Identify this antenna - Page 1
Forum section: electronics / rf-microwave

author: mark03
author_rank: Frequent Contributor
timestamp: June 07, 2020, 04:50:24 am
content: I took a stroll on some new trails adjacent to our local FEMA site today. Their HF antenna farm has always intrigued me but I've never had this view before (through the back fence, on the other side of the complex from the street). Multiple LPDA beams, including one with a section of tower for the horizontal boom. Then, this thing:The shape is similar to ahex beam, except there are a lot more wires, and the loop elements that I can see appear parasitic rather than driven. The overall appearance is of a giant funnel. There are at least 27 loops, spaced widely at the top of the funnel, becoming closer and closer together at the bottom. Then at the bottom there is some kind of transformer (?) inside the tower.Anyone know what this is? There is a loop analog to the log-periodic dipole array, but its loop elements are all driven, with alternating polarities like the LPDA. I don't see anything like that here.EDIT: Now that I look at it more carefully, it may actually be a spiral, possibly interleaved, and not disconnected horizontal loops after all. It's difficult to trace a single wire visually all the way around the tower, but it looks to me like it doesn't come back to the same place.

author: cdev
author_rank: Super Contributor
timestamp: June 07, 2020, 10:51:03 pm
content: Could you annotate the picture pointing out where these structures are?Looking at the picture I see a feedline that to me looks like it might be part of arhombicantenna farm? I could point some out on Google Earth from space. Rhombics are very high gain broadband traveling wave HF antennas.

author: Lord of nothing
author_rank: Super Contributor
timestamp: June 07, 2020, 11:00:21 pm
content: You can check if its Aircraft related. There are beacon out there. Baybe that is one?

author: mark03
author_rank: Frequent Contributor
timestamp: June 07, 2020, 11:18:36 pm
content: Quote from: cdev on June 07, 2020, 10:51:03 pmSorry, I opened it in an editor just now to try, then gave up. There are so many stacked hexagonal loops (at least 27) that it's very difficult to trace any one of them around all six sides on a 2D photo.However, the loops are very clearly hexagonaland also clearly separate, stacked one above the other like a wireframe version of a giant funnel which opens wider and wider as you move up from what is presumably the feedpoint (where that transformer sits inside the tower truss). If you're not seeing that, you probably didn't expand the thumbnail all the way. In Firefox, clicking on the thumbnail results in a "fit to screen" sized image. You have to click that image one more time to get 1:1 zoom.EDIT: See update at the top... I think it may actually be a giant spiral of some kind.It doesn't look like a rhombic to me.Very unlikely to be a beacon or aviation related. The site is run by our Federal Emergency Management Agency (FEMA). I think all the RF stuff is for emergency/disaster backup comms. Or part of the new world order, if you're into conspiracy theories

author: mark03
author_rank: Frequent Contributor
timestamp: June 07, 2020, 11:35:31 pm
content: Aha! "Spiral" was the magic keyword. Looks like a "conical logarithmic spiral":http://www.radiomanual.info/schemi/Surplus_Accessories/ASC_3000_Spira-cone_antenna_broch.pdfI would be very curious to know if similar antennas are in use by amateurs. It is not exactly compact, but the single-support design would seem to make it quite attractive as a back-yard all-band antenna. I wonder what drawbacks there are apart from the size.

author: cdev
author_rank: Super Contributor
timestamp: June 08, 2020, 01:33:03 am
content: Interesting, thank you for sharing this.Quote from: mark03 on June 07, 2020, 11:35:31 pm

author: mark03
author_rank: Frequent Contributor
timestamp: June 08, 2020, 01:57:45 am
content: I thought this part was interesting (from the linked pdf):QuoteAt first I was like:but the explanation at least sounds plausible:QuoteWeird, I've never heard anything like this before.

author: cdev
author_rank: Super Contributor
timestamp: June 08, 2020, 02:51:15 am
content: Circular polaization has very real advantages when it comes to immunity from fading.

author: mark03
author_rank: Frequent Contributor
timestamp: June 08, 2020, 04:57:17 am
content: Quote from: cdev on June 08, 2020, 02:51:15 amYeah, that part I get. It was the thing with needing clockwise in one hemisphere and counter-clockwise in the other hemisphere that was startling. At first blush it sounds like the urban legend that toilets flush one way in the northern hemisphere and another way down under, due to coriolis force