Posts Tagged ‘homebrew’
New MOPA Completed
April 3rd, 2015 | 
Author: 
Well ... my new 1930's style MOPA transmitter is finally finished after several months of construction. I have also put together a page on my website describing the project.
For many years, December's high winds here on the coast, have always arrived coincidentally with the start of the annual 1929 QSO Party, making my signal dance around even more than usual! With the new MOPA, I'll no longer have to worry about high winds upsetting signal stabilty ... although many of us do enjoy hearing these musical sounds of '29!
By the way, there's still plenty of time for you also, to put something together for this annual fun event. You can see a whole page-full of inspiring '29 homebrew magic here (scroll to the bottom half) ... and there is plenty of help available for your project in the AWA Yahoo Builders Group ... we are always looking for more new activity, particularly here out west.
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| courtesy: natubes.com/ |
I'm looking forward to working many of you in the next '29 QSO Party ... in the meantime, I'm thinking hard about what my next project might be!
A Versatile 630m Antenna
Mark, VA7MM, had come up with a nicely-designed antenna that will serve as his main 630m radiator. Not only that but it can be used on 10m, 15m and 30m and 160m as well!
The antenna consists of a mini-flat top dipole, with three resonant dipole legs all terminating at a common feedpoint.
This can be used directly on any of the three high bands. Shorting the end of the coaxial cable, dropping vertically down from the feeedpoint, the dipole feedline becomes the vertical element of a top-loaded 630m 'T' antenna, 100' in the air.
With suitable loading coils and appropriate switching, the 'T' will also be used on 160m, making the versatile antenna work on five different bands ... a nice demonstration of basic antenna principles put into real practice!
Low power longwave transmitter experiment
Many places in the world, low power transmitters in the medium wave band are allowed. I am talking about regulations like in the US where FCC part 15 allows up to 100 mW input.
In Norway we have a particular permission for members of the Norwegian Radio Historic Society to transmit up to 500 mW on 216 kHz. I’m not sure if this is output or input power. The permission is meant to cover a collection of historic radios. The frequency is the one used by the main transmitter north of Oslo from 1954-1995 running 200 kW. The frequency is still allocated to Norway, so I guess that is why we may use it this way.
I grew up close to this transmitter and have fond memories of my first homemade crystal set receiving this station.
There are several low power transmitters around that can be purchased, but most of them only cover the mediumwave band and not longwave (153-279 kHz). Further they are quite complicated as the frequency necessarily has to be user settable.
I looked for a simpler way to make a single-frequency transmitter and found that the function generator chip XR-2206 which I happened to have in my junk box could both generate this frequency and do the amplitude modulation. The RC-oscillator seems to be stable enough for this low frequency although I haven’t tested this much.
Here are the first results with images of the circuit on a Veroboard and the oscilloscope picture of the modulation with my Tandberg TP41 70’s radio on top of it listening to Dire Straits from Spotify streaming over longwave.
The circuit has very little output power, lacks antenna tuning and harmonic filtering, so there is room for improvement, but at least it works.
Too bad that the XR-2206 is obsolete and not recommended for new development!
RTL SDR Filters
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| SDR Sharp GUI |
DXer John Bellini in Colorado, and maker of those informative Low Noise Vertical videos, has been at it again.
This time it's a good demonstration of a pair of filters that he built to mitigate the front end overload experienced on his SDR receiver.
John is one of many radio hobbyists that have been playing with the very inexpensive RTL 2832 SDR dongle receiver. Being very close to several high-powered broadcast stations has been a bit problematic for him when using the low cost SDR but his new video shows exactly what was needed at his location to solve the problem:
It looks like John is using the SDR Sharp GUI (graphical interface) to operate the dongle SDR receiver. Those wishing to learn more about this might find this 'getting started' page of interest.
Good stuff John.
Crystals Go To War
Thanks to a recent posting in the Yahoo ParasetBuilders Group, I have a new-found respect for my small collection of prewar crystals! If you've ever wondered how a rough chunk of quartz gets transformed into an accurate frequency-generating device, this 'cinema-style' documentary shows exactly how it was done ... truly an amazingly complex, yet delicate, labor-intensive process.
After viewing the documentary, I can't help but wonder what later health effects some of these workers may have undergone after seeing them handling some nasty-looking chemicals and working directly beside desktop X-ray machines. If you've used old WW2 crystals before, I think you'll enjoy seeing how much work went into their production.
International Radio Restoration Contest
I have recently been made aware of the Socété Québécoise des Collectionneurs de Radios Ancien's / (SQCRA or Quebec Antique Radio Collectors Society) 'Radio Restoration Contest' and have been enjoying some of the published documents describing various refurburations.
