Balloon 915 MHz

Balloon 915 MHz Downlink.xmcd Helical Communication 9/3/2014
Technologies

634 Barnes Blvd
Rockledge, FL 32955
(321) 208-8978
[email protected]
www.helicalcommunicationtechnologies.com

The Quadrifilar Helical Antenna provides improved communications over a Half Wave
Dipole for High Altitude Balloon to Ground Chase Vehicle.
The purpose of this worksheet is to demonstrate how the Quadrifilar Helical Antenna is better
than a half-wave vertical dipole for Balloon Communications to Chase Vehicles.

The object is to provide technical support for the 915 MHz downlink from a high altitude
balloon to one or two mobile chase vehicles on the ground.

The purpose of the downlink is to send tracking data to the chase vehicles.

This worksheet will consider a 1 watt RF Output transceiver. An example product will be the
XTend-PKG 900 MHz unit. The range performance is stated to be up to 40 miles using a high gain
antenna. The RF performance is according to FCC Part 15.247 rules.

The corresponding Balloon and Ground Chase Receiver sensitivity is:

Receiver sensitivity of -110 dbm @ 9600 bps, or
Receiver sensitivity of -100 dBm @ 115,200 bps.

range 40mi The baseline range for useable communication is defined to be 40 miles.

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Balloon 915 MHz Downlink.xmcd Helical Communication 9/3/2014
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Let's get a view of the balloon path above the earth to get an idea of the range situation.

earth_radius 6371km
A circular view of the earth can be protrayed using the following mathematics.

i 0 180 "i", is a range variable indicating from 0 to 180 in increments of 1.

Balloon_height 100000ft 30.48 km 6.401 103 km
balloon_radius earth_radius Balloon_height

xi earth_radius cos(i deg) balloon_radiusxi balloon_radius cos(i deg)
yi earth_radius sin(i deg) balloon_radiusyi balloon_radius sin(i deg)
ri xi 2 yi 2

Earth and Balloon Radius

8

6
yi

1000km
balloon_radiusyi 4

1000km
2

0 10 5 0 5 10

xi balloon_radiusxi

1000km 1000km

From this view, the Balloon is hardly viewable above the earth's radius. This will look much different
for a small portion of the earth in the next view.

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Balloon 915 MHz Downlink.xmcd Helical Communication 9/3/2014
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The view looking at a small portion of the balloon orbit is shown below.

j 85 95 "j", is a range variable from 95 to 85 in -1 increments.

xj earth_radius cos(j deg) balloon_radiusxj balloon_radius cos(j deg)
yj earth_radius sin(j deg) balloon_radiusyj balloon_radius sin(j deg)

Earth and Balloon Radius from 85 to 95 Degrees

6.42

yj 6.404
1000 km 6.388

balloon_radiusyj 6.372
1000km 6.356

6.34600 480 360 240 120 0 120 240 360 480 600

xj balloon_radiusxj

km km

earth_radius 6.371 103 km Balloon_height 30.48 km
balloon_radius 6.401 103 km

For a high altitude balloon, the baseline height is 100,000 ft or 30.48 kilometers. In the graph above,
the height is indicated at the center of the X axis, or 0 lateral kilometers. For a balloon drifting to
the left, at a horizontal axis of 120 kilometers, the balloon distance to the center is over 45 km.

In order to cover a wide portion of the earth as the ballloon distance increasses from the chase
vehicles, the beamwidth would like to be quite wide. Thus the practical balloon 915 MHz GHz
antenna beamwidth might be in the area of about 140 degrees, or a solid angle of about 70
degrees.

We should establish a baseline, for initial calculating purposes, goal of 140 degrees beamwidth
for +1 dBirhcp antenna gain. Considering minimal losses, let's set an Effective Isotropic Radiated
Power of 1 Watt, or +0 dBW. +0 dBW may also be expressed as +30 dBm.

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Balloon 915 MHz Downlink.xmcd Helical Communication 9/3/2014
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Antenna Elevation Pattern for a Half Wave Dipole at 915 MHz.

gain_dipole270degEL 30dBi gain_dipole90degEL 30dBi

gain_dipole260degEL 18dBi gain_dipole80degEL 18dBi

gain_dipole250degEL 8dBi gain_dipole70degEL 8dBi

gain_dipole180degEL 2dBi gain_dipole0degEL 2dBi

Quadrifilar Helical Antenna Geometry and Elevation Pattern

The gain of the Quadrifilar Helical Antenna
is about 3 dBirhcp for the hemisphere.
For the balloon, the QHA would be mounted
upside down to maintain the 3 dBirhcp gain.

