Is that good SWR really good? Part 1

Is that Good SWR really Good? 

Lots of times I hear guys talking about their good SWR and I think to myself “that sounds too good to be true”. 

I want to caution anyone who has a low and especially a low flat SWR curve about a couple things.

The antenna has an impedance that changes with frequency.  If that impedance is a perfect match to your coax you will get a 1:1 SWR and that is good. However if there is any loss in the coax or in the balun or matching device between the coax and the antenna terminals you will not get the correct SWR reading at the transmitter end of your coax.

The SWR meter at the input or transmitter end of the coax can measure the forward voltage or power at that point. Let’s call that 100 watts of forward power. If you have only 3 dB of coax loss only 50 watts will make it to the antenna terminals. 

In all cases there will be an amount of power lost simply due to the inherent loss in the coax. In this example we will use 3 dB as the coax loss.  Assuming 100 watts to start with, if there is no reflected wave or power this will be a one time loss of 50 watts. With better or shorter coax there will be less power lost. At the antenna if there is a perfect match there will be no reflected power and the 3 dB loss or 50 watts lost is what we called the matched loss and that only occurs what the antenna is matched perfectly to the line. 

RG-8X coax that I use has a matched loss of 1.5 dB per 100 feet at 30 MHz. So for my example here it’s like we are using 200 foot of RG-8X on 10 meters. You would not ever want to do this but it makes a good example case.

So once our 50 watts makes it to the antenna, if matched it is simply radiated, however if there is any mis match at the antenna we will have reflected power.  If the SWR at the antenna was 3:1 we would have 12.5 watts of the 50 watts reflected. As it travels back down the line to the transmitter we will lose half of that power (coax loss is 3dB) or 6.25 watts. So far out total loss is the original 50 plus 6.25 more as a result of the reflection. It does not stop here. The full 6.25 watts will be reflected again back toward the antenna. On its way back again half will be lost. That’s an additional 3.125 watts lost and 3.125 reaches the antenna. Of that 3.125 reaching the antenna about 0.78 will be reflected. And again on the way back to the transmitter half of that ( 0.39) will be lost. If we stop here we can add up the loss. 50 + 6.25 + 3.125 + 0.39 = 59.39

We started with 100 watts and lost 50 due to the coax and an additional 9.3 watts because of the 3:1 SWR. 

That original loss was 3 dB. The total loss has increased from 50 to 59.39 watts. Expressed in dB that is 3.91 dB lost.

If we subtract the original 3dB we see that the SWR has increased the loss by 0.91 dB. This is by our calculation.  

The chart below shows the curves that depict this additional loss. We find the line loss when matched across the bottom and in our example it was 3 dB. If we go up to the SWR = 3:1 curve and over to the left we see that the additional loss caused by SWR is 0.9 dB. Pretty close. Remember that the 3 dB power loss is not exactly half the power but a very close approximation.