Hi Rich,
> 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
It better be a darned big loop antenna to reach 90% efficiency, and
far above earth!
> 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.
You should never interrupt the current path of a small loop with
pressure connections, or a feed system. You can gamma match,
or couple into the main loop via a small untuned loop that excites
the larger tuned main loop. Feed efficiency will be very high if loop
Q is high, the small (untuned) single-turn coupling loop will have
only feedline current and not the hundreds of amperes circulating
current the main resonant loop has.
The matching system is the least of the problems in efficiency,
feed system efficiency can easily get into the mid-90% range and
better with very little work.
> 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.
System loss depends on the ground system, capacitance of the
antenna base to earth, and many other things. Again the matching
system is the least problematic of anything. Feed system
efficiency can approach 100% easily.
73, Tom W8JI
W8JI@contesting.com
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