Data Stations Typically Require More Power
The typical data station requires more power than the typical voice station for several reasons:
- Higher duty cycle:
- Since data communications are faster, more accurate and provide better information workflow than a simple voice radio, data is the primary/preferred way to handle message traffic. Therefore, the data station will typically handle many more messages and be transmitting and receiving more frequently than the voice radio.
- Higher radio power:
- Ideal data throughput relies on higher signal to noise ratios. All other variables being equal, higher power produces a better signal to noise radio.
- Data transmissions typically use simplex communications, which are subject to the "hidden node" problem. The hidden node problem can usually be mitigated with higher antenna placement and higher transmit power. But, for a given antenna configuration, higher power is better.
- WiFi radios typically require about 24 Watts (1A x 24V) of continuous power, whether transmitting or receiving or idle. That's about the same power that a typical voice radio consumes while receiving or idle. And it's less power than a typical voice radio uses while transmitting (8-12A x 12V = 96-144W). But the WiFi radio is usually on 24x7 and power is in addition to the power required for a voice radio. The voice radio is still required for health and welfare and other command and control traffic.
- More components:
- In addition to the power for the radio itself, power is needed for the PC(s) and printer.
- For AX.25 (packet) stations: Power is also needed for the external TNC.
- Sites with WiFi connection often have more than one PC connected, which typically requires an Ethernet switch and (recommended) a firewall/router. Small, 5-8 port desktop Ethernet switches are usually low power. But even small firewalls can use significant power due to their higher pocessor power demands.
- If an inverter is used to power AC devices (firewalls, Ethernet switches, WiFi radio power injectors, etc.) from a battery, about 10-20% of power is lost due to inefficiencies in the inverter and the AC power supplies of the AC devices.
Grounding and Bonding
Proper bonding and grounding are important for optimal station operation, whether the station is a temporary field station or a permanent installation, such as at an EOC. And for permanent stations, proper bonding and grounding is required by building/electrical codes.
- Bonding vs. grounding
- The terms "bonding" and "grounding" are commonly misused.
- Bonding
- Bonding refers to making a low impedence connection between all devices (antenna, radio, power supply, PC, network equipment, etc.) such that they all remain at the same electrical potential (voltage).
- Proper RF bonding is done at a single point (not a daisy chain between devices) and uses a short, low-impedence conductor, such as a bus bar, copper strap or large guage wire.
- Low impedence ensures that the voltage difference between devices remains the same, even if the frequency of an applied voltage is high (such as RF) or really high (such as a pulse from a lightening strike).
- If the voltage difference between two devices is zero, then there will be no current flow between the devices. This eliminates audio hum, which can reduce signal to noise ratios and lower data communications throughput. It can also prevent damage due to lightening strikes.
- Grounding
- Grounding refers to making a low impedence connection to Earth ground.
- The connection to Earth ground safely routes potentially dangerous currents to ground.
- For lower frequencies, such as 60 Hz AC, it also means that other devices which are separately connected to a different ground connection are at approximately the same potential (voltage), which prevents hazardous potential differences from existing between the two devices. But, the higher the frequency, the higher the impedence between two Earth ground points. At higher frequencies (such as RF frequencies), the Earth presents too much impedence to properly bond between two devices in a station. So all devices should be bonded together and then that bonding point should be connected to Earth ground.
- The combination of bonding all equipment to a single point and connecting that single point to Earth ground is call a "single point ground".
- Improperly bonded and grounded stations can introduce several problems in any radio station (voice or data):
- Hum/noise during a transmission: Audio hum or other noise present during a voice conversation can be very annoying, and may require some repeats. Noise on a data connection can reduce the signal to noise ratio to the point where data throughput is significantly reduced, slowing down the radio channel for all. If the noise is bad enough, the connection may not be able to be established at all.
- Irratic device behavior: Common mode currents can cause erratic behavior of devices. PCs and Ethernet devices can exhibit strange behaviors, such as rebooting, losing connectivity, etc.. USB devices seem to be especially susceptable.
The ARRL publishes a good book on electrical grounding and bonding: "Grounding and Bondin for the Radio Amateur".
Calculating Power Requirements
The following describes how to calculate the power requirements for a radio and external hardware TNC used for packet communications. The assumptions stated below are appropriate for most situations. Most people can jump straight to the answer in the last bullet below. But for completeness, the detailed assumptions and calculations are presented so that anyone can adjust them to their own requirements.