Although the group has been sponsoring the refurb contest for over 10 years, this is only the third year that it has been open to international competition. The rules are interesting and are quoted from the SQCRA website:
" ... participants have one year to restore a basket case radio (the worst it is at start, the more points are awarded for the difficulty). Pictures must be taken before the restoration starts and at all steps of the process. One year later, the participants present their work to the international panel of judges. Pictures taken during the process will help judges better understand the challenges faced by the participants in order to finish their project.
The clubs that don't have a contest can nominate someone or make a group effort to represent their club at this contest.
A documented report containing photos and explanations and optionally a video of the working set from each contestant must be submitted to us. Then judgement and results are compiled to determine a winner and two runners up.
A documented report containing photos and explanations and optionally a video of the working set from each contestant must be submitted to us. Then judgement and results are compiled to determine a winner and two runners up.
The criteria's for evaluation are available in this document .
Our goal of course is to promote the conservation of the technological / historical heritage, to motivate our common interest, increase the general knowledge of ancient radio technology, gain restoration tips, increase club exchanges, and see what is done in other clubs."
Each project is judged on three basic criteria: difficulty (condition when found), restoration (chassis, cabinet, components, overall) and functionality when complete.
The present contest has just ended (March 15) but the judge's comments and project writeups from the previous two contests (as well as this year's project writeups) are available for reading ... and they are both instructive and inspirational as I found several new constructive hints embedded in the descriptions.
Particularly interesting to me was the sidebar in the writeup article presented by Gerry O'Hara of B.C.'s SPARC Museum. I have been struggling to develop a method of building this period-correct component for several years and the solution looks elegantly simple!
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| courtesy: http://www.sqcra.org/ |
There really is enough reading here to keep one entertained for days but the more I read, the more I want to find another old clunker and bring it back to life ... great stuff!
ET Call Home
Late last week I received an interesting e-mail from Thomas Pell in Winter Haven, Florida.
It seems that Tom has been doing some building after reading my series of blogs about my lightwave adventures and has started his own adventure. As he told me initially:
"... I'm retired with too much time on my hands, and I'm tired of the big gritty hobby projects ... after reading about Harvard U's optical SETI project, which looks for extraterrestrial laser signals with a 48 inch, then 72 inch mirror. I thought it would be fun to build a setup like theirs, but at audio frequencies instead of the RF laser signals ... my purpose was to get light signals from other amateur laser dx experimenters or even ET ... better than watching my wife's TV shows. Also a giant telescope that costs almost nothing is fun too."
Tom built up the PIN diode detector shown in my notes, which was a slight variation of the one developed by Roger, G3XBM (see his great lightwave notes here) and based on an earlier plan by K3PGP.
Along with the receiver, Tom built a substantial optical antenna ... a 48" Mylar-based parabolic mirror!
"... it is a wood structure ... round piece of 1/2 inch plywood, 48" in diameter ... around the perimeter a 4" wide strip of 1/8" plywood is wrapped and glued with epoxy putty. On the upper end of 1/8 ply is another circular plywood flange inside. This flange is to glue mylar sheet. The mylar is then tensioned with tape on the side. It is something like a round guitar in appearance... a box ... you suck the air out ... I use a shop vacuum with 1/4 in rubber tube taped to vacuum hose ... hose barb epoxied to hole inside of mirror ... not ideal but if you seal it up well with epoxy, it works..."
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| courtesy: Thomas Pell |
It seems that there is now an active movement amongst some SETI enthusiasts to search for optical beacons rather than radio beacons. When you think about it, it would seem to make just as much sense, if not more, to beacon with a modulated optical signal than with a radio signal ... and the optical signal might be far easier to detect. Some of the papers suggest that an optimum frequency would be in the near IR or deep red part of the spectrum, right where most optical amateur two-way work is presently being done.
During his first few tests of the new mirror, Thomas stumbled upon one signal (the only one) which came from just one single point in the sky ... almost directly overhead in Orion.
"At 9:30 pm 3/5/2015, using the amplifier circuit you use in your optical communication receiver connected to a 48 inch parabolic mirror, I received an apparently modulated optical signal originating in center of Orion constellation. Signal was audio frequency low to high pitch and lasted for more than an hour. I located the signal by moving the mirror back and forth across the sky for nearly an hour until I found a "blip", then focused the mirror exactly on the spot to listen to it, incredible experience."
"Received signal again last night from same location. I am becoming convinced it is information of some kind.Very irregular rapidly pulsed. This was only a test of amplifier ... I never expected this result, was totally unprepared. Meanwhile time is passing and I can't seem to contact anyone to have it confirmed before it disappears from the sky. It seems, no one has a setup like this ... this signal is either very important or it's nothing ... I think. Will let you know about outcome."
At this point, Tom is just trying to figure out what type of signal he has been hearing and will be attempting to get a better recording of it over the next few nights. I have heard his initial recording, just done with an I-pad held close to the amplifier's speaker output and it does sound suspiciously like a data train of clicking pulses. Hopefully Tom will solve the mystery soon!
