SWR bridge with noise source

In this demonstration we measure the standing wave ratio (SWR) of a load, in this case an UHF antenna. A constraint is that we want to do this in a cost-effective way up to 1.7 GHz. My earlier discussed VNA is not capable of doing this, you can get VNAs that can but they rapidly become rather expensive.

In the demonstration video we generate noise with a tool that essentially amplifies the noise of a zener diode, you could make it yourself, but if you google for BG7TBL noise generator then you will soon find out it is easier to order it than to build it yourself. Noise from the BG7TBL tool has the property that it is random phase by frequency and that the amplitude is somewhat constant over the entire spectrum. Put it on the scope and you get what you expect: something that resembles white noise. The tool needs up to 12 Volt to operate, it will get hot, but it also works on a 9 Volt battery.

If you insert noise in the SDR (I used my airspy) then it will fill the entire HF VHF and UHF spectrum, and you may wonder why you would like to do that at all. Please insert attenuators because the output of the BG7TBL tool is rather high, so I added 30 dB attenuation before I did the measurement with the SDR. When I measure the reflection properties of a load then 10dB goes before the bridge, and 20 dB goed after the bridge to prevent that the bridge is influenced too much.

There is a very good reason why you would like to insert artificial noise into your SDR. The reason is that it allows you to measure the performance of filters or loads connected in between the source and the receiver. In the video we inserted a RF bridge between the source and the receiver.

The RF bridge comes from the transverter store on ebay, see the movie, also there the conclusion is, it is easier to buy one than to make one. The bridge has an input, an output, there is a reference port and there is a device under test port. There are only SMA connectors on the PCB, the schematic is as follows:

On the input (left in the schematic) you insert a signal into the RF bridge, and on the reference port (top port in the schematic) you insert a nominal dummy load of 50 Ohm. On the output of the RF bridge (right port in the schematic) you measure the throughput with your SDR (an airspy). There are now several possibilities depending on what you connected to the DUT port (bottom port in the schematic):

  1. Nothing is connected to the DUT port, in this case the signal reflects back into the RF bridge and this is picked up by the receiver. The throughput is called the directivity of the RF bridge. (for details see the discussion on the transverter-store web page).
  2. The DUT port has a dummy load of 50 Ohm (or the same amount of Ohm as the reference port). In this case you will notice that no signal will pass the bridge and that everything is dissipated in the dummy load at the DUT port.
  3. Connect an unknown load to the DUT port, and now you will see that some of the signal is reflected back to the receiver, but that the rest of the signal is dissipated into the load on the DUT port.

With the first and the second measurement you calibrate the set-up, with the third measurement you can see who much dB is dissipated in the unknown load. You can measure this in the waterfall plot, and hence you can measure the so-called reflection loss RL in decibel caused by the unknown load. Next you need to convert the RL into a SWR value, the equation is as folows:

RL = 20 log10( (SWR-1)/(SWR+1) )

I measured on the waterfall a reflection loss of 20 dB near 440 MHz, this is compatible with a SWR of 1.2, the connected antenna is a miniature version of a 70 cm amateur band antenna.

Last update: 23-5-2018

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Fieldday 21-May-2018

Last day of the May vacation, let’s hike to the city park. Half of the time you are explaining what hamradio is to the general public.

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One bag, it weighs 20 kg due to the battery
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Glass fiber mast with tuner in the park
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The FT-857D, I love this radio, the battery is a bit heavy
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adorable
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Overview park
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The radials that you really need
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All packed together to transport.

Fading

6-5-2018: We see a lot of fading, and the waterfall plots contain the typical stripes:

Fading

Fading (or QSB) is a phenomenon where the strength (and phase) of  the received signal changes over time.

The first plot shows a waterfall on the 80 meter band recorded on May the 6th 2018, this is a UK station some 370 km West of me. Fading often shows as a pattern of stripes, typically the repetition interval is 30 seconds or so.

It is not uncommon that multiple ranges of the spectrum of the radio transmission are affected at the same time. You miss a part of the high tones, the middle and the low tones at the same time, it is as if three notch filters slowly slide over the audio spectrum.

During fading several wavefronts arrive at the antenna, upon arrival they can add-up or they may cancel one another, in physics this is called interference.

So what type of interference mechanisms are known? This is the HF and we are talking about long waves, with the station in the UK we can expect a direct wave over the ground and a reflecting wave via the ionosphere. Both waves will interact.

Another possible mechanism on the HF is that only the properties of the reflecting layer are changing during the transmission. This is known as Faraday rotation in the ionosphere causing a change in the polarization of radiowaves. Any antenna has a preference to polarization, changes in the Faraday rotation will affect the received signal strength.

Interaction between the ground and the skywave could explain what is probably going on in the first plot. The critical frequency is around 2.5 MHz and we are near 3.8 MHz, furthermore at 370 km you can still expect a groundwave.

In my experience the pattern of stripes is typical for nearby stations where the ground and the skywave interfere. QSB manifests itself differently for more distant stations where the radio signal may arrive after multiple reflections involving the ionosphere. In the latter case you see that the signal strength varies, but you wouldn’t see the stripes.

