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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