Well your erp on 160M is going to be pretty low. It is probably not worth the effort to model in NEC or an equivalent program. One can do a back of the envelope calculation that will point you in the right direction.
The efficiency of an antenna is:
efficiency = power radiated by antenna/total power fed to antennaq
The power radiated by the antenna is the (current squared * radiation resistance)
The total power fed to the antenna is the power radiated plus the power lost. The power lost is
power lost = ((current)^2) * loss resustances
To be precise, there can be a separate current associated with each loss resistance, but for now, lets say the ground loss dominates and the ground current is the same as the antenna current.
The radiation resistance can be calculated fairly straightforwardly. The radiation resistance is proportional to the the height of the antenna in wavelengths. The GADS is effectively about 20 feet high as the horizontal portion will not radiate much, but contributes somewhat to the loading of the vertical potion. It contributes to the loss, but lets ignore that for the time being.
The antenna is about 1/24th wavelength high. we know a 1/4 wave vertical has a radiation resistance of 36 Ohms or so.
So the radiation resistance, which varies as the square of he height will be ((1/6)^2)*36 Ohms, or about 1 Ohm.
Loss resistance is a pretty big variable, but I would guess in your situation that it would be hard to guess too high. :^)=
W8JI has measured a ground resistance with two 5 ft rods at 85 Ohms < http://www.w8ji.com/ground_resistance_measurements.htm >
so lets us that number. Your soil may differ, but probably not by a lot.
So if the ground losses are the only loss we consider, then the efficiency is roughly:
efficiency = 1/(1+85 = 1/86) = 0.012 = 1.2%
So your 5 Watts result in slightly less than 60 milliwatts radiated.
Now we have ignored other losses, which are likely to be significant. These include contact losses in the gutter joints and feed contact, losses in the adjacent brickwork, particularly when wet, and who knows what else. So you are probably putting out lots less on 160M.
You might want to put out some radials. They will help a lot to increase efficiency, even on the higher bands. - Dr. Megacycle KK6MC
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KK6MC
James Duffey
Cedar Crest NM