VK4ZXI

Japanese TRX use JIS not Phillips screws Ever wonder why it is so easy to damage a screw on a Japanese made radio using a Phillips head screwdriver? Obscure, but simple, they use JIS screws not Phillips! I haven't checked, but I think they are on made-in-Japan cars, certainly lots of other gear. JIS screws, which pre-date Phillips by 20 years, usually have a dot or other indent on the head. The solution is simple, buy a set of JIS screwdrivers, about $25 on eBay. http://themtblab.com/2016/01/tools-vessel-jis-screwdriver.html

RF tap for panadaptor/second SDR after IC-7300 bandpass filters (First draft) Introduction It is possible to tap into the IC-7300 after the bandpass filters, just before the ADC. There is a coax connector that allows an SDR tap to receive the filtered RX signal. It is shared with the low-level TX path, but the levels are low, giving a TX monitor too. Using CAT controls, the SDR can be controlled by the TRX. It can also operate as a second RX in the operating band. The RF tap is an alternative to the INRAD RX7300-receive antenna cable, with some advantages and disadvantages. Finding the RF tap In an SDR-based TRX there is no IF tap point, but on the IC-7300, there is a RF tap point after the band pass filter and RF amplifier, where an SDR has the good filtering of the IC-7300, but can access the whole operating band. The RF tap point J1431 is shown in a portion of the RF unit schematic from the service manual. The schematic shows the RX, TX and power paths in green, brown and ...

Power measurements of DVB-T transmitters- first draft Introduction I have been puzzled for some time about how power of DVB-T amplifiers were measured relative to other modes. This is an important issue in DATV as DVB-T has been criticized as inefficient compared to DVB-S. It would appear that some of the debate comes down to how the power is measured. Most amateurs use simple diode power meters that do not give an accurate reading for DVB-T. In this post, I outline the theory and practice of measuring DVB-T power correctly with either a thermal power sensor meter or envelope power using a spectrum analyser. It is important that the correct measurements be used otherwise it is comparing apples with oranges. Different power measurements are accepted in amateur radio, vis, CW (peak power, key down) compared to SSB (peak envelope power). I wonder if another measurement unit for digital modes may be more appropriate, such as the data rate to the DC power input and spectrum width, ...

Commercial DVB-T Amplifiers and filters and implications for DATV (first draft) Introduction Commercial solid-state DVB-T amplifiers use about 250 W "pallet" amplifiers; a pair of  amplifiers, each using a pair of LMDOS transistors in a single package. The circuit boards are mounted on a piece of thick copper then on a heat sink. To get higher power, 1 to 50 KW, many pallet amplifiers are used in parallel with a system of splitters and combiners. The amplifier output passes through a series of filters, to stay within a standardized spectrum mask to limit adjacent channel interference. To conclude I note some implications for DATV. Pallet amplifers A typical pallet amplifier is pictured below (bought on eBay). The gold rectangle on the left splits the input to a pair of Doherty amplifiers, while the one on the right combines the amplified signal to the output. The combiner uses a type of circulator to dump RF to a dummy load, the black rectangles on either side of the ...

Building a MMDVM multi-digital mode repeater- DRAFT 24/2/2017 Introduction Very draft. Will write more as I go. Everything has arrived and I am starting to put it together. Facebook and Yahoo https://www.facebook.com/MMDVM/ https://groups.yahoo.com/neo/groups/mmdvm How to do everything _Interfacing the CDM 1250 to MMDVM board.pdf in Yahoo files. For 1.01 board and updated files. https://www.f5uii.net/en/installation-calibration-adjustment-tunning-mmdvm-mmdvmhost-raspberry-motorola-gm360/ (Good but had trouble connecting via the network. Will go back to full desktop) DMR-UTAH MMDVM XFCE Raspberry Pi Image  * *        by Chris Andrist, KC7WSU         * * https://github.com/candrist/mmdvm-image * *           kc7wsu@dmr-utah.net    http://digiham.boards.net/thread/13/dmr-repeater Another, includes  CDM1550LS  code plugs https://www.facebook.com/MMDVM/ Posts...

Using notch filters for Rx and TX and extending the range of a notch duplexer (Draft) I have a six cavity notch duplexer for 70 cm. At the original frequencies, outside the amateur band, the RX and TX notch responses are a mirror of the other, as notch filter responses are asymmetrical, compared to symetrical for band pass.. The mirroring of the responses are achieved by adding a quarter wave line to each cavity in the TX half. Being a quarter wave length, it changes with frequency when the duplexer was tuned about 20 MHz lower from its original frequencies. This was enough to stop the mirroring, making the RX and TX curves much the same; high losses for TX. The desired TX shape can be restored by adding about 10 mm to the quarter wave lines on the TX side. The length was calculated as the difference in quater wave between the two TX frequencies, in this case about 8 mm. For this duplexer, it was quite easy as the join to the cavity was soldered. The cable was lengthened by addi...

DVB-T TX Filter: 23 cm filter using 70 cm duplexer Introduction I came across a commercial UHF bandpass cavity filter that was as long as a VHF cavity. I removed a coupling to see inside. The probe was nearly 500 mm long, rather than the 170 mm in most UHF filters. They were using the probe as a three quarter length, rather than the typical quarter wave length. I checked the cavity at VHF and, sure enough, there was another peak. The obvious question then was whether the 70 cm duplexer filter would have another bandpass at three times the frequency, about 23 cm? It does! With careful tuning it should be possible to use the ubiquitous 70 cm duplexer at 23 cm. Analysis Raw response With the filter set to 7 MHz at 446.5 MHz, I had scanned three times that frequency and a very wide span. Sure enough, there was a passband, although at a different frequency and three times wider. The frequency of these cavity tuners are determined by a fixed length probe (tube) and a capac...