I was about to order radios antennas and routerboards for a 10 mile point to point link on 5 gig. Before ordering I did the usual freespace antennas tx power calculation. As usual the thing looked great. So just for reference I went and done some calculations on existing shorter links same band same radios(but different locations) same antennas. WOW to my surprise every existing link was 9 db worse than the calculated value for its length. all the pigtails are the same. all the jumpers are lmr400 3 feet long.

I may just start throwing in -9 db in all my calculations if this holds true on the 10 mile link.

I really dont have a question but if anyone would like to comment it would be welcomed.

thanks

Depending what all you are calculating and what you are using to do the calcs, you could have an obstructed ferznel zone or simply at the wrong height. If you end up with a reflection your signal could be out by up to 20db or more. If you are not using it already download radio mobile its free and works great.

thats a very good point, i did the calculations tx-power(converted to db) + tx antenna + recieve antenna then subtracted from the distance freespace loss. all the links are in excess 120 feet with trees approx 50 to 60 feet and link distances of between 1/2 mile to 3.41 miles. No towers or tall buildings anywhere near. I exspected to see no more than 5 db due to a db or two for the connectors and the pig tail with jumper plus the lower transmit power at higher rates.

It would be interesting to know if others have had links match their calculations. All the links are stable and trouble free that i used for reference.

I downloaded radio mobile but after about 4 hours opted to use the hard drive space for something else since the documentation was vague and the learning curve was steep. I use National Geographics TOPO commercial application for finding the lay of the land. The learning curve was 5 minutes. It gives very similar graphs and data as radio mobile without the freznel zone acutally being on the graph. And it is extremely accurate using the US Army Corp of Engineers benchmarks that were available in my area for distance and elevations.

Keep the antenna height as low as possible and stay away from even freznel zones..... Radio mobile is a bit of a bugger to get started with but well worth the time invested.

I was about to order radios antennas and routerboards for a 10 mile point to point link on 5 gig. Before ordering I did the usual freespace antennas tx power calculation. As usual the thing looked great. So just for reference I went and done some calculations on existing shorter links same band same radios(but different locations) same antennas. WOW to my surprise every existing link was 9 db worse than the calculated value for its length. all the pigtails are the same. all the jumpers are lmr400 3 feet long.

I may just start throwing in -9 db in all my calculations if this holds true on the 10 mile link.

I really dont have a question but if anyone would like to comment it would be welcomed.

thanks

I have a link-budget calculator that of course incorporates all losses and power levels. I too have noticed I have to use an 'offset' in the calculation. Dont recall what it was, but it seemed consistent.

Since the time worn link budget calculation seems to really calculate a 'carrier' with a constant power level--therein lies the problem. Power levels on OFDM and any type of spread spectrum equipment have a different 'average' power level based upon the bitrate and other factors, it seems that it cant really factor a link-budget with varying average power. It seems to use the RMS value of the power output in dBm.

That has been my experience with attempting to calculate OFDM link budgets. Empirical evidence from known link configurations is the only way to
'dead-reckon' how a link might and/or should perform. RF being RF though will surprise you almost all the time.