>Hi Jim,
>
>Re your post on towertalk on the C-4, What does X stand for in your
>measurments.
Hi Ted,
The X is the reactive part of the impedance; the contribution from either
inductance, were the antenna is electrically longer than the test frequency
1/2 wavelength, or capacitve were the antenna is too short for resonance
for the test frequency. An antenna will have X=0 at only one, or a very
narrow range of frequencies. If X is measured to be zero over a broad
frequency range and/or the vswr is quite low over a wide frequency range,
immediately suspect too much loss at connections or in the feed line.
these conditions should only hold over a narrow span of frequencies.
When X=0 at a particular frequency, the antenna is "resonant" at
that frequency. That is, the energy will be entirely dissipated, read
radiated, into the purely resistive radiation resistance, read load, of
the antenna. All the energy will be radiated, or about 100% efficient
conversion of your signal reaching the antenna input terminals becomes
a radiated signal; this will be the transmitter output power less any
losses to I-squared-R heating in resistance at junctions, and in
the feedline's ohmic resistance, which is always finite -- that's the
best we can do.
One way of imagining, sort of, about what is going on, is to realize that
when there is some amount of reactance, X not being zero, then energy
can get "stuck" for a time in a reactive field; that is stored as an
electric
field in a capacitance, or stored in a magnetic field in an inductance,
this
condition does not match the output load which the transmitter wants to
see, nor the receiver, for that matter. So maximum power transfer in
either direction cannot occur! And, some transmitters, solid state
ones in particular, will go ahead and cut back their actual output
power attempt under such a condition.
However, antenna tuners can bring the entire antenna system to
resonance; that is, eliminate the reactive component so that the
antenna system: antenna, feedline, etc- appear to both the incoming
and outgoing signals to be purely resistive, with X appearing to the
receiver and transmitter to be zero throughout the antenna system,
and all the signal gets through, radiated, or received!
When this is the condition, then you get close to the most efficient set
up, which, of course, is what you want. That is why there are so
many different manufacturers offering antenna tuners to the
amateur market. Even broadcast stations use antenna tuners,
at least we did at KCOK in Calif. where I worked as engineer
for a time ('53-54)
However, you must buy an antenna tuner which itself is low loss, and
will cover all the nine amateur bands upon which you plan to operate.
Often the lower cost tuners will not cover 160 meters at all, and
some have troubles with both 80 and even 10 meters at the high
end of our nine H.F. frequency bands. One of the better ones has
just been introduced for a "reasonable" price, around $600 from
Ameritron; they call it the ATR-30. Have not seen one myself.
I went for another new one, Palstar's rather costly AT4K tuner,
but I often run legal limit power, and wanted a very low loss tuner.
The AT4K even has a blower one can turn on to cool things inside.
This sort of cooling would only be necessary for very high duty cycle
modes, such as RTTY, or the new PSK31 phase shift
digital mode, in which Walt, AH6OZ and I are quite interested.
There is a given amount of resistive loss in every conductor/
component in any of our gadgets, so at legal power max, there
are about 9.5 amperes of peak current (about 7 amps rms)
flowing around in a 50 ohm circuit!
So, even 2 ohms of resistance would result in 128
watts of power loss and heat dissipation! We want to
keep ohmic resistance very, very low in a full power set up, hi.
So you buy what you need for what you plan to do, or think you
might be doing, hi!
Interesting, right. Obviously, the use of an antenna tuner gives one
a lot of operating flexibility when you want to do a lot of QSY'ing
within a band, and make frequent band changes, should you be
using a multiband antenna--- tribander, G5RV, random length
doublet, etc. Of course you must take the time to reset your
tuner with a band or great frequency QSY, so takes a little
time. That is why, in a contest station, so much time is
spent with the antenna system -- to get it nearly "flat"
across the intended contesting frequencies, the
"run" frequency in particular! There is to be no time
wasted during a serious contest effort fooling with an
antenna tuner, hi!
73, Jim, KH7M
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