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Discone antennas
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Fleetinglimpse
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Post Discone antennas Reply with quote
http://www.northcountryradio.com/Articles/discone.htm

A UHF Discone Antenna for scanners


William Sheets K2MQJ Rudolf F Graf KA2CWL


The availability of scanners and wideband receivers covering the upper
UHF spectrum above 800 Mhz necessitates reasonably priced, wide bandwidth,
effective antennas. The most commonly used arrangements for wideband reception
on scanners using an outdoor antenna system are multiband trap antennas, a
discone, or a simple ground plane antenna. These antennas are usually non-
directional. They are generally designed for 50 ohms and are connected to the
scanner or receiver through a length of coax cable, most likely RG8 or RG58
type or similar. However, often this arrangement provides disappointing or
even poorer results at UHF than the indoor whip antenna mounted on the back
of the scanner. Here are several possible reasons for poor performance:

a) Poor antenna performance at frequency of interest
b) Very high feedline losses
c) Mismatches between antenna and receiver
d) Spurious signal pickup and interference
e) Lowered receiver RF gain at UHF
f) Poor receiver noise figure (>6 DB)

In all fairness to the equipment and antenna manufacturers, to cover 25
MHz to 1000 (or even in some cases to 2000 MHz) with one receiver, one antenna
and feedline system, no operator input, and yet still get good performance, is
a very tall order. Some compromises have to be made for economic reasons. Ant-
ennas that will perform well for the majority of communication frequencies of
interest to scanner users, say VHF High and 450-470 MHz, with good low band
reception of 30-50 MHz, will most likely not be anywhere near optimum at 800
MHz. There are no single solutions to all of these problems. Perhaps the best
compromise is the use of a separate antenna and feedline for frequencies above
400 MHz. Antennas for this frequency range are smaller and enable the use of
designs that are impractical at lower frequencies. The discone antenna is
quite useful if its dimensions are reduced so its lower cutoff frequeny is
around 400-500 MHz or so, yielding an overall size of six inches or less.
Actually, the reduction of antenna size is a simple approach but has several
advantages. Besides the obvious ones, such as cost, ease of construction,
simple mounting, foul weather survival, low visibility and other esthetic
considerations, there is another advantage. An antenna acts as a bandpass
filter, and tends to reject signals outside its design frequencies. This is
very useful in improving the performance of many scanners and wideband monitor
receivers. The majority of strong VHF signals, such as TV and FM broadcast as
well as strong local VHF amateur and commercial signals, will be markedly
attenuated by an antenna designed to receive signals at 700 MHz or higher.
The wide open front ends of many wideband receivers are quite susceptible to
cross modulation and intermodulation caused by the presence of strong unwanted
signals, resulting in all kinds of spurious signals, high background noise,
false signals, and other junk. It is amazing how this "crud" disappears
when an antenna designed for 700 MHz and above is used to receive the higher
frequencies. The discone antenna can work over about a 10 to 1 range, and
show a 2:1 VSWR (so can a resistor dummy load). Therefore this alone is no
guarantee of acceptable performance. VSWR, impedance, gain, pattern, and wave
angle performance are generally functions of frequency for practical antennas.
In the case of a discone antenna, the wide bandwidth needs to be qualified.
The upper end of the range has higher angle lobes than optimum for ground wave
and direct signal pickup, so the antenna is best over about a 3 to 1 frequency
range. Therefore a discone large enough for 100 to 300 MHz will not be really
optimum above 300 MHz. A smaller discone would solve this problem. Outside
antenna restrictions or outright prohibitions are very common nowadays. Those
with balconies or terraces may be able to install a small antenna that is
invisible or disguised in some manner. The small discone to be described is
easily hidden.

