Sunday, May 27, 2007

Mini field day at Dural

Attended the mini field day at the WIA NSW Dural site today. Beautiful weather for it, a good turn out, got some useful bits and pieces though nothing major. Quite a few car boot sellers oddly selling complete HF setups with older style rigs.

The home brew team got together with a bit of "show and tell", I brought along my MMR-40 and compared notes with John, another builder. Stephen, VK2BLQ, showed an experimental 80m short vertical which we tried to tune up by using the "pin through the insulation" technique of finding the right spot in the coil.

Alan, VK2ZAY, told me he'd built a tiny 2m AM transmitter Fredbox last night and when we got home I could hear it at my place, which is about 5Km away. Not bad for an estimated 30mW!

Gosh, during the writing of this story Blogger appears to be down for several minutes. Could Google be fallible?? It's giving 502 Server Errors.

Alan wrote up the day here, he scored some excellent bits and pieces I should have been more alert.

Friday, May 25, 2007

Built a 40m SSB transceiver kit: MMR-40

Since having a wisdom tooth out on Tuesday I've been laying low this week and took the opportunity to construct a kit.

Shown at right during construction, the MMR-40 from Hendricks QRP kits is a great little kit transceiver. Mine was missing a couple of diodes but was otherwise complete. I had a few problems getting the receiver going due to the fact that I hadn't read the errata and mistook the different zener diode for one of the missing diodes and experienced some very strange voltages that affected the transmit/receive switching circuit.

After a long night of hair pulling I joined the Yahoo MMR40 owners group and posted a question about my symptoms. Very quickly Tom replied from a Blackberry with a pointer to the problem. Great stuff.

I find building kits is a great way to get familiar with components and in particular diagnosing problems helps me greatly in understanding the circuit. I never dreamed that I could build an SSB transmitter. Still some work to do to get it all lined up but it seems to be putting out a few watts.

An interesting part of this design is the permeability tuned oscillator where part of the inductance is varied by screwing a brass bolt in and out of the coil. You can see it at the bottom right of the image above. Counter-intuitively, to me, is that frequency goes down as you wind the bolt in. The good thing is that you get slow change over turns of the knob and it seems remarkably unaffected by hand capacitance.

Update: After swapping out the mis-placed zener the receiver was still a little disappointing. Turned out that like others I had followed the layout diagram and installed a 22pF capacitor at C18 which caused T1 not to tune up. The only other oddity is that if I turn my variable supply up much above 12V the receiver mutes, not a big deal.

Anyhow, the receiver is working really nicely and is sensitive enough to hear all that bad band noise on 40m, which is all you need really. I'm not confident that my transmitted SSB is right yet, it puts out power when I talk but doesn't look like real SSB just yet.

A simple modification has been to install a stereo headphone jack that cuts off the speaker. Builders should note that the speaker really comes to life when you put it in the box and close it, very soft on the bench on its own.

I'm enjoying this but I think the kit errata needs an update. The design is very clever in that it re-uses lots of components for both receive and transmit but it lacks a block diagram so I've tried to create my own here.



(Click for a larger version). Please send me corrections!

Incidentally, wisdom tooth removal went very well - the wonders of modern dentistry.

Sunday, May 13, 2007

Built a roll-up 2m j-pole antenna

I saw Dom Bragge's posting about a 2m J-Pole antenna made out of 300 ohm TV ribbon and thought I'd give it a go.

Although the design specifies 51 inches in length, Dom notes that other builders have found this resonates too high in frequency and they suggest you start at 52 inches. I did that but found that the lowest SWR was at 148Mhz so even that's too short.

Perhaps 300 ohm twinlead material is different to the original designer.

I do have one contribution to make on this topic, it's really hard to strip the insulation from this twinlead. The insulation seems very strongly bonded to the conductors and they always rip out for me. The solution is to melt your way through with a soldering iron.

