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VK6 - QRN - Update



Many thanks to all those who replied to my post regarding the QRN we VK6's
have been receiving from the new 3G mobile phone services. 

As previously posted, the problem has been traced to inadequate image
rejection of some   2.4GHz AO40 down converters  when used with a 2m IF.  

So far we have found two solutions to the problem. 

1. Change the down converter local oscillator from low to high side
injection. I use a G3WDG converter and Charlie very promptly provided me
with a replacement crystal. This completely cures the QRN problem although
it  has the slight draw back that the down-link tunes in the opposite
direction. I consider that a small price to pay to be back on AO40. 

2. Choose a down converter with higher image rejection. In my particular
location, about 1km line-of-site from a 3G base station, I needed >65dB of
image rejection. After requesting image rejection figures from a number of
manufactures I selected an AIDC 3731AA from Bob, K5GNA. Using this
converter, and beaming directly at the base station, there is absolutely no
trace of the previous S9+20dB signal.

Others are looking at fitting various filters to either the input of their
down converters or  adding a filter part way down the RF chain.  More
details as these design materialize.

I hope this information is of use to others if and when they encounter the
new 3G phone service. 

It's great to be back on AO40!

73's  Phil... VK6APH  




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