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5/8ths antenna radial elements lowering radiation pattern?

 
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5/8ths antenna radial elements lowering radiation pattern?
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Titanium
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Joined: 04 Aug 2003
Posts: 27

Post 5/8ths antenna radial elements lowering radiation pattern? Reply with quote
I am interested to know if there no proportional value for the radials length with respect to the operating frequency or are the radials cut to a nominal length?

According to a link posted by 'Fleeting Glimpse' the duty of these radials is to lower the angle of reflections so you dont waste too much signal by it going up into the sky.
Mon Oct 27, 2003 12:13 pm View user's profile Send private message
Fleetinglimpse
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Post Comet Rewind-September 25,2003 Reply with quote
The Comet Antenna is Basically what is called a 5/8-WAVELENGTH VERTICAL ANTENNA.
This extended vertical design provides about 3dB theoretical gain over a ground plane antenna that is quarter wavelength long.
The realized gain of a 5/8-WAVE VERTICAL is practically related to the manner in which the field strength is achived and consequently radiated out as a usefull signal.

The field strength of a vertical antenna reaches a maximum gain figure when the vertical element is 5/8 wavelength in length or 0.625 Lambda.

The feedpoint impedance of a Regular Quarterwave antenna is in the order of 30-35 Ohms.The feedpoint impedance can be raised to about 50 ohms by drooping the radials down at an angle of about 45degs. which then makes it a lot easier to accomplish a match to Industry standard 50 Ohm Coaxial Cable which is available in various assortments,from various dealers.

The feedpoint impedance of a 5/8 Wave vertical which is what the Comet Antenna is,Has been proven to be REACTIVE in impedance measurements..hence a series inductance is required to establish a non-reactive termination or match point to that of the Coax cable feed from the transmitter end,which is 50 ohms..hence maximum Power should be transferred to the Antenna elements to be Radiated as a signal with a VERY LOW ANGLE OF RADIATION.

Hence that is the reason for the Combination PL259//MATCHING COIL BASE of the Comet antenna design.

The Vertical element is Cut to a length of 0.625 LAMBDA (5/8'S)AND THE RADIALS ARE USUALLY 0.25 LAMBDA(Quarterwavelength).

Now the real world applied aspects.

A)The mumbo Jumbo of Gains figures referenced to a isotropic radiator and Radiation gain can be translated thus:

1)The APPARENT GAIN OF THE COMET IS DERIVED FROM THE FACT THAT DUE TO FIELD STRENGTH PLOTS OF THE 5/8 WAVE PHENOMINON THE SIGNAL TAKES OFF FROM THE ANTENNA AT A VERY LOW ANGLE OF RADIATION....MEANING THAT VERY LITTLE IS RADIATED UPWARDS..THE USEFULL RADIATED ENERGY IS SPREAD DOWN LOW ON THE RADIO HORIZON.THE DISTANCE REACHED OR ABILITY TO PENETRATE THE RADIO HORIZON IS ALSO A DIRECT RELATION TO THE RF OUTPUT POWER OF THE TRANSMITTER.

An easy way to visualize this concept is to imagine a Soldier with a M16 Perched on top of a tower and systematically fires in a 360 degree of movement(rotation) and the angle that he maintains the rifle is pitched down to the ground so that he literally sprays all the bullets downward and out hitting the target area with 24/7 consistency.

2)There could never be any one consistent FLAT NORMALIZED GAIN FIGURE FOR THE COMET THAT TUNES A SPREAD OF 88-108 Mega_Hurtz.that is a 20mHZ. spread.Even though the tuning chart shows the Length that the Vertical two piece Aluminum taper Element..slip and slide length adjusment.

The radials are normally designed to be A QUARTER WAVELENGTH AT THE OPERATEING FREQUENCY..AS FAR AS THE COMET IS CONCERNED THE LENGTH OF THE RADIALS ARE PRETTY MUCH FIXED.

