> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
> Subject: TopBand: verticals
To: <topband@contesting.com>
> Date: Thu, 14 Aug 97 10:57:02 +0000
> >This is a long-held fallacy--that the higher impedance of the folded
> >unipole reduces the ground losses. The impedance transformation
> >of the folded unipole also transforms the ground loss part of the
> >equation in the same way and there is no net improvement
One could use the IRE definition of radiation resistance as
the NET current causing radiation. In that case the radiation
resistance would remain exactly the same. This is the definition used
to describe folded antennas on page 403 (Antenna Fundamentals
section) in Jordan and Balmain's "Electromagnetic Waves and Radiating
Systems".
The problem with using radiation resistance is several "definitions"
are floating around, and many of them are useless and the rest
often misused or misapplied.
> Interesting. The 'Services Textbook of Antennas and Propagation' by
> Glazier and Lamont defines radiation resistance as 'the resistive
> component of the input impedance of an aerial [antenna]'. On this basis,
> the radiation resistance must go up when the impedance goes up. But if
> the ground losses are not very low, why couldn't I measure any change in
> input impedance when adding radials - which you'd expect with alow
> radiation resistance? And why can't I measure any current in them?
Because input impedance often tells us very little about changes
downstream of that point.
Imagine a dipole built with full size elements in free space. Now
replace half of that dipole with a base loaded mobile whip. The
feedpoint resistance might decrease (if the loading inductor had
low loss), or it might stay the same ( if the loading inductor had
perhaps 30 ohms of loss). But in every case the antenna would work
poorer.
I obtained LOWER base resistance with resonant radials than with a
conventional ground system, yet field strength at a distance
DECREASED. The best way to measure anything is to measure what you
want to know, not some parameter far downstream of the end result.
No one in their right mind would tune a PA up by watching the AC
mains current, yet many very bright people insist they can measure
feedpoint resistance and determine output efficiency....all
without an output (field strength) meter.
At the feedpoint, you have NO idea how much of the power is absorbed
or radiated.... all you know is the power goes in the system. The
only thing that sorts that all out is a field strength meter in the
far field.
> Amongst professional antenna designers, I can find about a 50-50 split
> between those who hold that the radiation resistance goes up, and those
> that hold that it doesn't!!
That's because of the point of view. If they use the IRE definition,
R rad remains the same no matter how much the element is folded as
long as the current distribution and height doesn't change.
A folded dipole has the same radiation resistance as a regular
dipole of the same thickness and length, and both have almost exactly
the same radiation as an near end-fed wire. So whether the feedpoint
resistance is 2800 ohms, 280 ohms, or 70 ohms the radiation
resistance is about 70 ohms. Loss in every antenna, and efficiency,
remains identical. We can move the feedpoint all over the antenna and
radiation resistance remains the same.
But if we use only the feedpoint impedance as the "radiation
resistance", it becomes a nearly useless number. The resistance goes
all over the place and tells us NOTHING about system losses.
The impedance at the feedline connection will NOT include all ground
losses. We might add loss without changing the impedance at the
feedpoint, we might make the resistance LOWER while adding loss, or
HIGHER while adding loss. Once power leaves the point of
measurement, we have NO idea where the heck it goes without a FS
meter.
The only truly useful definition of radiation resistance is the IRE
definition. That definition is the TOTAL power radiated divided by
the square of the current causing the radiation, and it has nothing
at all to do with the feedpoint resistance.
> I always felt that I never could understand radio - how I've made a
> living at it for over 30 years I don't know!
Don't feel bad. One prominent editor of handbooks on the west coast,
who writes many antenna books, has no idea what radiation resistance
is and how radials work. He thinks folding the element decreases
ground loss and that using coax for a radial makes the radial "look
longer" and work better.
Try this Peter.
Picture the base current at the antenna. Whatever the NET current
flowing up is, and equal current has to flow at the ground system
connection. Herr Kirchoff won't allow anything else to happen.
Let's assume we have a giant clamp on current meter, and measure one
ampere. The ground current is one ampere (at the base, not
necessarily elsewhere). Now if we split the radiator down the
middle, and ground one half, each half of the antenna must carry one
half ampere. Our clamp on meter still reads one ampere, and the
ground terminal current is one ampere. The feedline current is one
half ampere, so the shield puts one half ampere into the ground just
as the other grounded half of the antenna does.
The only thing that changed is the feedpoint resistance, since power
remained the same and current halved....voltage doubled. The
feedpoint resistance seen by the coax went up FOUR times but the
ground system and antenna system NET current remained totally
unchanged.
If we used the IRE definition, radiation resistance remained
identical for both cases. If we use the nearly useless feedpoint
resistance "definition" of radiation resistance, it went up four
times. But in either case losses remain exactly the same.
Radiation resistance is commonly used to support false claims
and Voo Doo, because the term is so poorly defined and abused in
most text.
73 Tom
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