And when it gets one it will be about the same degree of sophistication as our WWII designs. They are an awful long way from fusion devices and a 15 or 20 kiloton device lobbed into space is NOT going to "blanket" the US from coast to coast. The expanding shell of electrons from a nuclear explosion will behave like radiant energy and follow the inverse square law. That is the electron density of the pulse will fall off by a factor of 4 for every doubling of distance between point of detonation and target. Thus you need to keep the blast close to the ground to have an effective EMP, but that will limit the area coverage. As you increase the altitude of the burst you get more area coverage (line of sight), but you reduce the electron density of the pulse (current). There is no way to have it both ways. An air burst of a 20 kiloton device is not TEOTWAWKI. To blanket the US from coast to coast would take something like a 50 megaton device detonated at 300 miles over Kansas city.
The USSR tested the Tsar Bomba at 57 megatons but never put them in inventory. I believe they really scared themselves when they popped that cork.
The reason I postulate a 15 to 20 Kiloton device for the Iranian's first device is as follows. There are two fuels that can be used to produce a fission bomb, U235 and Pu209. And there are two basic designs for assembling a super-critical mass, gun assembly and implosion. U235 is about 7% of natural uranium with the balance being U238. Enriching it by separating the two isotopes is a difficult and expensive task. Plutonium209 is a man made element that is the product of exposing uranium to the neutron flux in a nuclear reactor. Separating the plutonium from the uranium is much easier as it is a chemical process because they are two separate elements, not isotopes.
Why did Iran undertake the Uranium separation process and not the easier plutonium route? Because the spontaneous release of neutrons for plutonium is such that you only have micro seconds to assemble a critical mass whereas when using uranium you can do the assembly in milliseconds. Which brings us to the two assembly methods: The gun type bomb is a simple artillery field piece with a sub critical uranium bullet fired into a sub critical target. The process takes milliseconds and is simple and reliable. The implosion device takes a sub critical mass of either PU or U and surrounds it with high explosive lenses that create an inward moving shock-wave that compresses the core (pit), raising it's density in microseconds which drives it into criticallity. The trick takes very precise timing of the detonators firing the HE and sophisticated electronics that are generally tightly controlled.
In a nutshell, implosion can use either fuel but requires very sophisticated and precise electronics. The gun type assembly only works with Uranium, if you try to fire plutonium in a gun type device you will certainly get a fizzle.
Regards,
GtG
All you need is a small fission device: it is the gamma ray flux which causes Compton scattering (electrons knocked out of the atoms / molecules in the upper atmosphere) which travel at relativistic speeds, and interact with the Earth's magnetic field.
The design of the bomb, and the casing, can enhance the proportion of the weapon's energy which is released as gamma radiation: and by designing the case properly one can focus the gamma rays "downward" to the ground.
A fission weapon, according to these sources, will still be quite a good source of EMP: it is not directly a function of notional yield.
Cheers!