Well I am a little behind schedule! I started a new company: Global Technology Integrators…and I am swamped! But later this summer things will settle down and I am going to get to more projects! I feel bad because I like to crank out ideas and experiments as fast as I can! I am working on some really neat stuff and I am excited to share..so eventually I will inject some fresh DNA into the site. I intend to finish my antenna analyzer, I am also working on a 20-30 watt hybrid guitar tube amp with el84’s and an LND150 fet front end. Finally, I am doing more antenna experiments that look very promising.
A resistive bridge type SWR meter doesn’t actually measure SWR – it measures impedance mismatch or impedance(however you want to look at it). SWR is calculated by the ratio of two volatges on the bridge. The SWR and impedance only relate correctly if the bridge impedance and feedline impedance are the same(50 ohms typically). A resistive bridge works fine but you can do the same thing with a simple resistive divider. When using a divider, you can use any value resistor you want(50 ohms in my design) and as long as you know the characteristic impedance of your feedline, you can correctly calculate the results with any feedline impedance (not just 50 ohm coax).By measuring three voltages across the divider(Vs, Vr and Vz), one can easily derive: total Impedance, resistive impedance, reactive impedance, SWR, and return loss. A resistive bridge is most accurate when mismatched, but the resistive divider is most accurate when match is optimal. So ask yourself would rather know more accurately what’s happening between SWRs of 1:1..to 2:1 or when it’s 3:1 or greater? The only drawback of this method is that you cannot determine the sign of reactance(capacitive or inductive) directly, but you can derive all of the magnitudes. This is because when you convert the voltages to DC values by means of diode detectors, all phase information is lost.
I decided to build a small and reasonably simple antenna analyzer for 1 -30Mhz. I wanted it to provide accurate measurement of SWR, Total Z, R and X of the ANT. Simplicity in operation and moderate power consumption were also design goals. The schematic below is what I came up with.
Preliminary Schematic(not built yet)
I have etched a circuit board and will build the analyzer in early 2016. I will update the post as I make progress. Expect some values in the schematic to change. Below is a discussion of the math required to derive all of the analyzer measurements. Vr requires a floating measurement, Vs and Vz are ground referenced.
Diagrams for Analysis
Using the Voltages Vs, Vr and Vz, one can create a triangle related to the classic power factor triangle where the hypotenuse(Vz) can be seen to be shared and the cos(ang) allows one to find the reactive and real components of the impedance. One is required to find the cos(ang) using only the lengths of the sides of the Vz, Vr, Vs triangle. These values are the measured voltages from the diode peak detectors shown in the schematic. The law of cosines provides the solution.
cos(ang) = (Vs²+Vr² – Vz²)/ (2* Vs* Vr)
Using node voltage equations, the following relationships can be derived from the simple divider shown above. Note that the triangle legs:Vs, Vr, VZ , VZr and VZx are synonymous with Zs, R, Z, Zr and Zx respectively
The total impedance seen across the signal source (Zs): Zs = R*Vs/Vr
The absolute value of the complex impedance seen across the load connections of the resistive divider(Z): Z = R*Vz/Vr
The real part of the load impedance (Zr): Zr = Zs*cos(ang) – R
The complex part of the load impedance (Zx): Zx = SQRT(Z² – Zr²)
With these equations we now have total load impedance Z, the resistive component Zr, and the magnitude(but not sign) of the reactive component Zx.
If we know Zr and Zx, we can calculate the SWR and return loss as well:
First we calculate Γ: Γ = SQRT( (Zr-R)²+Zx²)/SQRT( (Zr+R)²+Zx²)
Now SWR: SWR = 1+Γ/1-Γ and return loss: return loss = -20 log Γ
Note: R used in Γ calculation is the characteristic impedance of the feedline not the R used in the divider (in my case they will be the same: 50 ohms).
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
QRP While Lounging Outside
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.
The built XCVR
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).
Link to design files:
My call sign is W4KIA … I am on 40 meter CW a lot these days! Would love to get some help testing my antennas.
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
Antenna mounted as a sloper
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).
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
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.
Antenna adjusted to resonance for 40 meter CW
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
Video of XCVR using the Filter