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Balloon 915 MHz Downlink.xmcd Helical Communication 9/3/2014
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Free Space Path Loss

fC 915MHz The downlink frequency is 915 MHz. The actual carrier is a spread spectrum
covering the 902 to 928 MHz frequency range.

λC c 32.764 cm The wavelength will be approximately 33 centimeters.
fC

40mi 64.374 km

range 64.374 km The useable range from the Balloon to the ground station receiver
is being stated as 40 miles or 64km.

The path loss equation in linear terms is:

Lp λC 2 This number is so small that it is usually expressed as a logarithm.
4π range

PL40mi 20 log λC 127.9 dB Thus, the free space path loss is about 128 decibels at
4π range 40 mile range.

Let's also consider the path loss when the Balloon is straight up from the launch area at 100,000 feet.

range 64.374 km 100000ft 30.48 km

PLstraightup 20 log λC 121.4 dB

4π 30.5km

The free space path loss goes from near 0 dB at the launch site with the transmitter and receiver
antennas very close by each other to about -120 dB when the balloon is straight up from the launch
site at 100000 feet. Then as the balloon drifts away, still at high altitude, from the launch site or
perhaps the chase vehicles, the free space path loss increases to about -128 dB at a range of 40
miles from the ground receivers.

The above path loss calculations will be used in the equations to determine
the power available to the ground chase receivers.

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Balloon 915 MHz Downlink.xmcd Helical Communication 9/3/2014
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Now, it is appropriate to consider the power delivered to the ground receiver under various conditions.
The power delivered to a receiver using a half wave vertical dipole for transmiting to the the
ground receiver. We will calculate the power delivered to the ground receiver.
Let's consider using a half wave vertical dipole on the chase vehicle, essentially under the
balloon.

TXdBW 0dBW

gainTX270 gain_dipole270degEL 30 dBiVert

gainRX90 gain_dipole90degEL 30 dBiVert

PLstraightup 121.362 dB

RcvrPowerunder_balloon TXdBW gainTX270 PLstraightup gainRX90 181.362 dBW

RcvrPowerunder_balloon_dBm RcvrPowerunder_balloon 30 151.362 dBm

Thus, when using half wave vertical antennas at the balloon and at the ground receiver there is not
sufficient power delivered to the receiver when the balloon is at high altitude directly above the ground
receiver.

This is a serious situation for the ground receiver. There is a 40 dB deficit for the 9600 bps data rate.
There is a null under the balloon antenna. There is also a null over the ground receiver.

By contrast, the Quadrifilar Helical Antenna would have positive antenna gain at the balloon and at the
ground receiver. The QHA case is studied later in this worksheet.

As the balloon travels away from the ground receivers, still at high altitude, the half wave vertical dipole
antenna gains increase. Let's calculate the situation when the ground receiver is at 70 degrees
elevation with respect to the balloon. Congruently, the balloon antenna is at 250 degrees elevation.
The actual range for this case may be a bit longer than 40 miles, but 40 miles will do for an
approximate calculation.

TXdBW 0 dBW

gainTX250 gain_dipole250degEL 8 dBiVert

gainRX70 gain_dipole70degEL 8 dBiVert

PL40mi 127.85 dB

RcvrPower70deg TXdBW gainTX250 PL40mi gainRX70 143.85 dBW

RcvrPower70deg_dBm RcvrPower70deg 30 113.85 dBm

For the balloon at 70 degrees elevation with respect to the ground receiver, the power delivered to the
receiver is about 4 dB below the minimum receiver sensitivity for the 9600bps data rate.

Thus, at high altitude, there is a serious null overhead using half wave vertical antennas. This null
extends from the zenith to 70 degrees elevation. Below 70 degrees elevation, the half wave vertical
antenna gain increases with lower elevation and the power delivered to the receiver increases.

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Balloon 915 MHz Downlink.xmcd Helical Communication 9/3/2014
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If the balloon stays at high altitude, let's calculate the approximate communication range.
long_range 200mi

PLlong_range 20 log λC 141.8 dB

4π long_range

TXdBW 0 dBW

gainTX180 gain_dipole180degEL 2 dBiVert

gainRX0 gain_dipole0degEL 2 dBiVert

PLlong_range 141.8 dB 137.8 dBW
RcvrPower0deg TXdBW gainTX180 PLlong_range gainRX0

RcvrPower0deg_dBm RcvrPower0deg 30 107.83 dBm

For the balloon at high altitude, the range could be as far as 200 miles and still have sufficient
power delivered to the receiver for 9600 bps data rate using the half wave vertical antennas at the
balloon and at the ground receivers.

