On 9/27/97 10:21, Webster D. Williams at kr4wm@sccoast.net wrote:
>This is my last posting to the Ten Tec reflector about
>antennas/feedline- and is concerning putting the tuner
>at the antenna vs. putting the tuner at the rig end of
>the feedline.
Well, this might be my last posting, if people would stop confusing
resonance with efficiency. It is an easy mistake to make, since the hobby
is full of myths about it.
>When you put the tuner at the antenna end, what you are
>doing, in fact, is NOT tuning the antenna to resonance,
>rather, you're either inductively or capacitively "loading"
>the antenna. Of course, this resonates the "system", but
>your "antenna" is not "necessarily" brought to resonance
>by placing the tuner at the antenna.
See, you're confusing the issue. Resonance is a condition where inductive
and capacitive reactances cancel each other. How can the antenna NOT
resonant if this is the case?
Let's go back to the example of a free-space dipole cut for
self-resonance at 7 MHz. Now, cut a meter off each end. Since it is now
short, it will show a capacitive reactance. Now, change the remaining
wire by coiling it up until you acheive resonance at 7 MHz again. See?
The capacitive reactance has been cancelled out by the inductance
introduced by coiling the wire.
Now, tell me, how is this resonance any different than the original
free-space wire? It's not!
Now, consider putting that inductance into a separate box. How is the
resonance acheived using the separate box any different than the original
free-space wire? It's not!
> Remember, conjugate
>impedance matches are a function of XC cancelling XL to
>equal Z0 (where Z0 is desired impedance, which in our case,
>is most likely 50 ohms).
Right!
> It's possible that the antenna
>still "could" not be resonant, even though your feedline
>sees a 50 ohm load and source at the ends.
How? If there's no reactance, it must be resonant.
> What happens
>with the tuner at the antenna is that you simply don't
>lose any (read- "as much", all coax has some inherent loss)
>RF as heat in the feedline. What will happen (as in mobile
>HF antennas) is that the inductor of your tuner you placed
>at the antenna end will radiate a portion of the RF you're
>sending to the antenna.
Now you're confusing resonance and efficiency. (And a well-designed
inductor should radiate next to nothing)
> If you don't believe this, try
>resonating a 5 foot piece of wire on 75 meters with a tuner-
>the inductor will get HOT!
OK, this is a great example to show the difference between resonance and
efficiency.
The 5 foot wire with a tuner is certainly a resonant antenna system - it
just isn't very efficient! Let's look at why.
That inductor is going to have to be quite large to properly load your 5
foot piece of wire to resonance. And it is probably going to have
considerable currents running through it. If we could make a vaccuum-core
toroidial inductor with superconducting wire, the inductor wouldn't get
hot. Imagine that.
The problem here is that the losses in the inductor have become
significant with respect to the RF currents flowing in it. While our
hypotetical inductor probably can't be built (yet), there are ways to get
more efficiency. We could make the inductor into a toroid shape, which
allows more of the magnetic field strength to be contained. Air-core is
more efficient than most iron or ferrite cores. Using larger wire helps
(perhaps tubing would be more appropriate), and coating it with a highly
conductive metal (like silver) would help even more.
Probably tbe BEST example of this type of antenna is the AEA IsoLoop.
This is a resonant loop made out of a strap of copper. To reduce losses,
the strap is welded to the tuning capacitor, and the feedline is
inductively coupled to the loop. This antenna has an extremely high Q,
but is resonably efficient.
>There's NO WAY the 5 foot piece
>of wire is resonant on 75 meters, rather, the combination of
>the tuner AND the 5 foot piece of wire is resonant.
What's the difference?
> If the
>tuner was enclosed in a metal box, then none of the radiated
>RF would add to your signal. It would just heat up the inside
>of the box.
If the inductor is getting hot because it has losses, then it will get
hot inside a box or out. The fact that the RF is heating up the inductor
indicates that some significant amount of power isn't being radiated.
>Now if you enclose the tuner (remember, we're
>now talking about placing it OUTDOORS, at the antenna!) in a
>plastic box, it would contribute "some" to your signal.
The lumped components in the tuner need not radiate.
>The
>same principle applies to larger (read- "more closely resonant",
>remember, we're talking 75 meters and comparing to a 5 foot
>piece of wire) antennas, although to a lesser degree (less
>inductive loading equals less RF lost as heat).
Wrong! Less inductive loading means that lossly inductors won't lose as
much heat. Two inductors with the same loss will radiate the SAME amount
of heat, regardless of their values.
And while you're poking fun at 5 foot antennas on 75 meters, I'd like to
point out that some people are using EXACTLY this kind of antenna in
their mobile installations. The problem they face isn't one of resonance,
but EFFICIENCY.
Let me put it to you this way: A sheilded dummy load is perfectly
resonant. It just is extremely inefficient.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
Quote: "Not in a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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