Attic antenna ideas, please - Page 1
Forum section: electronics / rf-microwave

author: tkamiya
author_rank: Super Contributor
timestamp: August 08, 2018, 11:03:32 pm
content: I want to set up VHF to SHF antenna in my attic. Receive only. My expectations are fairly basic. 0db gain is good enough. I'm going to use it for listening to AIR band traffic and WX fax satellite stuff. I currently have a discone and it works, but it is very cumbersome to maintain. Since I do not transmit, I'm wondering if any active antenna options are viable. Budget is about $300 or so.House is a standard American construction of the 80s. Attic to roof is about 1 meters (3 feet+) Supporting beam and struts everywhere. My lab is right under the area and I have short piece of hardline going to that area. (yes, I know it's an overkill)One of the ideas is active antennas used for TV reception.https://www.amazon.com/gp/product/B01MZ20Z2Z/ref=ox_sc_sfl_title_1?ie=UTF8&psc=1&smid=A26IMHULYH7PHYSpec wise, frequency coverage is not continuous but I am not sure it is filtered in some way. I like this Darth Vadar look..... yeah, coolness factor is high, too.If you have ideas, please share.I do not want an outside antenna as weather is unpredictable in central Florida.

author: cdev
author_rank: Super Contributor
timestamp: August 10, 2018, 10:48:00 pm
content: When you say "fax satellite stuff" do you mean NOAA APT sats? If you just need a wideband vertically polarized antenna for air band or land mobile, you could make any of a number of frequency independent broadband antennas which have proved themselves as solid performers. Very easy to make and good for almost any vertically polarized use over 130 or so MHz. Satellites are likely circularly polarized and you really should use a different kind of antenna entirely there. (which is less likely to be broadband, if you want good performance although its not absolutely impossible.) A discone has a higher angle or radiation than the antennas I am talking about but otherwise is similar in concept. Basically a flattened biconical antenna is a good choice with the gain being higher and more concentrated at the horizon as the width increases and the angle enclosed by the elements gets smaller. You should try to use either wire elements or solid metal triangles. You can make it using cardboard with a slot in the middle make four identical squares and cut a slot halfway up the middle so they can be pushed together. Solder it in the middle where the triangles all come to a point. The coax shield should be joined to the bottom part.

author: BrianHG
author_rank: Super Contributor
timestamp: August 10, 2018, 11:02:54 pm
content: 30 years ago, I once did a full outdoor antenna, with directional rotation control, installation in their summer cottage attic. As far as I know, it is still there to this day. So, it is possible and the antenna operated the full range from VHF to the top UHF, though, after a really heavy snowfall, if it was layed thick on the roof, the UHF suffered a little. You should be able to do the same. Depending on your circumstanced, directional rotation may be need. Since we were receiving TV from 2 different states and a city in Montreal, the rotation was necessary.As for what's available today, I cannot help you, but, we used a 200$ radioshack antenna similar to the one I attached a picture of below.Also, to get ideas:https://www.google.ca/search

author: tkamiya
author_rank: Super Contributor
timestamp: August 10, 2018, 11:36:05 pm
content: Yes, TV antenna is looking awfully attractive, both to use them directly and as cheap source of parts. By WX FAX stuff, I mean NOAA weather fax. I don't know what's legally called but the frequency is around 137Mhz and I thought it used circular polarization.

author: cdev
author_rank: Super Contributor
timestamp: August 10, 2018, 11:51:49 pm
content: Is everything you want to listen to near to 137 MHz? I bet a tall/skinny quadrifilar helix (yes, circular polarization) cut for 137 MHz would do an okay job on the air band. But it wouldn't be optimal. I would set up two separate antennas, one for vertical (land mobile and general VHF/UHF air band, hams and public safety, at least a few feet away from your antenna for APT sats. (radios can interfere with one another)As far as a TV antenna, Horizontal polarization is used for VHF/UHF DX (ssb) sometimes. But a TV antenna isnt such a good antenna for that because they are optimized for the TV bands, which might not even be around that much longer (there is pressure on the industry to give up spectrum to the lucrative wireless industry) A log periodic would be with a rotator. But pricey. Read up on antenna theory and specifically polarization.

author: tkamiya
author_rank: Super Contributor
timestamp: August 11, 2018, 03:26:16 pm
content: I'm sorry, I forgot to add the broadband requirement.Big picture:My main interest is RF, so I want a "system" to cover the whole spectrum. MF to SHF. Of course, that's impossible. So for low bands, I have a random/long wire stretched across my property at very low height. It's basically invisible. Usefulness of that antenna cuts off at around 50Mhz. So I need another that covers from there and up to about 2GHz. It does not necessary need to be one signle antenna. But I have been using a discone. So, no, it's not 137Mhz or near there but the whole spectrum.Further, since it is an attic, there are other structures. Rotating is not possible and not necessary. Omni-directional is desired. Gain expectation is NONE. 0dBi or 0dBd is just fine.Thanks again.By the way, since this is for receive only, active antenna would be great too.

author: cdev
author_rank: Super Contributor
timestamp: August 11, 2018, 08:03:36 pm
content: I would make a bi-conical antenna out of wire of the largest size practicable in the space you have. If you want a 50 ohm impedance biconical the angle of the elements in going to be narrower than 90 degrees. Think of a cylinder roughly the proportions of an old hatbox, but a bit taller. The feedpoint is going to be the exact center. The top and bottom elements will come to a point. The more elements you can include the better it will work up to a point. Eight elements is probably enough, six or four not enough. If you want to use an LNA, you could put it right at the feed point or a bit lower down. You could use semirigid coax which would be stiff and easy to manage. If you do that and use a flat metal bottom element the shield should be soldered to the bottom element (infinite balun) You should also decouple the feed-line from the antenna with RF chokes (clip on ferrites work well)The antenna doesn't have to be wire, you could use a flat conductor like aluminum foil. (what would work best would be a combination of copper tape for the points where the soldering needs to happen and aluminum foil where it doesn't.) the capacitance will be so high it doesn't matter if two different materials are used there. When I say you could use a flat conductor also it doesnt need to be shaped into a cone, it can be flat like a folder paper christmas tree..