Reading here and there that the signals of the on-going DX-expedition to
Glorioso Island are generally very low, I got the curiosity to simulate the
so-called "spiderbeam" antenna they are using (sized for the 10-meter band) on
EZ-NEC.

Doing that, I obtained an unexpected result. The simulated antenna shows a clear
SWR minimum at 29.0 MHz where impedance is 76 + j32 ohm.

I then checked SWR across the 24 - 34 MHz range with the following results:

- going up in range 29 - 34 MHz, the reactance steadily increases (+334 ohm at
34 MHz)

- going down in range 29 - 24 MHz, the reactance remains positive and steadily
increases up to 28.5 MHz, after which it starts to decrease, until it becomes 0
ohm at 27 MHz, and negative below that frequency. At 27 MHz impedance is 9 + j0
ohm (hence it is the resonant point).

I knew that the resonant point does not precisely coincide with the minimum SWR
point, but I would not have suspected such a big difference (2 MHz shift at 29
MHz!).

Any comment?

Tony I0JX
Rome, Italy

"Antonio Vernucci" wrote in message
.. .
Reading here and there that the signals of the on-going DX-expedition to
Glorioso Island are generally very low, I got the curiosity to simulate
the so-called "spiderbeam" antenna they are using (sized for the 10-meter
band) on EZ-NEC.

Doing that, I obtained an unexpected result. The simulated antenna shows a
clear SWR minimum at 29.0 MHz where impedance is 76 + j32 ohm.

I then checked SWR across the 24 - 34 MHz range with the following
results:

- going up in range 29 - 34 MHz, the reactance steadily increases (+334
ohm at 34 MHz)

- going down in range 29 - 24 MHz, the reactance remains positive and
steadily increases up to 28.5 MHz, after which it starts to decrease,
until it becomes 0 ohm at 27 MHz, and negative below that frequency. At 27
MHz impedance is 9 + j0 ohm (hence it is the resonant point).

I knew that the resonant point does not precisely coincide with the
minimum SWR point, but I would not have suspected such a big difference (2
MHz shift at 29 MHz!).

Any comment?

Tony I0JX
Rome, Italy


that is not surprising for an antenna that has a very low or very high
impedance at the resonant point. The SWR depends on the magnitude of the
impedances not the angle, so you could have a minimum SWR with a big
reactance and small real component.

On Sat, 19 Sep 2009 18:03:25 +0200, "Antonio Vernucci"
wrote:

I knew that the resonant point does not precisely coincide with the minimum SWR
point, but I would not have suspected such a big difference (2 MHz shift at 29
MHz!).

Any comment?

Hi Tony,

What did you expect it to be?

73's
Richard Clark, KB7QHC

Antonio Vernucci wrote:
I knew that the resonant point does not precisely coincide with the
minimum SWR point, but I would not have suspected such a big difference
(2 MHz shift at 29 MHz!).

There's a thread over on eHam.net dealing with this same subject.
Many complex antennas exhibit this effect to a certain extent. The
reason is obvious. Our SWR meters are calibrated for 50 ohms and
an antenna may be resonant with a e.g. 9+j0 ohm feedpoint impedance.

That's a 50 ohm SWR of 5.6:1 where almost 1/2 of the RF is rejected
at the antenna when 50 ohm coax is being used. If the 50 ohm SWR
drops below 5.6:1 somewhere else it necessarily must exhibit a
higher resistance and reactance than exists at the 9 ohm antenna
feedpoint.

Moral: There is nothing magic about 50 ohms. If you were using
a transmission line with a Z0 of 9 ohms with a 9 ohm SWR meter,
you wouldn't notice anything worth reporting.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Dave wrote:

. . .The SWR depends on the magnitude of
the impedances not the angle, so you could have a minimum SWR with a big
reactance and small real component.

