2009 $20 Ultra High Relief Double Eagle

Thanks, I did some more internet reading and this page gives lots of information:
https://auctions.stacksbowers.com/l...view/1-1DM2F

But I still don't understand why the modern version is a smaller diameter than the ultra-high relief of 1907. Couldn't they reproduce the same conditions? Both are 4mm thickness.

The original Double Eagles contain 0.9675troy oz of gold and are 90% gold & 10% copper. The 2009 Double Eagle contains 1troy oz gold and is 100% gold. Therefore, the original Double Eagles contain approximately 7.5% more metal by weight. Therefore, they can't be the same dimensions given that their weight and density is different.

Now the question that you might ask is why they made the 2009 version 100% gold instead of 90% gold like the original. That I do not know for sure, but my guess is that it has something to do with the difficulty in making an ultra high relief coin and the legality of minting a coin containing a weight of gold that the mint is not already authorized to produce.

I believe the answer to the question rests in the fact that the US Mint produced two different patterns of ultra high relief (UHR) St. Gaudens Double Eagles in 1907: first it used 34mm planchets and then later it tested 27mm planchets that were the same weight but thicker. A very limited number of each was struck but both proved unworkable for mass production; neither UHR pattern was issued for circulation.

The Mint then lowered the design relief and created what is referred to as the high-relief version of the coin on 34mm planchets and released 12,367 of them into circulation. Striking these coins also proved to be problematic, however, so the Mint reduced the design's relief even further and issued only standard or flat relief coins from late 1907 through 1933.

For its 2009 UHR Double Eagle, the US Mint decided to reproduce St. Gaudens' masterpiece using the smaller 27mm diameter planchet size. The smaller but thicker planchet selection was made to enable the coins to be struck with the extreme depth and relief that St. Gaudens originally envisioned.

Also, the Mint chose to use 0.999 fine gold planchets in 2009 to make it easier to strike the coins with full detail; 0.999 fine gold (i.e., 24 kt) is softer than a gold composition of 0.900 fine gold and 0.100 copper).

Hope that helps!