VK4ZXI

High power UHF DVB-T amplifier, filters and testing- very draft I have a used 150 W pallet amplifier from a scrapped DVB-T transmitter via eBay. It is bolted to a heat sink from a satellite transmitter. I am basically following the 1 kW CW UHF amplifier from W6PQL. I have the low pass filter for the amplifier. I temporarily soldered some SMA connectors to test its frequency response. Down about 1 db at 500 MHz, but -40 db at the third harmonic; very good. It has been suggested to use a pass band cavity filter duplexer. I had one on hand that I had just tuned for a repeater, trying to get a narrow pass-band. Just using three cavities, I varied two of the three cavities to try to get a wider response. No joy, little wider, but more importantly 10 db loss. To be continued...

Polyphase harmonic rejection mixer: AirSpy HF+ Introduction Can you get excited about a new mixer, usually boring devices that haven't changed in decades? Yes, the new polyphase harmonic rejection mixer in the AirSpy HF+ is almost as revolutionary as SDRs and will have a major influence on their design. The big advantage of a polyphase harmonic rejection mixer is that it acts as a RF filter for the selected signal, as well as suppressing harmonics and other aliases of the mixing process and local oscillator. It means that the mixer can virtually be connected to the antenna. Typically, a polyphase harmonic rejection mixer converts down to an ADC at base-band. It seems they can be used for both RX and TX. The post covers how the AirSpy HF+ works, and gives references to what I have been able to find out about polyphase harmonic rejection mixers. They are new and still covered by recent patents. A link to a PowerPoint gives general technical details of the mixer. AirSpy HF+

Modifying cavity filters for DATV TX or for repeaters Introduction I am currently doing further work on using notch cavity filters for DATV DVB-T transmitters. My earlier efforts were with what I had at hand and not knowing the solution; I (re)discovered that notch filters clean up DVB-T TX very well. However, it was at low power, 10 W, and high losses, >6 db because of the six cavities in a mobile duplexer. Here, I will report on modifying high power >100 W individual filters. In the next post I will report on using them and determining is just one pair are sufficient. The other goal of this post is to show how easy it is to modify older commercial filters for DATV or repeater use. Modifying cavity filters Old commercial filters are relatively easy to modify as the only thing that changes is the coupling loop, provided they are on frequency (not too hard to change that too!). Notch filters are the simplest as they use a single simple coupling, just a loop of metal. Old co