Geekcreit produces mini digital voltmeters for general purposes. Because the price of such a voltmeter is low (less than 2 euros) you can incorporate the voltmeter into each of your do-it-yourself projects. Range is about 2.5...30 VDC and the voltmeters, to be found on aliexpress or banggood.com, are offered in various colours: red, yellow, blue, green and white.
Discone antennes cover a wide frequency range. MFJ has introduced a new discone antenna: the MFJ-1868.
It is designed to receive radio signals from 25 - 1300 MHz and to transmit from 50 - 1300 MHz (up till 200 Watts RF power). Interesting because it covers 6m, 4m, 2m, 70cm and the 23 cm band. A discone antenna is omnidirectional, vertically polarized and has a rather narrow radiation pattern in the horizontal plane. This makes the antenna sensitive in the direction of the horizon. SWR remains below 1:2 for the 50...1300 MHz frequency range. Interesting if you do not want your garden covered with lots of antennes. The MFJ-1868 has a vertical whip affixed to the centre in order to extend the low frequency response. Price of this antenna: appr. $70
Some collections are special and deserve to be maintained. The collection of Mr. Teyler from Haarlem is such a special collection. This week we visited the Teylers museum and were able to see interesting items such as Leyden jars, 19th century light bulbs, ancient skeletons and the study room of prof. Lorentz. More info: https://www.teylersmuseum.nl/nl