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R: Popularity of 2m I.F.



Hi Rich

Receiving 2400 MHz  converting it to 144 MHz  IF the local oscillator LO is
2400-144=2256 MHz  but  your converter will convert to 144 MHz the image
frequency as well at 2256-144=2112 MHz

If  the selectivity of  RF circuits beginning from converter input and
mixer  is low it happen that 2400 MHz and
2112 MHz will be converted both in the same 144 MHz IF

The noise power contained in to the 2112 MHz band will degrade the noise
figure of the system while receiving at 2400MHz

Since the difference between the desired 2400 MHz frequency and the 2112 MHz
image frequency is 144 x 2=288 MHz,it is possible to get a good selectivity
using stripline filters on PCB or tunable elicoidal filters with center
bandpass frequency on 2400 MHz and high rejection at  2112 MHz

On the other side,if i use 28 MHz to directly convert   in  2400 MHz it
happen that the image frequency fall down  from 2400 MHz to only 2344 MHz
and  the above filters  are not able to attenuate the image band to such
high extent and so,receiving 2400 MHz the use of  28 MHz for the first IF is
not recommended because desired signal and image are only 56 MHz apart

On the other side there are very good HF receivers with desiderable
characteristics that can be used at 2400 MHz
If you like to use it is necessary first to convert  2400-2402 MHz in to
144-146 MHz first IF and than convert 144-146 MHz in to 28-30 MHz
 second IF using a 2 meters converter

Personally i use the above configuration with a 2400-2402 MHz transverter
antenna mounted running  the 144-146 MHz  IF down to the shack via  RG-213
coax cable.
The 144-146 MHz  band is sent to the input of a  high dinamic range
144-146 MHz converter with 28-30 MHz as a second IF

The 28-30 MHz IF is sent to a HF Drake R-4C via a 20 dB, 1dB step 50 ohm
attenuator by wich i can made many usefull measurements as sun noise,
transponder noise using the S-meter only as a 0 dB reference meter.

In addition i can switch the AGC ON and OFF and made the same measurements
using a linear dB meter connected to the R-4C audio output.
Much more,using a  good existing HF  receiver with the addition of a 2 meter
existing converter there is not risk to burn up the 2400  MHz converter and
is cheap

Best 73 de i8CVS Domenico


----- Original Message -----
From: Rich Dailey, KA8OKH <okh.npi@gte.net>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, May 30, 2001 4:21 AM
Subject: [amsat-bb] Popularity of 2m I.F.


>
> Was on the phone with a good friend the other day,  extolling the
> virtues of ao-40 (of course).  He has always been interested in
> satellite communications,  and is real receptive to the idea of getting
> a mode S downlink setup running.  But he posed a question that I had
> no clear answer for - "Why does everybody want to use 2m as an I.F.,
> given the proliferation of good used hf radios?  Why not use 10m?
>
> So,  what should I tell him?
>
> tu de Rich
>
>
>
>
> -------------
> Rich Dailey, KA8OKH - Phyllis Dailey, KB4NPI
> mailto:okh.npi@gte.net
> http://home1.gte.net/web22jfw/
> -------------
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>






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