Wednesday, August 3, 2022

POTA activation with TR-35 and 9V battery

Years ago - pre-POTA and pre-lithium - some intrepid QRPer (a W3-lander) decided to see how many states he could work with a single 9V alkaline battery during Sweepstakes. I don't remember his results but his radio was a new-at-the time Norcal 40A.

These days, any weekend POTA activation can serve as a test platform for this and other absurdities.

I decided to see if a 9V alkaline battery could power my TR-35 long enough to make the 10 contacts required for a valid POTA activation. The TR-35 draws 90-100 mA on receive. Transmit current (with 9V DC in) is 700mA and that results in an RF output of about 1 watt on 20m and 2.5 watts on 40m.

In all, I made 11 contacts and did a lot of CQing.This was on a Wednesday morning - more contacts and less CQing would be the likely result on a weekend.

The capacity of a Duracell 9V battery is a measly 300 mAH with a 50 mA load. The TR-35 draws twice this much current just on receive.

This cannot be correct
The battery voltage dropped to 8.5V just by turning the radio on. Key-down voltage decreased from 7.4V at the beginning of the activation to 6.8V at the end.

No one mentioned any chirp on my signal but I'd be surprised if the radio was able to produce a steady tone.

Eleven contacts and about 40 minutes later, I QRT'ed and the battery's voltage popped back up to 8V. The meter also shows 13mAH used for the activation - this is not possible and shows that these meters don't accurately measure small amounts of current when toggled on and off briefly (ie, 22 wpm CW).

This was a fun experiment. I may repeat it sometime during a CW contest just to see how many QSO's can be squeezed out of a tiny battery.



1 comment:

  1. Maybe two or three 9V batteries in parallel would work. They would all have to be from the same batch and without previous use to avoid paralleling batteries with different voltages which would be problematical.