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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