APRS on HF

Modulation

The dominant modulation scheme for HF APRS, is Frequency Shift Keying (FSK) with a frequency shift of 200Hz, and at the rate of 300 bits per second. (The same as ye olde HF Packet)

Note that, due to the NRZI nature of AX.25 packet radio, it doesn't matter which tone is the mark and which is the space; it's the transitions that matter. Importantly, this means that you can use an old ex-commercial HF transceiver which is only capable of USB operation, and still successfully operate on HF APRS. All you have to do is ensure your transmitted tones fall on the right frequencies (ie the suppressed carrier frequency for an USB transceiver will be lower than the displayed suppressed carrier frequency on an amateur LSB transceiver).

The exact two frequencies that you will transmit on are a function of your suppressed carrier frequency, whether you are running USB or LSB, and the audio tones generated by your TNC or modem.

Bell 103 based HF modems (such as the AM7910 used on VK6ZTN's Flash TNC) use two different sets of tones, depending on whether the modem is configured for "Originate" or "Answer" mode. The PK-232, and the Tigertronics TM-1, use different audio tones again. Overall, the dominant toneset seems to be 1600Hz & 1800Hz, followed by the 2110Hz & 2310Hz tones of the PK-232.

Australasian APRS HF Operations Guide.

As HF APRS is becoming more popular in Australasia I have put together a set of guild lines that we all need to adhere to. This will allow the successful long-term use of the ARPS HF 10Mhz network as it’s popularity continues to grow.

The Do’s and Don’ts for APRS HF.

Do’s

Do consider all other users of the HF network. Remember you are sharing this single frequency with hundreds of other operators.
Do use only a path of GATE,WIDE
Do Net (set your frequency) to the Primary Net server VK3MY-4. Or if you can not here VK3MY-4 use a secondary APRS HF Gate server.
Do user a SSID of -4 if your station is an HF to VHF (or IS) Gate. (Refer to the SSID Guide)
Do user a SSID of -15 if your Mobile Station transmits on HF. (Refer to the SSID Guide)
Do keep your packet size to a minimum. Use the Mic-e or APRS compressed format and no or minimal information in your status text.
Do set your beacon time to 10 minutes or more. ( The primary and secondary net stations are the Only station that will beacon at more frequent intervals)
Do use the path of ECHO Only for tuning your radio into the Net.
Do use the following TNC settings for your success. TXDelay 400ms, PACket LENgth 128, SLOTtime 300ms PERsistance 64ms.

Don’ts

Don’t Digipeat any packets on HF (the only exception is for tuning your radio).
Don’t transmit Home station positions on HF. Only Gates and Mobile stations need to transmit position information.
Don’t feed VHF or IGate data to HF
Don’t allow your station to operate as a digipeater

APRS HF NET Servers.

10Mhz

Primary NET Station – VK3MY-4

Secondary Net servers – VK4DMI-4, ZL(?)

The Primary Net Station will transmit the following messages at the indicated times:

Every 5 Minutes - / APRS 10Mhz Net Station (1)

Every 10 Minutes - APRS HF To VHF Gateway On 10.147.60Mhz USB

The Secondary Net Station will transmit similar messages at 5 minute intervals.

Eg; APRS 10Mhz Net Station (2)

7Mhz

Net Station – ZL3RX

(More details when available)

APRS HF Gates;

10Mhz

	VK3MY-4
	VK3WRM
	VK4DMI-4
	VK4CLM-4
	ZL3RX
	ZL3GR
	ZL2GTB

7Mhz

ZL3RX

NOTES:

Remember that an average length APRS Posit packet takes 3 to 4 seconds to transmit on HF. Assuming a channel efficiency of 30% for ax25 this results in a maximum of 7 stations being able to transmit per minute. At a transmission rate of one posit per 10 minutes the maximum number of stations that can be in a single APRS Gate’s coverage area is 70 stations. This will be less as the Net stations transmit frequently and messages and/or tuning may be under way on the channel. HF Propagation is also an important factor in limiting station numbers

Modulation

Frequencies

HF Digi Path