Grumble of the week: Why no one runs permanent PACKET/IRLP/ECHOLINK/WIRES Nodes!

After doing a bit of reading, there are two core issues:

  • The band plans don’t allow for much spacing between the aforementioned nodes & FM Simplex / Repeaters.
  • ‘Commodity’ Amateur 2 & 70 Radios have very little front end attenuation.

I haven’t tested it (and I won’t) but I can’t imagine running 50 Watts on an Internet Linked WIRES Gateway on 145.325 at the same time as trying to operate FM Simplex or even VK3REC 147.175 (1.85MHz Spacing Maximum) is a great idea, given that the Antennas at best would be within 20M of each other and with an average gain of 6dB each (aka you might as well transmit 5 Watts directly into the other radio).

I did actually calculate that*; on 2M the path loss of 2 antennas with 6dBi gain 20 Metres apart is 9.7dB.  That means if you TX 50 Watts, the other radio will receive 5 Watts of power.  I don’t design RF front ends for a living, but S9 VHF is defined as 5 uV (0.000005 Volts).  5 Watts @ 50 Ohms = 15.8 Volts (P=V^2/R)!!!  Simply put, without using cavities, you are looking at a ~S9+130 situation at best, but more realistically a blown front end stage.

*My calculations assume both radios are tuned to the same frequency.  That said, I’ve done a bit of reading regarding repeater building and apparently modern FM radios offer less than 1dB of attenuation across a 3 MHz spread (prior to the front end filtering).  The ARRL test reports disagree with this in a sense, suggesting 55dB of rejection with 20kHz channel spacing, but I’d imagine their test did not involve *significant* front end overloading – just a simple adjacent frequency test in the microvolt level! 

From what I have read, most 2M repeater builders find they need to use between 80 and 100dB of Duplexer / Cavity Attenuation to avoid de-sensing the receiver.  This makes sense, as ~80dB of cavity + 55dB of front end rejection would take care of S9+130.

Has anyone got any ideas of how I could operate a permanent Wires-X (IRLP/EchoLink Equivalent) node at my QTH?  The best I’ve come up with so far is setting up the Wires-X Gateway on 6M using a Yaesu FT-8900R, which at least is not harmonically related to 2M & 70CM.   

The only downside to that is whilst I have a 6M Handheld and a 6M Radio for the car (my Yaesu FT-857D), running the Wires-X Link on 6M limits the potential audience, as a lot of people just have 2M & 70CM rigs, especially mobile where it would be interesting.  No current manufacturer even makes a FM Mobile or HT that includes 6M!  Simply put, other than myself, I can’t imagine a 6M Analogue FM Wires-X Node would get much use!

All of the above leads me to one conclusion.  The reason no one runs permanent Packet/IRLP/EchoLink/Wires Nodes at their QTHs these days is that it requires one of the following:

  1. Investing in a full repeater grade duplexer / cavity solution, or:
  2. Giving up either 2M or 70CM at their QTH.

Given that FM Phone on 2M & 70CM is by far one of the most popular modes, and most Amateurs would lack the resources to build a repeater grade solution, it is little wonder no one bothers with permanent ‘esoteric’ modes such as Packet/IRLP/EchoLink/Wires.

 

Feel free to correct me if I’m wrong…

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