Another tool for planning LOS contacts

Following a previous post on tool for planning microwave (outdoor) activity, I would like to add a web service that eases the search of two spots in line-of-sight: https://link.ui.com .

Select a starting point in the map (single click), then "+ Add PtP" (Point-to-Point). Then drag the grey circle (receiver) around and pay attention to the color of the line: green means OK, red means NO-GO.

From the bottom of the page a slide-up appears which shows the terrain profile below the RF link and antenna heights. Don't forget to review antenna heights because the default value is 12 meters above ground!

Remember that the tool does not take into account buildings and vegetation!

My best DX on 23 cm so far

My usual DX on 23 cm is 136 km during the monthly activity contest. Last Sunday there was the 50th anniversary of the Vecchiacchi V/U/SHF contest and I wanted to pay my tribute to the event.

All I could do was to spend the last contest hour on the air on 23 cm SSB. I was logged to the on4kst chat and got a request from a station 368 km away.

My 23 cm setup consists of a transverter with about 15 W out, 5 metres of SAT-TV coax and a 23 cm Yagi on the balcony, with "hand-rotor". The DX station was checking aircraft positions so that we could attempt an Aircraft Scatter QSO. At the first try my antenna was beaming too North. Then I adjusted it to what should have been the right direction and ... BINGO! The QSO was completed. We used three airplanes, that made a very long opening on our path.

Luckily I guessed how much off-frequency my transverter is!!

It is amazing how well AS works. Much similar to MS, but much more predictable!

First 23 cm QSO

The first 23 cm monthly activity contest date came too soon. It was my first opportunity to test on the air a 20W transverter and 23 element beam. The antenna went up easily and I used the TV coax already installed plus two BNC-N adapters: losses everywhere!

The 23 cm antenna on the balcony, beaming 60°

Apart being away for a long time from an SSB contest, I immediately felt restricted to the 3000 Hz bandwidth of the FT817, while for the last 12+ months I was able to see 2 MHz of RF spectrum.

The RTLSDR does offer an advantage. Even though I have probably not missed a QSO, I was thrown back to the good old days of search and pounce with the VFO.

I was able to speak with all the people I have heard last year. All signals were very strong, with the ODX being S9+60. Curious band!

Next month I will try to arrange more room in the shack/lab desk so that I can switch the antenna to the SDR when done with QSOs: at last I will be able to compare the two RX solutions.

Packed up RTLSDR

A local HAM agreed to compare his 23 cm transverter with the RTLSDR in order to see if the little stick can be competitive in certain situations, like a contest. So I needed to do some adjustments to my typical home setup.

First of all, his cables terminate with N connectors. I initially though of changing the short RG174 cable end into a female N, but quickly realised how weak the whole system would be considering the weight of N and in-flexibilty of thick coax. I needed to protect the dongle with a box.

I picked a metallic chocolate box I kept aside some months ago and fitted a flanged N female. While I was at it I also shortened the RG174 cable to what was strictly necessary (1 metre to 10 cm): these two actions resulted in at least a couple of dB gain (or ... "less loss"). Note that 1m of RG174 at 1300 MHz looses 1 dB.

The USB hole is not something I am proud of, but it was realised in less than 5 minutes, so it is acceptable for this experiment. If the on-the-field comparison shows RTLSDR is a decent performer I will consider soldering directly into the PCB both the USB cable and the N or BNC female head.

Transmitting on 23 cm (thinking of)

After one year of SWL'ing on 23 cm I feel I would like to have a QSO up there. Besides buying a transceiver for that band, another option is a transverter. Current DB6NT catalog lists a 144 to 1296 transverter for 425€ built, or 199€ in kit (SMD). But how about a simpler solution? I came up with:

• a Si570 generator/kit, at 60€ or so, and it requires a power amplifying chain
• a frequency multiplier

While checking if any of my XTALs or canned oscillators would fall within 1296 MHz, it occured to me that 1296 is an integer multiple of 144 and 432 MHz. This observation led me to explore the almost forgotten world of multipliers and I met the varactor component.

