24 August 2019

Zagreb Hrelic flea market

Hrelic, at last! In August 2019 I could spend a whole Sunday in Zagreb, which means I could visit the large open-air flea market (every Sunday morning).

Zagreb Hrelic flea market, August 2019.
It is no different from other flea markets, of course! Lots of clothes and books (99.5% in Croatian, obviously), mechanical stuff, fishing equipment, very little electronics. I noticed that there was almost no stuff from 1970's, because Croatia at that time wasn't as wealthy as it is now and their borders (Yugoslavia) were closed to many Western products. Most sellers place their stuff on the ground, so be prepared to bend if you spot something that catches your interest.

It took us two hours to walk through 90% of the stalls, but consider that only 70% of them were occupied. And by noon they started leaving! Therefore plan you timing accordingly, if you are serious about finding something unique (for you ;) ).

I bought a working Canon VHS videocamera/camcorder for less than 100 kn and a vintage 4.5V flashlight for 1 kn. The first item will donate the CRT viewfinder to a clock/display project while the flashlight will need some restoration of battery contacts before being usable again.

How to get there.
You can get there by tram and walk about 1 km on the Western bank of the river Sava. Or you can go by car, pass the used cars market and enter a large parking area (10 kuna/stay in 2019). Note: throughout the 1 km walk there are occasional "pirate" sellers, that you'll miss if you go by car.

It is all in open air. Hot in a sunny Summer day, freezing cold in Winter. Dress accordingly! I was lucky to be there in a cloudy August day, which was perfect.

You can buy food and beverages at few kiosks spread around the area, if you aren't looking for a modern cozy romantic restaurant :) I didn't look for toilets.

It was a positive experience and I will pay another visit when I have a chance.

21 August 2019

LNB current consumption

My 10 GHz LNB draws 106 mA regardless of the voltage. And apparently it works down to 8V: battery power for /P is possible!

Actually there is a 5V regulator inside the LNB, so it is possible to bypass it and feed 5V from a USB power bank.

22 June 2019

A second dish for 3 cm

I got a surplus 40 cm parabolic reflector for my 10 GHz experiments. It doubles the caravan set where I will mount the LNB and use in the receive chain. Since the caravan set can be mounted with a suction cup, it will probably stay on the roof of the car, while the TX dish has to be adapted to sit on a photographic tripod.

The picture depicts the way I found to attach it to the tripod: a removable vice. The tripod is lightweight, but on a field without vegetation I don't need to extend it fully, so the center of mass will be as low as possible.

Then I needed to see how it performs, depending on the position of the TX signal vs focal point. Here things got interesting. We have installed locally up in the mountains a WebSDR equipped with just an LNB pointing towards the Pianura Padana. It covers 10368.0-10369.0 MHz and receives signals beyond 300 km. It is at about 40 km from my home, slightly behind a building, into the Fresnel zone I dare to say.

The "red cover" TX was put into service. Its TX patch antennas are quite off the focal point, closer to the dish and on a side. The resulting beam, as seen on the remote RX waterfall is quite wide, with strong sidelobes. I could not get a null.

Second TX head.
Then a second TX head "white cover" was tried. It is shorter and closer to the focal plane, slightly lower than the focal point. The beam is sharp now! Because of the RTL-SDR AGC I cannot estimate the gain, but any movement impacts the signal strength, move few degrees and it is gone from the waterfall!

I am worried that dealing with two dishes will make beaming twice as time consuming, but at least the HB100 TX has a form of passive thermal stabilisation that keeps it relatively still in frequency once it reaches equilibrium.

Unfortunately the dish weight makes it difficult to make smooth movements on the cheap tripod, but that is what I have for now.

Last but not least, going from about 25°C (home) to full sunshine (35°C, not measured) resulted in 2 MHz frequency change downwards of the whole system. Good to know when arranging for real QSOs!

20 June 2019

Thermal isolation for HB100 transmitter

The actual frequency of an "HB100 10 GHz radar doppler HAM RTX module" is very sensitive to external factors: supply voltage, proximity, temperature.

Voltage is easy to stabilize. Proximity too, and if the IF port it becomes less sensitive.

On the other hand, temperature has a devastating effect, both on short and long term. On the long term, the frequency will drift until the HB100 reaches a constant temperature. But even a quick breeze will produce a snake on the waterfall. So, in order to minimize the effect, I found my way to add short-term thermal inertia to my 10 GHz transmitter:

Transmitting end on 10 GHz, modulated on the IF port.

Variations on long term ("minutes") will still occur, but at a slower pace so that it's easy to do a manual AFC on the receiving side. And the trick is to let it reach a "working" temperature before use and avoid variations over the whole QSO.

15 May 2019

Tools for planning 10 GHz "expeditions"

Activity on 10 GHz means portable operations. If you live near some mountains, it is a matter of finding a panoramic spot, clear of vegetation, open to possible correspondants. Besides asking for advice to "who has already been there", technology and Internet help a lot.

Street View on Google Maps allows to survey many places directly from home. You can get an idea of the possible presence of vegetation, parking place, pic-nic table or grass field for stuffing the equipment.

Once you're done you can run a coverage simulation with Radio Mobile online. You'll need some practice in order to get meaningful and reliable maps, but I am sure there are many tutorials to choose from.

Last but not least, the wonderful www.peakfinder.org website lets you name mountains/peaks surrounding any location of the world: without running a simulation you will know if two remote locations can "see" each other and stand chances for a 10 GHz QSO. This is also useful if someone will operate from home and wants to know if the portable station is visible.

06 May 2019

Just another 10 GHz test transmission

Yesterday I wanted to test again my HB100 signal and the online receiver 30 km away. No dish on either side. This is the result of my WFM signal modulated with a square wave (which generates two parallel traces on the waterfall).

Three beacons and me on 10 GHz.

My transmitting setup.
I did not leave it transmitting for long given its strength on the waterfall and the frequency too close to beacons. Moreover the TX beam was 60 degrees off, at least according to the theoretical/datasheet radiation pattern. 

Simply amazing.

29 April 2019

32 km on 10 GHz

Still one-way, but it is quite outstanding: my bare HB100 signal could be received by a bare LNB at 32 km distance with a 13 dB S/N margin. I could increase the signal by adjusting the TX direction, but the RX side is fixed on a remote location (remote SDR receiver).

Also I think that the strong wind was waving some vegetation into the path because the signal would disappear with heavy QSB.

What is even more interesting is that the bare LNB is receiving a 3 cm beacon 300 km away! At least now we know a remote site where we could try a super-HB100-DX!