Antenna Testing thresholds

[yukondan]

Does anyone have some concrete benchmarks for antenna/cabling testing? I am looking for minimum acceptable levels for the following : Return Loss (in dB)
VSWR
cable loss (dB/m , dB/ft)
Basically I looking for general bencha=marks...values that should make me go "what's wrong here!" Thanks in advance

 

[VEBill]

The word 'minimum' is problematic since (for example) Return Loss and VSWR are essentially the same thing, but going in opposite directions...

VSWR - normally as 'bad' as 2:1 is considered acceptable for most non-critical communications work. Less critical applications might even accept 3:1, but maybe try to fix it when they had time. There are MANY applications where 2:1 would NOT be acceptable (one example: high power radar where high VSWR would cause arcing in the waveguide). Certain antennas can have extreme VSWR on certain parts of the feedline (as a matching system).

Return Loss is the same thing as VSWR but on a different scale. You can look up the conversion tables.

Cable Loss goes up with frequency. Most HF cables might have one or a couple of dB per hundred feet. Good TV coax can be several dB per 100' even at 1 GHz. To know the limits for a certain cable, you need to know the frequency and simply look it up in the specs for that exact cable.

The above is the short answer.

The longer and more accurate answer is: 'It depends...'

 

[Higgler]

For loss, you really need to specify the cable and frequency band.

Rtn loss for each connector of a cable should be -25 dB as good, -20 dB as mediocre, and -17 dB as unacceptable for an sma connector at Ku band (12-18 ghz). That's using time domain measured for each connector.
Typically, overall return loss of a good cable is -30 dB in the 200 Mhz range (-34 to -38 dB measured on 0.25 inch semi-rigid cable recently with two type N connectors), and closer to -20 dB at 18 ghz.

0.141" semi-rigid cable loss is typically 0.6 dB/foot at 18 ghz, good expensive ($600 for a 4 footer) cable is 0.25 dB/foot at 18 ghz. Loss is proportional to sqrt of frequency, so at 4.5 ghz, loss is about half that amount.

check out Storm cable or other cable manufacturers, they have alot of spec's for VSWR and loss.

kch

 

[logbook]

And of course if the cable has a loss of 10dB then you are not going to measure a return loss of less than 20dB. Likewise the VSWR is going to look good even if it is actually rubbish at the antenna itself.

 

[Higgler]

Only if your input connector is good will logbooks note apply. A 10 dB lossy cable with poor end connectors can still give you -5 dB rtn loss at one end while having -25 dB rtn loss at the other end of the cable where the good connector is.

kch