On Sun, 6 Aug 2017 12:17:16 +0100, ldagan
wrote:


Hello all. If we assume that the antenna's surrounding is air, then my
assumption is that dielectric losses for the antenna are rather
negligible. What type of antennas are usually showing higher effect of
ohmic losses?
I mean, I know that reflector antennas are usually little susceptible to
ohmic losses. But some types are not, my assumption would be loop
antennae,

Yes, loop antennas can be VERY sensitive to resistive losses thanks to
very high currents through the loop. The SLA (small loop antenna)
magnetic loop performance is highly dependent on having the highest
possible Q (to a limit where it becomes less than the modulation
bandwidth). Any series resistance would ruin the Q which is why
mechanical joints are soldered or brazed. Try playing with the AA5TB
SLA spreadsheet calculator at:
http://www.aa5tb.com/aa5tb_loop_v1.22a.xls
and watch what happens at low frequencies to the efficiency when you
increase the "added loss resistance". Notice that it's in milliohms.

Another thing is attenuation in waveguides. I assume that it would be
the case with many tens of GHZ, for rectangular device. But, as I am
only guessing, as I have been out of the game for long.

At GHZ frequencies, the skin depth is quite shallow. Therefore, only
the inside surface plating really needs to be low resistance. I've
worked with silver plated rigid molded PLASTIC WR90 X band waveguide
which worked but had mechanical rigidity problems. We also had
problems with cracked plating on outside 90 degree corners, such as on
flange to waveguide connections.

https://www.microwaves101.com/encyclopedias/waveguide-construction
http://www.eenewsautomotive.com/news/plastic-waveguides-future-communication-networks

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558