Simple low current mini-whip antenna

I decided to try using a small  wide band E field type antenna with my newest receiver  design…the Mini SDR and the results have been gratifying. There many useful articles describing this type of antenna; so I won’t go into much detail about how it works. More or less it functions as  a capacitive E field probe and therefore is very sensitive to EMI. However, if placed outside away from house wiring and provided with a modest local ground reference..the antenna is a good performer. The classic circuit uses a JFET source follower and a BJT follower stage to provide impedance transformation of the Hi Z capacitive terminal to a 50 ohm Z drive for transmission line. This circuit works fine but has some drawback, namely requiring 65mA of current and having a somewhat large input capacitance, which reduces performance with frequency. I decided to use a wide bandwidth op amp to simplify the circuit, reduce current, and provide a little  voltage gain. The op amp I chose was one I have used before for RF amplification..the LT1818. When choosing an op amp for such an application..there are a few important criteria to focus on:

1. current noise: Unlike low impedance  topologies where voltage noise and resistor thermal noise will dominate, having a Hi Z input will make the current noise the dominant source of amplifier noise.

2. Bandwidth: you need a wide bandwidth on the order of hundreds of MHz or more to provide the required frequency response up to 30Mhz. This is especially true for voltage feedback op amps, where the phase shift compensation rapidly reduces performance over frequency.

3. Slew rate: You want the largest slew rate you can get to reduce distortion and IMD products.

4. Input bias current/ input Z/ input capacitance: You need low input capacitance so as to not to create a lossy divider with the antenna terminal. You want low input bias current and high input Z to not load down the terminal. If the input bias current is too high , then you need a low value  bias resistor which loads the terminal.

5. Low output impedance: To drive 50 ohm Z and minimize distortion.

The LT1818 has excellent specs with regard to all of these criteria. It can operate on 3v-12v and requires only 9mA of current to operate. The Amplifier will operate from VLF to beyond 30Mhz with no change in performance.


E Field Ant

The antenna is powered via a power splitter connected between the receiver and antenna. This is the purpose of L1 to isolate the DC power from the RF output from the antenna.

I 3D printed antenna capsule  from HIPS, which is a low RF loss material and used a 3″ square of PC board to create the capacitive antenna terminal.  The printing was done at 50% density so it’s a very light, low dielectric loss enclosure.

Installed Antenna(with LED illumination)


Antenna Element



Complete 4″ x 1-1/2 inch antenna



Amplifier Circuit



Antenna Terminal



Efield Antenna Demo:


Si5351 based SDR Circuit Updates

I am going to be posting any tweaks, component changes and firmware/circuit mods here on this page.

02/02/2020: Changed the RF input buffer to a better performing amplifier with another option annotated as well. Also I changed some resistor values in this gain stage to  improve performance.

02/07/2020: Oops the integrating capacitors on the switching detector were shown as 1uF…it has been changed to the correct value of .1uF

02/07/2020: Updating the download link with the new schematic and an improved  AM demodulator for the FV-1.

Coming soon: New code for FV-1 based on 48Khz sampling…plus new schematic/layout utilizing the leftover SI5351 output as The FV-1 clk instead of a 40KHz xtal.

Newest Schematic here:

Updated Schematic


Compact Si5351 based SDR

Go here for the most up to date circuit /firmware mods: Design Updates

This is a revised version of my FV-1 based SDR. I replaced the CS2100 clk generator with the Si5351 clk generator. The Si5351 has some advantages over the CS2100, namely you can generate quadrature clks directly. This simplifies the hardware design and improves the quadrature accuracy. The sideband rejection in LSB/USB modes is impressive..somewhere around 60 db as best I can measure. The DSP processing is accomplished by the use of a FV-1 audio processor. The device makes the base band signal processing a snap. It requires some code to be loaded on a EEprom but the circuitry is simple and allows for up to 8 selectable programs. I created three: AM/USB/LSB . The FV-1 provides for three analog POT inputs to control any parameters you choose. Gain, variable filter bandwidth and depth, AGC are some examples of adjustable parameters if you desire. I kept it simple and created fixed band pass filters to taste. I did use one of the controls for AF gain. The design has no tuned circuits or band pass filters but they could easily be added.  It works just fine without them. Occasionally, I come across a ghost signal from harmonic mixing, when tuning, but not enough to matter. The design uses an OLED display and a rotary encoder for tuning. The frequency coverage is from 2.7 Mhz to 25Mhz. The bottom limit is created by the inability of the Si5351 to support quadrature below this frequency. Although I have improved my DSP programs for the FV-1 and have developed new display drivers and the new code for the Si5351, useful detail about using the Fv-1 can be found in my original design from a few years ago:

Schematic: Updated 05/17/2020

The design uses a LT1818 or THS4304 low noise op amp as an RF input with gain. It provides a constant and reliable resistive Rf termination for the sampling detector.  This allows for random antennas to be used without adversely affecting the input termination to the detector. All the code to operate the main processor(display/clk generator/tuning, band select and receive mode) was written in MikroC which is a C compiler for PIC and AVR processors. The generation of quadrature signals out of the Si5351 is not difficult to implement once you know how but..figuring that out took me a couple weeks of experimentation! You can connect switches, the encoder, volume pot and display directly to the main board for operation but I created a secondary board to mount the display and encoders. Instead of an analog pot and selection momentary switches, I used another microcontroller and two encoders(with one built in momentary push switch each) to create all of the switching signals, gain control, etc. This allowed me to have just two controls for all features.  The controls include: tuning, audio gain, mode, and tuning step. Tuning resolution is from 1Hz to 100KHz . For fun, I made the output of the FV-1 differential into the audio amp. This is not necessary.

