tune the circuit (the first schematic in this post and the second image down). I wanted to do this on a separate board, before connecting it to the rest of the receiver.  This is where it started. It sure was exciting looking at a bare board (blank canvas) with just an air-spaced variable capacitor. The variable capacitor was given to me by a friend and boy, what a great gift. Thank you – you know who you are. I was looking at this thing and thinking of all the possibilities – a signal generator, crystal set, or a regen perhaps?  Air-spaced variable capacitors are very inspiring to me –

Here’s the VFO circuit built – no buffer yet, and you’ll notice that I have not yet installed C2 and C3.  I wanted to see what the frequency coverage was without those capacitors first.  It was pretty wide, so I ended up installing C2 and C3 in the values suggested, and removing a few turns from L1 to achieve coverage of 3600 – 4000 KHz with one gang of the capacitor, which was about 330pF at full capacitance –

And with the buffer added, and temporarily terminated with a 51 ohm resistor for drift testing:

I noticed that on touching the output of the buffer with a small metal screwdriver, the frequency of the VFO changed by about 600Hz when terminated with the 51 ohm resistor. I wonder if this is the reason the VFO I built on the main board FM’s when receiving signals? The only difference between this one and the one that is incorporated into the rest of the circuit as pictured 5 images above, is that the one directly above is tuned by a variable capacitor, whereas the other one is tuned by a varactor. Either way, it suggests to me that I need another stage of buffering. Before I even look at the drift and figure out how to compensate for it, I need to tackle this issue.

To be continued……..(unless another project derails this and it ends up on the shelf, with the variable capacitor used for something else…..)

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