About feed-line loss and SWR

 Antennas systems have gain and loss. In general 3 dB gain is equivalent to doubling your power and - 3 dB gain or 3 dB loss is reducing your power in half. Usually the loss is in the feed-line and any other component connected to the feed-line. . 

Let’s assume your transmitter is capable of delivering 100 watts of output power and we will go through what can happen to that power.

First of all we need to get the 100 watts into the feed-line or coax heading toward the antenna.

Most radios need to have a load connected that is a match for the fixed output circuit of the transmitter. That will usually happen if the system SWR is 2:1 or less. If not some sort of tuner can be used to perform the required matching to whatever the impedance is at the coax. There could be a loss of power at this point if not done properly.

After that there will be a loss of power in the coax. This loss is determined by the characteristics of the coax and the length of the coax. The loss will be expressed in dB. For new  RG 8X coax this loss will be approximately 1 dB per 100 foot at 10 MHz. So let’s assume we will use 100 foot of RG8X on the 30 meter band. 

The first loss we will incur is 1 dB. Are there any more losses? 

That depends on how well the antenna is matched to the coax. With a perfect match there will be no more losses in the coax, but we could lose another 1 dB if the SWR at the antenna was 5:1. This is additional loss caused by SWR in coax that has a matched loss of 1 dB. A SWR of 5:1 will cause less loss in coax that starts out with less than 1 dB loss and more loss in coax that starts out with more than 1 dB loss. In other words a 5:1 SWR will not cause 1 dB additional loss in a piece of RG8X less than 100 feet on 30 meters but will cause more than 1 dB additional loss if the coax is longer than 100 feet. 

Some antennas will operate with a SWR of 5:1 and the operator will use a tuner at the transmitter to allow the transmitter to supply full power to the system. In that case there will be a 2 dB loss in the coax whereas there would only be 1 dB loss if the match was made at the antenna. If using a tuner at the transmitter or even using the built in tuner, I suggest determining what the matched loss should be for your type and length of coax. Then determine the SWR at the antenna. (There are a couple ways to do this.)

With that information the chart below will tell you what your additional coax loss is due to SWR.

If you know the matched loss of your coax, the second chart is used to find the SWR at the antenna given the SWR at the transmitter. 

Other things can also introduce loss in the system. Components like transformers, baluns, poor connectors, bad connections and improperly adjusted antenna tuners are the most common. 

So how much power gets to the antenna? In our 30 meter example the best we can do is lose only 1 dB with a perfect antenna match. That would mean the of the 100 watts leaving the antenna 79 watts gets radiated or lost in the antenna.  That 1 dB is a 21% loss of power. Remember 3 dB is a 50% gain or loss in power. 

I have figured my feed-line losses to be the following:

160 meters. Less than 0.5 dB

80 meters.  Less than 1 dB

40 meters  Approximately 1 dB

20 meters.  Approximately 1 dB

15 meters.  Slightly greater than 1 dB

10 meters.  Approximately 1 dB

The worst case additional loss would be approximately 0.2 dB when operating with a SWR of 2:1 as my matched line losses are probably never more than 1 dB except on 15 meters.  My additional loss on 15 meters may be 0.2 dB when using my 40 meter dipole. 

In order to reduce losses I try and place my low frequency antennas farther away and my higher frequency antennas in locations where I can keep the feed-line short as possible. 

 Now we know that of 100 watts from the transmitter the best I can do is to get 79 watts to the antenna. If I operated any of my antennas with a high SWR such as 5:1, I would only expect to get 63 watts to the antenna as I would have 2 dB loss total. Remember 1 dB is 21% power loss, 2 dB is 37% power loss and 3 dB is 50% power loss.

Also keep in mind that the SWR measured at the transmitter end of the coax is not the true SWR at the antenna. The SWR at the antenna will always be greater. See the second chart.