[amsat-bb] Re: Downlink issue with Icom IC-910H

[Jeff Mock]

Ronald Nutter wrote:
> After not having used my IC-910H for a while due to moving across 
> country, I unpacked it tonight and setup my antennas.  I was able to 
> make contacts on SO-50 and AO-51 with one small problem.  I can hear 
> signals on the downlink but not mine.  When I transmitted, all I could 
> hear was some white noise when I was transmitting.  I head several 
> exchanges but it was like I was in half duplex.  This was a working 
> system before the move.
> 
> I have good quality coax running from the IC-910H to an ICOM preamp for 
> each side (VHF or UHF) and then running to the antennas.  I believe that 
> I have used all of the original coax but I could be wrong.  The only 
> thing I havent put in place was my mode J filter on the UHF downlink. 
> Could this be causing my problem with not hearing my signal on the 
> downlink but hearing those who are calling me ?  Sorry for what is 
> probably a stupid question but I am getting all the kinks worked out in 
> my setup before Field Day.
> 

I think the lack of a filter on the 70cm downlink might be the problem. 
  I started out without any filter on the 70cm downlink, I added a DCI 
70cm bandpass filter because of local QRM from commercial radios and it 
also improved my ability to hear the downlink on my 910H.  As you 
suspect, the 2m transmit might be desensing the 70cm receiver or preamp.

I tested this by listening to a weak faraway 70cm repeater while 
transmitting on a clear portion of the 2m band.  You can do this with 
and without the 70cm preamp and hear if the receiver is desensing. I 
found that the filter made my troubles go away.

jeff
AD6EO