What really determines the efficiency of an antenna?
Is it Standing Wave Ratio (SWR)?
It is common knowledge that when an antenna has high SWR some of our transmitted power is wasted instead of being transmitted. But is this really true? The trouble with “common knowledge” is that it spreads without further scrutiny. “It must be true because that’s what everybody thinks”. But let’s consider another perspective.
What happens to our signal when it meets an antenna with high SWR? Some of the signal is radiated while the rest is reflected back down the transmission line to its source – the transceiver. What happens to the reflected signal when it reaches the transceiver? It is re-reflected back towards the antenna and the cycle repeats.
So does all the signal eventually get radiated? No. Energy is lost (RED ALERT from the physics department: Energy can neither be created nor destroyed, only converted from one form to another). Ok, my apologies to the physics department, some of the energy is converted to heat as our signal passes along the transmission line and through any ununs, baluns, impedance transformers or other devices en route. Further energy is converted to heat due to the resistance of the wires and the impedance of the transmission line itself.
Thus, on every trip between the transceiver and the antenna, some of our transmitted RF is converted to heat. If the antenna has a high SWR some of our signal travels back and forth between the transceiver and the antenna multiple times and becomes further attenuated on each trip. Therefore, if we can reduce the loss of RF (due to conversion to heat) as it passes through any devices along the journey between the source (transceiver) and load (antenna) we will improve the efficiency of our antenna system.
How can we do that?
One simple way to achieve that is to correct for the high SWR right at the antenna. A remote tuner can do that. A loading coil will compensate for the high capacitive reactance of a short antenna, but loading coils can be inefficient because of wire resistance. This is especially true in the case of base-loading coils on a quarter-wave vertical antenna. The current is highest at the base of the antenna so more RF energy will be lost to heat (P=I^2*R) than with a center-loading or top-loading coil.
So the real culprit is not SWR, but the insertion loss of ununs, baluns, impedance transformers, loading coils, transmatches and any other “energy conversion” devices, including the transmission line itself, through which our signal has to pass.
Insertion loss of Ham Radio Outside the Box’s 4:1 ununs
In the previous post I reported on my build of field test versions of a 4:1 unun and a 4:1 balun to compare how each would handle the task assigned to them. Now the job I set myself was to transform what might be called the “Ugly Sisters” builds into something with the good looks of Cinderella. And Cinderella had to be an unun tough enough to withstand rough treatment out in the Big Blue Sky Shack through all four Canadian seasons (Late Winter, Brief Summer, Early Winter, Deep Winter).
QRP 4:1 ununI built two versions of a 4:1 unun; one for QRP and another for what I like to call QROp. “QROp” is an unofficial label I have adopted to mean about 20 watts or so. Twenty watts will give a 1 S-unit advantage over 5 watts – maybe just enough for our signal to poke its nose above the noise floor when propagation conditions are not so good.
QROp ununThere are 2 main differences between the QRP and the QROp versions: The QRP unun uses a BNC connector and a 4:1 transformer wound on a tiny FT82-43 toroid. The QROp version uses an SO-239 connector and a 4:1 transformer wound on an FT140-43 toroid.
If we look at the tables below, we can see that the QRP version may have a little too much insertion loss. When we are trying to do as much as we can with as little as possible every milliwatt is wanted. As the wonderful friendly folks on the big Canadian island of Newfoundland like to say: “A little’s a lot if it’s all you’ve got”.
Insertion Loss effects of the Ham Radio Outside the Box QRP unun
BandQRP (5 watts) UNUN Insertion Loss (dB)RF Power Lost (watts)% RF Power Lost10m0.390.438.612m0.370.418.215m0.350.397.817m0.340.387.614m0.330.377.430m0.320.367.240m0.350.397.880m0.730.7715.4Insertion Loss effects of the Ham Radio Outside the Box QROp unun
BandQROp (20 watts) UNUN Insertion Loss (dB)RF Power Lost (watts)% RF Power Lost10m0.241.085.4012m0.231.035.1515m0.220.994.9517m0.210.944.7014m0.200.904.5030m0.200.904.5040m0.200.904.5080m0.220.994.95A little extra heat in winter
You would think Canadians wouldn’t mind a little extra heat in winter. It’s true, but not when the source of that heat is our precious transmitted RF. In case you were wondering, the amount of RF converted to heat by inefficient devices is mostly undetectable. If it can be easily detected the “magic smoke” can’t be far behind. When it’s 253 Kelvins outside you just ain’t gonna notice when the temperature rises to 254 Kelvins (note: the physics department advised me to use Kelvins to avoid confusion between degrees Fahrenheit and degrees Celsius).
Oh no! There’s more?
Yes indeed. An unun does not attenuate Common Mode Current (CMC). For that we need a Common Mode Current Choke (CMCC). CMC is the current on the outer surface of a coax braid. Differential mode current is carried on the core and inner surface of the coax braid. Does a CMCC also have insertion loss? Yes, but how much? Let’s take a look.
Insertion Loss of a QRP (5 watts) Common Mode Current Choke (CMCC)
BandQRP (5 watts) CMCC Insertion Loss (dB)RF Power Lost (watts)% RF Power Lost10m0.250.285.612m0.220.255.015m0.210.244.817m0.190.214.214m0.170.193.830m0.150.173.440m0.140.163.280m0.130.153.0 QRP CMCCInsertion Loss of a QROp (20 watts) Common Mode Current Choke (CMCC)
BandQRP (5 watts) CMCC Insertion Loss (dB)RF Power Lost (watts)% RF Power Lost10m0.180.814.0512m0.160.723.6015m0.150.683.4017m0.130.592.9514m0.110.502.5030m0.100.462.3040m0.090.412.0580m0.080.371.85 QROp CMCCThe (not so) grand total of RF going up the chimney
BandTotal QRP (5W) % RF power lost to heatTotal QROp (20W) % RF power lost to heat10m14.29.0912m13.28.7515m12.68.3517m11.87.6514m11.27.0030m10.66.8040m10.06.5580m18.46.80The white bearded man in the red suit and his flying reindeer might be grateful for a few watts of heat going up the chimney at this time of year, but those of us in the frozen barren tundra of the northern states and provinces, as well as licensed ham dwellers in other cold lands, may not see things the same way.
What can we conclude?
If we only consider the insertion loss – in this example – of the 4:1 voltage unun and the Common Mode Current Choke and ignore resistive losses in the transmission line, and possibly insertion loss in a transmatch (“tuner”), we can determine the potential efficiency of our antenna system.
- For our QRP devices the efficiency varies between 81.6% and 90% across the bands
- For our QRO devices the efficiency varies between 90.9% and 93.5% across the bands
This conclusion is based on the assumption that there is no loss in the antenna itself. We are treating the antenna, the transmission line, unun and CMCC as the “antenna system”. I have made no allowance for SWR losses for the reasons stated in the introduction to this post.
What a load of old codswallop!
I am an expert in the sense that “X” is an unknown quantity and “spurt” is a drip under pressure. I may be completely wrong; I may have fallen off my horse and bumped my head on a rock. I may have come to a fork in the road and taken it as Yogi Berra once famously said. If you would like to correct me on any wrong assumptions please do so. I receive a lot of direct emails from readers and, while they are most welcome, if you write a comment to this post instead it may trigger an interesting technical discussion here.
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