{"id":42122,"date":"2020-02-27T06:00:49","date_gmt":"2020-02-27T10:00:49","guid":{"rendered":"https:\/\/swling.com\/blog\/?p=42122"},"modified":"2020-02-22T07:56:45","modified_gmt":"2020-02-22T11:56:45","slug":"from-the-cia-archives-an-hf-spaced-loop-antenna-for-direction-finding","status":"publish","type":"post","link":"https:\/\/swling.com\/blog\/2020\/02\/from-the-cia-archives-an-hf-spaced-loop-antenna-for-direction-finding\/","title":{"rendered":"From the CIA Archives: An HF Spaced Loop Antenna for Direction Finding"},"content":{"rendered":"

\"\"<\/a><\/p>\n

Many thanks to SWLing Post<\/em> contributor, Grayhat, who shares the following:<\/p>\n

I stumbled upon a document I found in the “CIA reading room”:<\/p>\n

https:\/\/www.cia.gov\/library\/readingroom\/docs\/CIA-RDP76-00451R000200010013-1.pdf<\/a><\/p>\n

In short, the document, titled “HF SPACED LOOP ANTENNA” and dating back to 1967, describes the design of a “direction finding” antenna used to find an HF transmitter. The document describes a number of setups tried during initial experimentation and then explains the reasons for the selection of what’s called a “Coaxial spaced loop in vertical opposition”, basically two vertical loops connected in anti-phase; the document describes in detail the loop and some circuitry used with it, but the most interesting section is the one which deals with radiation lobes. if you look at that you’ll suddenly realize using such a configuration, the resulting antenna acts as a huge dipole with extremely deep nulls !!<\/p><\/blockquote>\n

Fascinating! Thank you, Grayhat.<\/p>\n

And, honestly, there’s enough material in that CIA document to please the most demanding of antenna nerds! Most impressive. I’m curious if anyone has ever built one of these. If so, please comment with your experience!<\/p>\n","protected":false},"excerpt":{"rendered":"