9-5-2018: when you think about theory to explain what you saw then it is always good to look again at the observations and ask yourself whether the theory is still valid. Here is the waterfall spectrum of a QSO with EA3BOX Juan who is too far away for me for a groundwave, but still you see the fading stripes, which is the faraday rotation in the ionosphere. The critical frequency was 3,475 MHz at this time, here is the waterfall plot:

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I also attached the ionograph of Dourbes (Belgium) at the time of this QSO, I don’t claim that I fully understand what I see in the ionographs, but the most important number is the foF2 which is the critical frequency, and other important ones are the MUF at the bottom legend.

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If someone has a manual or a better explanation of what you see in the ionographs then it would be appreciated.

Last update: 9-May-2018

Field day at Maasvlakte

Antenna: 10 meter glasfiber pole with an antenna tuner at the base. Couple of radials. It is an easy solution that always works: 17 QSOs in a 5 hours, conditions were mediocre.

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The weather was just too good to let this opportunity pass by, blue sky beach weather in April, what more do you wish. Here is the log, worked the 40, 20 and 17 meter, heard some Canadians and Americans, but no QSO was established.

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Last update: 20-April-2018

Field day Brouwersdam

Today we (Chris, John, Adri and I) went to Zeeland to check the QRM level near the Ouddorp lighthouse (it was ok), and I wanted to check the performance of a 80m vertical delta loop. Included are some images.

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The 80 meter delta loop consists of 80m of wire, and a 1:4 current balun. It has two functions, it does the common mode rejection from a symmetric antenna to an asymmetric transmission line, and it transforms the 200 Ohm impedance to the 50 Ohm. The measured SWR was around 1.7 at the resonance frequency of 3650 kHz. Still confused? This is what we made:

 

The 1:4 Guanella balun (invented by Gustav Guanella, 1909-1982) was discussed earlier in this blog: link Why this design? I never found an opportunity to test a delta loop in the field,  I didn’t have a good antenna for 80 meter, and I never tested my Guanella Balun. A significant advantage is also that the Delta loop only requires one mast rather than two (or even three) required for the dipole.

The radiation pattern of this design is interesting when you put it in NEC, it appears as if all power is radiated upwards and it has a little bit of directivity, so this is an antenna for NVIS (near vertical incidence skywave) activities. During NVIS you stay under the critical frequency which was about 4 MHz during the event.

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Azimuth pattern, the antenna has 7 dB gain apparently, but it changes by frequency (it sweeps over the full 80 meter band here)
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Charge intensity
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Elevation pattern, NEC suggests that most of the power is going upwards

Here is the log of the QSOs we made, all locally (of course because of the time of day and the frequency):

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For DX contacts on 80m you need to be there around sunrise and sun-set.

Last update: 20-April-2018

Is there any propagation? Yes of course there is on 40 meter.

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40m, 500mW WSPR TX mode, last 12 hours relative to Mar 9, 2018 10:26

Propagation mechanism

From this WSPR graph you can see that 40m radio waves propagate well over the ocean, that the F layers are likely reflectors at 250 km altitude and that the skip distance is around 500km. Here is the explanation in more detail.

The first WSPR fact is that you are only heard either within approximately 30 km via the ground-wave (over the surface direct line of sight) or at distances greater than 500km. Between 30 and 500 km you usually get no reports whereas there should be some listeners. I rarely get reports from for instance the university of Twente where there is a
permanently running WSPR receiver. Instead, the first signal reports seem more to arrive from southern Germany and the center of France, roughly at 500 to 800 km from Rotterdam. The mechanism that likely explains this is a reflection of the radio wave against an ionospheric layer at a height of probably around 250km, but this depends of course on the critical angle. An ionosonde can tell you what the height of the reflecting layer is, my guess is that it is around 250km, so this is a F2 layer. All of what you see in the WSPR plot is observed during the night time the D and E region descrease in intensity (number of free electrons). Within the EU we see single, double or maybe triple hop reflections, but nothing seems to go any further than that.

But now the long paths over the north Atlantic, these paths can only be explained by more than 3 hops, there should be tens of hops to explain a path across the Atlantic ocean. The first surface reflection towards the west seen from Rotterdam will be over a water surface, the north Atlantic in particular. There are no reports from the UK, my explanation is that it is too close to the transmitter. Over the ocean the propagation is much better, water (and ice) are very good reflectors, and the signal goes on until you hit the east coast of the united states and also Costa Rica in central America. The WSPR signal survives maybe one or two hops over the continental US, and then it dissipates because land is a worse reflecting surface than water or ice. This is the general tendency of HF radio propagation, on 20 and 40m it is very difficult to get any reports from the central US, Texas, Oklahoma and Kansas and northward. Most of my QSOs are with the
Eastern states because of the mechanism described before, land is not a good reflector, water is better.

Some logbooks like hrdlog.net allow you to plot the states that you have worked in the USA, this summarizes the contact mostly on 20 and 40m over the last several months (1000 QSOs back actually, on average I do some 8 per day). It also shows that the eastern states are confirmed, and this is in agreement with the WSPR propagation plot, propagation over land is less effective than over sea water or ice.

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Last update: 10-3-2018 9:15