Specifically, and I'm not sure of the correct technical term here I think it's saturation, the blanking of the receiver that occurs when a receiver is close to a transmitter on an adjacent frequency. This is especially true when using these low cost radios. They don't have the selectivity to reject close by, slightly off frequency signals. In this case, slightly off frequency is 600 KHz, which is the standard offset of 2 meter amateur radio repeaters. Why is that important?
Well, even though the stated goal of this project is a cross-band repeater, which would involve band seperation of several hundred MHz, my original desire and goal prior to this project was to assemble a portable special event/emergency deployment repeater system that could be used on 2 meters.
My desired configuration of these radios is shown in the picture below:
The picture here shows the two radios, positioned with the transmitter on the top and the receiver on the bottom. In actual operation, they will be separated by a center compartment of aproximately 8 to 10 inches. It was my hope that that distance combined with some metal shielding between the radios would eliminate this blanking. Thus eliminating the need for the costly and bulky system of caveties that are normally required in permanent repeater installations. Unfortunately, this doesn't appear to be the case. And to be fair, I'm not the first person to consider using HT's as a temporary repeater. I've seen on the web at least a couple "kits" that consists of a metal briefcase, pluckout foam, and a repeater controller. Many commenters on those pages ask the same questions I'm asking myself about the blanking and intermod without having the cavities in place. And so far I've not seen any real answers. That's why I'm experimenting.
In a different test, I turned the transmitter down to one watt and then stepped about 5 feet away from the receiver. That test produced good results. I was able to trip the repeater with the transmitter, and the receiver picked it up during the whole test without the blanking effect.
So the next question is, how far do the antennas have to be separated to allow the transmitter to work at full power? And rather than physical spearation, would it make a significant difference if a non-standard offset was used. Some research indicates that it would be possible to get a 2.4 MHz separation, and still stay within the published national band plan. (Iowa doesn't have a "published" band plan, at least not one that's readily accessible to regular operators like me.)
I would expect that as long as this is used on a temporary basis, is low power (5 watts or less), is within the established band plan, and in all respects in compliance with part 97 that the repeater would not have to be coordinated at the state level and could use a non-standard offset.
In order to test this with a non-standard offset, I'll need to go forward with modifying the radios to have a COR output and build a rudimentary repeater controller and use a couple additional radios to test with. Rather than a full-on build of the controller, I'll try putting something together on my breadboard. Anything I come up with I can transfer to something more permanent later.
That's all for today. The weekend is coming, so I hope to get started on more stuff.
73
Steve
KDØWSW
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