Thursday, January 17, 2013

Receiving VOR signals for the first time

My principal transceiver in the shack is an Icom IC-746. It's coverage is from 30 kHz up to 60 MHz and 118 MHz up to 174 MHz. As you can see, no VOR band (108-118 MHz). Because recently I gained interest in receiving VOR signals to continue my experiments about radio detection of meteors and radar techniques, I had to find a solution.

Interestingly if you look in Internet about IC-746 frequency coverage, you will find two groups of pages: One group saying the transceiver covers 118-174 MHz on VHF, and a second group saying the VHF coverage is 108-174 MHz. Maybe a typographic error?

There are several modifications for the IC-746's diode matrix to get expanded frequency coverage. I opened my transceiver to see what diodes were installed. The diode matrix is located at the bottom of the transceiver, near the HD6433042SFB24 IC, and consists of 14 diodes, but only five were installed in my transceiver

From top to bottom, diodes 1,2,3,9 and 14 were installed. None of the modifications I found had this combination of diodes, so I tried one combination I found who differs by only one diode from the one I had: 1,2,9,11 and 14: This is remove the diode at position 3 and resolder it at position 11. And voila:

In a very first band scan, I found several VOR signals, some of them quite strong!.The first impressions about VOR are:

They are not frequency standards: They are +/- 1 kHz from the nominal frequency.

The 30 Hz AM sidebands inherent to the VOR modulation are quite noticeable.

Some of them have slightly unstable carriers:

VJZ on 115.100 MHz

But others have rock-solid carriers:

SIE on 115.400 MHz

But all of them have nice doppler curves from airplanes!

Doppler from airplanes over SSY signal, on 117.850 MHz

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