Useful information regarding antennas for SDR products.
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i've been looking for a low profile antenna system for HF. i live in an apartment building and HF antennas are a bit problematic because of the large lengths of wire required. so i've been searching for something that's an ok antenna (doesn't have to be perfect, but has to be better than a dipole made of magnet wire hidden in the bushes 3 feet off the ground.... very poor s/n ratio). i happened across some articles about a rather odd type of antenna, and after seeing pictures in some of the articles, i'm thinking "we're allowed to have deck furniture on out the patio... one of these antennas might actually work, and i can make it look like a table..." to see what i mean about a "table" form factor, look here... http://www.kn9b.us/spiral-dipole i have two antenna tuners, an MFJ-941E, which i've been using with my magnet wire dipole and both an SDRplay and a grundig radio, the other tuner, sitting idle is an MFJ-989C (which is jokingly referred to as a "doorknob" tuner, because you can supposedly tune a doorknob as an antenna with it). after seeing another article about using a spiral coil as a ground plane ( http://www.n0lx.com/petlowany_ground.html ) i decided to test the idea with an MFJ multiband whip i have (MFJ-1899T) just to see how effective it is. i bought the antenna after moving into the apartment, looking for a low profile antenna, but even this antenna requires a ground counterpoise, which again means long wires. so i went to a thrift store and picked up a bunch of speaker wire that they happened to have (i've seen spools of various types of wire in the tool section there in the past, and that's where i bought the "doorknob tuner"). i grabbed a large square of cardboard and taped a spiral of about 50 feet of wire to it, stood a cardboard mailing tube up in the middle and mounted the whip on top of it, and connected the BNC ground to the center of the spiral. the whip has a bunch of jumper connections for bypassing portions of the loading coil, this is what makes it a multiband whip. with no jumper, the whole loading coil is in circuit, and i tested it with an antenna analyzer, and got a broad null (actually only dipped down to 2.5:1 SWR) at about 4Mhz, which using the big antenna tuner would be a piece of cake to match for the 80 meter band. with the jumper going from position 0 to 1, i got a perfect match at 7.6Mhz (the whip was up so high on the mailing tube that the telescopic section was only half extended and touching the ceiling), again very manageable with a tuner. with the jumper from 0-2, i got a perfect match at 15Mhz, and another one around 21Mhz. remember, this was using a random length (no calculations were done, wire length was selected by "how much wire do i have?", and the spiral didn't even look nice or evenly spaced). jumper from 0-3 yielded a match at 26Mhz, and i tried the jumper from 0-4, and got a broad 2.5:1 dip around 29Mhz. i will start gathering materials for my "deck table stealth dipole" and will probably use a design similar to KN9B's off-center fed multibander (with some modification to see if the 160M band is doable) with the "doorknob tuner" the whole HF spectrum should be easy, especially with the SDRplay's more than adequate gain. what i need to start with is a plastic deck table with a center post... as long as the management here doesn't notice the coax cable going to the table... i'll post details when it's done.
Last edited by N4TKO on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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