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FT-847 70mhz LNA Kit PDF Versão para impressão Enviar por E-mail
Escrito por CT1FFU   
Sábado, 28 Janeiro 2012 22:44

Yaesu FT-847 70mhz LNA Pre-amplifier


By CT1FFU- Tony





I decided to buy a Yaesu FT-847. Using an all mode all band radio, operation becomes much easier and give better accommodation on shack however, I soon got frustrated with its reception as the signals were much weaker then the ones received with my home-made X-verter (dx-70-G4DDK design).

I decided to look on web for a modification and Marc's, PA1O BPF, was a good to start. Marc’s article, convinced me with the good results on filter modifications as well as the simplicity of those changes.


After Marc’s modification the radio became better, but still not with a satisfying RX performance.


FT-847 has a build in preamp, but it seems to have poor gain at 70mhz

,so I decided to add another preamplifier stage.

I have looked carefully on circuit diagram trying to find the right place to insert a amplifier

which wasn’t however easy, but I realise that the band-pass-filter, a Chebyshev, could be rearranged and have the preamplifier inserted there.

Results were clearly better as sensitivity is concerned and in many comparisons with

transverter it was easy to see the improvement.


After I published a small article on www.70mhz.org these modifications were reproduced with success by others.

of the FT847 after preamp fit:

Since the radio has some 1.5dB loss on the PA filter and still some more few dB in the subsequent HPF filter on the daughter board the lowest NF achievable is still quite limited by those factors. NF at 70MHz is now about 7dB (note the 15.5dB NF the radio had originally) which makes perfect sense taking in consideration the circa 1.8dB NF of the new LNA 02186
Using a lower NF device would bring just a bit better NF but as per the loss limitations described above, without further changes into the radio,

The results are good

and hundreds of boards are now installed world wide on FT847s

Later, as per many requests, I decided to make a KIT available.

This KIT is very easy to install and anyone with a bit of radio/electronics experience can do the job easely.


Laboratory tests made by GI0GDP and CT1BXT shown that once fitted the pre-amp the 70mhz RX achieve similar sensibility as on 50mhz, even without PA1O BPF modification.



About the design:

I decided to use a LNA-02186
NF <1.8dB
50 ohm impedance;

stable amplification, 30dB

gain and noise figure low enough for the application. This amplifier uses the tipical configuration and bias and is placed in the in the BPF which was rearranged as seen in fig1.





performance would not get much better than this by improving the preamplifier alone. This 7dB NF could be also expressed as a sensitivity of about 0.1uV for S/N of 6dB at 2.8KHz BW.





     Download here the Preamp Manual: FT-847 LNA pre-amp Manual


Kit FT-847 LNA pre-amp   Price with shipping
Ready build total with Shipping 39€
Kit not builded total with Shipping 32€



Actualizado em Segunda, 06 Fevereiro 2012 15:24
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