TopBand: Radiation Resistance

[W8JITom@aol.com]

Hi Henry,

In a message dated 96-11-02 22:48:54 EST, you write:

>
>Average Joe Ham has no way to measure radiation resistance of an
>antenna, and even professionals can only measure antennas of
>small size.  Is this correct?

Radiation resistance is the TOTAL power radiated as an electromagnetic
radiation divided bt the square of the current at some defined point in the
system. It is never the feedpoint resistance expect by pure blind luck.

Often a word is added to define the point, like feedpoint, base, or loop
where the current is taken, but ALWAYS the entire power radiated as
electromagnetic radiation must be determined.

> If not, how can I measure the radiation
>resistance of my TH-7?

1.) By putting it in an enclosure of known loss that does not directly affect
the near field, but stops all radiation fields from escaping and measuring
the change in feedpoint charicteristics.

2.) By measuring enough data that you know the actual TOTAL amount of power
radiated in every direction, and knowing the current at a point you define
that causes that amount of radiation..
 
>What is the proceedure for measuring the radiation resistance of an
>antenna, step by step for us dummies?

Too long and out of context for topband. But in short, you almost certainly
can't.
 
>It seems that if I know the radiation resistance of the antenna, I
>can calculate the efficiency--Yes?

No. You need to know the loss resistance normalized to the very same point
where current was measured. Change in resistance at one point *by itself* is
absolutely meaningless for positively evaluating system efficiency. Remember
an antenna is a "transmission line transformer" that modifies resistances, so
what you measure may be due to a change in transformation rather actual loss
resistance changes.

This is the single most common reason voodoo antenna designs get published
and accepted as fact.

73 Tom


73 Tom
>73   Henry  WA0GOZ

Hi Henry,

Basicly ypu

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