Yes, in other words you need an ABX test (a true double blind test). Not
easy to do.
-Steve K8LX
On 5/20/2022 6:42 PM, Michael Tope wrote:
If you use a differencing method that inserts and removes the 1dB
transmit attenuator in a way that is not known to the operator and that
ensures the operators spends an equal amount of time at each power
level, then the impact of the 1dB "psych out" would presumably get
spread equally between the two power levels. The key is engineering the
attenuator control so that the system doesn't give off subtle clues that
it has changed state (e.g. change in VSWR, change in plate or drain
current, sound of vacuum relays clicking, etc). Of course, as N5OP
suggests, getting volunteers who are representative (i.e. highly
competitive individuals) who are willing to subject themselves to being
at a small power disadvantage 50% of the time, might be a challenge. 😉
73, Mike W4EF....................
On 5/19/2022 4:24 PM, David Hachadorian wrote:
Just knowing that you are wasting 21% of your output power in an
unnecessary 1 dB of feed line loss will play with your head and cause
you to perform sub-optimally.
Dave Hachadorian, K6LL
Yuma, AZ
On 5/19/2022 3:01 PM, Lux, Jim wrote:
On 5/19/22 11:38 AM, Jim Brown wrote:
On 5/19/2022 6:23 AM, Lux, Jim wrote:
I'm not so sure that it's out of reach. yes, trying to implement it
with gear from 1980 would be challenging. But with more modern
equipment, where the "radio" is a black box controlled by a "front
panel" or "computer" it gets easier.
The Elecraft K3 with second RX that is the same as the main RX, and
which can be synced with the main, allows diversity reception, and
I've been using it since 2008.
Diversity requires an antenna for each RX, spaced as widely as
practical from each other. It was invented in the earliest days of
radio to counter the effect of selective fading, which is the the
cancellation of two or more arrivals of the wavefront from the same
TX that have followed different paths, arriving at different times.
The time differences cause the arrivals to have a variable phase
relationship with each other, combining algebraically to cancel or
add, depending on the resulting phase relationships. Diversity works
best when the antennas have the greatest spacing, so that when
cancellation is occurring at one antenna, it is less likely to do
so, or even to increase, at the other.
And the diversity combining - doing it in analog is hard, but in
the digital domain it's much easier, and for the most part it can
be done at audio (or post down conversion to baseband or low IF).
As diversity has been practiced since the beginning, combination is
done in the brain of the operator, with audio from the two receivers
in opposing ears. That's how it's done in the K3. The result is a
sort of spatiality to the sound, a bit like the true stereo image
produced by a spaced pair of microphones dedicated to left and right
loudspeakers.
Combining the outputs of the two receivers to a single (mono)
channel is problematic, because the phase relationships at audio
have a good chance of cancelling.
For SSB, yes - a simple summing won't work. But it's widely used in
other systems where there's some processing or where the baseband
phase is reliable - For instance, on AM or FM, the instantaneous
audio phase will match, so you coherently combine - typically modern
diversity receive does some sort of weighting on the basis of SNR -
the stronger signal gets a heavier weight, and when there's fading,
it smoothly changes.
I will say that there are *bad* implementations - I had a car radio
that did diversity on FM, but the two paths were noticeably different
time delay (as in milliseconds) so you could hear an apparent "echo"
as it switched from one to the other.
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