The discone antenna is a vertically polarized antenna, and consists of a
flat disc of about 0.17 wavelength diameter at the lowest desired frequency,
with the disc plane horizontal, mounted atop but insulated from a cone that
has a length of 0.25 wavelength on the side. It has been around for many years
and is widely used where wide bandwidth is needed, as in aircraft band UHF
communications where 225 - 400 MHz coverage is needed with one antenna, and
unity gain (0db) is satisfactory. It has been used in modified form for
HF communications in the 2 to 30 MHz range where a wide range of frequencies
must be covered. It is popular for scanner use in order to receive the entire
VHF and UHF spectrum with one antenna. The discone antenna is fed with a 50 ohm
cable, the outer conductor connected to the cone, the center to the disc.
Actual impedance varies from 50 ohms depending on the cone angle, frequency, and
disc to cone spacing. Nothing is critical. A good cone angle (see fig 1) is 25
to 40 degrees. A 30 degree angle is generally used, so the diameter of the
base of the cone is equal to the length of the side in this case, although
not critical. The lowest (cutoff) frequency is that for which the length of
the side is a quarter wavelength. This would typically be 24 to 30 inches
for the reception of all bands 108 MHz and up to about 1000 MHz. The disc
and cone in this size would be a little impractical mechanically and also
would have considerable wind resistance, if constructed of solid sheet metal,
It is possible to simulate the disc and cone with metal rods called radials,
in order to realize a practical structure. 16 rods or more is desirable in
both disc and cone, but 8 can be used with a slight loss in performance, this
being 1 to 2 DB or so. Fewer than 8 as some cheaper antennas have is not very
efficient. It is possible to extend the low frequency limit by placing a
vertical whip above the disc, using the cone as a ground plane, but this is
dubious in its effectiveness and may cause poorer performance at higher
frequencies. Ideally the discone is best used over about a 3 to 1 frequency
range, since at higher frequencies (3X lower cutoff) the wave angle rises
somewhat causing a loss in peak effectiveness for desired ground wave signals.
The disadvantage of the discone is low gain (0 db) and large size for its
performance. However, if bandwidth is needed, it is a good choice. Low loss
feeder cable and if possible, a mast mounted preamp, (reception only) is
desirable. Overall, it is a highly recommended all purpose scanner antenna.
It can also be used as a transmit antenna if short range omnidirectional
coverage is desirable. A seven inch discone will work well on the 440, 900,
and 1300 MHz amateur bands, for both receive and transmit.

A discone for use at 700 to 2000 MHz will be described. It is also very
small, but effective. In tests, it outperformed a 24 inch discone at 900 MHz.
This is probably because it would have a lower wave angle and that it is small
enough to use solid copper elements, with resulting lower losses. Improvement
on reception and transmission was about 3 DB. Measured VSWR is better than
1.5 to 1 at 910 and 1289 MHz, and the discone was actually used as a transmit
antenna for some experimental amateur TV transmissions at these frequencies.
It is hard to believe such a small antenna can perform so well. Used with a
pocket scanner, excellent results were obtained on reception of 860 and 935
MHz commercial signals, much better than the 8 inch rubber ducky original
equipment. Although below the cutoff frequency, satisfactory 450 MHz reception
was also obtained. If optimum 450 MHz performance is desired, the cone and
disk dimensions can be increased 75%. The rest of the dimensions will be
satisfactory. A longer line section might also be desirable for mechanical and
mounting considerations. Also, a marked reduction in intermodulation and cross
modulation effects was noted. Strong FM, TV, and VHF High commercial signals
are severely attenuated by this antenna. As a test, a wide range scanner that
covers 500 KHz to 1300 MHz was tried with this discone antenna connected to
its antenna jack. The test was performed in a New York City apartment building.
Use of this antenna really reduced the interference often heard on weak
signals, and noticeably improved signal to noise ratios compared to the stock
whip antenna supplied with the scanner. In addition, commercial and municipal
2 way radio signals around 855 MHz were noticeably stronger. The wide open
front end used in the scanner was helped by the smaller antenna, as there were
fewer strong lower frequency signals (< 200 MHz) to deal with.

Referring to fig 1, the discone dimensions were calculated for 700 MHz
The actual construction must allow for feedline connections and mechanical
support. A piece of 5/8" brass tubing is used to support the discone and as
the outer conductor of the feedline. This tube has an ID of 19/32". With
commonly available 1/4" brass rod for an inner conductor, a section of coaxial
line results that has a 52 ohm impedance. The impedance of a coaxial line
with an air dielectric is calculated as follows:

Z = 138 log (b/a) Z = impedance ohms
b = ID outer conductor
a = OD inner conductor
log = logarithm base 10

The exact impedance is not too critical and a 10% variation in impedance
should not cause problems. Choice may be limited by the available tubing sizes
If exact impedance is needed as in measurement applications, custom tubing is
necessary and this is out of the question for a small quantity application.
Length of this line is at the discretion of the builder. Loss is negligible.
We used 5.5 inches, but up to about two feet will present no problems. Longer
lengths will require some mechanical modifications in order to ensure that the
line geometry remains concentric and reasonably rigid.

Courtesy Uncle Rudy
Sun Nov 02, 2003 8:22 am View user's profile Send private message
Fleetinglimpse
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Joined: 26 Oct 2003
Posts: 142
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Post THE 'SHAQ' VERSION Reply with quote
Outdoor Scanner/Ham Discone Antenna

$65.99 Brand: RadioShack
Catalog #: 20-043 Model: 20-043
(Pricing and Availability may vary outside
the contiguous 48 United States.)

Features:
• Omnidirectional, rugged stainless steel construction
• Wide 25-1300MHz coverage
• Also for transmitting on 50, 144, 220, 440, 900 and 1296MHz ham bands
• Resonator and tunable whip for best 50MHz performance
• About 44 inches high overall
• Fits mast up to 1.5 inches in diameter
• Accepts PL-259 connector
Sun Nov 02, 2003 8:25 am View user's profile Send private message
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