COAXIAL CABLE LOSS

[nokones]

I am curious if 5 dB of loss with 9 feet of RG-58/U cable with a "N" Male and SMA-RP Male connectors using 902-928 MHz is significant to sweat about or it is no big deal?  Any help would be appreciated.

[tweiss3]

I guess what do you determine significant? You should be able to make that up in antenna gain really easily, but 902 is more line of site than 440/GMRS, so you aren't going to be chasing skip.

[marcspaz]

5 dB is more than double what it should be.  It should be about 2 dB of loss.  I would think something is wrong somewhere along the setup.

[kidphc]

Self Edited/deleted due to useless info

[WSAM454]

It depends on what kind of distance you need to cover, and how much power you are running, and how sensitive your receiver (s) is/are. 

As an example, 10 Watts out of the 900MHz radio with the cable you mention will  result in about 7 Watts at the antenna.

Fill in the Line Loss Calculator here:

https://kv5r.com/ham-radio/coax-loss-calculator/

[WRYZ926]

Here is an excellent coax loss calculator: https://kv5r.com/ham-radio/coax-loss-calculator/

I plugged in the numbers using 912 MHz and RG-58 coax. RG-58 has a 97% loss at 100 feet. It has a 15.5 dB loss at 100 feet. Using 9 feet there is 1.4 dB of loss. This is with a 5 watt radio and an antenna with 3 dBd of gain. ERP is 7.2 watts. If you use an antenna with 1 dBd of gain then the ERP is 4.6 watts.

While RG-58 will work, it is not ideal for the 900 MHz band. You can overcome coax loss with a good antenna with higher gain.

I have found that calculator to be pretty accurate and if you are getting 5 dB of loss over 9 feet then I would be looking at the condition/quality of the coax and connectors.

 

[marcspaz]

3 minutes ago, WSAM454 said:

It depends on what kind of distance you need to cover, and how much power you are running, and how sensitive your receiver (s) is/are. 

As an example, 10 Watts out of the 900mHz radio with the cable you mention will  result in about 7 Watts at the antenna.

 

That is not correct.  A 5 dB loss for the total length of cable with 10w in would be 3w out.  That is a lot of loss.

[SteveShannon]

34 minutes ago, nokones said:

I am curious if 5 dB of loss with 9 feet of RG-58/U cable with a "N" Male and SMA-RP Male connectors using 902-928 MHz is significant to sweat about or it is no big deal?  Any help would be appreciated.

-5 db is a loss of 68% of the signal.  Almost any cable is better than RG-58 at 900 MHz.  You could do better with CATV RG6, even with the impedance mismatch. RG-6 has 6.1 db loss per 100 feet at 900 MHz compared to 21.5 db for RG-58.

But the real question is, given the 5 db loss you quoted, is the remaining 31% of the transmitted signal sufficient for the situation?

[nokones]

The transmitter is putting out unknown milliwatts, into a Yagi pointing to the receiver omni-antenna about 500 feet away.

[SteveShannon]

12 minutes ago, nokones said:

The transmitter is putting out unknown milliwatts, into a Yagi pointing to the receiver omni-antenna about 500 feet away.

So if the transmitter puts out 50 mw that’s 19 mw that reaches the Yagi.  The Yagi has a pretty high gain.  You don’t say what the spec is but it certainly is at least 6 db and probably more, which more than makes up for the -5 db earlier loss. At 500 feet I wouldn’t expect problems unless there are other issues, such as RF interference.

Is it being reliably received or is this just the design phase and you’re being conservative with the -5 db?

[Lscott]

1 hour ago, nokones said:

I am curious if 5 dB of loss with 9 feet of RG-58/U cable with a "N" Male and SMA-RP Male connectors using 902-928 MHz is significant to sweat about or it is no big deal?  Any help would be appreciated.

How did you determine you have 5db of loss?

[nokones]

1 minute ago, Lscott said:

How did you determine you have 5db of loss?

RigExpert Antenna Analyzer.

[nokones]

41 minutes ago, SteveShannon said:

So if the transmitter puts out 50 mw that’s 19 mw that reaches the Yagi.  The Yagi has a pretty high gain.  You don’t say what the spec is but it certainly is at least 6 db and probably more, which more than makes up for the -5 db earlier loss. At 500 feet I wouldn’t expect problems unless there are other issues, such as RF interference.

Is it being reliably received or is this just the design phase and you’re being conservative with the -5 db?

I don't know the antenna spec and I don't remember how many elements there are on the yagi.

 

The 900 Meg freqs are ISM band freqs and there are a gazillion warehouses nearby.  I'm not sure if any of them are using the ISM for wireless devices or the DLR/DTR radios for voice comm.

[WSAM454]

1 hour ago, marcspaz said:

5 dB is more than double what it should be.  It should be about 2 dB of loss.  I would think something is wrong somewhere along the setup.

Yes, according to the calculator I posted earlier, the loss should be about 2 dB, and my quote of 10 in and 7 out would then be correct according to the calculator.  If it IS actually a 5dB loss, then some other info is missing, or the cable is bad.

[SteveShannon]

17 minutes ago, nokones said:

I don't know the antenna spec and I don't remember how many elements there are on the yagi.

 

The 900 Meg freqs are ISM band freqs and there are a gazillion warehouses nearby.  I'm not sure if any of them are using the ISM for wireless devices or the DLR/DTR radios for voice comm.

Is it necessary for the receiver to use an Omni antenna or could it use a Yagi as well.  That might help if interference is an issue.

The other thing is that I think rg58 is more porous to the effects of interference.  Again, a better cable could help with that.

[nokones]

7 minutes ago, SteveShannon said:

Is it necessary for the receiver to use an Omni antenna or could it use a Yagi as well.  That might help if interference is an issue.

The other thing is that I think rg58 is more porous to the effects of interference.  Again, a better cable could help with that.

There are two transmitters with yagis from opposite directions to the Omni antenna for the receiver.  The transmitters are sending very small bytes of digital data.

[WRYZ926]

I redid the calculations using the kv5r calculator using 0.05 watts (50mw) and 9 feet of RG-58 coax. I don't know the actual dBd gain of the yagi antenna so I just put in 3 dBd of gain. Here is a screen shot of the results.

As you can see, with power in at 50 milliwatts and an antennas with 3 dBd go gain you get an ERP of 100 milliwatts.

Now if you are getting 5 dB of loss when checking with Rig Expert then I would be double checking the coax and antenna setup.

Did you check the coax by itself using a dummy load and open?

If so is that when you saw the 5 dB of loss or was it with the antenna connected?

I redid the calculations using RG-213 and LMR400. While both have less loss at 100 feet, there was no difference in the ERP using 9 feet of coax.

[SteveShannon]

48 minutes ago, nokones said:

There are two transmitters with yagis from opposite directions to the Omni antenna for the receiver.  The transmitters are sending very small bytes of digital data.

A bidirectional antenna in the center might be a better choice than the omnidirectional antenna. It has both forward and reverse gain without wasting power to the sides.