Søren Kjærsgaard<p>Spiral antennas are funny devices. They are known for their extremely wide bandwidth, the ones shown here, are specified to operate from 2-18GHz. </p><p>They’re not meant as transmitting antennas, these critters are receive-only, used by (largely non-civilian) pilots who have those needs ☺️</p><p>Their low frequency boundary is determined by its physical size, in this case, the circumference of the small plate where you can see the fine spiral traces. </p><p>Once you cross below that boundary, the structure basically collapses electrically, seen here as a steep SWR ‘wall’ to the left side of the sweep. <br>At this point it’s not an antenna anymore, but something else (whatever that is 🤷🏼♂️😂)</p><p><a href="https://techhub.social/tags/rfengineering" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>rfengineering</span></a> <a href="https://techhub.social/tags/rfengineer" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>rfengineer</span></a> <a href="https://techhub.social/tags/testandmeasurement" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>testandmeasurement</span></a> <a href="https://techhub.social/tags/hamradio" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>hamradio</span></a> <a href="https://techhub.social/tags/hamr" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>hamr</span></a> <a href="https://techhub.social/tags/antennaengineering" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>antennaengineering</span></a> <a href="https://techhub.social/tags/emso" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>emso</span></a></p>