Saturday, January 28, 2012

Radio Frequency Ground

A good Radio Frequency, RF, ground is as important to having a strong good quality signal as a good Safety Ground is to a safe operating environment. 

RF grounding is a very broad subject.  It includes but not limited to topics such as coaxial shield, antenna elements, ground planes above, on and below the ground, in-shack grounding, baluns, coaxial chokes, bead chokes, sometimes called bead baluns, wave guides for the microwave folks, and the list goes on.  However, for purpose of brevity this post will be limited to some approaches for preventing or solving RF in the shack.

First, how might you know if you have RF in your shack?  Here is a list of common problems that indicate that you might need to work on improving RF grounding.

                Getting RF burns from metal surfaces of the equipment

                SWR fluctuations when you get near or touch metal surfaces

                Reports of poor quality audio, usually a buzz or distortion[1]

                Poor receiver reception or noticeable drop in receiver reception

                A change, usually for the worse, in SWR[2]

                High SWR on one band but not others

So what to do? 

First, if this is a new problem, check all your ground connections to be sure that they are all secure.  This means check that all coax connectors are screwed down snugly[3] and the same for all ground lugs on station equipment, ground rods, and all safety ground connection points. 

There is one thing that I’ve observed many new hams doing that often is at the root of RF grounding problems.  They fail to provide any type of RF ground at all in the mistaken idea that their safety ground will serve as an RF ground.  This is not completely in error.  The truth of the matter is that safety ground does contribute in some part.  The problem is that safety ground is seldom sufficient for radio frequencies especially on HF.  On higher frequencies this is not as often a problem in some part because most antennas for VHF/UHF provide built-in RF grounds. 

RF grounding problems often show up at a particular band while other bands are fine.  This situation is because something in the station installation is providing the necessary ground for all the frequencies except the one that is causing problems.  This is particularly true for tuned random wire antennas that often used on HF.  Almost all end feed wire antenna must have a tuned length counter poise[4] for each frequency of operation.  I will have more on this later when I discuss random wire antennas.

In cases of balanced antennas fed with coax, a balun, be it wire wound, coiled coax or ferrite beads, will usually resolve RF in the shack problems.  This point is often neglected when balanced antennas, such as dipoles, loops, and yagis are discussed.  For best results with these antennas something should be done to accommodate the interface between an unbalance feed and a balanced antenna.

If you have installed a balun and still have RF in the shack, you might measure the feed line to see if it is a ¼ wave length[5] on the frequency that is giving you problems.  Sometime a feed line can act as a “receive” antenna and pick RF.  And it does not have to be the feed line that is in use.  This is especially true if the feed line runs parallel or even close to parallel to the antenna that is in use.  (Running feed lines in parallel to antennas should always be avoided) 

The suggestions in the posting are by no means exhaustive.  However, if you are having problems with an existing installation or are building a new installation the things discussed here should be followed as a minimum.

[1] - On VHF/UHF this might be multi-path.  If the problem come and goes it may be multi-path.  Multi-path will be the topic of a future posting.
[2] - This may take place slowly over time.
[3] - There was a recent article in QST where the author promoted using pliers for tightening PL-259 coax connectors.  I would recommend using caution.  This may be appropriate if you expect a lot of vibration such as a mobile installation.  However, I cannot remember ever having a connector back off on its own.  I recommend you use fingers to tighten connectors.  If you expect vibration or encounter this then a turn or two of electricians tape will go a long way toward keeping things in place.
[4] - A counter poise is a length of wire cut for a specific frequency that provides a RF ground.
[5] - A quarter wave or multiple of quarter wave (¾ - 1 ¼ - 1 ¾ etc) can cause this to occur. 

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