Introduction: Discone

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This guide will teach you how to make a durable discone antenna aimed at the rtlsdr crowd. I will show you how to build a complete discone antenna with just materials from home depot and radioshack in one afternoon for about 100 to 150 dollars.

This antenna is going to be massive, so you better get some friends to help you mount it and have the room on your roof (and the permission) to do it. Granted it may not look that great and your wife might behead you, so use reason before attempting this project!

Step 1: Purpose


The discone is a popular design for the RTLSDR but i wasn't happy with most of the designs spread around on the message boards as they were quick and dirty solutions that wouldn't last long at all mounted outside (as a proper antenna should be). I used this discone calculator to make my design. I have found that on the net most call for a more compact design starting at 100-150 mhz but I've found that my larger design (55mhz) may work better for wider ranges in the low vhf as well as HF. Although the frequency cut off for discones is pretty sharp I was surprised when I started to pickup CB radio in the 27mhz range even though the preamp itself says its cut off is exactly 50mhz!

Step 2: Construction and Tools

My design calls for eight five foot radials. Originally i tried to use 16 elements but it became difficult mechanically to mount all of these properly so i settled for 8, if a full discone is really what you are going after i would recommend finding some thin metal mesh (like used for old sat dishes) and weave it between the radials. But for me just eight radials is fine.

Tools

  • adjustable butane torch and fuel (you need this to solder, don't even think about using an iron)
  • two c clamps (to clamp pieces while soldering)
  • dremel tool with cut off wheel, sanding wheel and a fine drill bit
  • heavy pliers
  • small hammer
  • sharpie
  • heavy wire cutter and wire stripper
  • safety goggles and gloves (do yourself a favor please)

Step 3: Shopping List

Radioshack (and optional amazon) parts

  • 6$ large roll of solder (get one that has some thickness to it not the thin stuff for electronics, also make sure its not the lead free stuff trust me)
  • 4$ so-239 chassis plug plug (radioshack quality isn't the best, I'm using it now and it seems to be fine but if you can buy the better one)
  • 5$ pl259 to f adapter (you may not be able to find this so instead get a pl-259 plug and crimp a short length of rg6 to it)
  • 8$ rtlsdr stick (optional, if you have one already or have another radio to use)
  • 38$ inline preamp

    (optional but strongly recommended!) unless your receiver is extremely close (possible with dongles but durability and grounding from interference is questionable) you really need an amp as the discone has no gain and you will lose most of your signal, unless you go crazy with expensive low loss coax. However since this attaches right to the top of the antenna so it maybe too thick for the support pipe so you will have to gauge which diameter of pipe you need and perhaps file down the edges of the amp to make it fit

    *the one i bought is round and fits in perfectly so shop around and keep this in mind when choosing you mast supports

  • RG-6 quad shield coax (this can be terribly overpriced so look around for a high quality one and try to figure out exactly how much you need)
  • 16$ f to f antenna lightning arrestor

    (optional but recommended!)

    if the antenna is going to be the highest part of your structure you really need to be super careful with lighting, mine is way above my roof on a mountain top and i get a lot of lightning in the summer months. this really is a necessary thing but depending on your situation you maybe able to compromise

  • 5$ fm trap (this is optional) but where i live i have an fm station broadcasting so close that i don't even need an antenna on my home theater receiver, so this cuts out the high power 88-108mhz to prevent over driving your signal but its not strictly necessary

approx 30-60$

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Homedepot parts

  • 1$ two copper plumbing caps (these maybe hard to find but keep looking you want the paper thin flat ones)
  • 12 or 10 guage solid copper electrical wire (romex is good and straight but buy the shortest length you can measuring for the disc length and number of radials)
  • 1$ pvc cap and a pvc threaded bushing (make sure these two fit together and have enough room for your copper disc inside!)
  • 15$ nine 5 foot shark bite thin flex pvc pipes (these give the wire weather proofing and help support it a bit from flexing)
  • 4$ bundle of 6 foot flag wires (find the bundle of wires used for gardening, these are 6 feet long and they come in large packs for a couple of bucks and make great antennas!)
  • 12$ two 3/4 inch floor flanges (the main supports for the whole antenna you want to make sure you get ones that fit the pipe you will be using and keep in mind how the amp will fit inside)
  • 2$ four bolts/nuts for flanges (i had to drill the holes in my flanges out to fit the bolts properly, keep this in mind)
  • 3/4 inch threaded pipe (you may already have some lying around, depending on how you will attach it to your mast you may want to buy a longer length)
  • 2$ two large flat washers, two small flat washers (these must be as thin as possible if you hope to be able to screw your coax cable to them)
  • 9$ roll of rubber mastic tape 2228 type (necessary to weatherproof the connections)
  • 6$ epoxy glue (to permanently fix the top pvc parts together)

approx 70-80$

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you may need to buy a ground rod (a copper one costs about 18 dollars) and mast clamps (4 dollars each) and grounding wire (#8 stranded copper is good but don't exceed 20 feet) as well depending on your situation

Step 4: Cone

Prepare the washers

  1. polish the two large flat washers on one side with a dremel tool sander to aid in soldering
  2. draw eight lines for positioning the wires
  3. flatten the ends of flag wires with a hammer
  4. use two c clamps to secure to two large flat washers against the wires

Solder the wires *IMPORTANT* when soldering always heat the thing you are soldering to before heating the solder itself or else it will spill out!

