I have seen several webpages describing home construction of discone antennae from inexpensive materials.
Wire version:
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Sheet version:
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Combined with the Mini Circuits RF amp below, I would like to cover the range of 50MHz to 2GHz as a temporary (indoor) input antenna for my HP8495E spectrum analyzer.
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The sheet version of the antenna would be easier for me to construct, i.e. out of foil covered poster board. Or an aluminum disc for the top plate and flyscreen for the cone.
On 09/03/14 23.34, snipped-for-privacy@keystone.com wrote: ...
Hi Mark
You could read these papers:
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Oak Ridge National Laboratory: Type: Discone antenna 850-6000 MHz:
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Quote: "... At 2 GHz, the gain of the antenna varied nearly 10 dB as the antenna
... Next, after a variety of ferrites were tried, the cavity between the cone and the base was filled with Carbon Foam absorber, which greatly reduced feedline coupling. Modified as described, the antenna was tested, and the following data generated. ..."
Nice collection of dissimilar metals. How long do you expect such an antenna to survive outside? With what looks like about 1/2" of exposed center conductor on the coax, I would not expect 2GHz performance to be very good.
I couldn't find an HP8495E spectrum analyzer. As near as I can tell, it's an attenuator, not a spectrum analyzer. What's the real number so I can lookup the overload specs on the front end?
I've done exactly that with my antique HP141T and HP8555A front end. Instead of a $130 MCL amplifier, I used a $20 TV antenna amplifier. My guess(tm) is that the performance is similar since both probably use a simple MMIC amplifier. 3.5dB NF is quite good for such a device and quite suitable for reducing the effects of the coaxial cable losses.
However, you're going to have a problem depending on your location. With 17dB of additional gain, the front end of your spectrum analyzer is not going to appreciate the overload from nearby strong signals. In my area, I'm line of sight to several mountain tops infested with various RF sources. Trying to see weak signals under all the other junk is difficult. Add intermod and mixes to the exercise. Indoors, you're probably ok, but outdoors might be a problem.
Lots of examples to follow:
I like to build the discone out of copper roofing flashing. Solder the ends of the cone together. Silver plate, electroless silver, or just shine up the copper and spray paint it with clear acrylic. Most of mine are on microwave frequencies and made with semi-rigid coax. If you want to use aluminum, use aluminum rivets, Pop-Rivets, or have the local HVAC sheet shop make you a proper seam. I don't have any examples handy of my butchery, but mine method look fairly close to this design:
However, that's if you want the antenna to last. When I'm in a hurry, I've built them using a stryofoam cone covered with aluminum foil, and a pie tin or aluminum pizza pan for the disc. Aluminum duct tape for assembly. The difficult part is making the connection between the copper coax cable pigtail and the aluminum parts.
When calculating the dimensions: the only semi-critical dimension is the spacing between the cone and the disc. You'll need a sweeper and some kind of impedance measuring device (return loss bridge) in order to get it perfect. I have yet to get this right from the calculations and find it easier to tweak the distance adjustable using spacers. Don't use nylon washers at over about 100 MHz.
A big problem will be the size. A discone that works down to 50 Mhz will require a cone that is about 1.6 meters on the side and a disc with a 1.2 meter diameter. That's a rather large antenna. The way antenna vendors get around the size problem is to design it for a higher frequency, and then attach a lower frequency antenna to the center conductor. For example, the Diamond D130J that goes down to 25 MHz:
I won't offer any alternatives because I don't know your exact application. If you find yourself troubled by front end overload, intermod products, mixer artifacts, and broadband crap, you might consider using directional, narrow band antennas, or filters to help reduce the problem.
Good luck.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
My typo. It's an 8594E. Yes, please advise. I don't want to damage the front end.
It will be indoors as per my OP.
How about silver loaded paint as used in PCB repair? I take it the coax does not need to be heavy gauge.
How could I simulate the 50 Ohm if using a lighter type of shiekded conductor?
What material do you recommend?
You can buy poster board with a metallized finish. I might try that with aluminum duct tape for both the cone and plate just to get an initial feel for the whole thing.
You can relax. The front end is well protected. There's no way you're going to burn out anything with a MMIC amplifier. I was worried about too much signal creating spurious signals and junk. Incidentally, these are easy to detect. Just temporarily switch in about 10dB attenuation between the antenna and preamp (or SA input if you're not using a preamp). If the suspected spurious junk drops more than 10db in level, it's being generated in the spectrum analyzer and can be safely ignored. If the signal drops exactly 10dB, it's real.
The data sheet shows a -127dBm to +30dbm (1 watt) input range (Pg 7): With -127dBm sensitivity (assuming you have the high sensitivity option), you don't need a preamp if you keep the coax cable lengths reasonably short. However, if you're using tiny or long coax, you'll need a preamp, probably near the antenna, not near the spectrum analyzer. Adding 20dB of gain to the system is going to reduce your dynamic range by 20dB and probably create a few spurs at the high tend. I suggest you try your antenna without a preamp and see if it's acceptable.
You're probably just fine for indoors. It's outdoors that worries me.