At some range as the balloon loses altitude the curvature of the earth and other factors would limit
the communication range. For this situation, the gains of either the vertical antennas or the QHA
would be about the same. As long as either vertical antennas are used at each end or the QHA's
used at each end the antenna would perform equally.

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Balloon 915 MHz Downlink.xmcd Helical Communication 9/3/2014
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Quadrifilar Helical Antenna Calculations

The QHA antenna gain can be considered +3 dBirhcp for all of the angles considered for the balloon to
ground receiver application. This can be seen by inspecting the QHA radiation pattern earlier in this
worksheet.

Following are calculations for a Helical Communication Technologies QHA used at the balloon and at
the ground receiver.

For the balloon straight up from the ground receiver:

The power delivered to a receiver using a Quadrifilar Helical Antenna for transmiting to the
the ground receiver. We will calculate the power delivered to the ground receiver.

Let's consider using a Quadrifilar Helical Antenna on the chase vehicle, essentially under
the balloon.

TXdBW 0dBW TXgain_QHA 3dBirhcp RXgain_QHA 3dBirhcp

PLstraightup 121.4 dB 115.4 dBW
QHA_RcvrPowerunder_balloon TXdBW TXgain_QHA PLstraightup RXgain_QHA

QHA_RcvrPowerunder_balloon_dBm QHA_RcvrPowerunder_balloon 30 85.4 dBm

Recall, the Balloon and Ground Chase Receivers sensitivities are:

Receiver sensitivity of -110 dbm @ 9600 bps, or
Receiver sensitivity of -100 dBm @ 115,200 bps.

The power delivered to the ground receiver now using HCT QHA's at the balloon and at the ground
receiver for the balloon straight up over the ground receiver is now -85 dBm, versus the half wave vertica
antennas at -151 dBm. There is more than 60 dB improvement using QHA's over the half wave dipole
antennas at 100,000 feet high altitude.

There is no null to overcome using the Quadrifilar Helical Antenna. At 70 degrees elevation:

TXdBW 0 dBW TXgain_QHA 3 dBirhcp RXgain_QHA 3 dBirhcp

PL40mi 127.9 dB The range might be less than 40 miles, but would be a minor
QHA_RcvrPower40mi adjustment to the value.

TXdBW TXgain_QHA PL40mi RXgain_QHA 121.9 dBW

QHA_RcvrPower40mi_dBm QHA_RcvrPower40mi 30 91.9 dBm

Using the HCT QHA's there is about 18 dB margin for the 9600 bps data rate and
8 dB margin for the 115,200 bps data rate.

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Balloon 915 MHz Downlink.xmcd Helical Communication 9/3/2014
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CONCLUSION

This work sheet has compared the use of half wave vertical antennas with Quadrifilar Helical
antennas. The half wave vertical antennas have a significant null overhead that is not over come
until the elevation angle of the balloon with respect to the ground receiver is lowered below 70
degrees elevation.

This might indicate the futility of chasing the balloon, trying to get under the balloon, when the
balloon is at high altitude using half wave dipole antennas.

At long range, where the relative elevation angles are much less than 70 degrees, there is no
significant advantage of the QHA over the Vertical antenna.

The Quadrifilar Helical Antenna developed by Helical Communication Technologies can provide
significant communications improvement over half wave vertical dipoles.
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The following are global MathCad defined units. MathCad handles decibels in an awkard manner. My
method is to provide a global worksheet definition as below. This imposes a responsibility on the user
to properly use these unit definitions. This is the only situation where Mathcad does not understand
application of the unit. My method states decibels in the proper context for electrical engineers.

dB 1 For Mathcad, we define a "dB" as 1. The writer must use the unit
dBi 1 properly.

"dBi", is a unit definition for a decible with respect to an
isotropic point source.

dBirhcp 1 This is a unit definition for
Mathcad.

dBW 1 For Mathcad purpose only, this becomes a dimensionless unit.
dBm 1
dBiVert 1 For Mathcad purpose only, this becomes a dimensionless unit.

dBiVert, refers to Vertical Polarized antenna gain relative to an isotropic
antenna.

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