Re: [144MHzEu] Out of band QRM. Worst case ?

["Mike Tubby" <mike@xxxxxxxxx>]

The most successful mast-head pre-amplifiers that we have
used are based on the "muTek" front-end design from the
old FT221/FT225 board, but with a few slight changes:

- use BF988 because cannot get BF981 easily any more

- put 2k2 resistor across first coil in output filter (at drain
  of fet)

- increase Q of front-end coil (more turns, less C) and use
  0-6pf air spaced trimmer

- put 12dB attenuator on output side

When correctly set up you can get a simple, reliable, low
cost mast-head amplifier with NF ~ 0.7dB gain (before
attenuator) around 22-26dB (depends how you tune it)
and if you tune it carefully 1dB crompression point at around
-3dBm.

We use these pre-amplifiers on the M0BAA/P and G8T
contest station in line-of-sight of 153MHz radio paging
transmitters and have no problems!


Mike G8TIC/M0BAA/M0VHF/G8T



> 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
> 
>

LINRADDARNIL