[CQ-Contest] Multiple direction antennas on one radio...why?

[David Gilbert]

Hi, Tom.

I have no first hand knowledge of such a setup, but these were my 
thoughts after watching the ZX5J video.

I assume that having antennas simultaneously pointed in four directions 
means that they have transmit gain in all four directions, although that 
would suffer a bit if you have all four amps cranked back to 1/4 legal 
power.  I suspect it would be awkward to readjust all amps for those 
times when propagation is clearly dead for one or more of those 
directions, assuming they couldn't afford to waste transmit power.

Having four antennas simultaneously pointed in four directions 
sacrifices the S/N you would otherwise have with just one antenna, but I 
assume that it does give you the ability to at least hear callers that 
would otherwise be off the back or side of a single yagi without having 
to switch antennas to do so.

I think can understand simultaneously using more than one antenna.   
What I cannot understand, assuming total power output meets all rules, 
is how using multiple amplifiers simultaneously to do so is not 
considerably more expensive, slower, and operationally awkward than 
simply feeding a single amplifier into a high power splitter as is 
commonly done with stacks or 4-squares.

It is entirely possible, though, that one or more of my assumptions is 
invalid.

73,
Dave   AB7E



On 10/18/2011 9:30 PM, Tom W8JI wrote:
> This has my curiousity.....does anyone know what stations doing this really
> do?
>
> When I have a splitter, power divides from the exciter. Each amp has fixed
> gain. If I have one 10 dB gain amp and a 100 watt exciter I have 1000 watts.
> If I split the power into two 10 dB amplifiers, I have 500 watts per amp
> minus splitter losses. If I split it four ways, I have less than 250 watts
> per amp. It doesn't matter if I split before the amps or after, except for
> the size of the splitter.
>
> I would either have to have a variable gain amplifier driving the splitter
> that would increase power into the splitter just over 3 dB  for each two-way
> split, or I'd have to have dummy loads and replace amplifiers with dummy
> loads as I pulled antennas out of line.
>
> How do they do it?
>
> On receiving, summing receive signals from four antennas in different
> directions would hurt MDS, because at minimum every additional pair of
> antennas would decrease receiver S/N by at least 3 dB. There would be at
> least 6 dB of S/N loss and probably more in most cases.
>
> Why would someone do that?
>
> Or do they really use four radios with four amplifiers and four antennas,
> and transmit on all and only listen on one???
>
> Does anyone actually know how these systems really work, because it seems
> pretty strange to me on the surface. Are these systems planned, or is a case
> of just throwing something at the wall and hoping more sticks to the wall
> than falls off?
>
> One way it would work well is with four listening operators on four separate
> receivers so there is no S/N loss, and one transmitter at a time driving all
> four amps through one additional amplifier so there is minimal phase
> distortion if someone hears more than one TX antenna at once.
>
> I'm just trying to learn something useful here. I would never think of
> running four antennas in different directions into one radio for receiving
> except as a last possible resort.
>
> 73 Tom
>