[Elecraft] K3, VHF Diversity

[David Anderson]

That should work well Chuck. I know that SM5FRH had a similar system many years ago when he had his huge EME array. No computer required, just the brain doing the combining. I never tried that when I had my previous dual polarity array, I just had simple H or V switching, but I am planning to have polarity diversity this time on digital at least with a LinkRF IQ+ system. I may also try using it on CW with LINRAD and the same hardware. 

I agree that you have to be very careful with choice of computers and power supplies when doing EME on VHF, or make the choice not to switch them on unless absolutely necessary.

Before the advent of digital modes for EME I nearly always had all things switched off apart from an old NEC laptop similar to the Tandy Model 100 with an LCD display which was totally silent RF wise. The NEC was just for giving me Moon position, tracking of the moon being done manually. 

Gradually computers crept into the shack as we had nice tools available like FFT waterfall displays, the first popular one being AF9Y FFTDSP and later we had various DSP audio filtering programs added to the suite of essential utilities.  Now of course we have the Internet and all that entails, including distractions while operating. 

I see photos of shacks with multiple computers and monitors and wonder how much the noise floor must be raised by all of that on VHF. I know Leif SM5BSZ wrote an article on the extreme measures he had to take with his computer to make it quiet for LINRAD use. I remember changing the clock crystal in my old PC to a lower frequency one so that the harmonic fell below the bottom of the band. Similarly I replaced the switched mode power supply in the PC to a model that had better filtering and the noise fell 10dB. Even a dB of extra noise can kill weak signal EME.

The bottom ends of our VHF and UHF bands are now so full of birdies from computers etc that CW weak signal users have to move up the band from their traditional place. For most people there is no possibility of working in the bottom 20 or 30 kHz. I contrast that to when I (and much more so Chuck, who is one of the pioneers of EME) started on EME and there was no QRM at all, if you heard something it was another station.

Sometimes it is nice to get away from it all and just have a radio, headphones and key.

Good luck.

73

David Anderson GM4JJJ 

> On 18 Feb 2015, at 19:25, Chuck Smallhouse <w7cs at theriver.com> wrote:
> 
> I am in the process of implementing full diversity reception of 2M CW EME signals.
> 
> I will be using 2 each quad arrays of 2M long yagis,  One horizontally polarized and the other vertically polarized.  At and after the antennas' power dividers, there will be installed equal gain and NF LNAs, followed by equal lengths of Heliax Super Flex down to the shack.  In the shack will be two identical phase locked (together) down converters that drive the K3 and it's Sub RX.  This will result in one polarization in one ear and the other in the opposite ear.  The TX upconverter and K3, will be locked to the same source.
> 
> It will be an interesting on going experiment, to see if/how this affects and hopefully somewhat overcomes the polarization rotation and faraday/libration properties of EME received signals.
> 
> Also planed, will be to feed both polarizations simultaneously, during TX, with a QRO+ PA.
> 
> BTW, to date I don't use any computer control of the K3 !   I don't even allow any computers to be on when performing 2M EME operations, due to the residual broad band noise and birdies emitted by most .  The availability of a none computer(external) controlled SDR receiver, among other features, is what sold me on the K3/P3SVGA system.
> 
> Chuck,  W7CS
> 
> Message: 21
> Date: Tue, 17 Feb 2015 22:09:08 -0800
> From: Wayne Burdick <n6kr at elecraft.com>
> To: Vic Rosenthal <k2vco.vic at gmail.com>
> Cc: Elecraft Reflector <elecraft at mailman.qth.net>
> Subject: Re: [Elecraft] [Elecraft_K3] Who said anything about removing
>    diversity? That would *never* happen...
> Message-ID: <835CB799-17AF-4106-A3B5-4BC9AFADBFC9 at elecraft.com>
> Content-Type: text/plain; charset=us-ascii
> 
> Diversity reception is characterized by perceived "phase precessing" between the two receivers' audio streams due to varying phases of the main and sub RF input signals. You don't want additional, unpredictable phase precessing on top of that due to the receivers themselves. Additive phase changes would sometimes make diversity reception less pleasant to listen to. Better to have the receivers locked together, eliminating a variable.
> 
> Wayne
> N6KR
> 
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