[TowerTalk] Impedance Matching Losses

[K7GCO@aol.com]

In a message dated 8/31/01 6:09:38 AM Pacific Daylight Time, 
RCARIELLO@si.rr.com writes:

<< 
 Impedance matching losses.
 
 The efficiency of the antenna is not being questioned.
 Only the loss of power when coupling to it.
 
 When attaching the transmission cable to the antenna we try to have equal
 impedance values with no reactance. A good example of this is the center
 feed half wave Dipole antenna. The antenna's 72 Ohm feed point couples to
 50-Ohm coax with a simple balun. Of course using 72 Ohm coax would be better
 but then the mismatch would be at the radio. In any case there is a slight
 loss of power but nothing to be concerned about.
 
 When we try to use small antennas the feed point impedance generally becomes
 very low. An example to this is a small magnetic loop transmitting antenna.
 By adjusting the antenna size and material the efficiency of the antenna can
 be made to around 90%. But even with this example the feed point impedance
 will only be around 1 ohm with some reactance. Coupling the 50ohm coax to
 the antenna is done easily enough but what is the amount of power being
 lost.
 
 A ground mounted wire half wave vertical fed at its base. Its feed point
 impedance will be in the thousands of ohms with some reactance. Coupling to
 the 50-ohm coax can be done easily enough but what is the amount of power
 being lost.
 
 Rich AA2MF
  >>
Rich:  One test is to run KW for 10 minutes and if the inductor of the L 
network is cool or barely warm, the transfer of RF from the coax to the 
antenna is very efficient.  That's not the full story of end feeding.  How 
much RF Spill Over is there down the coax shield and its affect on the 
pattern?  Or how much goes into a ground rod or raidial system what ever it 
is and what affect is it. Since you can't see RF and it can be difficult to 
measure, run comparisons using a FS meter and distant hams with center 
feeding running the coax out at right angles for away vs end feeding.  That 
will give you some idea.  k7gco

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