Posts by DH2VA

    Michi HB9WDF had for quite some time a MKU UP 2424 A without any output power in storage and was kind enough to give it to me for analysis and possible repair. Symptoms were only minor current increase in TX (some 240mA vs. 150mA RX) and no output at all.

    TLDR: bad solder joint in the p-channel FET at the 28V switching regulator for the final stage.

    Long version: All of you who ever dared to open the MKU UP 2424 A will remember the stacked approach of the design with one HF module sitting on top of the final PA (see HERE). As I had nothing to lose (I was expecting a blown final stage), I disassembled the module and opened the lower shielding. Following a rule from my mentor Volker DF7IT(sk):

    "Even in RF circuits, most problems are found with a simple multimeter."

    I started checking the voltages and sure enough, the 28V for the PA were missing, regardless of the PTT TX/RX state. If you follow the large traces from the final stage, you cannot really miss the two large 2512 case SMD chip resistors (R022=22mOhm) routing the 28V to the PA.

    The switching regulator IC is a LT1268B from Analog Devices (former Linear Tech.) and is supposed to have its 12V input at pin 5 but nothing to be seen there. But the FET just above had a funny looking tab..

    Careful visual inspection and some gentle pushing with tweezers confirmed that the tab was NOT soldered to the pad below and was moving around. As this FDD4243 is a p-channel FET which provides the TX/RX switching for the 28V of the final amplifier and the tab is the drain (and the only drain connection of this device) the switching regulator effectively was disconnected.

    Resoldering with a powerful soldering iron and plenty of flux does the job. Don't even try with a weak soldering iron: the PCB is mounted on a heatsink because of the PA and using a weak tool may result in a solder joint worse than before.

    It does not look like it on the picture but the tab is actually now soldered to the pad below AND to the cap next to it, which is the regulator's input cap.

    Result: TX current without drive power is now just above 1A and 20W are easily reached when appropriate IF power is applied.

    I hope this is useful for anybody with similar problems.

    73s Achim, DH2VA/HB9DUN

    Does it have more or less 360 deg view or mounted to a side of a building or next to some vertical wall? I am asking because multipath effects could potentially throw off the GPS timing information (which is what drives the PLL), depending on the fidelity of the control loop.


    The amplifier is tested at VSWR >20 , 20W output power at all phase angles and it survives without damage and degradation.

    Which only states, that the will keep it's performance without getting worse. But that does not mean, that you will have actually 20W at a mismatched load. As an example: if you have an open coax cable end without any feed, the load is ca. infinite (expect for some minor reactive parts). I am pretty sure the power in this 'load' will be close to zero. If you then attach 50 Ohms again, the PA will deliver 20W again, as it was neither damaged nor degraded.

    2E0ILY your feed is not a standard POTY but one with a horn on the 10 GHz part, so get rid of all lenses. The horn is taking care of the illumination angle.

    G8PJH VSWR of 2.7 to 1 is really bad (return loss of around 7dB). While would result in 80% of the power radiated, it is very well possible that with such a mismatch, the output stage of the PA does not deliver its rated power (power gain is depending on proper matching). Typical return losses for POTY feeds are 17dB (VSWR 1.3). I assume you measure the output power with a attenuator and power meter so this will be close to perfect 50 Ohms but for some reason the POTY currently isn't.

    Obviously, the 2.4 GHz itself cannot enter the LNB due to waveguide cutoff. 4x2.4 GHz = 9.6 GHz, which is a theoretical possibility.

    But assuming you stick to the legal power limit on 13cm and drive the final amplifier not too recklessly, the harmonics should be far below any danger limits for the LNB. You want to use a 4 W amplifier presumably for SSB so you would need to back off a bit anyways. And then, I don't think harmonics are a problem.

    Have fun!

    David G0MRF will there be a technical analysis of the mishaps? I have tons of questions but for these to be answered a more detailed breakdown of the sequence of events and the lessons learned would be helpful.

    Thanks for supporting this education project, as always!

    The case of Arecibo is a sad but classic case of negligence and budget cuts. The hurricanes did not killed it, they just gave it the final blow. Working with them in 2014 for the ISEE-3 recovery, they were already struggling then for basic maintenance.

    oh2uds it seems you have a severe ground loop problem and as USB is not galvanically isolated, this will lead to future defects unless fixed. You have to break the loop somewhere, either by inserting a capacitor in the SMA GND input or using an OTG USB-Ethernet adapter (Ethernet is galvanically isolated).

    The Pluto is rather power hungry so you have to be super certain about the PSU for the Pi4 (which itself requires a bit of power). Have you tried a USB power monitor dongle to measure the voltage? You can also use a separate USB line to power the Pluto via its second USB socket (from a 5V /1A charger, NOT from another Pi4 port).

    DD4YR is correct.. the any bad matching on the PA output will not (or only at a tiny degree) backpropagate on the PA input.. This is actually related to the S12 S-parameter and can be n the order of 20 dB. So even with a full short on the output, your input return loss is still -20dB and therefore close to perfect.

    I did have a high SWR issue in the past but the bag cured it, at least until today.

    That sound already very odd and should be the first thing to be checked. It could be a faulty cable.. if this is not 50 Ohms all the way (including connectors) it can transform even a perfect 50 Ohm match into anything else triggering the SWR warning of the PA.

    you should not look on the Es'hail-2 footprints on the WWW as they only list the spot beams for the TV programmes. The AMSAT payload (QO100) has dedicated global beams which are not listed in the usual places but here:…and-operating-guidelines/

    Be aware, that actually two transponders are used for hamradio: one for TV and one for narrowband (SSB, CW, digimodes), check out the website for more information (frequencies, polarisation). Don't bother looking for the TV programmes of the main satellite.