Insertion Loss, Mismatch Loss, Transmission Loss
A correspondent having read Exploiting your antenna analyser #12 asks whether the measurement provides evidence of loss of the connectors, and referred me to (Arther nd) where he reports some measurements of UHF series adapters and conclusions.
Let’s deal with interpretations of my own measurements first.
Measurements of input impedance only for such an electrical short transmission line will not give useful data for determining TransmissionLoss which is the result of conversion of RF energy to heat. The measurements do give ReturnLoss and given that InsertionLoss=MismatchLoss+TransmissionLoss, they set a lower bound for InsertionLoss.
Above is a plot of ρ and ReturnLoss for the DUT. ReturnLoss curiously is plotted ‘upside down’ as ReturnLoss increases downwards… a quirk of AIM software, but remember that ReturnLoss in dB is +ve.
Taking 438MHz, ρ=0.1295 which reconciles with ReturnLoss is 17.75dB. MismatchLoss=Pforward/(Pforward-Pref)=1/(1-ρ^2)=1/0.983=0.073dB for the pair, or 0.037dB each if we allocate equal loss to both adapters. (InsertionVSWR for the assembly calculates to 1.3, and most people would not be too concerned at overall VSWR below 1.5 for say a mobile radio installation.)
ReturnLoss does not imply conversion of energy to heat.
MismatchLoss does not imply conversion of energy to heat.
(Arthur nd) measures a different assembly with a two port VNA which does provide the basis for assessing TransmissionLoss as well as ReturnLoss and MismatchLoss.
Above is his plot of S21 from which he draws the conclusion:
The real bad news is at 432 MHz where we see a loss in the order of 1.0 dB, this equates to a transmission loss of around 6 Watts with 25 Watts input.
You cannot draw that conclusions from S21 alone, S11 has a bearing.
Above is his S11 plot, and by eye, S11 at 438Mhz is approximately -7dB, so we can calculate MismatchLoss=0.97dB.
Though S21=-1.21dB, TransmissionLoss which is due to conversion to heat is 1.21-0.97=0.24dB which would result in 5.7% conversion to heat for his cascade of connectors or 1.42W heating for 25W input rather than 6W as calculated by VK3JEG.
What does this mean for a transmitter application?
Transmission Loss calculated from (Arthur nd) does give rise to conversion of RF energy to heat and so reduces EIRP.
Mismatch Loss reduces power absorbed by the load to that extent IF the source is an ideal source with Zs=50+j0Ω, the value is premised on that assumption… but transmitters are not necessarily so and they may deliver higher power into a different load, including one resulting from the use of more or more UHF series connectors in the chain.
Of the total InsertionLoss (1.21dB in Arthur’s 438MHz case), only the TransmissionLoss component represents conversion of RF energy to heat and explains the inconsistency between loss figures bandied about and actual heating… the loss figures are probably InsertionLoss which of itself, does not imply conversion to heat.
- Arthur, C (VK3JEG). nd. The UHF type connector under network analysis. http://www.qsl.net/vk3jeg/pl259tst.html (accessed 01/02/2016).