Real-time antenna comparison system based on WSPR and the Ultimate3 kit

An article by Antonio EA1CDV


Every amateur radio operator wants to reach as far as possible or with the best signal, and this depends on the radiating system.

We always want to have the best antenna possible within our limitations: economic, available space, permission, etc. For this, we use our knowledge of antennas to perform the best installation or the one that best suits our QTH and circumstances.

With the antenna design tools that exist today, such as Eznec, Mna and so on, a world of possibilities is available. Through software design, we can adapt to our conditions, the most suitable antenna or that will surely perform better in our location. This simplifies the task of choosing between one or the other, and saves on costs and time. Undoubtedly, a great technological advance within reach of all today.

Even so, we always hear on the bands: "... now I using antenna 1, now antenna 2 ... Which do you think is better?". A classic in our radio hobby. Despite having designed and emulated such antennas, doubts always remain. Which one will work better? Maybe the computer simulation is not 100% applicable to our location for God knows what reasons. Hence the above-mentioned real-world tests are always made.

What I propose here is a low cost system, capable of transmitting on any HF frequency, autonomously (without a computer), automatically, and using the two antennas we want to compare in each transmission. These transmissions are received at the same time by hundreds of colleagues around the world. These signal reports are automatically uploaded to an internet database, which we can access, download, and interpret comfortably whenever we want.

What if we had thousands of monitoring stations all over the world QRV 24 hours a day, ready to give us an accurate SNR report? We have them! For example, using the digital mode called WSPR (whisper). These monitoring stations upload our reports via the internet to a web database, automatically and in real time.

What if we had a low-priced device that could transmit on any HF frequency? We have it!! The QRP Labs U3 Kit for a whopping €20,

With these two technical issues largely solved for this project, we would only have to do one thing, and is that our low cost HF transmitter must operate with one antenna, and then with the other, in cycles configurable by us. The signal reports received by our correspondents need to be stored in a database that we can consult online, or download for further processing.

The simple modification presented here, will make the QRP Labs U3 kit transmit with the antenna that we want, and at the moment that we want, however we configure it. Thus we will get a QRP transmitter with two possible antenna outputs, that can transmit in WSPR, JT9, Olivia, FSK, CW, QRSS ... etc. Since there are many radio amateurs today that when they are not using their equipment, they collaborate with the community by leaving them in QRV in digital modes, we have a huge network of possible real-time signal reports from anywhere in the world.

What I propose next is the construction of a comparison system for two antennas in real time, for transmission, and that these reports can be seen instantly on the internet, so that we can draw conclusions about the behavior of both antennas in the same location. Which antenna is better? What behavior does one antenna have over the other? Soon we will know.


We will use as a base, the Ultimate3 kit designed by our friend Hans and marketed by QRP Labs. There is a review of the first beta version in my blog: http: //

So that the signal reports that we want to obtain are as accurate as possible and can be consulted on the internet, we will configure the U3 for operation with the WSPR mode that best meets these requirements. This mode (WSPR, or whisper) is based on a single frame transmission in FSK modulation, which occupies a bandwidth of only 6hz and lasts 2 minutes. This transmission is synchronized exactly with world time, so our U3 has to have an accurate clock, or have GPS (highly recommended, although synchronizing the internal clock could work also, but is more cumbersome).

These WSPR frames that we broadcast, of 2 minutes of duration, encapsulate the following data:

  • Our Indicative,
  • our QTH Locator and
  • our transmitter output power.

When receiving us, the monitoring stations send the received data automatically to the official database of WSPR where by filtering by our callsign we will be able to know who has received us and with what signal to noise ratio (SNR). To give you an idea, SNR of -30 are tiny weak signals, -15 weak, -10 acceptable, and 0 to +10 very powerful. With the minimum spread and power used, in this case the 150mW of U3, we can reach the other side of the world thanks to the use of this mode. Joe Taylor K1JT, Nobel Prize winner in physics, invented WSPR and also the famous digital mode JT65.

Added to Kit U3, we will use the following simple auxiliary circuit:

This circuit, automatically, will allow us to switch between two antenna outputs as we configured in the menu of the Ultimate U3 Kit. This simple switching scheme also allows us, apart from the antenna comparison system, to use our WSPR beacon with several antennas depending on the band used. For example if we have a dipole for 40m and 80m, and a Cobweb for 6, 10, 12, 15, 17 and 20m, we can configure the U3 so that when transmitting in 10, 15 and 20m, for example, do it with antenna 1 (Cobweb in this case) and when using 40 and 80m, use the corresponding dipole antenna 2.

In summary, we can transmit on any frequency or several sequentially, and QSY from one antenna to another automatically according to our wish, by configuring the menu. The Ultimate U3S is capable of transmitting on any frequency between 100khz and 200Mhz with a power of about 250mW (band dependent; can be increased by adding more MOSFETs and more voltage). Which is more than enough for our signal to reach the antipodes in the WSPR mode.

Constructing the Circuit

The circuit is very simple to assemble. It has few components and can be done on a perforated circuit board. It consists of a micro relay DPDT (5Vdc 2A type NA-5W-K), that is to say, a double-pole relay that will switch from one antenna to another, and in parallel will light one LED or another, and in this way Indicate the antenna used for each transmission.

List of components:

  • Micro Relay 5Vdc 2A Type NA-5W-K
  • 2 diodes 1n4148 or similar
  • 1 BC548 or similar transistor
  • 1 red LED
  • 1 green LED
  • 1 10uF capacitor
  • 1 150 ohm resistor
  • 1 10K ohm resistor
  • 1 piece of 3cm x 2cm perforated matrix board
  • Pieces of fine wire, tin, scissors, solder... etc.