Here is a link to all the files used to build this radio in a zip file(updated 2/07/20):

Tech File Download

The schematic and PCB was done with express pcb freeware. The C compiler used was MikroC, and FV-1 assemble was built in SpinAsm which is free and available from Spin Semiconductor(who makes the Fv-1). The gerber files provided were created for OSH park. I had my boards etched by them.  If anyone is interested in building this radio or leveraging elements of the design. I can answer questions.

Misc Notes: I use a 16650 3.7v lithium  rechargeable battery to power the radio. The current draw is about 100 mA with audio.  The radio works even when the regulators drop out so it will work at 3 v.

The enclosure is a machined aluminum 1590A style hammond box which you can buy on Ebay from alpinetech. They are $14.00 which is pricey but they are not cast. The quality is much nicer and you can anodize them.  It’s a different topic but home anodizing of aluminum is easy…and I do it with all my enclosures now. In this example, I anodized twice to create the base blue color and then the labeling as well. It looks really clean with this method. The nice thing about anodizing is if you make a mistake, it’s really easy to go back and redo the process.

Designers will note that the resistive terminations on the input RF OP amp contributes to the noise figure of the radio. As a practical matter. a negative impact on performance is not noticeable because of  atmospheric noise in the shortwave bands. For the best performance…no front end circuitry or a different front end input amplifier should be considered. Note that the op amp serves to bias the analog switches to half supply; so this bias must be provided to the sampling detector if the input termination is modified.  R10 set the impedance of the sampling detector, conversion gain, and low pass roll off. The schematic shows a value 0f 210 ohms…I think I am using 100 ohms actually now…which works well.

If you want quadrature out of the Si5351 below 3MHz you can create two outputs with 0 deg offset with one output at F and the other at 2F. You can then drive an analog mux with those signals and generate quadrature sampling for low frequency applications. Just note the output sequence of the samples change so you have to flip two outputs of the detector.

Top view of the circuit board:


Bottom View Showing FV-1 circuitry



Display board


Completed Radio



Demo Videos:


A very efficient half size 40 meter Vertical

I had an old broken 20 foot fishing pole that only had 16 feet of length so I decided to see if I could make an effective half size 40 meter antenna with it. Here is what I did. I used a capacitive hat to increase the radiation resistance of the antenna considerably. There are 8- 15 foot radials suspended a little less than a foot above ground. Finally, I made a small loading coil to tune the antenna up to the center of the band (7.100 MHz) for CW.  I first tested  a full size vertical to optimize the ground plane. By suspending the radials by only a few inches above ground, significant improvement was achieved . With 8 half size radials 10 inches above ground (compared to 4 on the ground), the measured loss went from about 20 ohms to  4 ohms. My loading coil has a measured Q at 7Mhz of about 300…with about 1 ohm of loss. The form was printed on my 3D printer out of HIPS..which is a low loss RF material. So my total system loss was about 5 ohms. With the HAT top load, the 17 foot antenna had a total impedance of 20 ohms.  This means I had a total system loss of only 1.25 db…which is not bad. the full size antenna had a loss of about .5 db … so really the difference is negligible. Finally I matched the whole antenna to 50 ohms with a little L network connected at the base. I like to use crimp style bullet connectors for all wire  connections because they provide quick disconnect and you can field repair(crimp) without need to solder anything(nice for portable setups).

I use it for QRP work on CW at about 1.1 watts. The antenna is a solid performer and it is very portable and easy to breakdown/setup. I can hear my signal on the many of the web based SDRs around the country. and have made numerous casual QSOs with it. Six or Seven radials work well start seeing some more  loss when you go to four radials but it is still usable even then.


View of Top Load( 2 -8 inch strips of thin aluminum)

ant with loading coil

View of loading coil and base assembly

loading coil

The Loading coil

unmatched ant

Unmatched Antenna Z at Resonance


The Final Matched Antenna Z

Forget the Bridge! Determine SWR with a Resistive Divider.

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 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)

Antenna Analyzer

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

example pic

Voltage dia

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).

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


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.



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).

Link to design files:

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

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).

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

IMG_20151031_170006399 IMG_20151031_165858898

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.


Antenna adjusted to resonance for 40 meter CW


Ultra Simple 8 pole Low Pass CW filter

I wanted to add more audio filtering to my 40 meter CW transceiver  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


Filter Schematic

Simple CW filter

Video of XCVR using the Filter

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


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.