Sorry. You are in error. A conducting structure need not be resonant at
the operating frequency to be an efficient radiator. The function of an
'antenna tuner' is ONLY to establish a 50 ¡Àj0 ohm match at the input to the
transmitter from whatever the antenna / transmission line at the shack end
presents to the input of the matching network. It does, *N O T*....N O T
(!!!) 'tune' the antenna!!! It is just an impedance transforming network
consisting of adjustable reactive circuit elements. Become a bit familiar
with the Smith Chart and you will better understand.
I can adjust my matching network (a.k.a., antenna "tuner"), for example, to
produce a 50 ¡À j0.0 match on 7.2 MHz with the feedline connected to the
input of my matching network (a.k.a., antenna "tuner"). The measured
impedance at 7.200 MHz of my HF doublet is 10 - j80 ohms. The function
of the matching network (a.k.a., antenna "tuner") is to transform in a
passive manner that 10 - j80 ohms to 50 ¡À j 0.0 ohms. It in no way 'tunes'
the antenna. However, adjustment of the matching network in NO WAY, alters
the basic resonant structure of my doublet. The matching network does NOT
effect the antenna structure, itself. It only establishes a match of
whatever the doublet at the end of my parallel conductor transmission line
at the shack end presents to the input of the matching network. The
resonant structure of the antenna does not change with the adjustment of
the matching network! ! ! ! ! Even after transforming the measured 10 -
j80 ohms to 50 ¡À j0 ohms, the lowest 1/2-wavelength resonant frequency of
the doublet still remains at 950 kHz.
Dave - W?LEV
toggle quoted message
Show quoted text
On Sun, Aug 16, 2020 at 6:39 PM KENT BRITAIN <WA5VJB@...> wrote:
The wire may be non-resonate without your home brew matching network,but
with the network it has to be. To be an efficient radiator, you haveto
resonate. Kent WA5VJB
On Sunday, August 16, 2020, 1:34:08 PM CDT, David Eckhardt <
davearea51a@...> wrote:
How many hams actually match *at the feedpoint* to a 50-ohm coaxial line?
I'll bet very few. At VHF and UHF, yes, at the antenna match is practical
and usually accomplished, but not at HF.
That's why I use open wire feeders where SWR losses are *far* less than in
coaxial cable and the feedline is not stressed even with SWR at full
power. And......I use a single set of wires for 630 through 6-meters with
that system with home brew matching network.
Antennas do not require being resonant to do a good job of radiating. My
system is not resonant (¡À jX = 0.00, the definition of resonance) in any of
the HF ham bands, but it does very well, both in practice and in the 4NEC2
model. It's lowest 1/2-wavelength resonant frequency is 950 kHz, the lower
1/3 of the AM BC band. I seriously doubt I'd do any better with resonant
dipoles for all the individual bands.
Dave - W?LEV
On Sun, Aug 16, 2020 at 6:07 PM Chris Wilson <chris@...> wrote:
Hello David,
Sunday, August 16, 2020
Without knowing what impedance the antenna itself presents, how do I
know the best way to match it to the coax impedance?
Best regards,
Chris mailto:chris@...
DE> What's important is the load the antenna plus feedline present to our
DE> modern transceivers. So, why the concern for 'at the antenna'
DE> measurements? Sure, its nice to know, but the coax contributes it's
own
DE> impedance transforming properties. If you must, make the measurement
at
DE> the shack end of the feedline, and use a tool such as SimSmith to
take
out
DE> the impact of the coaxial line.
DE> Again, what is important is the load presented to the transmitter,
not
DE> necessarily what the antenna impedance is at the feedpoint. One must
STILL
DE> consider the transmission line between the transmitter and the
antenna
DE> feedpoint to obtain this result.
DE> Dave - W?LEV
DE> On Sun, Aug 16, 2020 at 5:31 PM Jerry Gaffke via groups.io <jgaffke=
[email protected]>> wrote:
Roger wrote:
You can clearly see that the two plots are nearly identical.
What kind of coax?
I bet it isn't RG174.
I have seen significant differences when checking out an HF antenna
over
100 feet of RG8X.
I have a 20M dipole with a current balun attached to it which for
this
test I consider to be the "feedpoint" of the antenna.
Not totally clear which side of the balun you consider to be the
feedpoint
of the antenna
I assume this means you leave the balun attached to the antenna, and
only
calibrate out the coax.
Jerry, KE7ER
On Sun, Aug 16, 2020 at 09:47 AM, Roger Need wrote:
I have a 20M dipole with a current balun attached to it which for
this
test I
consider to be the "feedpoint" of the antenna. 55 feet of coax runs
back
to
the shack. First measurement with NanoVNA Saver was made at the
feedpoint and
stored as an s1p file. The coax was then "calibrated out" and a
second
Saver
measurement made back in the shack. The first s1p file was then
loaded
and a
comparison was made. You can clearly see that the two plots are
nearly
identical.
Roger
--
*Dave - W?LEV*
*Just Let Darwin Work*
--
*Dave - W?LEV*
*Just Let Darwin Work*
