>I need some educating. I always believed that it was the
>LOAD at the
> end of a coaxial line that established the SWR and phase
> shift. With
> a receiving antenna, that load is the receiver or, in the
> case of the
> 4 square, the phasing box with its included circuitry, and
> ultimately
> the receiver. If this is true, why is it necessary to
> match the elements
> to 75 ohms other than for power transfer?
Because the source can also shift phase and change the
levels. Think of the antennas as small generators in series
with reactances and resistances before the signal reaches
the feedlines and phasing lines. It is not imporatnt the
elements be matched except for signal levels, but it is
critical they all be equal. When the antenna is high-Q
reactance will change different amounts with very slight
changes in tuning, temperature drift, or even with with
moisture.
>Mark, WA1ION, has done some
> experiments using just transformers to "match" various
> antenna types.
> I also read that an antenna could be "matched" by using a
> transformer
> that steps up the impedance to the same value as the
> antenna reactance.
> In other words, if a short vertical had a reactance
> of -1200 Ohms, it
> could be "matched" to 75 Ohms with a 1-16 ratio
> transformer.
No, it cannot. 75 ohms reactance terminating a 75 ohm line
is an infinite SWR.
A reactance can only be converted to a resistance by
applying an opposite sign reactance. say you had a short
whip with 35 ohms of ground terminal resistance. If you
applied 35-j1200 to a perfect 16:1 transformer the result
would be 2.19 -j75.
35 -j1200 is a 75 ohm SWR of 551:1
2.19 -j75 is a 75 ohm SWR of 68.5:1
> word match in quotes because this is not matching for SWR,
> just for
> reasonable signal transfer. Why would this not work for a
> 4 square
> system?
Because a reactance neither sources nor dissipates energy.
It simply shifts phase between voltage and current.
> I agree that all elements have to produce the same amount
> of signal
> and introduce the same relative phase shift to the signal.
> But, if
> you are not required to maintain 75 Ohms at the feedpoint,
> it would
> seem that any conductivity changes in the ground would
> affect all
> elements equally obviating the necessity for a low loss
> ground. '
That is not true for several reasons. The two most important
reasons are:
1.) With a very poor ground the feedline is involved in the
grounding process. It will act like an antenna unless it is
isolated from the feedpoint. The better the ground, the less
isolation you need.
2.) With a very poor ground it is unlikely each element will
have the same signal level as ground conditions change.
When I developed these short vertical systems I put
considerable thought and time into the best elements to use
in the array.
The best broadband element is an active element.
The best narrowband element is a reasonably tall element
with a large capacitance hat and base loading to cancel
reactance, with a series or shunt resistance to bring the
remaining resistance to 75 ohms. The reactance values in
relationship to the resistance mainly determine Q, and you
want a low Q element. This means the element should require
a minimum amount of series reactance and have a reasonably
high feed resistance. If you use a 300 reactance and the
feed impedance is 75 ohms, the element Q is 4. If you use a
short coil loaded whip the Q of the system might be several
hundred to a few thousand and tuning will be very critical.
> would only need a minimal ground to compensate for changes
> in ground
> conductivity in the vicinity of the elements themselves.
Correct. You need a stable ground, not a low loss ground.
The only other concern is the coax shield has to be isolated
from the ground so it is not part of the radial system in
cases where the ground is very poor.
73 Tom
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