Mt Donna Buang SOTA VK3-VC002

On the 26th September, the weather was looking to be good, so took a few hours off to do a SOTA activation at Mt Donna Buang, VK3-VC002.

One of the main reasons for the activation was to test out the home brew backpack linear amplifier to see how practical it was to use in the field.  The amplifier draws about 10 amps at 50W out, so it would be powered from a Lipo battery.  I chose a 2500mAh 3S pack that is initially 12.6V.  The transmit duty cycle on an activation is hard to predict, and SSB of course is not 100% duty cycle, so despite the modest capacity of the battery, it should last an hour or so of operating.

A simple 10metre high vertical antenna was set up for 7MHz with four 10 metre long ground radials.  The VSWR was reasonable without an antenna tuner which is good because the Elecraft T1 ATU would have cooked.

After 90 minutes of operation and 19 contacts both the FT-817 pack and the linear amplifier pack had a reasonable work out.  I probably should have kept a closer eye on the linear battery as it had gone down a little far and the IMD was probably not so good.

Had some great signal reports and worked a few QRP stations in NSW and Victoria.  One local operator commented that the copy would have been difficult without the linear amp running.  The vertical antennas radiation angle does not favour close in stations.  One station worked was maritime mobile south of Hobart.  The highlight was a summit to summit contact with Peter VK3PF on top of The Twins in the North East.

After the activation, the batteries were recharged to assess energy used over the 90 minutes of operation.  The FT817 consumed 1032mAh of the 2500mA Lipo’s capacity.  Not too bad.  The linear amp had consumed 2300mAh of the 2500mAh capacity.  Oops, probably should have kept a better eye on that one.  Not bad though for a battery that weighs 200grams and was generating up to 50W TX power.

To see how the power would have dropped during the discharge curve, I plotted the power out of the amplifier against supply volts (5W in @ 7MHz):

Volts:    9     9.5     10     10.5     11     11.5     12     12.5     13     13.5     14     15     16     16.5   Watts:  27    30     33      36       39     42       45      47       50      >53     >53  >53  >53   >53

The output was tested up to 16.5Volts out of interest in case a 4S Lipo is used.  In an ideal world, it should survive at 17V as the fets are 55V and if the VSWR is good, the fets should only see about 2xVdd or 34V.   The power meter only goes to 50W so couldn’t measure above 53W (full scale).

So, in conclusion, the next activation will probably use a larger battery for the amplifer, maybe 4000mAh for a 90 minute activation.  The 2500mA battery inside the FT-817 is probably fine.  The extra weight penalty of the amplifer is 418gm(amplifier) and 200gm(battery).

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