Thursday, March 10, 2011

Boosting the VX-8GR on APRS

A few weeks ago I did an analysis of the audio levels of different APRS radios and lamented the low level of the audio transmitted by the Yaesu VX-8GR. An Italian ham wrote to me enclosing a copy of a document he obtained from Yaesu showing how to increase the transmit deviation. I decided to give it a try. You can find a copy of this document in the Files section of the VX-8R Yahoo group, but I will describe the process here.

Note that performing this adjustment will increase the transmit deviation on speech as well. There is no way to increase the packet deviation independently. Note too that while you are in the alignment menu it is possible to change other settings as well by accident. This may be undesirable, especially if you don't have the test equipment to realign the radio properly, so be careful and perform the adjustment at your own risk! Finally, note that these instructions will work only for the VX-8GR. There are instructions for accessing the alignment menu of the VX-8R on the web. They don't work for the GR and these instructions don't work for the R. I have no idea if any of the instructions work with the DR.

To avoid entering the alignment menu accidentally, Yaesu has made accessing it quite difficult. First you must enable the CW ID (main menu item 16) and program a password AH041M into it. The manual explains how to do this. You must then set the transceiver to single band mode on the A band, in VFO mode (not memory) on a frequency of 430.000MHz. Now switch the radio off.

Press and hold the HM/RV key and turn the VX-8GR back on. If all the above steps were carried out correctly the radio should start up in alignment mode showing the first alignment setting. Rotate the control knob clockwise a few clicks to select the MAX DEV adjustment, then press the V/M button to select it. A pointer symbol should appear to show that adjustment is selected.

The control knob now adjusts the deviation setting. Make a note of the original value in case you want to reset it, then turn it up to 254. Yes, I know this sounds like a CB "screwdrivers to the max" tweak but as you can see from the spectrograms below, even at that setting the packet deviation won't quite match that of the Kenwood rigs. Press the V/M button again to exit the adjustment, then press HM/RV to exit the alignment menu. The radio will restart in normal operational mode. Don't forget to clear the CW ID once you're happy with the new setting.

The spectrograms below show the difference made by the adjustment, with the Kenwood TH-D72 shown as a reference.


As you can see, the peak deviation of the high tone is now within 1dB of that of the Kenwood, though the Yaesu still has more low-frequency roll-off. Nevertheless, this is as good as it gets with the Yaesu. If you still can't hit the digis you think you ought to, perhaps you'd be better off with a Kenwood.

As I said earlier, this deviation adjustment makes your audio louder as well. You, your local hams or your local repeaters might not like this. The VX-8GR has a "Half Deviation" menu option which will reduce the deviation back to approximately what it was before this adjustment, but it works across the whole radio and not per band, so you can't have the wider deviation only on the APRS band.

It is interesting to note that in the Yaesu alignment document the deviation alignment is performed at 435MHz. I found that the deviation on 70cm is higher than it is on 2m. This appears to be a consequence of the way the radio is designed, as there are not independent deviation adjustments for the two bands. But this does explain to an extent why the deviation on 145MHz is lower than it should be.

Where I live, 70cm is completely dead and 2m is quiet so we can operate using 25kHz channel standards with no problems. Therefore I have not found this adjustment to cause any adverse effects and it certainly has improved the reach of my APRS packet beacons. Your mileage may vary.

1 comment:

Julian said...

Robert, VA3ROM, found a VX-8GR service manual (no I don't have a link to it) which offers some additional information that suggests it may be possible to adjust the deviation for the APRS band on 2m only. He wrote:

"Start off as you described, and then press the Band button to switch to the 144 band (you usually have to dial off the default setting menu, first to get it to shift to 144). Then, just dial down to the deviation menu and adjust as you described. Mine was set at 147 and you can dial from 0 to 255. My 440 band deviation setting was 109 and 2m was 147. So they are in fact separate and different.

The main VFO and sub VFO can have different menu settings from each other and therefore you can turn on Half Deviation just for the main VFO ONLY (to offset the deviation increase for voice use) and the sub VFO (APRS band) can stay at the default Half Deviation OFF. Since you won't be using voice on the sub band, as you'll lose your APRS ability, this fixes the over-deviation problem for the main VFO.

Just select the VFO that you want to use for setup (A/B keys) then enter the main menu setup and adjust accordingly. Some functions are shared but Half Deviation is separate for the two VFO's and two bands. Select the other VFO and do the same. You'll see that each are totally separate in their settings.

Yaesu was smart enough to have both UHF and VHF use separate menu settings and system) and ditto for the main and sub VFO's."

Due to feeling unwell I am unable to test this or modify my description of the process at the moment, but I felt I should mention this for anyone who was thinking of trying it.