Build my own Discone (Imgur album in text)

[shardsofaperture]

As I've gained more equipment (now an SDR which I use for ADS-B and ACARS) and my pro 2006 for NOAA APT i've started experimenting more with building my own antennas (Such as a double cross for APT).

Well, I decided I wanted to do a little more, get a little bit more technical. I looked at the off the shelf stuff and thought that I should be able to build something for about as much as a new diamond, plus building teaches me more, and it half the fun.



**ABOUT THE ANTENNA**

I have all the images hosted in this album here >> Discone Antenna - Imgur


I created the antenna out of 6061 Aluminum round stock and threaded 3/8" rods.

* The disc is 1.50" and the cone is 2.25" stock taperd to 60 degrees. * Each disc element is 18" and the cone rods are 24"
* The insulator is made from a 1/4 fernco cap which I cut off the sides (plumbing fitting). 16 elements in each section.
* A uhf connector is run though the bottom of the cone and it's put together with machine screws, running from disc to cone section but insulated by flexible pvc inserts to keep it from shorting.

I plan on painting the entire unit to seal it best I can, as well as locktight threads in place. Discones are very high gain from what I can gather, and I know this needs tweaked. I plugged in to my SDR receiver just to get a base and it rose the noise floor quite a bit more than it is right now with my RS discone on it, but that is possible because it's in the house instead of outside. Also, I plugged in my pro 106 scanner, the p25 from the local PD came in much more clear than I've heard it before.

I know the insulator has a large effect on the so that's something I'm considering modifying before it goes to it's final place on my roof. Additionally I'm thinking about adding a center element on top tuned for CB. I'm also not sure if the disc being .75" smaller than the cone is an issue. Thoughts?


TL;DR - Aspiring rf engineer (ha) builds first major antenna, wanted to share.

 

[davenlr]

Nice project. I also want to make one (or modify my radio shack one) to get rid of the hollow aluminum cone elements using threaded stainless steel rods that wont wobble.

 

[prcguy]

A Discone disc to cone ratio is about .67 to .70 diameter of the disc to the cone length, so for 24" long cone elements the disc diameter would be around 16.5". That would make each disc element about 8.25" minus the diameter of the hub, and so on.

Your 24" long cone elements are probably a good length and the hub will add a little to that and it should work down to the bottom of the air band voice frequencies at 116MHz. With the typical 8:1 frequency range of a Discone that should put you in the 900MHz range at the top end.

The distance between the disc and cone can affect the match, so hopefully you can vary that distance and tune it with an antenna analyzer.

Otherwise, nice work!
prcguy

 

[shardsofaperture]

A Discone disc to cone ratio is about .67 to .70 diameter of the disc to the cone length, so for 24" long cone elements the disc diameter would be around 16.5". That would make each disc element about 8.25" minus the diameter of the hub, and so on.

Ah, so the disc elements I have 18" each element to should actually be cut down quite a bit then. It should be around 18 inches from tip to tip across the center including the actual disc not 36 inches across plus hub tip to tip? Also that 116-900ish is what I was going for so I'm excited for some confirmation!

 

[br0adband]

Not sure this will help but, it's a simple discone "calculator" I found and it seems to be fairly accurate compared to some other such dimension calculators for making discones. Using 116 MHz as the low end frequency, this is what it shows. I'm guessing that with the 24" cone radials you've got made + the spacing of the cone mount gives a tiny bit extra to get it closer to that ~116 MHz and it should work great in that respect (it's not like it's totally deaf and cutoff at whatever frequency it is actually functioning at for the low end.

As prc mentioned, the disc radials you've got are a wee bit too long considering the size of the disc mount you've created. The diameter of the disc (visualized as just that, a flat dish or plate) needs a diameter dictated by the formula you used to calculate the lengths in the first place.

The main issue with the picture below (meaning the actual diagram itself, the thicker/thickest black lines, is that it's not to the proper scale of the dimensions either. Obviously if the skirt diameter is 28.2 inches then the disc diameter at the top being 19.75 inches means the very bottom line and the very top line as drawn shouldn't be the same width. Also, it doesn't mean you need to go redo the cone radials and replace them with longer ones, no, they'll be fine just as they are. It's a good looking discone for sure and gives me some ideas on how to get the one I intend to build going here at some point this summer - a 16 radial one built from scratch myself.

You'll figure it out. If you want the discone calculator (tiny little thing but it's proven useful to me), you can get it from the author's site at:

Antenna, VE3SQB Ham radio antenna design software programs and sstv,qha, amateur radio,sstv,antennas,j-pole,quad,dipole,discone,antenna program, super scanner, pdl, omni, rhcp, bazooka, coaxial dipole,skyhopper)

and click the yellow diagram of a Discone for the download (discone2002.exe, 260KB)

 

[shardsofaperture]

Thank you a bunch! That gives me a great overview of what I need to do. I used an online calculator somewhere with 118 mhz as the cutoff and tried going a little over, but didn't take into consideration the hub as part of the total length of the elements so it worked out pretty well to drop the cutoff a little bit. Basically what I'm going to do is trim those disc elements a bit to match up best I can to what my total cone element length is and then work with the insulator spacing and see how it functions!

