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[linrad] Out of band QRM. Worst case ?

As you know, I have been wrestling for some time with the front end
problem for 2M EME here. My plan is to drive an RX2500 direct
with the front end. 

I think you overestimate what is commercially available and well suited
to Linrad implementation. The second stage you mention that has
only 10db gain and a 2db noise figure probably does not even exist
even though it is easy to homebrew with 3x J310's. The first stage
with only 15 db gain does not exist either. Most commercial preamps
will have between 23 and 28 db gain at 2M with the smaller gain number
ususally being the result of loss in the output circuit.  There will be a great temptation
to use some of the commercially made preamps by itself to drive the RX144
or worse yet, many will try to cascade two of these fairly high gain preamps,
perhaps with a commercial bandpass filter between them. This would be 
a disaster for IP3  make mediocre a potentially great system.
I suggest as a minimum your RX144 should have a system noise figure of 2db.
1db nf or less would even be better. In that case some EMEers in some very
harsh environments would likely use no external preamp at all.

The first stage preamps that would seem to be called for in a Linrad system
would be in the ATF33143 and MGF1801 class. Neither of these are known
for being unconditionally stable. When you throw in the requirement for 
only 15 db gain in the first stage, then lossless feedback seems to be called for.
As far as I know there are NO commercial manufacturers successfully using
enough lossless feedback to reduce the gain down to anywhere near 15db.
An oscillating preamp is often the result. Perhaps someone with the knowhow
and proper design simulation software will step up to the challenge.
One could use a  normal commercially available preamp with an attenuator
after it, but the result would be poor output power from that stage and due
to a lack of input selectivity, the "damage" may already be done.

In the US, there will be no signals of any consequence below 144 Mhz until
the FM BCB. Starting at only 150 Mhz there are many two-way FM radio signals.
250 watt paging transmitters are often present at 152 and 157Mhz. These paging
signals here are -35dbm. There can be two of them at once at 152Mhz. I am
in a town of only 40,000 people, but adjacent to a town of 60,000 people.
Certainly large cities will present a bigger challenge. Commercial preamps 
generally lack substantial selectivity AHEAD of the active device.
Two Meter repeaters are present from in the low 145 Mhz range to 148Mhz.

73, Jim Shaffer, WB9UWA.

On 13 Mar 2004 at 1:10, Leif Åsbrink wrote:

> Hi All,
> I am going to make a front end for 144 MHz that will fit
> the WSE converters and Linrad.
> There will be two RF amplifiers, one preamplifier for
> mounting in the tower and I will probably not make them
> since such amplifiers are available from many sources.
> The noise figure of RX144 will be about 11 dB and there 
> is a need for a second RF amplifier with high IP3.
> The idea is to have a tower mounted preamp with a 
> gain (including cable losses) of about 15 dB followed 
> by a high level amplifier with a noise figure of 
> about 2 dB that will add about 10 dB gain.
> A good preamplifier may have an output IP3 of +25dBm.
> The input IP3 of the RX144 is about +20 dBm but if
> both signals fall within the passband of the RX70, the
> input IP3 is only +15 dBm.
> After adding the second RF amplifier, the IP3 mis-match
> between a +25 dB IP3 preamplifier and the Linrad system
> amounts to 20 dB for in-band signals.
> The in-band input IP3 (at the antenna) will be -10 dBm
> while compression will start at something like -20 dBm. 
> The second RF amplifier could incorporate a filter that
> converts in-band interference to out-of-band interference.
> Are there any signals within 10 MHz from the 2 m band that
> could produce more than -30 dBm so they would be near blocking
> or is there any risk for a signal pair that could produce 
> third order intermodulation on 144 MHz ? Two equally strong
> in-band signals at -50 dBm would give IM3 at -130 dBm which 
> would be well audible.
> As far as I know there is nothing here in SM but I do know 
> there are pagers just below 144 MHz in the UK. It will be 
> fairly easy to design filters that make performance limited
> by the mast mounted preamplifier for signals that are more
> than 4 MHz outside the band edges if one wants full sensitivity
> from 144 to 146 MHz. Likewise it would be fairly easy to make 
> the system limited by the preamplifier for signals below 142.5 
> or above 146 MHz if one wants full sensitivity from 144 to 
> 144.5 MHz only.
> My question is: Is there a real need for better than this?
> 73
> Leif / SM5BSZ
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