Content provided by: Gerald, K5SDR
When do you need to use the RF Preamp?
On HF, rarely, unless you are using preselectors that have insertion loss.
Before the instructions on how to set the preamp, a description of how signal strength is read and the effect of RF preamp gain on signals is warranted.
FlexRadio transceivers measure the signal strength at the antenna terminals in the same way that a spectrum analyzer does. If you change the RF preamp settings (turned on or to a higher gain value depending on the transceiver), if improvement occurs, the noise floor will DECREASE, and the signal level will stay the same.
Although this may seem counterintuitive, it's the way RF Preamps function. Remember that when you engage the preamp, you have not changed the signal level at the antenna terminal. You have, however, improved the signal-to-noise ratio (SNR) with the preamp, and therefore, the noise will go down by a fixed level. The previous statement holds in all cases except one. That is, if the radio is antenna noise limited.
There is a small caveat to adding RF preamplification gain; it reduces the dynamic range of the radio. As noted, if you enable the preamp (or increase the gain by n-dB) and the noise floor in a FLEX-8000/6000 decreases, the preamp will likely improve the SNR because it is exposing signals that were below the noise floor, which was NOT the atmospheric noise floor -- it was the noise figure of the preamp preventing optimal SNR. If the noise floor increases when the preamp is enabled, this means that you were already receiving at the atmospheric noise floor. You just added unneeded gain and noise, and decreased the dynamic range of your radio by the gain of the preamp.
For each 10dB of RF gain you add, it decreases your overload on the "high end" (large signal) by 10 dB. If the noise floor only goes down by 1dB when you enable an additional 10 dB of RF preamplification, you decrease your dynamic range by 9 dB. This is usually not an operational issue because if you are trying to optimize the SNR, you are most likely trying to receive weak signals where a high dynamic range is not needed.
In general, no RF pre-amplification is usually needed on most HF bands due to environmental and atmospheric noise at most QTHs versus the radio's internal noise. The one exception is if your radio has a preselector enabled, which has some level of insertion loss that can be overcome by using additional RF preamp gain.
If you add 10dB of attenuation and your noise floor does not rise, it means that your noise figure is well below the atmospheric noise floor. By adding 10dB of attenuation, you just added 10dB to your overload point and therefore an additional 10dB to your dynamic range.
As a rule of thumb, you want your antenna noise to show an increase in your S-meter by 8-10 dB and no more. When this condition is met, it means that your receiver is not adding additional noise to the signal and that you have the correct RF preamp gain to maximize reception performance and optimize your signal-to-noise ratio (SNR). If the noise increases more than 10 dB with the RF preamp and antenna, you have too much RF gain, and the SNR is degraded.
- Here is the process to optimize receiver sensitivity for your given operating conditions:
Find a clear frequency on your VFO. - Set the slice receiver mode to CW and the receiver passband filter to 500 Hz. Turn off any noise reduction, APF, or RX EQ that will add gain to the receive audio.
- Disconnect the antenna - You can select an antenna input with no antenna on it, like RXA or XVTA.
- Note the dBm reading with no antenna connected.
- Reconnect the antenna.
- Note the reading in dBm. This is where an accurate dBm-calibrated S meter counts. Ours is truly 6 dB per S unit and measures the actual receiver sensitivity in the selected bandwidth.
- If the noise goes up about 8-10 dB, you have the optimal RF preamp setting for the noise on your antenna for the given time of day and propagation.
- If the noise with the antenna connected increases significantly more than 10 dB, you have too much preamp gain, which will limit the dynamic range for large signals. Reduce preamp gain.
- If the noise increases less than 8 dB, increase preamp gain to get it closer to the 8-10 dBm increase.
If you are not working weak signals near the noise floor, then less sensitivity is better because you want to use the least amount of preamp gain that allows you to hear the signals of interest. Even attenuation can be helpful on the lower bands. This maximizes a large signal dynamic range.
You may wonder why an 8-10 dB signal (noise) increase is the target range for adjusting the RF preamp.
With a 10 dB difference, the receiver noise figure adds only 0.1 dB of noise, which on HF has no impact on the received signal whatsoever. What you are trying to accomplish with this setting is to match the receiver noise figure with the propagated noise coming from the antenna to maximize dynamic range for given band conditions. An 8-10 dBm differential is about right if you are working ultra-weak signals right at the antenna noise floor. Otherwise, you should turn the gain as low as possible while still maintaining the appropriate signal-to-noise ratio for the stations you are contacting. In other words, don't apply RF preamp gain unless you need it.
Why are the FLEX-8600(M), FLEX-6600(M) noise levels +10dB higher vs. the FLEX-6500, FLEX-6700, FLEX-6400(M) or FLEX-8400(M)?
Now to the difference between the FLEX-6500/FLEX-6700/FLEX-6400(M) and the FLEX-6600. The FLEX-6600 incorporates 7th-order bandpass filters on all of the HF contest bands. The 6500/6700/6400(M) incorporate 3rd-order filters on those bands. The 7th-order filters provide greater than 50 dB of band-to-band isolation between the contest bands at the cost of higher insertion loss. This is excellent for Field Day, SO2R, and other contesting configurations. This additional loss is easily compensated for with additional preamp gain.
Let's take a look at the default preamp gain strategy for the 6400 and 6600 radios. We used the "Rural" noise floor numbers for our calculations to set the default gain for each band. Note that the FLEX-8400/6400 filters have a lower loss on the contest bands because they use 3rd-order preselector filters on all bands, just as the 6500 and 6700 do. Below, you can see the ITU typical noise power in dBm in a 500 Hz bandwidth with S unit callouts in red.
Given the Rural noise floor for each band and the receiver noise for each band, we calculated the target sensitivity to be 10 dB below the ITU typical. We then picked the preamp or attenuation setting. The table below shows the gain setting and the expected MDS with no antenna attached for that setting on the respective radio. If you need more or less gain for your location, antenna, and operating conditions, use the procedure above to adjust accordingly.