Homemade Discone Antenna

[mne...@keystone.com]

I have seen several webpages describing home construction of discone
antennae from inexpensive materials.

Wire version:

http://imgur.com/a/nKTQW#NyuZX18

Sheet version:

http://www.qsl.net/n/n9zia/portant/index.html

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.

http://www.minicircuits.com/pdfs/ZJL-4HG+.pdf

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.

Any advice along these lines?

Mark Newman

[Glenn]

On 09/03/14 23.34, mne...@keystone.com wrote:
...

> Any advice along these lines?
>
> Mark Newman
>

Hi Mark

You could read these papers:

http://www.hipoint.ca/whitepapers/WhitePaper_DisconeAntenna.pdf

Oak Ridge National Laboratory:
Type: Discone antenna 850-6000 MHz:
http://360antenna.com/pdf/360-Antenna_SAMPLE_Discone-Testing.pdf
Quote: "...
At 2 GHz, the gain of the antenna varied nearly 10 dB as the antenna
was rotated 360°.
...
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.
..."

/Glenn

[Jeff Liebermann]

On Mon, 10 Mar 2014 09:34:04 +1100, mne...@keystone.com wrote:

>I have seen several webpages describing home construction of discone
>antennae from inexpensive materials.
>
>Wire version:
>http://imgur.com/a/nKTQW#NyuZX18

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.

>Sheet version:
>http://www.qsl.net/n/n9zia/portant/index.html
>
>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.
>http://www.minicircuits.com/pdfs/ZJL-4HG+.pdf

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.

>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.
>
>Any advice along these lines?

Lots of examples to follow:
<https://www.google.com/search?q=discone+antenna&tbm=isch>

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:
<http://www.northcountryradio.com/Articles/discfig2.htm>

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:
<http://www.changpuak.ch/electronics/calc_11.php>
<http://www.ve3sqb.com/discone2002.exe>
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:
<http://www.diamondantenna.net/picts/D3000N_L.jpg>

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 je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[mne...@keystone.com]

On Sun, 09 Mar 2014 16:35:14 -0700, Jeff Liebermann <je...@cruzio.com>
wrote:


Thank you for your response and great links.

>I couldn't find an HP8495E spectrum analyzer.
>

My typo. It's an 8594E. Yes, please advise. I don't want to damage the
front end.

>Indoors,
>you're probably ok, but outdoors might be a problem.
>

It will be indoors as per my OP.

>The difficult part is making the connection between the
>copper coax cable pigtail and the aluminum parts.
>

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?

>Don't use nylon washers at over
>about 100 MHz.
>

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.

Mark Newman

[Jeff Liebermann]

On Mon, 10 Mar 2014 17:40:29 +1100, mne...@keystone.com wrote:

>My typo. It's an 8594E. Yes, please advise. I don't want to damage the
>front end.

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):
<http://www.teknetelectronics.com/DataSheet/HP_AGILENT/HP__8590ESeries.pdf>
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.

>It will be indoors as per my OP.

You're probably just fine for indoors. It's outdoors that worries me.

>>The difficult part is making the connection between the
>>copper coax cable pigtail and the aluminum parts.

>How about silver loaded paint as used in PCB repair? I take it the
>coax does not need to be heavy gauge.

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.

>How could I simulate the 50 Ohm if using a lighter type of shiekded
>conductor?

I don't understand the question.

>>Don't use nylon washers at over
>>about 100 MHz.

>What material do you recommend?

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.

>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.

I didn't know that was available.

For some of my antennas, I use foil backed urethane insulation board.
<http://www.specjm.com/products/foamboards2/apfoil-facedpolyisocyanuratefoamsheathing.asp>
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.

[artie]

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...


