QRP On Vacation

I used my new XCVR and my short portable dipole down in Ft Pierce Fl, over Thanksgiving.  The rig, antenna, feedline and tools all fit  in a camera bag and it was easy to setup. I made some 800- 1000 mile contacts on 1 watt and drank a few beers and smoked a stogie while doing it!

Short Dipole strung between two Palm Trees

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QRP While Lounging Outside

 

 

40 meter XCVR size of an Business card with 1 Watt output, 7.000 to 7.150 MHz coverage

I tweaked my previous XCVR design to use push button tuning and made the board layout extremely compact. I improved the side tone injection to be absolutely  perfect – not too loud, full break in and no clicks. I went ahead and integrated my switch cap 8 pole CW filter. The schematic is fairly simple with 65 parts or so.. and the whole design fits into a 3D printed enclosure the size of an business card. The only penalty in the design is  signal on both side of zero beat(direct conversion). There is no AM leakage though and the filter rolloff is very good. The circuit is easy to build as it required no adjustment, or alignment, but uses some fine pitch surface mount parts requiring a very precise soldering station. When I got my boards in I literally just built it – applied power and starting using it. In this one I used a ferrite toroid for the Sorta-Balun, but I have used scrap bobbins from HF power inductor for this with great success . The push button tuning works well, it has freq (up/down) buttons which shift through at 10 Hz increments for 5 sec then goes to 1 Khz. It can run on a 10 to 24 volts supply. The 8 pole audio filter is adjustable to taste, by changing one capacitor(C27), and the side tone injection level with one resistor(R12). The choice of band only requires a firmware change and 3 inductors and three capacitors – which make up the RF matching filter. Its really a unique little transceiver. Its very small, reliable and has excellent performance. The TX frequency is offset 700Hz above receive frequency, but this could be made adjustable if desired(using the push buttons and the Key) – I just haven’t done it yet.

Schematic

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The built XCVR

QRP UltraLite     QRP Ultralite2

I am going to provide a link to all the design files in the next few days including the 3D print files for the enclosure. I am also going to sell a couple units at cost(get rid my extra boards!) as partial kits( I will put most of the surface mount components on) and provide the switches and connectors that fit the enclosure (which I will also include). Will probably be around $70 for everything for a complete unit(except wire).

 

Efficient Half Size Dipole for 40 meters

In my continuing saga of antenna experiments, I have designed  a 30 (a little less than 1/2 normal size) foot long 40 meter dipole that is a solid performer. The goal was to make it short enough that it would work well in portable applications as a vertical or sloper hung from a tree.  It is center fed with one continuous loading coil tapped at the center two turns to a SO-239 type jack for coax. A balun is not necessary because of  the tapped connection to the loading coil. Along with the loading coil, there are two cylindrical capacitive hats which  replace about six  feet of wire each. The hats improve current distribution, bandwidth and efficiency,  allowing for a smaller loading coil. The hats are lightweight and flatten out for very easy transport. The antenna uses carabiners and inline connectors to allow quick connect and disconnect for setup and removal.

Link To STL Files

https://www.adrive.com/public/F8Fsz4/Antenna%20Forms.zip

Antenna mounted as a sloper

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The Loading Coil and Hat frames were printed on my 3D printer using a low RF loss plastic material – High Impact Polystyrene(HIPS). Here is a link to the STL files which can be used to print these forms. An STL file is the standard format used by almost all 3D printers. If you do not have a 3D printer – there are online services available to 3D print the files as well as some print shops and office stores. Other plastics could be used such as ABS but use HIPS if you can. Because the antenna is relatively small you can mount it vertically or nearly vertical with the center relatively high above ground for better efficiency so besides being small, the antenna has some benefits with respect to ground losses and radiation angle (depending on how you mount it).

Loading Coil

Coil has 11 full turns(plus half a turn) to center from each side(3 inch diameter and 3/16ths turn spacing). It can use 12 gauge or smaller wire. The mounting holes can be tapped for 6/32 or 8/32 screws for the coil terminal and connector flange. Total coil inductance between 16-17 uH

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Capacitive Hat

The hats are 6 inches in diameter with the wire soldered together in the center and then pinned with a 8/32 screw and washer.

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Antenna adjusted to resonance for 40 meter CW

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Ultra Simple 8 pole Low Pass CW filter

I wanted to add more audio filtering to my 40 meter CW transceiver https://circuitsaladdotcom.files.wordpress.com/2015/08/qrpxcvr2.jpg  but didn’t want to put much effort into it, so I used a MAX7401, 8 pole switch cap filter IC. This IC requires just a few discretes and has an extremely low passband ripple and group delay, as it is a Bessel configuration. I have used this IC before in some of my guitar effects pedals. It has worked very well in my other designs. The knee of the filter is adjustable via a capacitor(C4). To integrate into my receiver, I simply connected the input and output of the filter across the input capacitor(C27 removed) connections to the final audio amp. It may be desirable to put a single RC low pass filter stage in series with the output of the switch cap filter to remove clk artifacts on the output.(clk is 100X greater than the roll off frequency). Performance is very good with clear tone and no ringing.

Below, I have a video demo of the XCVR utilizing the filter. This version of the XCVR is tuned via a POT connected to the microcontroller A2D converter instead of a rotary encoder. It also has a push button to shift in 5Khz increments. When the button is depressed for longer, the frequency in KHz is sounded in morse code. Only the KHz is sounded, so for 7.100 Mhz for example, only 100 is sounded in morse code. In the video you can also see my cool 3D printed Code Key. It uses rare earth magnets instead of a spring for key action. It’s a really delightful bug.

Picture of filter Daughter Board connected to XCVR

8polefilt

Filter Schematic

Simple CW filter

Video of XCVR using the Filter

Improved Power Toggle Latching Circuit

Awhile ago, I posted an example of a Power toggle circuit using a handful of components that I  use all the time to allow a momentary switch to control Power On/Off function.

https://circuitsalad.com/2014/04/12/simple-momentary-switch-latching-circuit/

The circuit works well but can require a series diode with the output to prevent  stored charge or magnetic energy from re-energizing the lating function such that it won’t turn off. The diode drop may be undesirable, so I modified the the circuit  as show  below.

Alternate Power Toggle

The addition of R7 and C8 eliminates this problem in every circuit I have tried and is sure-fire in reliability. Normally, C8 can be omitted, but I added for one application with very strong RF fields near by.  As before, the circuit draws no current when off and can toggle within a second or so.

Of course you might ask why not just use a toggle switch?  Well of course you can but you can get momentary switches cheaper, they can be much smaller and generally I find switch bats ( toggle levers) to be un- aesthetic and sometimes cumbersome. So now I usually do something like this.

More Spiral Experiments

I have come to the conclusion that my large(1 meter diameter) spiral loaded vertical is really an asymmetric dipole with the spiral being the other radiating element.  For fun I made a much physically smaller spiral and attempted to use that.

New Small Spiral

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I found it was extremely difficult to tune and narrow band as well. It did work but the receive  level went down slightly  so I have concluded it is not  as efficient as the large spiral. Feedpoint impedance measurements were better matched(higher R) than the other spiral but this is surely IR loses in the dense coil.

My conclusion is a moderately sized spiral of a few turns(> 1/20 wavelength) is very effective as its area makes it a respectable radiator. If the spiral coil becomes small the approach doesn’t work so well.  My large spiral is a solid performer and it is my opinion that it is preferable to a modest radial array.