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Re: [linrad] Out of band QRM. Worst case ?
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
>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:
>>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?
>>Leif / SM5BSZ
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