|70 MHz -
OK1TEH's photos & hardware info
Since 25th July 2010 I've been QRV 4m with a new 2el HB9CV 4,1dBd. Any QROO station for cross band EME test? :-)
new 4m dipole on the West side of shack since 14th June 2009
old 4m dipole in the ham-shack below parasol
new 4m PA made by OK1VPZ for 10W out in WSJT with LNA & filters - PWR can be adjusted on front panel
4m PA schema (btw. in OK we have allowed maximal power of 10W ERP)
FT-847 RX improvement - why is better to use an external LNA?
In Dubus issue 4/2009 you could find following: "Now you can have easily a good RX on your FT847 with a new low noise preamplifier. It’s designed with a BF981 or MAR-6. It is very easy to mount into the radio in 5 minutes. The FT-847 gets a sensitivity of 0,1 uV on 4m with this unit. The preamp / kit is available from CT1FFU. More on the web at www.ct1ffu.com".
However in next Dubus issue Keith G4FUF showed how it's in reality "I was somewhat appalled once more to find reference to this so called “preamp” from CT1FFU. Lets put this in perspective: (I) An unmodified FT-847 has a noise figure on 70MHz of typically 15,5dB. This is because there is some 1,5 - 1,7dB lost in the PA filter and some 3dB in the HPF on the daughter board. (II) Putting a so called preamp, even if it had a ideal 0dB noise figure on the RF board would still give you a noise figure of typically 4,5dB. (III) The CT1FFU “preamp” has a device with a claimed noise figure of typically 3dB, so when installed on an FT-847 that has had ALL the BPF mods done we still see a noise figure of typically 8dB at the RF input. (IV) His own claims are 0.l uV, for how many dB S/N ????, correspondence indicates this is a measurement actually outside the FT-847 environment. (V) I have measured his preamp alone, let alone in an FT-847 and it is more like 4,2dB. (VI) An independent measurement by GI0GDP showed the sensitivity was increased to -111dBm from -120dBm for 12dB SINAD. A simple bit of maths (or use APPCAD) show this is going from 15dB to around 7dB noise figure. (VII) I have attempted to discuss this with FFU but his fundamental understanding of RF is somewhat lacking. And yes, I do produce a preamp kit and it does do what I claim. So please stop this advertising nonsense because a few people have been parting with good money for a crap product. 73, Keith G4FUF”
So if you you use CT1FFU's "on board LNA", the FT-847's Noise Figure can improve from 15,5dB down to 7dB. While FT-847's efficiency of 4m PA is pretty low and more you have to do something with outband signal, better solution then make some nonreversible changes in radio is to do all necessarily changes in external device. That's why I used external PA with BPF and LNA. Especially with WSJT it's much better solution due to overheating problems and Noise Figure close to 1,5dB (better improvement has no sense due to wide antenna lobe and high ambient city-noise). The change in better Noise Figure you can prove by VK3UM's EME Calculator.
If you haven't possibilities to measure Noise Figure and made such a LNA by yourself, maybe you would like to know where to buy it. At ZS2EZ's 4m web I found note of The Spectrum Communications (UK) RPS4 Preamplifier, more at http://www.spectrumcomms.co.uk/Four%20Metre.htm#RECEIVE_PREAMPS.
Some info about our time sequencer at: http://www.ok2kkw.com/00003016/sequencer/sequencer_en.htm
73 & hope to meet you on 4m