K1TTT wrote:
On Jun 9, 10:38 pm, Jim Lux wrote:
In fact, the ration between that stored energy and the amount flowing
"through" (i.e. radiated away) is related to the directivity of the
antenna: high directivity antennas have high stored energy (large
magnetic and electric fields): the ratio of stored to radiated energy
is "antenna Q" (analogous to the stored energy in a LC circuit leading
to resonant rise).

So, high directivity = high stored energy = high circulating energy =
high I2R losses.

this is a relationship i haven't heard of before... and would be very
wary of stating so simply.

I should have used arrows rather than equals signs.
But it's basically a manifestation of Chu's idea combined with practical
materials.

Chu proposed the concept relating directivity and stored energy and
physical size. A passively excited multi element array (like a Yagi) has
to transfer energy from element to element to work, and it follows the
characteristics outlined by Chu. And anything with circulating energy
that gets carried by a conductor is going to have high(er) I2R losses
than something that doesn't.


it may be true for a specific type of
antenna, MAYBE Yagi's, MAYBE rhombics or or close coupled wire arrays,
but some of the most directive antennas are parabolic dishes which i
would expect to have very low Q and extremely low losses.

Interesting case there. Loss isn't all that low (typical parabolic
antennas with their feed have an efficiency of 50-70%), although it IS
low compared to the directivity. And, in fact, there's not much stored
energy (so the Q is low). On the other hand a parabolic antenna is
physically very large compared to a wavelength, so the Chu relationship
holds. I'd have to think about whether one can count the energy in the
wave propagating from feed to reflector surface as "stored", but I think
not. Probably only the E and H fields at the reflector surface.


you could
also have an antenna with very high Q, very high i^2r losses, but very
low directivity, so i would be careful about drawing a direct link
between the two.

Yes.. you're right.. the relations set an upper bound on what's
possible.. That is, for a given directivity, you can get either small
size and large stored energy (the Yagi-Uda or W8JK), or large size and
small stored energy (the parabolic reflector and feed). As you note, a
dummy load has very low directivity.