G4AKW

Following on from the construction of the Norton amplifier, the real question to be answered is 'is an HF pre-amplifier necessary on 14MHz and if so what should its specification be'? The full system noise figure factor is given by f=fa + (Lc-1)(Tc/To) + Lc(Lt-1)(Tt/To) + LcLt(fr-1) where fa is the external noise factor given by: fa = pn/kTob pn = available noise power from a lossless antenna Lc = antenna circuit loss Tc = temp of antenna and ground surrounding (in range) Lt = transmission line loss (1dB in our case) Tt = transmission line temperature (288k as its quite mild today) To = reference temperature (go for 288k) fr = noise FACTOR of receiver Fr = noise FIGURE of rx = 10logfr (14 dB in our case) so the noise power in watts from all sources - pre detection is n = f k To b where f is as above the system noise factor which is a theoretical quantity taking account of all noise sources. For the purposes of finding out whether we really need a pre amp we will simplify the ma

The 20m vertical antenna looks good, VSWR < 1.3 : 1 but RX might be a bit deaf. RX details: 1dB antenna cable loss: + 14MHz to 144MHz SBL-1 mixer (straight 6dB loss) :+ IC 202 144MHz receiver (8dB NF). Hence total receive noise figure is at least 15dB. Built a Norton HF preamp (2n5109) to try and improve situation. (Is this necessary given the noise level at 14MHz? see following. Photo below shows the circuit and the measured cbe voltages resulting from a 13.9v supply. First audible results were however not particularly impressive. The dominant noise is the external noise? and this is in excess of any receiver contribution - even at 15dB noise fugure? But I will look into this and quantify the position. OK, this is a simple circuit and the 50 ohm output load is transformed by the broadband auto transformer to the collector load. The actual turns ratio used was 3 to the tap and then 11 to the collector. The turns ratio is then 14/3 or 4.6 which is the voltage transformation.

Antennas are always an issue with neighbours, family, town planners etc. So to date I have limited them to either a) discrete long wire or b) 'TV' like antennas e.g. 2 el band 1 TV antenna for 6metres. However, I had a need for a decent 20m DX antenna and the obvious choice was a vertical. The ideal design was for a full 1/4 wave vertical on the gable end with two 1/4 wave radials sloping down the gable ends. So with the following raw materials: 1) glass fibre fishing pole (7m) 2) 3 x 17ft lengths of 2mm wire 3) Self amalgamating tape 4) 10m UR67 5) 'N' type bulkhead connector 6) chimney brackets and u bolts 7) 2 x dogbone insulators and 2 x standoffs The vertical wire was run up the inside of the fishing pole and soldered to the N type bulkhead. The two radials were then soldered to the N type and the fishing pole shoved over the vertical element. Self amalgamating tape then sealed the bottom and top of the fishing pole. Initially the top section was left off