I was thinking more in the way of a staple or clip. The silver paint is fairly conductive and can easily be used with high impedance traces. At 50 ohms, it can easily look like a resistor and produce some loss. For this exercise, I think some aluminum duct tape will suffice. There might be some corrosion if the humidity is high, but that's easily fixed by replacing the aluminum duct tape.
I don't understand the question.
I use Teflon (PTFE), machinable ceramics, styrene, white PVC, and blank PCB material.
The material needs to NOT be hygroscopic (absorb moisture) and to NOT be lossy at operating frequencies. The first is easy enough. Avoid using nylon, ABS, acrylic, polyurethane, polycarbonate, PET, PBT and wood, all of which gobble water. The good plastics are polyethylene, polypropylene, polystyrene, and PVC. Also glass. Some plastics are lossy at RF frequencies. Rather than lookup the loss tangent at 2GHz, just shove a piece into a microwave oven. Give it about 15 seconds at full power. If it gets hot, don't use it. If it's cold, it's good at least up to 2.4GHz. Watch out for anything black, as the filler is often carbon based and very lossy.
I didn't know that was available.
For some of my antennas, I use foil backed urethane insulation board. It's not really suitable for a discone, but it makes a dandy patch antenna. Foil on one side forms the ground plane. The other side forms the antenna element. Other structures are possible. The downside is that the foam absorbs some moisture, which detunes the antenna.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
The venerable RSGB VHF/UHF manual has a section on discone construction (Section 8.37 in my Fourth Edition, published 1983) with the characteristic impedance set by the end diameter of the cone and its spacing to the disc.
Construction is one issue. Operationally, the discone has the interesting behavior that the take-off angle for the radiation pattern increases with frequency. Think about it -- this may not be what you want...
Nope. The pattern on a discone doesn't quite do that. As the frequency increases, the pattern falls apart into a mess of minor lobes more closely resembling a crude hemispherical pattern than a broadband dipole. It's truly ugly.
To illustrate the point, I borrowed the NEC file for the Diamond D130 discone without the top section: Note that the model uses a real ground and a realistic mounting height.
I ran vertical radiation patterns from 100 Mhz to 1000 Mhz. 4NEC2 doesn't have an easy way to produce a sequence of plots by frequency, so I just plugged in various frequencies and saved the plots. Things look fairly sane from 100 to 200 Mhz. At 400 Mhz, the pattern starts to break up, where the takeoff angle is still low, but most of the RF is at much higher angles. At 500 to 600 Mhz, the major lobe is almost straight up, with almost no RF at the horizon. At 800 to
1000 Mhz, that main lobe moves back down to the horizon, but the bulk of the RF is still at higher angles. If you want an animated GIF of the changes with frequency, I can do it, time permitting. Along with the takeoff angle, the gain at the major lobe also changes radically with frequency. However, this is meaningless because most of the RF is NOT at the major lobe (which defines the takeoff angle).
At 146MHz (middle of ham radio 2m band) it doesn't look too horrible: but at 445, it's rather ugly with much of the RF going upwards:
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Done: Note that the discone takeoff angle changes wildly with changes in frequency. I just noticed that there is a scale change in some of the patterns, which then fails to show the relative changes in gain with frequency. I'll fix that later.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
What exactly are you trying to measure? Generally for test purposes, they use those spiral cone antennas. A discone has a funny near field pattern supposedly.
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You are doing damn good if you get 10:1 on a discone. They usually just cheat and add an extra vertical element for the low frequency.
You're not going to get any reliably conductivity THROUGH the aluminum duct tape. The adhesive side is not easily penetrated unless you melt it away first with something hot. I've had moderate luck with heat gun. Note that the bulk of the strength is not in the aluminum layer, but in the adhesive layer.
Then burying a connection under a layer of aluminum tape, the electrical connection is made mechanically, between the coax cable and the aluminum antenna. The tape just holds them in place. You could use insulating electrical tape for that, except that most such tapes stretch too much. Aluminum electrical tape stretches much less and conforms somewhat to the lumps and bumps.
If you need a better electrical connection, which can be soldered, I suggest copper adhesive tape, as sold by stained glass supply stores. If you need to electrically connect a seam or surface which might flex, I've used copper and brass "wool" and scrubbing pads.
Oops. I probably should not have included PVC. PVC will work, but the problem is the microwave oven test. PVC breaks down to vinyl chloride, which is a known carcinogen. Some contain bisphenol A (BPA). You don't want to be eating those. I have a small dedicated microwave oven for testing materials, accelerated drying, and dangerous experiments that might create hazardous substances.
The problem with PVC is the additives for things like UV resistance, color, stability, etc. They can make PVC somewhat lossy at RF frequencies. In my limited experience, PVC is good enough at microwave frequencies for receive antennas, but will get hot if used for transmitting. I don't expect your discone to be used for transmit, so you're ok.
Looks flimsy but might work with reinforcing or over a styrofoam form. I usually use copper or aluminum roof flashing material for prototype antennas.
The aluminum layer on paper might also be too thin. The skin depth
foil layer is less, you may experience problems.
Good luck.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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