  1. solder one wire at a time, heat the washer with the torch for 20 seconds before feeding solder between them
  2. once the eight wires are secure sand the outside of the washers with the dremel tool
  3. take a pair of heavy pliers and crimp the middle of the washers down so the whole thing is flat/straight
  4. take two more washers (the diameter of the so259 plug) clamp them down with the c clamps solder around the washers but be careful not to undo the flag wire's solder, (you may have to re-solder them as you go around and make sure everything stays even)

  5. Now take the flanges and place the wire washer assembly in the middle, mark where the bolts meet the flange and cut those notches out of the washers with the dremel tool so the bolts can slide all the way down.

Then once everything is cooled down. Do a test by flexing each wire. Make sure everything is secure and even.

Prepare the amp and plug assembly

  1. Take your so-259 plug and solder a 4 inch copper wire to the center
  2. place the plug inside the washer assembly and secure it with the UHF connector to F connector adapter as far as it goes. (if it doesn't go in enough take it apart and use the pliers to crimp the washers together more but don't deform them!)
  3. now screw in the inline amp to the f connector (if you are not using an amp just screw a short length of coax)
  4. carefully wrap the plug assembly once with mastic tape (you need to stretch it tightly or else it wont fit inside the flange!).
  5. leave an f connector bare at the bottom (i made the mistake of pre-attaching the whole coax wire and it was a total bitch to untangle while trying to mount!)

Flange assembly

  1. Slide the preamp attached to the washer down the flange
  2. Slip the pvc flex pipes, one over each end of wire right up to the washer
  3. Bend the wire at the ends to keep them from falling back out
  4. Secure the flanges down with bolts so it will press down and secure both the washers and the pvc pipes
  5. measure and cut the ends of the cone radials per the frequency calculation

Step 5: Disc

Prepare wires

measure and cut your electrical wires for each length from the center of the copper disc
strip the wires bare and straighten them out

Create the disc

  1. cut four notches in copper caps then push both together
  2. carefully clamp down to table without deforming edges
  3. solder both together all around edge
  4. polish the edge with dremel sander bit
  5. draw eight lines from center, continue lines down each side
  6. drill hole in the center of the disc through both sides
  7. drill holes eight holes all around marked dead center
  8. solder wires into each hole feeding ends to the center of the disc

Cut the cap and thread

Draw lines on pvc cap lined up for each wire then cut the lines with dremel tool down each side file inside until the copper disc fits perfectly in the top of the cap

*IMPORTANT* depending on what frequency you calculated you MUST keep in mind the EXACT distance between the flange and the top disc!

  1. measure the distance between the copper disc and the flange
  2. cut the threaded pvc bushing down to meet this distance
  3. sand the end of the bushing to complete a tight fit without sharp edges
  4. cut the cap enough to cover the bushing but not extend down too far past it
  5. drill the holes in the cap wide enough to fit the pvc pipes
  6. cut the pvc pipes to the same length as the wire before attaching

Step 6: Final Assembly and Conclusion

At this point the bottom cone and the top disc should be complete

  1. Screw the pvc threaded bushing to the top flange,
  2. Place the copper disc on the top copper wire
  3. Solder and cut this wire,
  4. Fit the cap and bushing
  5. Attach the flex pipes to the cap
  6. epoxy the cap bushing and flex pipe together

*once you are sure everything is right, you may want to fill the gap between the flanges for weather proofing

Mast mounting

Depending on the thickness of your mast you will need to adapt the bottom flange diameter with a bushing

  1. Feed the rest of the coax cable down the mast pole
  2. Screw the cable up to the bottom connector in the antenna flange
  3. Twist the antenna onto the mast

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I use two 15 foot lengths of well pipe threaded 1 inch with one end driven three feet into the ground with cement filler. Then with two riser bars I can adjust the height of one pipe against the other to raise and lower my antenna. Since the mast runs through the bottom of my elevated deck horizontal swaying is accounted for. If its a free standing mast you need to anchor rods into the ground and run guy wire depending on your situation I would seek advice.

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Grounding

Grounding is very important, a whole other topic in itself. It depends on your individual situation, however i will share what I did for mine.

At the bottom of the pipe (where the coax cable feeds out of) I have a grounding clamp on the mast with an 8 gauge stranded wire going to my ground rod. the coax itself is then attached to a surge arrestor with the ground wire of this going to the rod with a 10 gauge stranded wire

Both wires then attach to an inter-system bonding clamp with a 30 foot 4 gauge aluminum wire (always use cu to au connectors) feeding back to my main panel neutral/ground. this is important to bond these two right when you use a separate rod, its strongly advised to mount the giant antenna on the other side of your house away from the incoming power line for obvious reasons!

Final thoughts

My design omits a bottom support for the radials, I'm probably going to create a revision to this that might include another set of flanges and a cross support to hold them up as they look terribly bent right now. The performance doesn't seem to suffer too much but I'm planning to do it none the less.

So far I've been able to pickup clear signals from 25mhz to about 900mhz. My dongle caps out at 1.7ghz and i think i saw a few signals from cellphones up there but nothing remarkable.

I hope I was clear enough with my instructable and this helps someone out there. This is the first time I've completed one so give me some feedback!