Mt Nelse (VK3/VE-004) first attempt: Or, how to blow an IRF510 on a 10 point summit

Day 3 of the Australia Day long weekend (Saturday 28th) was to be a 30 point day up on the summits of the Bogong High Plains, not far south of Falls Creek, starting with Mt Nelse (VK3/VE-004), Mt Cope (VG-001) and Mt McKay (VE-007). I chose Mt Nelse, a 5km walk each way, to start the day, parking at the second sign and car park after Rocky Valley Dam (first Heathy Spur, then Watchbed Creek).  The walk up is easy over rolling hill-like grades. After passing Marum Point track on the right, then Johnson’s and Edmonson hut turnoffs, the summit came up on the right, about 300m off the track. At the top the wind was buffeting. I found a threesome of hikers eating their lunch in the leeward side of the cairn which provided some limited shelter. These three mid-age walkers had met at Bogong summit 30 years ago and had got together for a high country walking weekend nearly every year since. It was their 30th anniversary walk this year.

After farewell-ing them I decided to strap the squid pole to the base of the bent trig marker and brave the wind. It bent over wildly and for the first time I thought it possible that a squid pole could snap or rupture in a particularly strong gust. I have never seen it bend as far.

I spotted and on air struggled to make a scratchy contact with Steve VK7CW, he gave me 3×1, I gave him 5×6.  I called CQ multiple times with no answer. Thinking that the issue might partially be due to the trig pole de-tuning my ground plane antenna, I disconnected the coax and retired to the cairn where I walled in the lower segment of the squid pole with loose rocks. But after more spots and dozens of calls still nobody was hearing me.

At first I put it down to poor propagation. When Peter 3ZPF called at 59 and couldn’t hear my reply, then no-one else answered my calls, I got suspicious.

What finally confirmed a problem was my inability to work Russ VK2BJP/P Mt Tawonga, about 60km north. He was 57 to 9 and he could barely make me out. As an aside, I heard Russ qualify as a SOTA Goat — congrats Russ!! I was listening to your entire activation! Russ could not make out my call. If any RF was going up the pole it must have been milliwatts of leakage from the driver across the blown FET and into the LPF.

In response to my spots I heard calls from VK4AAC/3, VK3ZPF and VK3SQ…there may have been others while I was fiddling with the vertical.

Next I checked the antenna and found the outer sleeve of the PL259 not screwed on. I am not sure if it was properly attached when transmitting from the trig marker.   Fixing this made no difference.

Time to stop and think. 

  • Is the rig powered and is the power holding up on transmit? Check.
  • Is the receiver working? Yes, everything sounded normal, the antenna was receiving.
  • Is the transmitter working? I don’t carry an SWR meter and the homebrew rig doesn’t have such luxuries built-in. Nor does it have SWR sensing or any form of PA protection. But the RF power LED was blinking on voice peaks.

My final attempt was to reconfigure the antenna. I removed the center loaded ground plane element and plugged two of the 1/4 wave radials into the junction box to reconfigure the antenna as a dipole. When this made no difference it was clear my activations were over. I had spent 2 hours on the summit.

First and last spots:
Sat 02:34 VK3HN/P on VK3/VE-004 VK3HN/P on VK3/VE-004 – [edit] 7.09 ssb
*Mt Nelse qrv [SOTA Spotter] (Posted by VK3HN)

Sat 04:17 VK3HN/P on VK3/VE-004 – [edit] 7.095 ssb
*Ant reconfigured, final try qrv 7095 [SOTA Spotter] (Posted by VK3HN)

On the walk back I had plenty of time to think it through. I devised the hypothesis that I had transmitted into a high SWR and damaged the IRF510 final. This theory did not explain why the LED winked when I spoke, but it may have been detecting an amount of drive leaking into the PA past the damaged FET. Everything else made sense.

The next night, on the bench at home, I confirmed that the the PA stage was dead. In the photo below I am whistling loudly!

A new IRF510, another whistle, healthy RF output… about 10 watts on the Oskerbloc, and an amp at 12  volts. 

Homebrew rigs do not have the protections that an FT-817 or KX-1/2/3 have, so an antenna slip-up can destroy the PA transistor. David VK3KR reminded me that some PAs have a 24-40 volt zener fron collector to earth, to shunt high voltage peaks in the event of a high impedance load. Wayne Burdick’s 2 watt PAs on the Wilderness SST and Sierra transceivers have these, but I have not noticed them on an IRF510 PA stage.

I searched up some digital SWR meters using couplers and analogue pins on an Arduino. This can result in a neat on-screen SWR indicator. But a level detecting circuit and a feedback path back to the bias on the PA transistor would be needed to provide actual protection. At this point it starts to get complicated.

If I had been carrying a back-up rig in the car boot I could have continued on to Mt Cope and Mt McKay. I did have the SST (5 watts CW on 30 meters), but I did not like my chances of 4 CW QSOs on 30 meters on two successive summits, given the time left in the afternoon, and recent propagation. And besides, I need to be fresh to work CW, not weary.

In SOTA, there is always another day.

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One thought on “Mt Nelse (VK3/VE-004) first attempt: Or, how to blow an IRF510 on a 10 point summit

  1. […] last mod to my rig added a built-in SWR meter to help detect antenna problems.  I did this after blowing the PA transistor on Mt Nelse by hurriedly transmitting into a partially erected vertical.  And I even used my new inbuilt SWR […]

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