Some toxic vapour comes off but it seems pretty easy to tin the wires after the plastic melts away. Shown above is the bottom of the antenna where the co-ax connects and the shorting bar at the base. The melting works quite nicely as you can see.

Thanks to Dom for the pointer to this simple and highly portable VHF antenna design.

Saturday, May 12, 2007

The Nature of Photographs - mini book review


Spotted this wonderful book this morning. "The Nature of Photographs" by Stephen Shore, subtitled "A Primer".

The reproductions of photographs is fantastic in this high quality publication from Phaidon. Works are by the author and many others including Robert Frank, Walker Evans, Cindy Sherman, Lee Friedlander, André Kertész, William Eggleston, Garry Winogrand, Bernice Abbott, Dorothea Lange, Alfred Stieglitz, Diane Arbus, Eugene Atget, and even the great John Szarkowski is in there.

Well structured and simply stated, this book has given me a good structure for appreciating photographs. Packed with insights that in retrospect seem obvious. For example, the artist starts with a blank page and must fill it, the photographer starts with the clutter of the world and must simplify it.

Progress on the 80m challenge transmitter

Finally making progress on the NSW Home Brew group's 80m challenge project to build a 20W 80m AM transmitter.

I'm a bit daunted as I have never made anything that puts out more than about 500mW and have very little RF experience, but progress is finally being made.

A design has been cobbled together based on the Hendricks QRP TwoFer design, built for 80m.

So far I have the oscillator and a buffer stage running nicely and I'm looking for an RF power transistor to get to 2W. I can't seem to source a 2N2553 or 2SC799 so I've ordered from the U.S. but disappointingly they say it won't ship for up to 3 weeks.

The smoke from an MPF102 was released along the way due to wiring it in reverse.

In other news, I picked up 20m of RG213U co-ax from the MWRS at a great price. I've run this through a hole, up between the double brick walls, through the ceiling and to my newly stretched out 40/80m dipole upstairs.

Reception is great but when I transmit all hell breaks loose from RF getting into audio amplifiers. Once again, my long wire antenna looks like being the most practical transmitting system here.

Thursday, May 10, 2007

MacMorseTutor version 0.00001

As part of my elaborate procrastination about learning morse code I've started writing my own tutor program for MacOS X. So far it sounds out the text you type in a field, displaying the character it just played after the audio.

It is available for your evaluation pleasure here.

This is just the start, planned features are:
  • Drill characters by playing and then displaying them.
  • Test by playing and then having me type the letter.
  • Give positive and negative feedback after typing the letter.
  • Controls for speed and spacing.
I know almost nothing of Cocoa programming but I find it a very easy environment to work in. Can't wait for WWDC.

Wednesday, May 09, 2007

Python decorators a simple example

Python decorators are a wonderful syntax for wrapping one function in another. I had a bit of trouble understanding the documentation, which often seems to involve talking about the history of how the syntax evolved rather than cutting to the chase and giving a simple example.

Specifically, I wanted to know how to get access to the arguments to the function I'm decorating in the decorator. Here's my example, at the top is the output:


"""
Example decorator that gets function arguments.

>>> decotest.py

>in mydecorator
>Function decorateme has been wrapped
> args = hello this is request
> kwargs = {}
>request = hello this is request
decorateme got request = hello this is request
>finished wrapped function
"""

def mydecorator(f):
print ">in mydecorator"
def wrapper(*args, **kwargs):
print ">Function %s has been wrapped" % f.__name__
print "> args = %s" % args
print "> kwargs = %s" % kwargs
request = args[0]
print ">request = %s" % request
f(*args, **kwargs)
print ">finished wrapped function"
return wrapper


def test():
request = "hello this is request"
decorateme(request)

@mydecorator
def decorateme(request):
print "decorateme got request = %s" % request

if __name__ == "__main__":
test()

Monday, May 07, 2007

Teac DV-B420 receiver creates bad RFI

Ever since I started hanging up antennas here, I've been plagued with a terrible, pulsing, interference across 40m.

