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VK6 - QRN - Update




Many thanks to all those who replied to my recent positing regarding the
widespread QRN that many VK6 AO40 users are suffering.  

It turns out that the QRN is due to the commencement of trials of the new 3G
mobile phone services in Perth.  There are a handful of such test sites over
the metropolitan area, one being 1km from my QTH. 

The 3G services operate in the range 2 - 2.2GHz which  falls into the image
response of our S-Band down converters when using a 2m IF. 

There are a number of ways to overcome the problem. In the long term we are
going to need converters with higher image rejection. The JA's, who have had
the same problem for some time, are using interdigital filters to notch out
the image frequencies. 

Rather than modify our down converters we could fit a high-pass, or notch,
filter in front of the converter. Unfortunately, the filter will have some
loss that will add directly to the overall system noise figure.  

A simpler alternative is to use high side local oscillator injection. This
shifts the image response to 2.7GHz, which hopefully will be QRN free.  High
injection will mean the converter will invert the tuning direction, but this
is a small inconvenience to be back on AO40.  I'm working with Charlie,
G3WDG, to test this solution with  his make of down converter. 

Based on being 1km line-of-site from a 3G base transmitter it would appear
that I need an additional 60dB of image rejection.  This should be quite
achievable. 

A few VK6 operators did not report any interference despite having a 3G base
station near by. It would appear that some makes of down converter already
have acceptable levels of image rejection.  It would be useful to make a
table of the image response of converters and if you have the ability to
measure yours then I would be happy to collate the results.  

An Australian  manufacture - www.minikits.com.au
<http://www.minikits.com.au>  -, VK5EME, has already modified his design to
meet the rejection figures required  

As annoying as the interference is at least it's not in-band and with
suitable engineering can be overcome. With the projected explosion in the
use of 3G mobile phones this could be one problem that many of us will face
in the future. 

73's Phil... VK6APH 
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