Friday, 2 March 2018

Friday, 9 February 2018

Discone antenna

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


Friday, 19 January 2018

LM3886 for lower bands

DIY audio fanatics appreciate this ‘chipamp’ a lot. See for example

                                 LM3886 as 65 Watt audio amplifier

It provides 65 Watts audio into 8 ohms. Audio quality is more than all right. PCB’s with this chip are available for about 10 euro’s (eBay, Aliexpress). 

Bunch of low-cost LM3886 PCB's (10 euro each)

The frequency response curve shows that we are dealing with a broadband amplifier. @ 200 kHz the attenuation is only 1.7 dB !

Frequency response

Therefore, this chip should be able to be used as an RF amplifier for the 2200 m band (135.7 – 137.8 kHz) and lower bands.

A challenge for the designer is of course to adapt the 8 ohms output to the antenna impedance.