A varactor is a sort of high power varicap diode which is able to return a signal tripled in frequency with a 35% efficiency, all with passive components. FM or CW only. Since I own a 30W UHF transceiver, I potentially have a 10W CW 1296 MHz transmitting chain. I think this technology has been almost forgotten, while today it would allow many amateurs to give 23 cm a try.

A quadriband transceiver will transmit on 70 cm (or at least 2m if using a 5x multiplier), and they are very popular => no additional cost.
An RTLSDR stick will provice wideband 23 cm coverage for 20 USD.
A computer is then needed as part of the receiver, but I would say most HAMs own one => no additional cost.

So, for the cost of a RTLSDR and a varactor multiplier you can be active on 23 cm (CW/WBFM). Not bad.

23cm biquad and coax

After a long a careful theoretical planning, I moved the 23cm RX-only biquad antenna in a more permanent location, not obstructing the view out of the balcony.
Since the antenna is now closer to the coax entrance into the shack, I could shorten the cable: what a better chance to take a couple of measurements?

After fixing the antenna in place I fired up SDRsharp and tuned the local 23cm beacon. Its carrier was peaking -40dB (relative). Then I cut at least 4 metres off the coax cable (unknown 75 ohm), re-soldered the TV-plug at the shack end and measured again: carrier now peaks at -35 dB (relative). That's about 5 dB S/N improvement.

Lacking proper instrumentation I cannot certify the gain is due just to the shorter coax, or to any other factor like: impedance match (remember my antenna R+jX was never measured), better coax-to-connector(s) junction, ... In any case apparently now I have 5 more dB of RX "power" for the next 23 cm event.

I am still after a simple method to remotely turn my antenna over a 90° range (max). I have few ideas but they are mechanically too complex for my time and tools.

A container for 23cm biquad antenna

There is a bit of local activity on 23cm and the RTLSDR dongle allows me to receive that HAM band. An improvised indoor dipole brought in few interesting signals, increasing my interest for 1296 MHz. I have already built a biquad antenna, which needs a (cheap!) housing before being installed outside.

My biquad size is about 20x30x6 cm and I have had troubles locating suitable plastic containers in the kitchen department of local supermarkets.

During a visit to IKEA lower floor I spotted the SAMLA series of plastic transparent containers. The 11 litres one is large enough to host my biquad (38x28x14 cm) and it costs 1+1.75 = 2.75€ (lid+box).

The antenna will go on the internal side of the cover, which is easier to work on and cheaper in case the experiment fails. In that case the SAMLA container will be repurposed in the house.

New RTLSDR dongle has arrived

Simply amazing. The sensitivity difference between my first RTLSDR dongle based on E4000 and today's with R820T is about 3 times, in favour of the latter. Even with the stock DVB-T antenna I could pick up more signals than the E4000 and the external GP. Then I "sacrificed" the stock antenna to use the cable with MCX connector for building an adapter to the TV plug: with the GP antenna outside I could receive airplanes as far as 300km/160nm. The ADS-B software was showing an average of >50 signals per second, against less than 10 for E4000.

So, as many others have reported, the R820T stick is more sensitive above GHz than the E4000 counterpart. I also noticed a much stronger frequency drift in the first minutes of operation, which also result in a warmer RTLSDR dongle.

Two experiments will follow:
- retry 23 cm SWL'ing during the monthly activity event
- test the ADS-B reception in a location with a 270 degrees wide horizon

ADS-B ground plane antenna

This is not rocket science: dipole and ground plane are the most basic antennas you can build. And they can be good performers too! Here is a picture of the 1090 MHz GP I assembled for my ADS-B reception tests.

It is built around a panel-mount BNC socket. Each of the three solid copper wires is about 6.6 cm long and there is a 10 kohm 1/4W carbon resistor across "hot" and "cold" points to discharge static electricity right at the source.

