Distance vs. Throughput?

We are in the process of aligning a 45km link. We tryed aligning it at own end (it got to REGISTERING) but never registered. So we went to the other end to try and align there but we couldn’t even getting a SYNCING out of it. At this point I got thinking about the whole 2x thing and the affects it has, so I disabled the 2x at that site and then drove back to the other site and disabled the 2x there. As soon as I did that it registered with a -76, jitter of 1 and RSSI of 600ish. Now we are still going back to the second tower tomorrow to do some more aligning but I was wondering if anyone else has come across this on long links? Is there a problem with 2x mode and long links or could it just be that we still have it slightly miss aligned? Or could there be a setting that I’ve missed?



in 2x mode the modulation is changed, when this happens effectively they are trying to pack twice as much info in the same pipe. In doing so it decrease the margin for error, hence the required S/N ratio increases.

It is the same with the 20M BH, sometimes you get poor link in 20M but a perfect in 10M.

It could be due to alignment, you really need to have people at both sides to conduct a good alignment. It could be that you are peaking on a secondary lobe. Scan up and down past where you are peaking to see if you find another lobe.

Thanks VJ. Your right it would be easier to have guys at both ends but it’s not an option. There’s only 2 of us.

We did a full sweep today on one end once we finally got it to Register. We are going to do the other end tomorrow and hope it works out. If it doesn’t, I guess we’re gonna have to live with a 10MB pipe for the time being.

Thanks for the help VJ

2x rate has some limitations and some requirements.

you need a minimum of 10db carrier to interference

the recommended margin is another 10db to maintain 2x rate.

-76 is only 10db above the RX threshold of the radio so 2x rate is not likely possible.

2x rate halves the distance.

what are you using for reflectors?

wow… 10db… thats high… normal canopy works on 3db and OFDM on 2.5db

from the sounds of it the are just doubling the modulation, without enhancing the FEC

as Jerry mentions… look at the reflectors, see if you can get extra db by changing the reflectors…


We got the link working in 2x mode now. We ended up with a -71 with a jitter of 2. Link test is sold 100% both ways with over 6Mb each direction as well. But from what you are saying, that doesn’t give alot of clearence for 2x mode to run in, Correct? It would need to be around -65 to work effectively? How trust worthy do you think the link will be?

Thanks again guys.


will depend on noise during the course of the day… log the jitter values during the course of the day and see if there is an issue.

-71 with jitter 2 sounds pretty solid to me.

I checked the Sprectrum for noise in the area, there wasn’t much in either direction so noise shouldn’t be a problem unless someone throws up a BH on the same channel as us near by.

Thanks again vj

Sorry Jerry I missed the part of your post about the dishes. We are using the standard canopy reflectors on both ends.