[Moon-net] 2 receivers with the same antenna

[Mark GM4ISM]

The best way of splitting the antenna to 2 receivers is to use a hybrid 
coupler. This represents the lowest loss, and importantly provides isolation 
between the 2 receive ports so that if the input of one RX does not present 
a good match, it does not affect the other port. Placing such a hybrid after 
a preamplifier will usually not degrade the performance of either path 
significantly
To get the same (or at least adequate) isolation, requires a resistive 
splitter with significantly more loss. (not necessarily bad, but may require 
more front end gain.)

On my webpage  www.dc2light.co.uk are some simple designs for hybrids that 
will do this job.
I also plan to use these for combining 2 transmitters, into a single 
amplifier to do direct comparisons of different digital modes, with the same 
power to the same antenna simultaneously.. you do have to watch the peak 
power rating of the amplifier, or intermediation will occur. My 300W capable 
amps on 2m and 6m produces 2 very clean signals at the 50W level 
simultaneously.  All I need is someone the other end to have 2 receivers 
running from the same preamp!
When using these hybrids to combine different frequencies, the ballast load 
obviously dumps half the power of each input, and the isolation must be 
good!
regards
mark GM4ISM

----- Original Message ----- 
From: "Edward Cole" <kl7uw at acsalaska.net>
To: "Gabriel - EA6VQ" <ea6vq_ml at vhfdx.net>; "Lista-MoonNet" 
<moon-net at list-serv.davidv.net>
Sent: Saturday, November 17, 2007 7:00 PM
Subject: Re: [Moon-net] 2 receivers with the same antenna


> At 09:38 AM 11/17/2007, Edward Cole wrote:
>>At 07:44 AM 11/17/2007, Gabriel - EA6VQ wrote:
>> >Hi,
>> >
>> >Might be this is a silly question, but I can't find very much 
>> >information
>> >about this subject using Google.
>> >
>> >What is the right (lowless) way to connect two 144 MHz receivers to a 
>> >single
>> >antenna?
>> >
>> >I want to make some tests comparing the reception of my usual receiver
>> >against an SDR-IQ receiver. What kind of splitter should I use?
>> >
>> >73. Gabriel - EA6VQ
>>
>>Gabriel,
>>
>>I tee the output of my second preamp (which is in the shack) and run
>>two RG-58 lines: one to the FT-847 and the other to the 2m/28 MHz
>>converter in front of my SDR-IQ.  The mismatch doesn't bother the
>>sensitivity of the radios due to 31 dB gain of the two cascaded
>>preamps that precede them.  My mgf-1801 (0.15 dBNF, 16.8 dBgain)
>>preamp is at the tower-top and the ARR P144VDA (3SK48) (~1 dBNF, 14
>>dBgain) is in the shack with 170 feet of LMR-400 between the two
>>preamps.  My 2m converter is the DEMI 144/28 xvtr. with about 1 dBNF
>>and 15 dBgain, followed by a 18 dB gain wide-band IF amp before the 
>>SDR-IQ.
>>
>>
>>73,
>>Ed - KL7UW
>>======================================
>>   BP40IQ   50-MHz - 10-GHz   www.kl7uw.com
>>144-EME: FT-847, mgf-1801, 4x-xpol-20, 185w
>>DUBUS Magazine USA Rep dubususa at hotmail.com
>>======================================
>
> One more comment, Gabe,
>
> I measured the sensitivity of both radios in this setup by inserting
> a signal generator in the input of the second preamp with a 66 dB
> attenuator so I could get low enough signal levels to resemble
> eme.  I found the FT-847 system sensitivity to be - 152 dBm using
> SpectraJT to display the signal.  The SDR-IQ sensitivity was -166 dBm
> with similar RBW running winRad.  But, operationally, I do not run
> the SDR-IQ RBW that narrow for eme.
>
> This was the subject of my paper for CSVHF this year:
> http://www.kl7uw.com/SDR.htm
>
> So in general the two receiving systems are equally sensitive to eme
> signals.  I generally find my lower signal threshold is about -28 on
> JT65B and this is typically produced by a single-yagi station with
> 600w.  This is about equivalent to my station ERP.  My "normal" eme
> reception limit is about 1kW for single-yagi stations, (typ -27 to
> -25 on JT65B).
>
>
> 73, Ed
>
>
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