HI Leif,
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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