20/40 BITX Build Update #1: VFO Success

I've started construction on my second BITX transceiver, this one for 20 and 40 meters.  As with the previous rig, I decided to take the VFO first, but this time I pledged not to surrender, not to wimp out with a VXO or (worse!) a DDS.   No, this time it would be a real LC VFO.

I took seriously all the admonitions in the tech literature about the fickle permeability of ferrite and iron powder, so this time I used an old-fashioned air-core coil.  It is wound around a cardboard tube.  The tube was previously the bottom portion of a coat-hanger from the dry cleaner.  I wound 40 or so turns on this core, then measured the inductance:  5.6 uH -- that looked about right.

For the oscillator stage I used a Colpitts circuit very similar to the one in the original BITX schematic (but I am hoping I won't need the varactor diode fine tune mechanism).   I had on hand a nice Heathkit 19-146 pf variable cap with an internal 4:1 reduction drive.  Not wanting to pluck rotor places out of this beautiful part, I had to calculate the series capacitance that would yield a frequency spread of about 175 kHz.  It turned out to be 40 pf.  Then I had to figure out how much capacitance to put in parallel with the variable.  Well, it all ended up like this:

There was a bit of trial and error in the process of getting the VFO to cover the desired range.  A big help in all this was an on-line reactance calculator.  I found this one to be very useful:

  
http://www.1728.org/resfreq.htm

I found that the VFO is more stable if I reduce the voltage from 12 volts down to about 8.  Also, I found that when evaluating the stability, it is better (psychologically!) just to use a stable superhet receiver instead of the frequency counter.  The counter may appear to be jumping around a lot, especially if the signal you are monitoring is not very strong.  Just listening to it at zero beat on the superhet is very reassuring.

I followed the Colpitts oscillator with the FET buffer and two stage BJT amplifiers from page 50 of Doug DeMaw's QRP Notebook (page 50).  I now have the requisite 7 dbm signal.  And it appears to be quite stable. 

I plan using this with an 11 MHz IF, with the VFO running at around 3.875--3.700 for 40 meters and 3.175 -- 3.355 for 20.   I plan to use a small relay to switch in some additional capacitance to move the VFO down to the range for 20 meters.


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University of Twente's Online Receiver

Wow, I played with this on-line SDR receiver a few years ago and thought it was fantastic.   It is now even better.  I'm sitting here listening to 40 meters in the Netherlands.  Great stuff.   Check it out.  It is really amazing:

http://websdr.ewi.utwente.nl:8901/

Thanks to Bob, KD4EBM for letting me know about the updates. 

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Circular Polarity and The Water Wheel in Dale's Moonbounce Amplifier

Bill:

 

I'll attach some pix of the feedhorn and LNA for you.

The importance of circular  feeds is that as a linear wave passes through the ionosphere, it undergoes Faraday rotation. So it may arrive at  the station you are talking to having been twisted 90 degrees. This  is a slow progressing process and  on  all bands except 23cM, may cause EU for example to be locked out for hours at a time for linear stations.

With circular polarity, Faraday is a non issue. The feedhorn almost all of us use is a VE4MA that has separate TX and RX probes. The circular polarity is synthesized as the linear wave propagates down the circular waveguide and encounters sets of  capacity stubs. The exact opposite occurs for waves entering the waveguide. The result is we get CW and CCW without having to use any relays (loss) and phasing  lines (loss).

My LNA has a noise figure of under 0.24dB and uniquely connects to a protection relay with no cable or adapters (loss).

The position of the feedhorn and its scalar ring is tediously adjusted by measuring the difference between sun noise and cold sky. W4SC developed a very accurate and repeatable process that uses an SDR RX for this.

I use  a modified C band satellite drive system known as a polar mount so I only need one motor drive to track the moon.

Anyway, hearing my own echoes off the moon was and still is the highlight of my amateur career. 