Walking around with a short wave radio seemed to show that it was coming from somewhere near my house. It seemed to come from the mains, it was very strong near our meter box which has some suspiciously new meters in it. I could hear it at the next power pole down the street but not at my neighbor's meter box.

I decided to turn off the power to the house and while walking around looking for things that should be shut down before turning off the power I noticed the signal got very strong near a TEAC DVB receiver, model DV-B420.

When I unplugged it, the broadband, pulsing signal vanished.

What's worse is that this box makes the interference even when turned "off". As with many modern appliances, there is no true "off", it just stops putting out video. My guess is that it's a little switching power supply that just keeps going. Thankfully, this device isn't required at our place any more.

Some searching reveals that I am not alone in noticing interference from these devices.

Saturday, May 05, 2007

Antenna activity

To maximise my procrastination on the morse code front, I've spent the weekend improving the antenna situation here. There are two HF antennas, a 40/80m trap dipole, and a recently improved (by lifting it high into a tree with a squid pole) long wire.

The long wire was used with an excellent Emtron matcher but with just a wire counterpoise in place of a real earth. I had a lot of RF in the room so I've now run that wire down to an attachment on a copper water pipe in the yard pretty close to the house. See picture above.

The dipole was in a sharp V shape from the middle of the house to two trees in the back yard, but with the help of the squid pole and a daughter, I've swung one arm right around, over the house and hung it from a tree in front of the house. So now the dipole is nicely in line and up pretty high.

The feed point is on a TV gutter mount on the second floor, the arm that goes over the roof just grazes the peak of the roof and the ends are both about 3m up. Pretty much as good as I can manage without a tower.

Disappointingly, a comparison with the long wire shows that the long wire antenna performs much better. The coax feed line to the dipole is RG58 but it's crummy stuff with aluminium foil mixed with only a few strands of copper in the shield. Next step is to upgrade that coax.

Friday, May 04, 2007

Morse code at Manly Warringah Radio Society

Attended a Wednesday meeting of the MWRS. Great fun, a lovely group of blokes. I asked about learning Morse code and was treated to a live demo which included a contact with someone in Texas.

Pictured here is (if I've got this right) Russel Clarke VK2BYN and, on the iambic keyer, Yves Bernier VK2AUJ.

Russel confirmed that there's no easy way to learn Morse, I was so hoping for a pointer to the tape I could play under my pillow while asleep that would have me wake up with the knowledge.

On another topic, my thanks to Mike Bell from the home brew group for very kindly posting me some 3.6884Mhz crystals so I'd have no excuse not to build the 80M challenge.

Tuesday, May 01, 2007

Learning Morse Code, MorseBlog anyone?

I'm learning Morse Code. It's hard getting started.

Amazingly, given that it's no longer required for Amateur radio, there is a lot of morse to be heard on the HF bands.

Last week I bought a key (pictured) on eBay. There is a ton of software around to help and that seems to be a good way to go. For now, I'm using Koch Method CW Trainer from G4FON along with the companion program KochRx.

In these programs it sends from a given number of letters and you type in what you hear. When you start going mad, you stop and it compares what you typed with what was sent and tells you how far you got before an error.

I'm keen to learn the code at high speed (but with big gaps for now) so I can scale up in the future.

What I'm looking for is software that:
  • Teaches a character by playing the sound and showing (or speaking) the character
  • Plays the sound and I type the character
  • Rewards me if I get it right
  • Corrects me when I get it wrong
  • Reviews characters already learnt
  • Repeat, adding more characters
So far I haven't found this, any pointers would be greatly appreciated.

When I learned to touch type, I did a course. It was tedious but the group helped me get through it. There's lots of on-air practice, but I'm no where near that stage yet.

This post was brought to you by the letters K, M and R. -.-, --, ._. (which is all I have so far).