- Determine the percentage of time the station is sending or receiving messages
- It takes approximately 20-25 seconds to send the average packet message seen in recent incidents and public service events. For conservative purposes, assume 25 seconds per message.
- Assume the field station sends approximately 4 messages per hour (i.e. one every 15 minutes). This is probably far more than most field stations will send. But it serves as a conservative estimate for power requirements.
- Assume the field station receives about the same number of messages as it sends. For example, headquarters asks for a report; field station sends a report. Or field station sends request for supplies; headquarters responds with availability info.
- Therefore, the station will send approximately 4 messages per hour and receive approximate 4 messages per hour
- For each message the station sends, it will eventually receive an acknowledgement
- For each message the station receives, it will send an acknowledgement
- Acknowledgements take about 5-15 seconds, on average, to send or receive, depending on if they are sent during an existing session. For conservative purposes, assume 15 seconds per acknowledgement.
- Calculate transmit and receive time
- Sending messages:
- When sending a message, the radio spends more time in transmit mode than in receive mode. Assume about 70% transmit, 30% receive.
- [(4 messages/hr) x 25 sec/message)] + [(4 acknowledgements/hr) x (15 sec/acknowledgement)] = 160 seconds/hr sending messages
- TX time for sending messages = 160 sec x 70% = 112 sec
- RX time for sending messages = 160 sec x 30% = 48 sec
- Receiving messages
- When receiving a message, the radio spends more time in receive mode than in transmit mode. Assume about 70% receive, 70% transmit
- [(4 messages/hr) x 25 sec/message)] + [(4 acknowledgements/hr) x (15 sec/acknowledgement)] = 160 seconds/hr receiving messages
- TX time for receiving messages = 160 sec x 30% = 48 sec
- RX time for receiving messages = 160 sec x 70% = 112 sec
- Totals
- Total TX mode time per hour = (TX time for sending messages) + (TX time for receiving messages) = 112 sec/hr + 48 sec/hr = 160 sec/hr
- Total RX mode time per hour = (RX time for sending messages) + RX time for receiving messages) = 48 sec/hr + 112 sec/hr = 160 sec/hr
- Note: Since the assumption was that the station sends the same number of messages as it receives, the TX and RX times are expected to be the same, and they are.
- Total Idle time per hour = (1 hr) - (Total TX time) - (Total RX time) = 3600 sec - 160 sec -160 sec = 3280 sec/hr
- Percentages
- Percent TX time = (160 sec/hr Total TX time) / (3600 sec/hr) = 4.44%.
- Percent RX time = (160 sec/hr Total RX time) / (3600 sec/hr) = 4.44%.
- Percent idle time = 100% - (Percent TX time) - (Percent RX time) = 100% - 4.44% - 4.44% = 91.12%
- Calculate current capacity requirements
- Assume a typical mobile radio operating at 50 Watts with the following current draw: TX = 13 Amps, RX = 1.0 Amps
- Many radios have a lower current draw when idle (not receiving packets) than they do when receiving. For conservative purposes, assume idle current = receive current.
- So average Amps use by the radio = [(TX%) x (TX Amps)] + [(RX % + Idle %) x (RX Amps)] = [(4.44%) x (13 A)] + [(4.44% + 91.12%) x (1 A)] = 0.58 A + 0.96 A = 1.54 A
- Assume a TNC with a current draw of about 250 mA
- Total average current for radio and TNC = 1.79 A
- Assume an 8 hour shift
- Amp*hr = 1.79 A x 8 hr = 14.32 Ahr
- Determine battery capacity requirements (AGM-type battery)
- Battery capacity is listed in Amp*hours (Ahr)
- The Ahr rating is typically based on 20 hr test which uses a much lower current than required above and uses a final cut-off voltage that is much lower than what most radios can tolerate. A conservative rule of thumb is to derate the AGM-type battery by 1/3 (for new batteries) to 1/2 (for old batteries) to account for the higher current draw, higher cutoff voltage, and battery aging. For more details, consult the packet training classes. Note that Lithium Iron Phosphate batteries have different discharge curves. Consult the battery data sheet to determine the right size battery.
- In order to support a 14 Ahr requirement, the AGM-type battery needs to be rated at about 21 to 28 Ahr
- SCCo ARES/RACES recommends a 26 Ahr battery (if using an AGM-type battery) for powering many types of field stations. As shown in the above calculations, that size will work well for powering a radio and TNC at full power for an entire shift. For other battery chemistries, use a size the provides the equivalent capacity.
The SCCo Packet Operations training classes cover the above information and much more. Consult the training course materials for more detail.
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