That's not true. For example, impedances of 50 + j0, 35.36 + j35.36, and
0 + j50 ohms all have the same magnitude (50 ohms), but a 50 ohm cable
connected to loads of those impedances will have SWRs of 1, 2.41, and
infinity respectively.

Correct formulas for calculating SWR can be found in the ARRL Antenna
Book, the ARRL Handbook, or any respectable transmission line text.
Incorrect ones can, I'm sure, be found on the Web and elsewhere.

Roy Lewallen, W7EL

Antonio Vernucci wrote:
Reading here and there that the signals of the on-going DX-expedition to
Glorioso Island are generally very low, I got the curiosity to simulate
the so-called "spiderbeam" antenna they are using (sized for the
10-meter band) on EZ-NEC.

Doing that, I obtained an unexpected result. The simulated antenna shows
a clear SWR minimum at 29.0 MHz where impedance is 76 + j32 ohm.

I then checked SWR across the 24 - 34 MHz range with the following results:

- going up in range 29 - 34 MHz, the reactance steadily increases (+334
ohm at 34 MHz)

- going down in range 29 - 24 MHz, the reactance remains positive and
steadily increases up to 28.5 MHz, after which it starts to decrease,
until it becomes 0 ohm at 27 MHz, and negative below that frequency. At
27 MHz impedance is 9 + j0 ohm (hence it is the resonant point).

I knew that the resonant point does not precisely coincide with the
minimum SWR point, but I would not have suspected such a big difference
(2 MHz shift at 29 MHz!).

Any comment?

Tony I0JX
Rome, Italy

Check the Alt Z0 option button at the upper left of the SWR display.
What happens to the minimum SWR frequency? Then change the Alt SWR Z0
value in the main window to some other value, say 300 ohm. What effect
does that have?

Interesting, isn't it?

Roy Lewallen

"Antonio Vernucci" wrote in
:

....
I knew that the resonant point does not precisely coincide with the
minimum SWR point, but I would not have suspected such a big
difference (2 MHz shift at 29 MHz!).

Any comment?

VSWR is not defined in terms of the conditions for resonance.

The characteristic of some kinds of antennas (including half wave dipoles
and quarter wave monopoles over ground) with resonant impedance near 50
ohms is that the R component of feedpoint Z varies slowly with frequency
around resonance (X=0) and X varies relatively quickly with frequency
around resonance. Because of this, in the region of resonance (X=0), X
tends to dominate VSWR(50) and the VSWR(50) minimum will be quite close
to where X=0.

Whilst many folk equipped with MFJ259Bs or the like, and with less
understanding, tune such an antenna for X=0, it is likely that the higher
priority for system efficiency is to tune for VSWR minimum. Worse, they
often do it at the source end of some length of transmission line.

I canvass the issues in the article "In pursuit of dipole resonance with
an MFJ259B" at http://vk1od.net/blog/?p=680 , you may find it
interesting.

Owen


Tony I0JX
Rome, Italy

"Cecil Moore" wrote in message
...
Antonio Vernucci wrote:
I knew that the resonant point does not precisely coincide with the
minimum SWR point, but I would not have suspected such a big difference
(2 MHz shift at 29 MHz!).

There's a thread over on eHam.net dealing with this same subject.
Many complex antennas exhibit this effect to a certain extent. The
reason is obvious. Our SWR meters are calibrated for 50 ohms and
an antenna may be resonant with a e.g. 9+j0 ohm feedpoint impedance.

That's a 50 ohm SWR of 5.6:1 where almost 1/2 of the RF is rejected
at the antenna when 50 ohm coax is being used. If the 50 ohm SWR
drops below 5.6:1 somewhere else it necessarily must exhibit a
higher resistance and reactance than exists at the 9 ohm antenna
feedpoint.

