Robin has added some very important considerations for a 160 IF link so i am
forwarding them to the reflector.
73
Herb Schoenbohm, KV4FZ
From: Robin <wb6tza@socal.rr.com>
Reply-To: Robin <wb6tza@socal.rr.com>
Subject: Re: Re: Topband: RX Link via Gunnplexer
To: herbs@surfvi.com
Hi Herb & Ken
I agree...
pertinent comments separated out below
> Robin,
>
> I am forwarding to you Ken's comments to you as he appears optomistic the
link idea might work and be will be helpful, for some stations plauged by high
local noise on 160.
>
> 73
>
> Herb Schoenbohm, KV4FZ
> ----- Forwarded message from ken.d.brown@hawaiiantel.net -----
>> To: herbs@surfvi.com, raoulc@smmcape.co.za
> "Pay attention to the levels to prevent I/M issues". Your remote front
> end will need the have the dynamic range of a good receiver AND the
> microwave link also needs to have as good a dynamic range. The IF level
> modulating your microwave transmitter has to be carefully set so that
> weak signals are above the microwave modulation/demodulation system
> noise AND strong signals do not over modulate the microwave transmitter.
> You will probably want to limit the front end or IF bandwidth to as
> narrow as possible to reduce the total peak signal level that needs to
> be modulated onto the microwave carrier.
this paragraph spells it out,. the dynamic range and S/N of the baseband
in the microwave are the controlling factors
one day soon Ill set a pair of radios up and make some hard measurements,
the 60-80 dB numbers Ive been tossing around may be off, I just dont think
they are... Only way to know is to do it
>
> Some of the possible schemes are as follows:
>
> 1) The simplest, but maybe not the most effective. Just have a bandpass
> filter and a preamp for 1810 to 1850, and have the output of this front
> end modulate the microwave oscillator. When the microwave receiver
> demodulates it, feed the the FM discriminator output directly ( at the
> appropriate level) to your 160 meter receiver. The frequency readout and
> all the controls of your receiver should work just like usual (over the
> frequency range the remote front end covers).
This isn't as bad as it seems. Many current receive systems have low
signal level antennas requiring preamps,> there are a number of preamps
around, and they manage to handle the whole band pretty well, most of the
time. Having enough gain blocks so that the atmospheric noise is a minimum
of 3-6 dB above the system noise level will be necessary... and the
critical factor is that specific number..
While moving the IF sounds simple, its not quite so simple as the LO that
makes the IF has to be of the same grade as the one in your primary
receiver... and the complexity goes up from there
I actually think that the SDR packages will end up being how we do this, if
it turns out that this gets done on a more than extreme occasional basis.
The SDR package solves most of the control issues, and as long as the
software defining the radio defines it to the precision needed to satisfy
one of the toughest crowds, the topband CW aficionado.
You have me thinking about this again, and Ill be poking at the details as
my projects go forward over the next couple years. the paths the club is
building will not support the 1.9 MHz modulating frequency needed for
direct grabbing the band, so I will have to do it differently
from the point to point standpoint, the SDR with separate TX and RX
hardware will be far and away the easiest as all that's needed is a decent
data path and a standard voice grade audio path with no unusual level
controls, sounds simple. Isn't. and is NOT cheap
73
Robin
----- End forwarded message -----
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