Many thanks to SWLing Post contributor, Grayhat, who shares the following: I stumbled upon a document I found in the “CIA reading room”: https:\/\/www.cia.gov\/library\/readingroom\/docs\/CIA-RDP76-00451R000200010013-1.pdf In short, the document, titled “HF SPACED LOOP ANTENNA” and dating back to 1967, describes the design of a “direction finding” antenna used to find an HF transmitter. The document describes […]<\/p>\n","protected":false},"author":15,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[3607,3,43],"tags":[6452,8338,8337,7880,8339],"class_list":["post-42122","post","type-post","status-publish","format-standard","hentry","category-antennas","category-news","category-shortwave-radio","tag-cia","tag-df-antennas","tag-direction-finding","tag-grayhat","tag-hf-spaced-loop-antenna"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/pn3uc-aXo","jetpack-related-posts":[{"id":43511,"url":"https:\/\/swling.com\/blog\/2020\/05\/guest-post-kk5jys-porch-loop-receiving-antenna\/","url_meta":{"origin":42122,"position":0},"title":"Guest Post: KK5JY’s Porch Loop Receiving Antenna","author":"Thomas","date":"May 25, 2020","format":false,"excerpt":"Many thanks to Matt Roberts (KK5JY) who has kindly given me permission to re-post the following article he recently published on his website KK5JY.net. Many thanks to SWLing Post contributor, Grayhat, for the tip! \u00a0Note: The Porch Loop project below is a re-configured Small Receiving Loop (SRL) antenna. For SRL\u2026","rel":"","context":"In "Antennas"","block_context":{"text":"Antennas","link":"https:\/\/swling.com\/blog\/category\/antennas\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2020\/05\/40m-10h-Porch-Loop-results.png?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2020\/05\/40m-10h-Porch-Loop-results.png?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2020\/05\/40m-10h-Porch-Loop-results.png?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2020\/05\/40m-10h-Porch-Loop-results.png?resize=700%2C400&ssl=1 2x"},"classes":[]},{"id":40778,"url":"https:\/\/swling.com\/blog\/2019\/11\/coastal-dxing-with-the-airspy-hf-discovery-and-a-homebrew-passive-loop-antenna\/","url_meta":{"origin":42122,"position":1},"title":"Coastal DXing with the AirSpy HF+ Discovery and a homebrew passive loop antenna","author":"Thomas","date":"November 25, 2019","format":false,"excerpt":"Last week, we packed the car and headed to coast of South Carolina. The trip was a bit impromptu but through the creative use of hotel points, we scored a two bedroom ocean front unit with a fantastic little balcony. The vacation gave me an excuse to test the new\u2026","rel":"","context":"In "AM"","block_context":{"text":"AM","link":"https:\/\/swling.com\/blog\/category\/am\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2019\/11\/Beach-Ocean.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2019\/11\/Beach-Ocean.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2019\/11\/Beach-Ocean.jpg?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2019\/11\/Beach-Ocean.jpg?resize=700%2C400&ssl=1 2x"},"classes":[]},{"id":53845,"url":"https:\/\/swling.com\/blog\/2022\/08\/small-unidirectional-loop-antenna-sula-part-3-questions-answers\/","url_meta":{"origin":42122,"position":2},"title":"Small Unidirectional Loop Antenna (SULA) Part 3: Questions & Answers","author":"13dka","date":"August 31, 2022","format":false,"excerpt":"Many thanks to\u00a0SWLing Post\u00a0contributor extraordinaire,\u00a013dka, who brings us Part Two of a three part series about the new SULA homebrew antenna project.\u00a0This first article describes this affordable antenna and demonstrates its unique reception properties. The second article focuses on construction notes. This third and final article will essentially be a\u2026","rel":"","context":"In "Antennas"","block_context":{"text":"Antennas","link":"https:\/\/swling.com\/blog\/category\/antennas\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2022\/08\/SULA_alt_LNA_pos-scaled.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2022\/08\/SULA_alt_LNA_pos-scaled.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2022\/08\/SULA_alt_LNA_pos-scaled.jpg?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2022\/08\/SULA_alt_LNA_pos-scaled.jpg?resize=700%2C400&ssl=1 2x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2022\/08\/SULA_alt_LNA_pos-scaled.jpg?resize=1050%2C600&ssl=1 3x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2022\/08\/SULA_alt_LNA_pos-scaled.jpg?resize=1400%2C800&ssl=1 4x"},"classes":[]},{"id":49992,"url":"https:\/\/swling.com\/blog\/2021\/08\/grayhats-ncpl-noise-cancelling-passive-loop-antenna-tweak\/","url_meta":{"origin":42122,"position":3},"title":"Grayhat’s NCPL (Noise-Cancelling Passive Loop) antenna “tweak”","author":"Thomas","date":"August 15, 2021","format":false,"excerpt":"Many thanks to SWLing Post contributor, Grayhat, who shares the following modification he made to a Noise-Cancelling Passive Loop antenna last year. He's kindly allowed me to share his notes here, but apologized that at the time, he didn't take photos of the project along the way and recycled many\u2026","rel":"","context":"In "Antennas"","block_context":{"text":"Antennas","link":"https:\/\/swling.com\/blog\/category\/antennas\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2021\/08\/moebius.jpeg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2021\/08\/moebius.jpeg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2021\/08\/moebius.jpeg?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":61335,"url":"https:\/\/swling.com\/blog\/2024\/12\/here-we-go-loop-de-loop-in-which-your-humble-correspondent-tests-a-hypothesis\/","url_meta":{"origin":42122,"position":4},"title":"Here we go loop de loop . . . in which your humble correspondent tests a hypothesis","author":"Jock Elliott","date":"December 10, 2024","format":false,"excerpt":"By Jock Elliott, KB2GOM It was this post by Alan -- https:\/\/swling.com\/blog\/2024\/11\/bbc-rd-how-to-test-a-loop-antenna\/ -- that set me off. After downloading the BBC report on how to test a loop antenna, I noticed the following in it: \u201cPolarisation Illustrations of the antenna usually show it mounted upright; that is, with its axis\u2026","rel":"","context":"In "Antennas"","block_context":{"text":"Antennas","link":"https:\/\/swling.com\/blog\/category\/antennas\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2024\/12\/MW-amp-001-1024x431.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2024\/12\/MW-amp-001-1024x431.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2024\/12\/MW-amp-001-1024x431.jpg?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":42582,"url":"https:\/\/swling.com\/blog\/2020\/03\/the-airspy-youloop-is-a-freaking-brilliant-passive-loop-antenna\/","url_meta":{"origin":42122,"position":5},"title":"The Airspy Youloop is a freaking brilliant passive loop antenna","author":"Thomas","date":"March 30, 2020","format":false,"excerpt":"Before I start talking Youloop, I have a little confession to make up front: At the Winter SWL Fest when I gave a presentation about Portable SDR DXing, not only did I give attendees the wrong name of the Airspy Youloop antenna, but I also configured it incorrectly, hence the\u2026","rel":"","context":"In "Antennas"","block_context":{"text":"Antennas","link":"https:\/\/swling.com\/blog\/category\/antennas\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2020\/03\/Red-Oxx-Lil-Roy.jpeg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2020\/03\/Red-Oxx-Lil-Roy.jpeg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2020\/03\/Red-Oxx-Lil-Roy.jpeg?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2020\/03\/Red-Oxx-Lil-Roy.jpeg?resize=700%2C400&ssl=1 2x"},"classes":[]}],"jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/posts\/42122","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/users\/15"}],"replies":[{"embeddable":true,"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/comments?post=42122"}],"version-history":[{"count":0,"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/posts\/42122\/revisions"}],"wp:attachment":[{"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/media?parent=42122"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/categories?post=42122"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/tags?post=42122"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}