VK4ZXI

Friday 18 May 2018 The Black Art of Duplexers: Demystifying Cavity Filters I prepared a presentation for the Wireless Institute of Australia (WIA) annual general meeting practical day on cavity filters and duplexers. Others may find it useful. Links to the files at the end. "A practical guide to the black art of cavity filters for repeaters and digital TV transmitters. Cavity filters are a mystery to most, but at a practical level, not that hard to make or tune. Drew has developed an inexpensive way of building filters using common materials. There are three basic types of filter, pass-band, pass-reject and notch. It is down to the coupling design, which again can be modified or home built. Each has particular applications. All three can be used in repeater duplexers at VHF and UHF. A Notch filter can be used to block unwanted signals like paging TX. Drew will demonstrate a unique use for notch cavities for cleaning the spurious skirts of DVB-T television power amplifiers and the ...

Earth Moon Earth communications- very draft Introduction Josh, a new call,  VK4JNA, suggested doing EME. I quickly went from uncertain to keen as it is very similar to satellite tracking, a long term interest. This  post is on EME thoughts, mainly as a record of what we have found; I have a bad memory. It will be updated as we proceed. There will be spin-off posts on things of particular interest. Antenna systems Mast and rotators Locate the antenna at the club; keep partner and neighbours happy. The club has a new tilt-mast for UHF/VHF. Needs to be erected, about $600 to drill hole and concrete. https://www.nbsantennas.com.au/ A priority to get foundation and mount done as the concrete takes a month to cure. The club has a new G-5500 Yaesu EL/AZ rotator. It needs the AL and EZ rotators to be mounted separately to suit the carriage of the mast. The club has a computer rotator controller. https://ea4tx.com/en/tienda/antenna-rotator-system/ars/...

Cavity filters for Earth Moon Earth communications- working draft Introduction Re-inventing wheel? Been done, but not well explained or understood. Google "Cavity filter LNA" or "cavity LNA" https://oz1ct.dk/konstruktioner/144mhz-cavity-lna-mgf1801.html http://www.wa5vjb.com/references/preamp-Cookbook.pdf Good. Impedance matching http://www.ntms.org/eme/presentations/w5lua/WD5AGO_70cm_EME_TCC.pdf AGO design detail http://www.qsl.net/om7aq/lna1.html Cavity filter for EME I haven't seen a cavity filter used in EME, but few know about them. It could be quite desirable to have a pass band cavity filter very close to the antenna and before the broadband low noise amplifier. The LNAs are usually mounted at the antenna, but the loss of a couple of metres of coax (>-0.3 db?) to a cavity filter (>-0.5 db?) may be justified to give 30 db of out of band noise attenuation. This would greatly help in reducing overload of the LNA by such noise. With a...

"Plastic Fantastic" Magnetic Loop- Linear actuator drive Introduction I have constructed a linear actuator drive version of VK5JST's trombone capacitor tuned "Plastic Fantastic" magnetic loop antenna for 20 m. The linear actuator drive considerably simplifies the construction and waterproofing. The linear actuator was too fast at 10 mm/s. A 12V pulse width modulation (PWM) was used to slow it down to a usable speed. The SWR is dependent on the coupling between the drive loop and the main loop. While 1.1 is meant to be possible, I achieved 1.5 with some adjustment. Tuning the antenna while operating a SDR TRX is possible as the noise is visibly higher in a spectrum scope waterfall; Win4IcomSuite on IC-7300. The noise on the S meter rises from S2 to S6. . The design The antenna uses the underground plastic gas pipe now widely available, including Bunnings. The pipe has a PTFE inner, a layer of aluminium then an outer of yellow plastic. With the PTFE ...