3)So from a practical point of application,as to whether the lack of being able to adjust the radial length to the theoretical 0.25 LAMBA at the particular frequency of Choice... will improve the signal strength drastically to a point where the your Audience would be able to diffrentiate a marked improvement in coverage and signal strength would be miniscule.

4)The Comet is a Clean..Nasty (nasty=coloquial eqivalent of Awesome performance) and Cheap LOW POWER COMMERCIAL RENDITION OF THE STANDARD ANTENNA PACKAGEING THAT HAS BEEN VERY POPULAR WITH THE LPFM COMMUNITY FOR QUITE SOME TIME.

5)Feeding more that 150 watts into the Comet would not be advisable..Over a long period of time one will experience some caststrophic failure in the transmitter Final Amplifer...and OR a Burnt Coil in the Base of the Comet..I am looking at one right now that i removed from a System years ago on the West Coast of Florida..Seen Quite a few with those Coils "Fried" and melted open circuited coil turns.

6)For high power users who happen to like the Comet that Much..and have the Height advantage..they can consider Phaseing two Comets seperated laterally on top of a tower.

7)An alternative for the high power users would be to consider a design that is a derivative of a 5/8 WAVE COLLINEAR VERTICAL CONCEPT.

8)The main areas to be concerned with as far as longevity of the Comet Antenna is to ensure that the Weather Insulateing Cover Cap(orange in colour) is always in place a top the small adjustable vertical tubeing..This prevents any water ingression into the Base below..and also use seal any openings (except the drain hole at the base of the Comet)that might be evident.

9)Use nothing less than a 9913 or RG213 COAX CABLE TO AVOID ANY RADIATION FROM THE COAXIAL LINE..ESPECIALLY IF THE USUALLY TECHNICALLY CHALLENGED LPFM STATION OPERATOR DOES NOT HAVE THE PROPER EQUIPMENT TO ADJUST AND TUNE THE ANTENNA OTHER THAN FOLLOWING THE TUNING LENGTHS FROM THE MANUAL.
Fri Oct 31, 2003 6:34 am View user's profile Send private message
Fleetinglimpse
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Joined: 26 Oct 2003
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Post Comet SPECS ACCORDING TO MANUFACTURER Reply with quote
CFM-95SL
88-108MHz Tunable Antenna
Gain: 3.4dBi
Band-width: 4MHz after tuning
VSWR: 1.5:1 or less
Max Power: 200 watts
Height: 5ft 6 inches min (88MHz)
7ft 8 inches max (110MHz)
Weight: 2lbs 7oz
Connector: SO-239
MSRP: $115.95
----------------------
Comet Antenna FAQ!

I've heard that stacking antennas will give me more gain. How do you stack?

When yagi antennas are stacked a 30% maximum gain can be realized. When using two Omni (vertical) antennas a maximum gain of 20% can be realized. The antennas need to be placed at least one wavelength apart.

How far from the tower should a vertical antenna be mounted?

A side mounted antenna should be placed one wavelength from the tower.


High VSWR can be caused by loose connections, a bad mounting location, pinched coax or incorrect ohm coax.
========================================

comet beam-cd95y

CD-95Y
88-108MHz Directional Antenna
Gain: 7.8dBi
Elements: 6
Polarization: V or H
F/B ratio: 15db avg
-3db E-Beamwidth: 60-70 deg
-3db V-Beamwidth: 110-120 deg
VSWR: 1.5:1 or less
Max Power: 500 watts
Boom Length: 45 inches
Weight: 7lbs 7oz
Mounting Mast Dia: 1.5-2 Inches
Connector: SO-239

==========

The Comet DS15 is what is called a discone antenna

DS-15
RX: 25-1300MHz
TX: 6M/2/M/440/900/1200MHz
Length: 4 feet 8 inches
Max Power: 100 watts
Weight: 3 lbs. 3 ozs.
Connector: SO-239
MSRP: $99.95
Sun Nov 02, 2003 7:54 am View user's profile Send private message
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