In article <orpph9puisv3ihvie...@4ax.com>,

--

[Jeff Liebermann]

On Mon, 10 Mar 2014 17:52:44 -0700, artie <art...@gNOSPAMmail.com>
wrote:

>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:
<http://www.qsl.net/kp4md/modeling.htm#discone>
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.
<http://www.LearnByDestroying.com/jeffl/antennas/Discone/takeoff-angle/>
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:
<http://www.learnbydestroying.com/jeffl/antennas/Discone/146Mhz-3d.jpg>
but at 445, it's rather ugly with much of the RF going upwards:
<http://www.learnbydestroying.com/jeffl/antennas/Discone/445Mhz-3d.jpg>

[Jeff Liebermann]

On Mon, 10 Mar 2014 18:51:54 -0700, Jeff Liebermann <je...@cruzio.com>
wrote:

>If you want an animated GIF of
>the changes with frequency, I can do it, time permitting.

Done:
<http://802.11junk.com/jeffl/antennas/Discone/discone-animated.gif>
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.

[miso]

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.

http://www.ets-lindgren.com/3103

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.

[mne...@keystone.com]

On Mon, 10 Mar 2014 14:54:19 -0700, Jeff Liebermann <je...@cruzio.com>
wrote:

>You can relax. The front end is well protected. There's no way
>you're going to burn out anything with a MMIC amplifier.
>

Thanks. That's all good news.

> 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 would be concerned about the conductivity of the tape's adhesive

>I use Teflon (PTFE), machinable ceramics, styrene, white PVC, and
>blank PCB material.
>

OK. PVC is easy. End caps from the plumbing shop.

>>You can buy poster board with a metallized finish.
>>

>I didn't know that was available.
>

Something like this.

http://www.grafixarts.com/product/Metallic_Foil_Board_Paper-2

Or foil-backed builder's paper might be stiff enough.

Time to get something built and see how it works.

Mark Newman

[mne...@keystone.com]

Here is a widely sold, inexpensive discone antenna that claims a
25-2000MHz range. Is this credible?

http://www.ebay.com/itm/Skyscan-Discone-ULTIMATE-BNC-Magmount-Desktop-Scanner-Receiver-Antenna-Mobile-/221187848491

Mark Newman

[Tauno Voipio]

Look at the array of whips at the top. It is not a pure discone.

--

-TV

[mne...@keystone.com]

On Tue, 11 Mar 2014 11:47:44 +0200, Tauno Voipio
<tauno....@notused.fi.invalid> wrote:


>> http://www.ebay.com/itm/Skyscan-Discone-ULTIMATE-BNC-Magmount-Desktop-Scanner-Receiver-Antenna-Mobile-/221187848491
>>
>> Mark Newman
>>
>
>Look at the array of whips at the top. It is not a pure discone.

Whipping it into shape one might say.

Still, whatever works.

Mar Newman

[artie]

Jeff--

Thanks for the clarification and the very interesting graphics!

In article <6r9th9h534sqgfl6d...@4ax.com>, Jeff

Liebermann <je...@cruzio.com> wrote:

> On Mon, 10 Mar 2014 18:51:54 -0700, Jeff Liebermann <je...@cruzio.com>
> wrote:
>
> >If you want an animated GIF of
> >the changes with frequency, I can do it, time permitting.
>
> Done:
> <http://802.11junk.com/jeffl/antennas/Discone/discone-animated.gif>
> 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.

--

Namaste--

[Jeff Liebermann]

On Tue, 11 Mar 2014 18:29:42 +1100, mne...@keystone.com wrote:

>> 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 would be concerned about the conductivity of the tape's adhesive

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.

>OK. PVC is easy. End caps from the plumbing shop.

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.

>Something like this.
>http://www.grafixarts.com/product/Metallic_Foil_Board_Paper-2
>Or foil-backed builder's paper might be stiff enough.

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
for aluminum at 50 MHz is about 12 痠eters or 460 痣nches. If the
foil layer is less, you may experience problems.
<http://www.microwaves101.com/encyclopedia/calsdepth.cfm>

>Time to get something built and see how it works.

Good luck.