This GP performs equally well with and without resistor, so leave it in place. This antenna is omnidirectional, and depending on its horizon it can bring up signals as far as 100 km away (E4000 tuner, airplane above 10'000 m). In this screenshot my receiver is below the ISSxyz airplane:

ADSB antenna

I am not a fan of aircraft monitoring, but their ADSB 1090 MHz transmissions represent a widely available and geographically distributed beacon for testing new antennas.

My antenna test-field is the usual balcony open to N-NE.

First I tried a biquad without reflector. This is somehow bidirectional in an "8" shape. The advantage of this antenna is the intrinsic short-circuit, which avoids the problem of static electricity build-up (and frying the RTLSDR dongle).

Then I read many people suggest a collinear antenna. Projects documented online do not agree on sizes of straight elements (13 vs 19 cm) and coils (2 vs 4 cm diameter), so I opted for a simpler ground-plane: one vertical element, and 2(4) radials. All 1/4th wavelength long = 300 / 1090 / 4 = 6.8 cm (a bit less, in practice). It is short, simple to tune and easy to handle. The 1090 MHz GP antenna can be made of 1mm dia. solid copper wire, even keeping the insulation. In order to discharge static I inserted a common 10kohm 1/4W carbon resistor across antenna terminals.

Performance? They receive signals from the same distance, the GP being slightly better (but I would need a side-by-side realtime comparison) probably due to the biquad needing some form of tuning, making it a bad match outside resonance window.

Conclusion. Unless you are absolutely sure in your interest in ADSB monitoring or you have access to proper instrumentation to measure antenna impedance above 1 GHz, go for the GP antenna. According to my observations, ADSB signals propagate in line-of-sight, so look for a good, open, position: I can draw my antenna electrical horizon by looking at what/where it can receive.
Use a low-loss coax (known quality SAT-TV coax is a good choice) and keep it short: this trick will easily save those couple of dBs so hard to achieve with a different antenna without tuning instrumentation.

Two beacons in one shot

I should consider myself lucky since I have a local 23 cm beacon to test my homemade biquad antenna. While aiming it, trying to discern reflections from the actual line-of-sight signal, the waterfall spread over 2 MHz showed another beacon, 500 kHz below the known one:

Without SDR I would not have noticed IZ1ERR/B. It was 10 dB S/N, even with the 23 cm dipole inside the house. We are in full line of sight, but it is pointing away from me (I am on his side)

I have recorded 50 seconds of baseband IQ signal with SDRSharp: if anyone wants it I will share a download link of the 404 MB file.

Troubleshooting the biquad

First thing I checked on the biquad was the continuity of the coax+antenna setup. Electrically the quad is a short circuit, so measuring resistance across the antenna plug in the shack should be as close to 0 ohm as possible.
Instead my DVM was reading ~10 ohm "round-trip", but measuring at the bare coax cable end returned a fraction of ohm...
The antenna plug has a screw for ensuring electrical contact, which I promptly secured with a solder joint. Electrical resistance dropped to a more meaningful value and incoming signal from the local beacon increased. The biquad is still beaming away from it, I need to move the antenna to ther side of the house so that it will be facing the beacon, but currently my only properly terminated coax cable is running through a wall :-)

Next weekend there will be a 23cm contest, so I will have other on-the-air signals coming through.

Nothing beats the good ol' dipole(?)

Last night I monitored 1296 MHz with the RTLSDR receiver and the homebuilt biquad antenna, fixed in one direction.

Apart from a strong station, I could hear nobody else. Then the usual HB9 placed a CQ and he was weaker than expected, so I switched back to the indoor 23cm dipole and his signal was at least 5 dB higher: something is wrong with the biquad! Last night's test confirmed my suspicions.

The outdoor antenna should gain about 10 dB over the dipole. The cumulative coax and connectors loss could be 3 dB, leaving a 7 dB boost in favor of the biquad. But according to my late night observations the difference is 7+5 = 12 dB in favor of the (indoor) dipole!