Glad it gave you some ideas! I wanted to document how I did it best I could because when I wanted to build I saw some plans (one using flat stock comes to mind) but none really a more detailed walk through of the build. I was hoping if I did it correctly it could be a guide to others. As far as 16 elements goes, I looked at the off the shelf 8 element models and thought I could build better for less (my total cost is right around $100 plus my time, but what I've learned is worth more than that!

 

[br0adband]

Since a true discone would literally be a solid disc and cone (with the cone being hollow, of course, but the same thickness as the disc) it stands to reason that the more radial elements you can fit into a given design the better. 16 radials should definitely outperform anything with less, 12, 8, doesn't matter: more radials more signal to work with on the receiver end (given you have great low loss cabling or a short cable run to the receiver, or some high quality RF amp inline to counteract any potential line loss).

Regardess, I'm sure it'll pull in stuff like mad when you get it installed/mounted.

 

[shardsofaperture]

That's my hope. With my psr 300 (or was it 320) I was able to pull North Canton from Youngstown. I moved to a different house and I have terrible fm interference from a local tower so an fm trap helps a ton but my distance is cut down quite a bit using my new pro 106. I hope a new antenna improves it quite a bit. I'll be using Belden 50 ohm to run this when all is said and done

 

[LtDoc]

Just something to keep in mind...
A discone has no dBd gain. It does have some dBi gain, but that's not saying anything, all antennas have that (almost). So if you are looking for distance, or weak reception, try something else. A discone's primary benefit is it's usable frequency range. They certainly are a 'multi-band' antenna, but like all multi-band antennas they are only -usable- not optimized for every band.
- 'Doc

 

[prcguy]

All very true. However when comparing a Discone to another scanner antenna that is say tuned to the various public service bands like 155, 460, 850Mhz, a Discone can have 10 or as much as 20dB gain over the public service tuned antenna when receiving signals that are out of band like VHF or UHF mil-air.

For the typical Discones sold for scanner use the only real problem is stuff above 500MHz where the antenna pattern starts pointing upward and at 800MHz there is a null at the horizon, which is not so good.
prcguy

Just something to keep in mind...
A discone has no dBd gain. It does have some dBi gain, but that's not saying anything, all antennas have that (almost). So if you are looking for distance, or weak reception, try something else. A discone's primary benefit is it's usable frequency range. They certainly are a 'multi-band' antenna, but like all multi-band antennas they are only -usable- not optimized for every band.
- 'Doc

 

[KD4UXQ]

Discone antennas do not have gain

Discone antennas do not have any gain over antennas designed for a specific frequency, unless that antenna is poorly designed. Tuned vertical omnidirectional antennas will have unity gain in reference to a dipole, or some gain over a dipole if designed as a vertical configured to flatten out the omnidirectional pattern and concentrate more of the radiated energy broadside to the antenna and less energy off of it's ends. Dipoles will have radiation patterns closest to a sphere, and verticals with gain will be more like a flattened ball extending the radiation out instead of up or down.

If I too had such a machine shop I might try building one, but I find many on eBay for $50 or less made with good materials so I would except myself and buy one instead. In fact I was just looking at one and wondering what the real useful bandwidth is for aviation monitoring. They list the length of the radials so I should be able to back out the design limits with a discone calculator.

The advantage of Discone antennas is that they are very broadbanded. When speaking about antenna gain there are 2 references. dbm and dbi. Unless otherwise stated, antenna gain is in reference to a dipole antenna which is considered to have unity gain or 0dbm in reference to itself. dbi referes to a thoretical antenna called an isotropic radiator, therefore dbi. An isotropic radiator has a uniform radiation patern all the way around it in the shape of a sphere. No such physical antenna exists. Gain in dbm is the measured gain in db over a physical reference antenna which is usually a dipole in general terms of antenna gain.

Isotropic radiators have a gain of 2.15 dbi in reference to a physical dipole which is unity gain or 0dbm. So if you see an antenna advertised as having a gain of 2.15 dbi then it is actually a unity gain antenna in reference to a dipole antenna.

Discones will not have a flat gain in reference to a single dipole over the entire advertised bandwidth, but they likely will be nearest to unity gain overall or much less depending on their exact design and whether they are advertised to cover a bandwidth exceeding a given negative gain. Depending on advertised bandwidth they could be 0dbm to -6dbm or as low as 0dbm to -24dbm.

This manufacturerer has advertised the bandwidth of this discone in a respectable limit of a VSWR of 1.5:1 for a bandwidth of 118-400 MHZ. Aviation Band Antenna, AWD-118-400, 118 - 400 MHz., 2 dBi. Gain - Antenna Experts Notice that the gain is advertised as 2dbi which means it is essentially unity gain and its advertised bandwidth is much less than the advertised bandwidth of the discones sold as scanner antennas with a bandwidth of 25-1300MHZ. Sure they will receive over that entire band, but gain will be a great deal lower on the extreme limits of the band. Having an advertised overall SWR of 1.5:1 is not a measurement of a loss of gain as there is no direct relationship, but an atenna with an overall VSWR of 1.5:1 is usually pretty flat in overall gain. It may be about a negative 1dbm gain on the band edges.