After trying some different NiMH cells, freshly charged, my H-520 seemed to be happy using my longer whip antennas, though it still wouldn't work with the supplied mini-whip. But after only a little use of the batteries it began cutting out again when using a 2ft centre-loaded whip. The only antenna it would still accept was the 4ft whip with counterpoise. I felt that something wasn't right, so I decided to investigate.
I measured the battery voltage and current. The batteries were not freshly charged but had been used on Sunday to make numerous calls though no contacts. With the radio turned on and receiving the current drain is 90mA and the battery voltage was 7.71V. This is substantially above the 7.2V nominal voltage which suggests that there should be plenty of life still left in the batteries. This was confirmed by the battery state indicator.
I then transmitted into a dummy load. On the 1W setting (with slightly under 1W output) the current drawn was 750mA. On the 4W setting (with about 3.5W measured output) the current drawn was 1.3A. The voltage across the batteries whilst full power was being transmitted dropped to 6.75V, significantly less than the 7.2V nominal voltage. The current consumption is not excessive in comparison to an FT-817 which is specified to draw up to 2.0A at 5W output. One factor contributing to the voltage drop is the use of separate AA cells. Most handheld radios use battery packs in which the cells are welded together to minimize resistive losses.
The radio still worked fine at 6.75V, but this was transmitting into a dummy load. On most of my 10m antennas - and as observed by M3XEM - the battery voltage would fall away after a couple of seconds and the radio would cut out and restart. I tried to measure the voltage across the batteries when using the two short whips and it fell right away to 3 or 4 volts! This explained why the radio cut out and restarted. It must be drawing a lot of current to cause such a large drop, but I was unable to measure exactly how much as the radio's voltage sensor cut the power to the TX too quickly for my meter to catch the reading.
My conclusion is that when presented with a load that is something other than 50 ohms, the PA of the H-520 draws an increased current. This effect is significant enough that the radio cuts out even on the 1W setting using the mini-whip antenna. This current draw may be more than rechargeable AA cells are designed to sustain, quickly pulling the voltage below the "low voltage" sensor level.
The transceiver isn't really "eating the batteries" as some users have claimed, as the cells quickly recover their original voltage when the load is removed. The problem is purely and simply poor design which causes the PA to behave in this way and draw too much current when presented with a mismatch. As can be seen from the schematic (which is included in the manual) there is no SWR protection to back off the power.
It is very unlikely for a whip antenna on a hand held radio to present a perfect match, so the H-520's performance in this respect is lamentable. As I noted in an earlier post, the supplied whip antenna was found to give its best match at 28.5MHz. It would present a high SWR on the UK and EU CB frequencies that most buyers of this radio would use and I doubt if any buyer of this particular radio would have found it possible to use the highest power setting with this antenna.