Last month a brief definition of a decibel was explained, along with the formula for power gains or losses.

 

We normally keep track of our Standing Wave Ratio. Some meters have a scale that indicate Reflected Power Percentage. Such as a 3:1 SWR indicates reflecting 25% power. Sounds like quite a loss, but how much is it in what we hear?

 

To give you an idea of how much difference you will actually hear, we will use a simple example.

 

Our Load is a resonant antenna or a pure resistive load at values other than 50 ohms. Either way the Load looks the same if we are right at the resonant point of the antenna.

 

Now we can look at a nomograph. What is that? Ho ho, it makes life simpler. It has forward power (horizontally) compared to reflected power (vertically). There are several straight lines at a 45 degree angle across the graph that represent various SWR values.

 

We will pick an unacceptable SWR like 10:1. On the SWR meter it will appear like nothing is going out. However on the nomograph at 10 watts forward, it shows reflected at 5 watts. Only half the power measured is reflected. How does that calculate in dB?

 

Using our formula from last month it figures at 2.3 dB loss. With most receivers this will be a little over 1/3 of an S unit. However at the same time, our ears will detect the loss.

 

This figure will vary with loads that are not a pure resistance and will increase loss with frequency. However it gives you an idea of what your actually loosing. So why is it at a 10:1 SWR you are not getting heard at all?

 

This is not due to reflected loss. The solid state radios will only provide a small amount of power out at this high of SWR.

 

If you are at 10:1 SWR due to moving frequency from where your SWR is 1:1, you will find by using a Tuner to cancel the reactances, you will allow the radio to put full power out.

 

Many have been surprised that communications can do well with this high mis-match when compensated by a Tuner.

 

This is a very controlled example, just to give you an idea of what a loss may sound like. It can get quite complicated, however, unless we are into antenna design, it is not necessary to be overly concerned.

 

If you are dealing with a high SWR, use the Tuner to reduce it. This will allow the radio to put maximum power out and it is a protection to the transistorized power amp. In many cases performance will be fine.

 

73,

 

Ralph WD0EJA

June 2022