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Re: [linrad] Out of band QRM. Worst case ?

In Australia, pagers have been a real problem, with typical EIRP of 1KW and some being on frequencies as low as 148.2 MHz. Not only that, several transmitters may use the same tower, with antennas only a few metres apart. Intermodulation may actually occur BETWEEN TRANSMITTERS as they dont tend to use isolators or output cavities. (they dont have to listen off the same antenna!).
I have pointed a 2M beam at a local site and connected it to my spectrum analyser. I have been able to observe intermodulation products when a particular combination of pagers come on together. (No, it wasnt my spec analyser overloading as I could step the signals down 10 dB at a time with the attenuator and the inputs were less than -20dBm)
Could this issue be important in your area?

Richard


Stan wrote:

Hello the Net:

Preamplifier problems I have had in the past were due to :

1. the 1/4 KW pages on 151 Mhz and the nearby FAA station at 136 MHz

2. low noise preamp in the 2M band but also a very low noise figure, with low + gain, in the 850 to 950 MHz cellular/garbage band.
like the SSB model LNA 145.

I really like the LANDWEHR mast mounted preamplifiers , that I use for terrestrial and satellite work
decent NF and gain, tightly around the 2M band, NF ~1db so not spectacular for eme. Many preamplifiers are broadband
with poor performance in a dense electromagnetic environment.

for 2M EME and terrestrial I am considering the cavity input product offering of:

http://www.lnatechnology.com

Stan, WA1ECF




wb9uwa@xxxxxxxxxxx wrote:

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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