[TowerTalk] 1 or 2 dB

[Steve Maki]

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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