Moral: There is nothing magic about 50 ohms. If you were using
a transmission line with a Z0 of 9 ohms with a 9 ohm SWR meter,
you wouldn't notice anything worth reporting.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com


Actually, there is something 'magic' about 50 ohms. An air-dielectric
co-axial cable has minimum loss per metre when its characteristic impedance
is 76.7 ohms and the relative permittivity of polythene is 2.26 so a
polythene-dielectric co-axial cable has lowest loss when its characteristic
impedance is 76.7/SQRT(2.26) = 51 ohms, which is most often rounded down to
50. This is on the basis that the conductor loss greatly exceeds the
dielectric loss, which is true over most of the frequency range for which
solid polythene dielectric is appropriate.

Maximum power handling, for a polythene-dielectric cable, occurs at a much
lower impedance: 30/SQRT(2.26) = 20 ohms.

Chris

Check the Alt Z0 option button at the upper left of the SWR display. What
happens to the minimum SWR frequency? Then change the Alt SWR Z0 value in the
main window to some other value, say 300 ohm. What effect does that have?

Interesting, isn't it?

Roy Lewallen

Yes, changing the Alt Z0 makes a dramatic effect, and setting it to 9 ohm
obviously causes the minimum SWR point to shift from 29 to to 27 MHz (reaching
1:1).

Interesting to note that, using a 75-ohm cable, one can get a perfect match to
the simulated spiderbeam antenna in two possible ways:

- either cancelling the antenna reactance using a -32 ohm series-capacitor. One
then gets a (nearly) perfect match at 29 MHz, where antenna impedance is 76 +
j32 ohm

- or using a 9:75-ratio transformer. One then gets a perfect match at 27 MHz
(where impedance is 9 + j0 ohm)

Another interesting observation is that, at 29 MHz (i.e. where the antenna
impedance is 76 + j32 ohm and the SWR on a 75-ohm cable shows the minimum value
of 1.95) one can find a cable length at which the impedance appears to be purely
resistive and equal to 1.95*75 = 146 ohm (or 75/1.95 = 38.5 ohm). This fact is
deceiving as, seeing a purely resistive impedance, one could be led to
concluding that the real antenna resonant frequency is 29 MHz, whilst in reality
it resonates at 27 MHz (although knowing what is the real antenna resonant
frequency may not be so important).

I raised the above arguments just as a confirmation of the fact that
understanding what to do before attempting to adjust antennas is not that easy.

73

Tony I0JX

Actually, there is something 'magic' about 50 ohms. An air-dielectric
co-axial cable has minimum loss per metre when its characteristic impedance is
76.7 ohms

I presume that the 76.7-0hm figure comes from a trade-off beween RF current and
conductor resistance. In other words, increasing the impedance value, the RF
current would become lower (for a given RF power), but the inner conductor
resistance would become higher because of the lower diameter needed to obtain
the higher impedance value (for a given outer diameter cable). And viceversa.


and the relative permittivity of polythene is 2.26 so a polythene-dielectric
co-axial cable has lowest loss when its characteristic impedance is
76.7/SQRT(2.26) = 51 ohms, which is most often rounded down to 50.

Under the assumption that dielectric loss is negligible, a permittivity 2.26
time higher than that of air results in a lower inner conductor diameter, for a
given outer diameter cable and a given impedance. Probably, lowering impedance
from 75 to about 50 ohm, the loss advantage one experiences thanks to the higher
inner conductor diameter needed for the lower impedance value is higher than the
loss disadvantage caused by the higher RF current (for a given RF power).

Maximum power handling, for a polythene-dielectric cable, occurs at a much
lower impedance: 30/SQRT(2.26) = 20 ohms.

I do not succeed to understand that statement. Maximum power handling is bound
to maximum temperature which is in turn bound to dissipated power. If 50 ohm is
the impedance at which minimum loss occurs (for a given RF power), why lowering
impedance to 20 ohm should result in a loss reduction. In the equation
30/SQRT(2.26) = 20 ohms, which is meaning of the figure 30?

I wonder whether you could indicate us a reference where all those trade-offs
are mathematically discussed.

73

Tony I0JX