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Re: 70cm preamp



There are two potential issues with Mode J:
1. The 2m TX overloading the preamp on 70cm.  This problem can only be 
addressed by reducing the amount of TX energy input to the preamp.  That means
         a. bandpass/highpass filtering on the 70cm side
         b. increased separation of antennas
         c. reduction in TX power
A 70cm cavity bandpass filter is pretty easy to rig up.  I have 
step-by-step pictures at http://members.cox.net/nx7u/mobile/4x3x5.htm
Now the SP7000 has bandpass filtering in it, but I don't know if it's 
before or after the preamp.  If it's not before the preamp, then it won't 
help the Mode J problem.

2. The 2m TX is transmitting a significant 3rd harmonic (145x3=435, a most 
unfortunate combination).  Similarly, that means
         a. bandpass/lowpass filtering on the 2m side
         b. increased separation of antennas
         c. reduction in TX power

Even if the 3rd harmonic is -60dBc, with a 100W (+50dBm) TX that's still 
-10dBm radiated--a big signal.

The most prudent (to me) solution is filtering on both ends.  When I ran 
Mode J mobile--where the antennas had no directivity and necessarily were 
very close together--using the 70cm cavity filter on RX and an ICE 2m BPF 
filter on TX allowed for no-worries LEO operation.  Plus you have the 
benefits of good amateur practice (filtering TX), and a more 
intermod-resistant receive path on 70cm.  Nearby 460MHz paging/two-way 
radio systems can really wreck your RX.






>-----Original Message-----
>From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On
>Behalf Of Bill Bruno
>Sent: Wednesday, September 27, 2006 7:04 PM
>To: Amsat BB
>Subject: [amsat-bb] 70cm preamp
>
>
>On my V/U setup I have a SP7000 70cm preamp. Working FO29(JAS2) my 2 meter
>signal overloads the preamp, my signal splattering like an adjacent strong
>station. With preamp off the problem is not there. In testing I found the
>overload happens in any combination of 70cm 2m frequency pairs. I can
>eliminate the problem by lowering the tx power, but by doing so I have a
>hard time copying my own signal. How can I eliminate the 2m sig overloading
>the preamp while still maintaining  adequate power?
>Antennas m2 436cp30,  Gulf Alpha 2m5el, both right hand polarization.
>I could use some suggestions.
>Thanks Bill
>
>
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>
>
>_______________________________________________
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