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Connecting 2 modems via FM

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Newbie level 6
Mar 30, 2002
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Hi everybody!
I would like to connect two PC modems together via FM transceiver to working like a radio modem.I have some schematics to connect 2 modems with two wires,but not via FM.
Anyone tried this?


problems involved

This will work in theory, but the problem is splitting the modem output into receive and transmit audio paths. If you just connect it to the two sets of radios required for to and from paths the radio portion will oscillate.

Your best bet would be to use the RS-232 port and radios designed for data.

Hi joco_b!

I have made such connection by FM tranceivers 5 years ago. As Flatulent says, you will need two independent channels with full duplex. Also you need balanced circuits to split your output and input from modem to avoid oscillation in closed loop. In my case I have used radiostations with 10 kHz audio band that allows me to connect on 14400.

If you have only one channel equipment (half duplex) then you need to look at KISS modems. There are a lot of KISS modem circuits on Internet.

Best Wishes! klug.

Sorry joco, but that's not easyly done for the quickest protocols ( > 2400bps).
I think (I'm not sure now, need to review some old stuff) that the
slowest protocols (300-2400bps) were half-duplex. In this case, you could
do the job but it is so slow and still a pain in the ass to implement (you need to simulate ringing tones, etc). There are much simpler ways to connect 2 computers via a radio link. The easiest is to attach an FM transceiver to the serial port: you get nice 115200 bps BPSK modulated with almost no effort. If you want much more throughput, you can use the parallel port but then you must serialize the output of the port and use previos method or use a 8PSK digital transceiver. Just one point, you should check your local law because you can be using more bandwith than allowed with such methods.

Now, the reason why modern modem protocols can't be directly radio linked. High-end protocols are, mainly, simetrical full-duplex. This means some bandwith is reserved for the uplink and some for the downlink. The problem is that what is cleanly separated at low-frequency it is not in
high frequency and then both channels interfere each other. Other considerations are the inconvenience of a MQAM modulation used in this protocols. It is not a good idea to use such in a radio-link without using any corrective methods.

Summarising, it is not feasible to connect 2 modems at high data rate
via a radio-link by just connecting them a a RF transceiver.

By the way, why not using a standard solution like WiFi or WiFi5? Do you have line-of-sight path between your computers? How far are them from each other?

One question, Klug, how about performance of such a system? In special I'm interested in what a BER do you have with protocol for 14400bps (if you ever measured that). Interested by de MQAM stuff, just curious.

Hi Darkseed!

I have found instruction and circuit diagram of station that I have used. It was longrange radiotelephone "Altay AS-3S" made by Belorussia. Working frequency is near 350 MHz with split 36 MHz between RX and TX. As I see on circuit, filter of main selection has 15 kHz band, air channels are with 25 kHz step. So I think real bandwidth in air is not more than 20 kHz. As I see now in instruction, audioband is 300-3400 Hz. Power of TX is 8 - 15 W.

First time I have connected with 9600 modem speed, but later I decreased modulation level and obtained 14400. Distance was near 5 km, but I put my simple 1/4 vertical antenna at my room in house with steel reinforced concreet walls. I think, that in open conditions it easy to make such connect on 20 km distance.

Sorry, but I have never used such term "BER", I am using Russian in my work and never meet it. Please explain it to me.

Good Luck & 73! klug.

I'll look for this Altay. Anyway, using 8-15W you shouldn't have much problems even with the worst modulation. That's simply a lot of power!

In respect to your other question, "BER" stands for "Bit Error Ratio". Anyway, as said, with 8-15W should be very low.



hi guys ...

i tried to do this but not a big success :0(. I connect 2 normal modems fortunatelly they had 4wire output so it was easy but to get highr seed is not easy. Do not forget that full duplex is also quite a problem for higher power you need quite good rf PA. For FM dat i suggest to use chips from page bellow
**broken link removed**



Hi guys...

Thnx a lot .

Dear klug!
How can I separate the input from output with balanced circuits?


Hi Joco_b!

I have had a problem in screen grabbing, so sorry for some additional advertisment on snap picture of this trial version program. (If anyone has some good screen grabber, please PM me, thanks in advance).

Look at attached picture, there is what you need. These transformers are identical. Every transformer has two balance coils L3 & L2 for first transformer, and L5 L6 for second. These coils are the same. Pay attention on big red points - they mark beginning of coils. I have not information about original transformers of this radiostation. But I have used such circuit for emulator of phone line. I have used low frequencies ferrite pots for transformers. Sorry, I have not turns numbers for them (it was at 1996), but I will try to ask my fellow worker tomorrow - may be he has noted this info. As I can recall L2=L3= near 1200-2000 turns, and L1 = 300-500 turns. Balance this circuit by R1.

Best Wishes! klug.

i`v lost the link where i have foun this but save the files ina a folder :lol:

the guys who built this made a LAN in austria (?)

so here there are .download them to know what you are dealing with.the modem has a xilinx fpga

do a google search .i think the project was named 56k radio modem

Hi joco_b!

I just have unwound one such transformer.

L1=500 turns of ~0.1 mm wire.

L2 & L3 coils are winded together, L2=L3= 500 turns of ~0.1 mm wire.

(R of L1) = 85 Ohms

(R of L2) = (R of L3) = 96 Ohms

For core i have used low frequency ferrite pot 22 mm diameter and 13 mm height.

Best Wishes! klug.

I've made a radio module (Modem) with cc1000 and pic, see at my homepage,
Orders from elektroda fans, 10% discount.

Why do not you try GSM modems?

**broken link removed**


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