When do I need to use the FLEX-6000 RF Preamp?

Tim - W4TME

by Gerald Youngblood, K5SDR Answer: On HF - almost never. I had the opportunity in April to take a pre-production FLEX-6700 to Rob Sherwood's (NC0B) low noise remote shack East of Ault, Colorado. [Ed: For those who don't know who Rob Sherwood is, please check out his extensive research and unbiased reports on amateur radio receiver performance at: http://www.sherweng.com/table.html ] Rob had been telling me that his site matches the Quiet Rural measurements shown in the ITU-R P.373.7 Radio Noise charts for HF. He also has an amazing antenna farm with single band antennas for each band mounted on 70 foot towers, as well as, a Marconi 160m antenna. Rob had also told me that many radios have problems with inter-modulation on his 160m antenna due to large medium wave broadcast stations in the area. My first goal was to understand 'if' and 'when' RF preamplification is needed on HF with the FLEX-6000 series. The second goal was to understand if the two -7 dBm (S9 +66 dB each) AM BCB stations would cause any adverse affect to the radio on 160m. We arrived at his QTH around 10PM and quickly got the radio up and running on his network after Rob remembered to connect his fiber to copper network bridge. Fortunately, there were openings over the North Pole into Siberia and Russia on 15m and 10m late into the evening. These were weak signals at his location so I am sure you would not be able to hear them at any urban location. The way Rob determines whether a preamp is needed is to connect and disconnect the antenna. If the noise floor rises by 8 dB or more with the antenna connected, then you don't need or want a preamp. The preamp would actually reduce dynamic range on any radio. What we found was that no preamp was needed on any HF band until we got to 10m. On 10m, 10 dB of preamp gain was almost enough and 20 dB only improved SNR slightly. Based on testing at Rob's lab in Denver, I learned that the preamp noise figure was actually 4 dB higher than its specified performance. Upon my return, I found and easily fixed the problem before we went into full production. That probably means that 10 dB gain would have been sufficient on 10m. An important phenomenon that most people don't realize is that even in the quietest locations on earth, noise propagates with the band opening. In other words, noise is RF and RF propagates just like intentional signals. We saw this easily on 15m where pointing the antenna away from the opening caused the antenna noise to drop below the radio's internal noise floor. When we pointed the beam toward the opening, the noise floor rose more than 8 dB above the receiver noise showing that a preamp was not needed on 15m. The next thing we did was to connect the 160m Marconi. I placed one receiver slice in the AM broadcast band to intentionally disable the 1.8MHz high pass filter. That means that the radio was wide open from DC to 70+ MHz on a very efficient antenna. The bottom line was no overload or inter-modulation from the two -7 dBm stations. Since no preamps were needed, that means that one could simultaneously receive AM broadcast and all but the very weakest signals on 10m without a preamp. In fact, the radio is capable of receiving up to +9 dBm (S9 +84 dB) signals without overload.