AMATEUR RADIO HIGH ALTITUDE BALLOON
LINE-OF-SIGHT RADIO COMMUNICATION LINK BUDGET
Ralph Wallio, WØRPK   WØRPK  at  netINS.net

Amateur high altitude balloon payloads are lightweight packages rarely exceeding 6-pounds. This weight limit is part of FAA regulations that place more stringent requirements on heavier payloads. Included in this weight are telemetry and other radio transmitters and the batteries they use for power. This weight limit usually results in the use of transmitters that are small and consume little power. These goals usually result in transmitter power output of 1 Watt and less.

The vast majority of amateur high altitude balloon payload downlinks have used 2m (144-148MHz) Amateur Radio communications modes. These low power narrow band transmissions successfully carry digital telemetry, voice repeater output and slow-scan images. Line-of-sight communications range is directly proportional to payload altitude with maximum distances approaching 650Km (400 miles).

Occasional experiments with low power wideband amateur television (ATV) links have not been as universally successful. Hams have successfully experimented with ATV for several decades but they have typically used high power transmitters and high gain antennas. When limited to approximately 1 Watt and a low gain antenna in a high altitude payload results are often disappointing especially over longer distances. This disappointment is often caused by the lack of adequate antenna gain and/or low noise preamplifier gain and/or low loss feedline at receive sites.

Fortunately there is a way to predict link performance before money is spent and/or information is lost. All of the factors that impact the quality of a received signal can be brought together in a LINK BUDGET which predicts signal-to-noise ratio and compares it with required SNR for given modes. These factors include

Link Frequency and Bandwidth
Transmitter Power and Transmit Antenna Gain
Path length and Resulting Loss
Receive Antenna Gain and Noise Temperature
and Required Signal-to-Noise Ratio

These factors can be brought together in an EXCEL spreadsheet template that is available as an email attachment (contact WØRPK  at  netINS.net). This spreadsheet was originally used to design spacecraft links but here it is applied to high altitude balloon links with no significant modification. The spreadsheet is divided into three sections of transmit, path and receive issues. The combination of these three sections result in a predicted Eb/No (signal-to-noise ratio) and, taking required Eb/No into account, a predicted link margin.

(See detailed explanation of this link budget analysis at bottom of this page.)



Required inputs to this EXCEL template are shaded in Fuchsia:

XMTR Power (Watts)
(XMTR System Losses can usually be ignored)
XMTR Frequency (GHz)
XMTR Antenna Gain (dBi)

Maximum Slant Range (Km)
(Polarization matching loss can usually be ignored)
(Atmospheric losses can usually be ignored)

Receive Antenna Gain (dBi)
Receive Noise Temperature (dK) see http://showcase.netins.net/web/wallio/SNT.html
Receive bandwidth (Hz)
Required receive Eb/No (dB) also known as signal-to-noise ratio


Starting with familiar examples, the first column, NARROW BAND - 2m FM - VOICE, we have a 1 Watt 2m FM transmitter and a 0.0dBi gain omni-directional vertically polarized antenna in the payload. We set the maximum slant range at 650km (400 miles) which is the maximum theoretical path length for a roughly 100,000ft altitude. We will attempt to receive voice repeater output on an HT and rubber ducky antenna (dummy load). Receive Noise Temperature is as calculated with another EXCEL template discussed at http://showcase.netins.net/web/wallio/SNT.html. Receive bandwidth for FM transceivers is typically 15KHz. Required Eb/No (SNR) could be as low as 12dB for reasonably comfortable copy.

With these inputs the calculator predicts a link margin of 14.8dB which means the downlink would be full-quieting the vast majority of the time for all listeners out to maximum distance. Assuming a true line-of-sight path the only factors that would degrade this performance would be, 1) transmit or receive antenna orientation away from vertical and resulting cross-polarization losses of as much as 30dB and/or, 2) the receiver almost directly under the payload with antenna nulls looking at each other.

The second example is for a 2m FM packet radio telemetry link. All inputs are the same except for required Eb/No (SNR). Performance of our inexpensive TNC modems has been characterized and is discussed at http://showcase.netins.net/web/wallio/BER_Packetradiobiterrorrate.html
 including performance data this table:

Assuming we would like to have close to perfect copy of long packets we will need an Eb/No of 24dB. The calculator predicts that we would have only a 2.8dB link margin to attain this performance. We would probably want to apply receive antenna gain and low loss feedline to improve this margin.

Finally we have the third example of moving up to 70cm for our telemetry link. This higher frequency has almost 10dB more path loss than 2m so, all other things being equal, our link margin for perfect packet copy is now -7dB. Participants at maximum range would receive no error free packets. Fortunately we can significantly improve the signal level by applying antenna gain, low loss feedline and a low noise preamplifier at the receive antenna.





