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TCP-group 1995


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Radios for higher speeds,



> Date: Wed, 8 Feb 1995 22:37:50 +0600 (CST)
> From: Doug Holcombe <holcombe@mc.edu>
> Subject: Radios for higher speeds,
>
>    I would like to know what radios are being use for the higher speed TNC.

You can probably forget about using TNCs at high speeds (esp 56k -
128k). An option is the Ottawa PI-2 card, which is a DMA driven high
speed HDLC card.

> I have heard that you need more bandwidth the higher the speed you go. I
> think its something like 15 kHz foor 9.6 kb, 30 kHz for 19.2 kb, 60 kHz for
> 38.4 kb, and almost 90 kHz for 57.6 kb! I would like to know which radios
> are modifable for these wide bandwidths! I have heard of the TEKK radios
> and how the can be modifed, but aren't they only capable of only a band
> width of 30 kHz wide? someone correct me if I'm wrong, because I would buy
> one if they could be used for 57.6 kb! What are the bands (2m, 220, 440)???

It's true that the 56kbps radio-modems have a bandwidth of 70 KHz or
so, at least this is true of the WA4DSY modem that the GRAPES people
sell. If you used a different modulation (the WA4DSY uses MSK) you
could get 56kbps into 35kHz or so, but at the price of needing a
better signal (higher S/N)

You won't be able to modify regular narrow band FM radios to handle
56kbps, and you don't need to because the radio is built into the
modem, instead of hacked on as an afterthought. The 56kbps modems have
an RF output around 28-29 MHz (anyone tried running that into a 1kW
linear? :-) Since you can't really run off the 28 MHz IF, you send it
to transverters which up/down convert it to whatever frequency you
want. Typically people have used 220 MHz and 440 MHz for transmit and
receive - that makes it easy to build a repeater, no duplexers needed.
In Vancouver, we're using strictly UHF - we had duplexers, and it
seemed like a good idea at the time, since you only need one antenna.

> I would like to know about the radios in the 220 band. who makes a radio
> that would work for high speeds with wide bandwidths.

Probably noone. You could try your luck with Hamtronics transverters,
but their stuff is pretty crappy. A local wizard has designed UHF (70 cm)
transverters and 30W PA - practically no-tune. Mail me for info, I
don't know if he's ready to announce it yet, we're still in the beta
testing of the first run of boards. I think he said he'd be doing
220MHz RF stuff as well.

For the modem you have to go with GRAPES. This is a problem because
they're the only source, and theere's a long wait for the kit. The 56k
modem is badly in need of a redesign, it's too expensive, has far too
many parts, and requires too much tuning and tweaking.

>
> I'm just another lost sole in this big game of knowledge.
                        ^^^^
well just ask a few questions, we don't want to see anyone flounder :-)


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