Few notes focused to the FT847 modifications for 70MHz band

Hi to all

I already made several upgrades of FT847s for 70MHz band. Let me suggest short note:

Spurios abt 3,6MHz up and down from 70MHz on the level 40dBc and worse (reported here by IZ8DWF] are products of combination of 3rd. harmonic (210MHz) from PA and 3rd harmonic of IF (45,58 x 3MHz) and these products are created in TX mixer.

To improve a spurious problem:

a) increase gain of PA modul on 70MHz, lower drive power from mixer will help you to decrease IF level and consequently the spurious will drop down. PA gain increase is manage by removal of C5051 (470pF) from PA. Such modification will decrease the above spurious to aprox. -50dBc.

b) C5123 in LPF (in EU version of FT847 is this capacitor omitted) should be added by 6,8pF SMD 1206 capacitor. It improves 3rd harmonic from PA a bit and decrease inserted attenuation of LPF by abt. 0.3dB without any substantial influence to 50MHz band,

c) coax cables and power wires incoming into PA block "cavity" should be thread through high permeability absorbing ferrite rings to prevent transfer of 210MHz harmonics over the cables surface back to RF unit board

d) parallel to TX LO connector on RF unit board (J3007) insert 210MHz trap made by series LC parts - 82nH coil (5turns on 3mm dia) and 10pF small trimmer. This trap will decrease the above spurios by next 6-10dB

Spurios are then below -60dBc in case of 10 W out, abt, -53 to -56dBc in case of 20W and abt -50dBc if 50W RF out  is set. Max output in my case is piece by piece between 50 to 70W RF with abt. 13Amps DC, but due to spurious, not more, than 10 - 20 W has recommended to use.

TX BPF mods by PA1O works great and IF spectral line on output is below -60dB. For best BPF curve is needed increase C3082 and C3097 to 10pF. On my pics is visible additional 97MHz product from local FM transmitter, hi. This is not
product of FT847 TX.

RX mods uses G4FUF kit, which works OK. However, due to request to keep MW broadcasting ability, there was added 470Ohm resistor across 1,7MHz HPF. Such mod gives almost no additional att. of HF RX HPF and on freq. below 1.4 MHz the G4FUF filter has abt. 20dB att, what is good enough for MW radio reception.

73 de OK1VPZ www.ok2kkw.com



l3024 - 3029 coils


847's trap


G4FUF's LNA 20dB modification


ferrits - hf pa





lpf 4m                                                                                      lpf 4m

FT847's spectrum before - 10 W out                             FT847's spectrum before - 10 W out 



FT847's spectrum after                                        FT847's spectrum after - 20 W out