Now we will consider the problem with wideband AM ATV in three cases that start with 1 Watt transmitters and antennas with 3dBi gain such as 1/2-wave dipoles. The first case is at maximum distance with a medium gain Yagi receive antenna, no preamplifier and 3dB feedline loss. Note that bandwidth is now 6MHz. Resulting receiver Eb/No is only 1.1dB which produces nothing better than a P0 image per this table:

(See http://www.ussc.com/~uarc/utah_atv/psandqs.html for a more complete discussion)

The second case makes three substantial improvements, halving the distance to 325Km (200 miles), using a longer Yagi with 13dBi gain and using a low noise preamplifier at the antenna. Resulting Eb/No has improved to 15.7dB which improves our image from P0 to P2. Finally, our third case halves the distance again to 162Km (100 miles) and uses an even longer Yagi with 16dBi gain. Eb/No has now improved to 24.8dB with a watchable P3 image.

Image quality with 70cm AM ATV is now P3 and certainly worth our improvement efforts (not the least of which was to move our receiving station substantially closer to the payload). Could we make further improvements by using FM ATV and its much lower SNR requirement?



Our final analysis looks at FM ATV on three bands, 33cm (922MHz), 23cm (1250MHz) and 13cm (2400MHz). We are still using 1W transmitters and 3dBi transmit antennas. Our path length has been limited to 162km (100 miles). Receive antenna gain is set progressively higher with frequency and low noise preamplifiers are placed at the antenna feed point followed by low loss feedline. Required bandwidth for FM ATV is roughly 18MHz but required Eb/No is only 12dB for a P4 image. Predicted results are encouraging for this limited distance. Images on 33cm and 23cm are predicted to be P4 and 13cm would require only 2.7dB more antenna gain to also be P4.

This analysis technique allows adjustment of many variables but we have taken time for only a few of the many possible combinations. Send a request for an email file attachment of this template to WØRPK  at  netINS.net to explore other possibilities.

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SSOK Packet Radio Link Budget:

GIVEN Payload Transmitter Power [W] 0.3
CALCULATED Payload Transmitter Power [dBW] -5.2
GIVEN Payload Transmitter System Losses [dB] 0.5
GIVEN Frequency [GHz] 0.144
GIVEN Payload Transmit Antenna Gain [dBi] 1.5
CALCULATED Payload Transmit EIRP [dBW] -4.2

GIVEN Slant Range [Km] 420
CALCULATED Maximum Path Loss [dB] 98.2
GIVEN Polarization Matching Loss [dB] 0.0
CALCULATED Isotropic Signal at Receive Antenna [dBW] -102.4

GIVEN Receive Antenna Gain [dB] 7.2
GIVEN Receive Noise Temperature [dK] 1483
CALCULATED Receive G/T [dB/K] -24.5
CALCULATED Receive C/No [dB-Hz] 101.7
GIVEN Receiver bandwidth [Hz] 15000
CALCULATED Receive Eb/No [dB] 55.9
GIVEN Required Receive Eb/No [dB] 24.0
CALCULATED Link Margin [dB] 31.9


Discussion of each budget value:

GIVEN Payload Transmitter Power [W] 0.3

Pete is using an ALINCO DJ-S11T VHF transceiver powered with regulated 5VDC and modified for antenna feedline attachment

CALCULATED Payload Transmitter Power [dBW] -5.2

LOG(TRANSMITTER POWER [W])*10

GIVEN Payload Transmitter System Losses [dB] 0.5

Estimated loss between transmitter output and antenna input

GIVEN Frequency [GHz] 0.144

Downlink transmit frequency, 144.34MHz

GIVEN Payload Transmit Antenna Gain [dBi] 1.5

Pete has designed and optimized a 1/4-wave ground plane antenna that is suspended on its feedline in an inverted configuration.

CALCULATED Payload Transmit EIRP [dBW] -4.2

TRANSMITTER POWER [dBW] - SYSTEM LOSS [db] + ANTENNA GAIN [dBi]
(EIRP, Effective Isotropic Radiated Power)


GIVEN Slant Range [Km] 420

Distance between observer and payload

CALCULATED Path Loss [dB] 98.2

20*LOG(FREQUENCY [GHz]) + 20*LOG(SLANT RANGE [Km]) + 92.4

GIVEN Polarization Matching Loss [dB] 0.0

3.0dB for circular to linear, only if it applies

CALCULATED Isotropic Signal at Receive Antenna [dBW] -102.4

TRANSMITTER EIRP [dBW] - PATH LOSS [dB] - POLARIZATION LOSS [dB]


GIVEN Receive Antenna Gain [dBi] 7.2

Three element Yagi

GIVEN Receive Noise Temperature [dK] 1483

See discussion at http://users.crosspaths.net/~wallio/noise.html

CALCULATED Receive G/T [dB/K] -24.5

RECEIVE ANTENNA GAIN [dBi]-10*LOG(RECEIVE NOISE TEMP [K])

CALCULATED Receive C/No [dB-Hz] 101.7

ISOTROPIC SIGNAL LEVEL [dB]+RECEIVE G/T [dB/K]-(-228.6)

GIVEN Receiver Bandwidth [Hz] 15000

FT-726R in FM mode (15KHz @ -6dB)

CALCULATED Receive Eb/No [dB] 55.9

RECEIVE C/No [dB-Hz]-10*LOG(Bandwidth [Hz])

GIVEN Required Receive Eb/No [dB] 24.0

See discussion at http://users.crosspaths.net/~wallio/ber.html

CALCULATED Link Margin [dB] 31.9

RECEIVE Eb/No [dB]-REQUIRED Eb/No [dB]

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