I created this web page, not as much for you as for me. I wanted a place to store all my ham radio information, links, etc. This page is the result of my selfish effort. But, isn’t that the essence of the web – to share information?

Feel free to explore, comment and link to this blog. Or, better yet, contribute something!

Wednesday, March 20, 2013

Antenna Tuners - Necessary Evil or Just Evil? - Part I

Nearly every ham who operates HF has an antenna tuner, some have more than one. I do. There is a useful purpose for them, but, they have to be used properly to be an asset. Used improperly, and they can be more trouble than they are worth.

Let's start with the basic idea. If you have a properly designed and tuned antenna, you don't need a tuner. You might need a matching network, but not a tuner.

MFJ 949 Antenna Tuner
What's the difference?

The difference is radiation. What is important is getting your signal out of your radio and into the atmosphere. The objective is to push as much of your signal in the direction you want it to go as possible.

So, the antenna system has two functions. First, to efficiently induce electromagnetic waves into the air. And, second, to couple to the transmitter in a way that will make it happy. No tuner can help with the first function; that is dependent on the interaction of the metal components of the antenna and the atmosphere. But, a tuner can help keep the transmitter happy by allowing it to maximize the transfer of its power to the antenna metal.

Make no mistake, you cannot determine the effectiveness of an antenna by measuring the SWR. You could tune a lightbulb to 1:1 SWR pretty easily, but you won't make many contacts on a 100w table lamp.

Let's look at another example. What about using a tuner to allow a 6 meter dipole to be used on 80 meters? You could probably use a tuner to match the transmitter to the antenna, but a 6 meter beam may not be able to couple 80 meter RF into the atmosphere very efficiently. Just because you can match it doesn't mean it is going to radiate. In fact, in this example, the feed line may be doing more than the antenna elements themselves.

As an former broadcast engineer, I miss the Field Strength Meter in tuning antennas. SWR is not an important measurement in a broadcast transmitter, especially in one operating under 30 Mhz. There are things other than SWR that will indicate mismatch problems. But, measuring exactly how much of your RF is going in what direction is critical. So, broadcast engineers get off their duff, and head out into the world with a meter that measures the relative strength of your signal in various directions at various distances.

I will go into some techniques for measuring your radiation pattern in Part II, but suffice it to say that the key to using field strength measurements is maintaining a database of measurements. For now, just be aware of the value of knowing how your antenna systems interacts with the environment.

No comments:

Post a Comment