Either the biquad I have built has a weird radiation pattern or something is really wrong. Troubleshooting begins.

23cm biquad simulation

Before assembling my 23 cm biquad antenna I wanted to know what happens when loop-to-reflector spacing is reduced. In theory, at least.

The standard spacing is 1/8th of lambda, while I want to halve it to 1.5 cm which is about 1/16th lambda.
Why? Because it simplifies building the antenna since the double loop can then be soldered directly to the back of a BNC female head. :-)

3D model of biquad and wire mesh reflector

I use MMANA to simulate antennas. So I drew a biquad with a reflector spaced 1/8th lambda and simulated it (the .maa file). The result was encouraging; both Z and gain were within expected values.

Each simulation run takes 36 seconds in "free space" and 48 seconds if the antenna is said to be on "real ground", so I chose not to increase reflector density.

After saving to file the "far field" data, I reduced spacing to 15 mm (0.015 m) and re-run the simulation. In both cases the antenna was set to be over real ground.

Comparison of 1/8th vs 1/16th lambda in free space.

The comparison of far fields for both antennas (1/8th is in black, 1/16th in green) show a reduced gain and F/B ratio for the shorter antenna. Gain difference is 0.5dB (theoretical), which is negligible for my final, RX-only, application. F/B looses 3dB and doesn't worry me too much, even if I will mainly use the front lobe beaming out of the balcony... I am going to build the biquad right off a BNC female head screwed to the reflector panel (a copper clad board about 20x30 cm).

Next, since I had some spare CPU cycles, I ran an MMANA overview over +/-40 MHz from the center frequency (1295 MHz). Z/Gain/F-B were computed in 5 points, SWR is interpolated. Apart from an impedance discontinuity at 1315 MHz, all other relatively flat values give hope for a normally performing antenna ... especially taking into account all my mechanical bulding errors.

Gain and other parameters comparison at various frequencies.

SWR interpolation over 80 MHz span.

Someone may notice in screenshots that SWR is computed for 75 ohm and not 50. The fact is that the RTLSDR dongle is meant for TV reception, whose impedance is 75 ohm. Also I will use a SAT-TV coax to reduce losses, so why not reason in 75 ohm terms?

23cm biquad spacing

As 1296 MHz receive only antenna I have chosen to try a biquad (or dual-quad). It is composed of two full-size quadriangular loops (23cm perimeter each), set electrically in parallel and "touching" at one corner, the feedpoint.

Mechanically speaking, a 23cm biquad is simple to build, requiring a thick copper wire (1mm diameter or so) and a large reflector, say, 20x30cm, like a copper clad board. Loops-to-reflector distance should be 1/8th wavelength, which is about 2.9cm.

2.9cm ... that's the tricky part for someone like me that is not equipped to do mechanical work. I prefer what can be done with the soldering iron. So I wanted to understand what would happen to my biquad if the element separation would be of ~1.5cm, that is the height offered by a BNC female head screwed on the PCB/reflector element.

Online resources suggest that a change in spacing modifies forward gain and F/B ratio, but impedance doesn't vary too much. Anyway I have no means to measure SWR at 1200 MHz, so all I am interested in is a confirmation of the gain, amount and direction.

An MMANA simulation is on the way...

23cm activity

These heard stations on 23cm SSB are some of the local big guns. Well, not so local given the DX distance of 136 km and my antenna consisting of an indoor dipole! But the HB9's were using 200W or more...

HB9BCD at 136 km

I1NDP and another QSO at 1'296'250 kHz

HB9SV at 136 km.

I do not like those aliasing products (or intermod?! or receiver overload?!), which then disappeared when I fiddled with SDR# settings. That effect requires further investigation.

My RX setup was not QRP either, since the i5 core laptop eats up a good deal of energy too (50W or so). I have already shaped up a biquad antenna. Now it needs a reflector and a support, then I am ready for the next 23cm event. Unless I manage to get a signal out of that SAT-TV PLL...