We will first install the relay on the perforated board, and then the rest of the components. The LEDs will go off the board using wiring, so that we can instal them in the box where it suits us best. The circuit will be supplied with +5v, and ground, as is the entire U3 kit. As can be seen in the circuit diagram, the switching signal of the relay is activated by the signal marked in the diagram as PD4. This signal comes from the U3 kit PCB, specifically one of the pins that goes to the kit's display (see U3S assembly guide for details of the U3S's behavior). This PD4 signal is configurable from the individual configuration menu of each transmission of the U3 kit, forcing it to transmit a signal to PD4 of 0 (Antenna 1 activated) or a signal of 1 (Antenna 2 activated), according to our Requirements. By carefully studying the behavior of the U3 and the PDxx signals, we could, without much difficulty, activate a series of multiple relays, and further expand the system for other more complex uses. 

Configuration and startup

Once we have the circuit completed, tested switching, and connected the signal PD4, we will enter the U3 menu to configure our transmissions in WSPR. For a better understanding I will present a real example that I have used. This example would be valid for any band that the U3 covers. Antenna 1 is connected to the antenna connector 1, and Antenna 2 connected to the antenna connector 2 in our U3, which is already modified with two antenna connectors.

I want to compare two antennas for the 40m band. Antenna 1 is a monoband antenna for NVIS; antenna 2 is a wire antenna 34m long with a coupler at its base, set up for 40m.

Antenna 1: We enter the menu mode of the U3, and we set the transmit frequency, WSPR mode, the LPF used (4 in this case), and the value of 30 dBm as the output power (this will fool the WSPR reporting website by saying that we are using 1W, i.e. antenna 1). To the right of the screen there is a value that we will set to 0 (this indicates a signal 0 on PD4, and the green LED is on).

Antenna2: We enter the menu mode of the U3, and we set the same transmit frequency as with antenna 1, WSPR mode, the same LPF used with antenna 1 (4 in this case), and the value of 33 dBm as the output power (this will fool the WSPR reporting by saying that we are using 2W, i.e. antenna 2). To the right of the screen there is a value that we will set to 8 (this indicates a signal 1 on PD4, and the red LED is led on).

The values ​​marked in bold are essential so that after consulting our reports on the web, we can easily distinguish if we are receiving the transmissions of antenna 1 or antenna 2. In the web data query wspr at, there is a field that says "Pwr" and the value 30dBm corresponds to 1W, and 33dBm to 2W. The value of 0 or 8 causes our PD4 to activate or deactivate our automatic antenna switching circuit.

When the U3 is in calibration mode, or stand by, one of the LEDs will always be on, but when set to transmit mode, it will automatically switch to the antenna that we have previously configured, indicating by means of the LED, which of the two antennas is being used for that specific transmission. 

This configuration can be done for any band, and it is valid for the WSPR mode as well as any other that the U3 is capable of transmitting: Hell, CW, FSK, JT9, Opera, etc...

Setting the power value to 30 or 33 dBm does not mean that we transmit with different powers in each antenna! To clarify, we are always transmittingwith the same power, all we do is "fool" those who receive us into reporting that we are transmitting with 1W or 2W, so that in the list of reports we know which antenna was in use at that time. 1W = Antenna 1, 2W = Antenna 2.

Treatment and interpretation of data

This is a very important subject that gives us a lot to play with, since in the correct interpretation of all data and its subsequent processing, resides the conclusions of the behavior of both antennas, which is the goal of this project. One of them could be very good for DX, the other better for short hop, one having a signal peak at 200 degrees where another one is more attenuated by shielding due to buildings, nearby masts, etc...

From the automatic reports stored by the monitoring stations in the WSPR database, we will know immediately and in real time what antenna was used in that particular transmission, the SNR with which we were received, by who, from where, and at what exact distance. It is therefore a method more effective and reliable than the "ear" or the receiving station looking at his S-meter. Above all, we are alone with our WSPR beacon, automatically during the hours we need, and without giving the job to anyone!

Someone may think that since we have 2 minutes of a transmission with one antenna, then another 2 minutes later with the other antenna, we may not get identical propagation conditions for both transmissions. True! There are many ups and downs that can occur in 2 minutes, but even so, and since the system is fully automatic, we can leave it "cooking" for hours or days, and draw very accurate conclusions by means of a statistical treatment of the data extracted. Being analysed later in an Excel sheet, Access, or any computer system capable of data processing.

You go on a field trip, you set up your 2 antennas to test, and while you take a snack with your radio colleagues or the family, the system is comparing the transmissions, and storing them in the web database. With your mobile phone you can see more or less the behavior of both in real time on the web. Finally after a good day of enjoyment of the field, you have the data stored on the WSPR website, available for analysis later and quietly from your home with graphics.

I specifically use an Excel spreadsheet programmed by me, with dynamic tables in which through a script and other Excel tools, I can automatically extract the data from the WSPR web site, obtaining individual and joint graphs over periods of hours, days or weeks. I can see the actual behavior of an antenna by itself, or with respect to another reference antenna in the same location. It would be better to have a web site in PHP that would analyse this data and could be consulted online, without using Excel. But I have neither the time available nor the knowledge necessary. If someone is encouraged... I am happy to collaborate on such a project, it would be very useful for everyone.

The data are what they are, and are in the WSPR database where they remain stored for interpretation. In the correct interpretation reside the conclusions about which antenna behaves better or worse at our location for our purposes. With these conclusions we will have more knowledge, which antenna to use for that next contest or that new installation at the QTH. I hope that this simple assembly will be useful to many users of the Ultimate U3. If not an owner, it may also encourage you to get hold of it, and to experiment. This is our radio hobby, endless experimentation.


Questions and comments to ea1cdv-at-gmail-dot-com

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