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+--- Thread: V/U/Microwave panoramic display (/thread-442.html)
V/U/Microwave panoramic display - VK2JDS - 15-11-2017
How to add a spectrum display to your vhf/uhf/microwave station.
A spectrum display is really handy when you are looking at a band segment where you dont know if anyone is on, and even if they are on you don't know what frequency they are on due to errors in local oscillator frequency references on the millimetre wave bands, or compensating for doppler shift for moonbounce.
So, lets use a RTL 820t2 SDR dongle. These things are $14 from ebay. You can plug it into your existing laptop , or just add another old klunker laptop to the shack to run it. The software is SDRSharp which is free. If your existing pc has enough horsepower to run WSJT software and keep the cpu below 40 percent then it should handle one of these dongles ok.
How do we hook one up ? That the easy bit. If you are using a remote mounted LNA at the antenna then you just add a simple splitter onto the output of the LNA and run a separate cable down to your shack to the SDR. I used a scrounged commercial splitter for 23cm, but they are a snack to make for 2m and 70cm.
Lets look at some pictures.
here is the splitter right at the output of the low noise receive amp at the feedpoint.
So , what does it look like and how meaningful is the display.
well, once you have used WSJT you will have seen just how handy the waterfall spectrum display is. This just adds more to the display.
Getting the display to show the right frequency of the carrier you want requires an adjustment of the PPS of the RTL dongle. mine is +71 pps. You adjust this in the settings box of SDRSharp. Even if you don't fiddle with this setting it will still work, its just the cursor wont show the right frequency when you click on a carrier.
Here is the Dural beacon on the second laptop on 1296.420 MHz. its over180 KMs away over the blue mountains so is a great source of reference.
Here is what WSJT shows of the same beacon at the same time.
As you can see its only a narrow view of the signal, thats why the SDR makes a huge difference to finding those off frequency signals.
If you are using a transverter then a simple splitter added to the RX output before your TR relay to the radio will provide a tap point at the IF to attach your dongle. Set your frequency to the IF output and then view the whole segment.
You can adjust the width by the zoom control , or better is by the settings for sampling rate. i have mine set lowest so it covers only 100 KHz of the 1296 MHz band
73 Dave vk2jds