[TowerTalk] 1 or 2 dB

[David Hachadorian]

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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