 

The photos are the feedhorn + LNA, My first water cooled 500W  tube amp, my previous 400W solid state amp (mounts right at the dish). My current design is 600W solid state and will also mount at the dish.

BTW, that little circle in the middle of the tube amp is a paddle wheel that turns as long as water is flowing. A tachometer on the wheel sounds an alarm and shuts down plate voltages   should the wheel stop turning.

I'll keep  you up to date on my BB RX progress- thank you again Bill.

Dale W4OP

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The NASA Juno Fly-by "Hams say HI to Juno" Video (very nice)

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The QRP Mojo Ceremony

More from Lobstercon (from N2HTT): 
 http://n2htt.net/2013/11/24/there-are-lobsters-in-the-woods/
And thanks to Pete, VE2XPL, for alerting me to all this. 

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Need help with part construction...

Please take a look and see if you can help me figure out how to make this thing.  The diagram is eschpecially illuminating.

 

 

THE DIE-CAST DROP-FORGED PLASTIC

TRICHOTOMETRIC INDICATOR-SUPPORT

          Inquiries concerning the mounting of the Trichotometric Indicator

          Support indicate that some difficulty is being experienced with the

          brackets which attach the support. As an aid toward fabricating the

          support brackets, the accompanying illustration is provided to show

          the type of material as well as the dimensional data needed. It will

          be noted that in attaching the bracket to the support a special ambi-

          melical hexnut is used. The application of this nut is unique in that

          any attempt to remove it in the conventional manner only tightens it.

          Because of this design, the nut must be fully screwed on before it can

          be screwed off.

 

DRAWN BY	FRED SPOON	SCALE ¼”=1 MILE
NO. 43906		FILE – BAR NONE

 Thanks to ZL2DEX for bringing this to our attention.

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On the Wavelength of Santa's Sled (with a nod to FA Wilson)

About a year ago, David, M0VTG, was reading "From Atoms to Amperes" by FA Wilson.   His thoughts turned to a seasonal application of Wilson's lessons....  Thanks David! 

WAVELENGTH OF SANTA AND HIS SLEIGH

We're going to need to know the following:

Total mass of the sleigh, reindeer, Santa and his presents, and the speed at which the sleigh travels. Assuming that the sleigh travels relatively close to the surface of the earth, the mass can be regarded as the same as the weight. Now assume each reindeer weighs 100 kg. Traditionally there are nine of them so total mass of the reindeer is 900kg. Say the sleigh also weighs 500kg and Santa and his presents weigh 400kg; then the total mass is 1800kg. To get round all the children who believe in him in one night would mean him travelling at a speed of say 1000kph (277.8 m/s). (Note that scientific notation for numbers used is that used on some calculators)

So mass (m) is 1800kg

velocity (v) is 1000kph (notice the not so subtle change from a scalar quantity to a vector quantity).

Max Plank came up with a formula: E = hf relates Energy to frequency using h (Plank's constant = 6.626 E-34 Joule seconds).

Albert Einstien (as everyone knows) says that E = mc² (where c is the velocity of light).

So it follows that hf = mc² and therefore, m = hf/c².

The momentum of a photon (p) is defined as mass x velocity or p = mv and as every radio amateur knows, wavelength (λ) is the velocity of light divided by frequency or λ = c/f (or f/c = 1/ λ).

So mv = hf/c² x c (velocity of a photon is c don't forget); so p = hf/c

Since f/c =1/ λ, then p = h/λ i.e. λ =h/p

Also since p = mv then λ = h/mv (This expression is known as the de Broglie wavelength - Louis Victor de Broglie)

Notice that the velocity of light does not directly feature is the expression. We can, therefore use it the calculate the wavelength of anything!

So the wavelength of Santa's sleigh is Plank's constant divided by the mass times the velocity or

λ = 6.626 E-34 J s / (1800 kg x 277.8 m/s) which equals 1.325 E-39 m - an unbelievably small number! Make up you own mass and velocity for the sleigh if like.

I defy NORAD to track such a short wavelength.

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