fire value for mortars

[Mowgli]

"Artillery in Panzer Battles uses the ‘Alternate Indirect Fire Resolution rule’ as standard. In simple terms, you can only fire once, but that fire is doubled in strength and will impact all defending units in hex proportional to their strengths. Mortars and direct fire artillery (infantry guns) do not use this rule and can fire several times in a turn. "

I read this differently: namely that the impact of the fire is divided across the units proportional to their strenghts (i.e. when a large unit and small unit are stacked together, the losses are divided so that the large unit gets more losses than the smaller). Not that the fire value itself is impacted. (besides from the fact that mortars are excluded in the rule wording )

edit: did some testing with artillery. In the case of firing with artillery, the expected attack value seems doubled, and then each unit is attacked proportional to their strength (so indeed also for small units, the fire value goes to zero). For example firing at one infantry unit of 33 men has fire value 16, firing at a hex with three infantry units of 33 men has fire value 16 also, but applied to each unit separately.
(this example is for an attack which would have raw fire value 48, which is then multiplied by 2*33/200 per infantry unit of 33 men).

Yeah, I just started a PBEM for PzCampagins France '40 that also has both alt. fire resolution rules switched on. 
Indeed the wording is not entirely clear. 

"Alternative Indirect Fire Resolution – when this rule is selected, Indirect Fire of non-mortar, non-heavy weapon units is conducted under different rules. First, each such Indirect Fire is doubled in fire value, but the cost of firing is doubled. Second, such Indirect Fire is applied against all units in the target hex, proportional to their strength, instead of being applied against a single target unit."

Just like you, my first thought was that the total artillery fire value would be "split up/spread out" over all the units, with the larger units getting more of the fire than the small ones. But this doesn't make a lot of sense (all units have individual defense values!). Indeed your explanation (and testing) seems much more plausible. So "proportional" does not refer to the size of the unit in relation to that of other units in the hex, but rather just to the overall size of the unit (compared to some pre-determined levels/thresholds). 

I guess it works in a similar way as the "density factor" in Squad Battles: In Squad Battles, fire is always directed at a hex, not at a unit. All units in the hex are affected individually by the same base "fire value" of the firing unit. So each unit in the targeted hex individually applies its factors to that base fire value (cover, protection value, being unspotted, etc), including what's called the "density modifier": The smaller the unit, the greater the reduction of the base fire value. In the case of Squad Battles, the factor is (number of men in the unit /10) --> So when the fire affects a unit with just 1 man, the fire value is reduced to *0.1. If another unit with 9 men is in the same hex, it will also be affected, and more heavily so: by *0.9 of the fire. Just like it seems to be the case in Panzer Battles (which does feature a stacking bonus though!), the "density factor" can never be "positive" (greater than 1). Your numbers suggest that the density threshold in PzBattles is 200. Good to know! :)

But still the question why this affects mortars remains unanswered. Do you have the overall impression that "density" is also applied to all direct fire? (In Squad Battles, it is!) Does it take more direct firepower to achieve casualties against small units than against large ones?