Hi to all,

I read somewhere that, in the case of Yagis, ''in the range of 25-35
Ohm you get the
best balance between gain, pattern, bandwidth and element currents.'''

Is that true? And if so, I would like to have the theoretical
explanation behind this.

Thanks and 73 de Pierre

Sorry, there's no theoretical basis for declaring what the "best
balance" of those parameters is. So there's no theoretical basis for
deciding what the feedpoint impedance will be for the "best balance".

But. . .

If the Yagi impedance is very low, it indicates very strong coupling
between elements and high element currents. This indicates a sharply
tuned antenna which might have high gain if the losses are minimized,
but also narrow bandwidth. This is a common result of trying to squeeze
too much gain from too small an antenna. To understand why, try googling
"super gain" or "supergain" antennas or look this topic up in an antenna
text.

If the Yagi impedance is high -- close to that of a dipole -- it means
that there's very little coupling from the driven element to the
parasitic elements. Consequently, the parasitic elements won't have much
current with which to produce fields, and they won't do much. The
antenna won't have much gain relative to a dipole, and its pattern won't
be much different from a dipole.

So while a Yagi having an impedance outside very roughly the 25 - 35 ohm
range can still perform well in one or more respects, you should look
carefully at it to see what tradeoffs have been made.

Of course, this applies only to the resonant feedpoint resistance at the
center of the driven element, which can be transformed over quite a wide
range by various structures and networks.

Finally, the above comments are pretty broad generalizations, so they're
subject to numerous exceptions. But they're a good starting point for
understanding some basic properties of Yagis, and hold often enough to
be reasonable rules of thumb.

Roy Lewallen, W7EL

ve2pid wrote:
Hi to all,

I read somewhere that, in the case of Yagis, ''in the range of 25-35
Ohm you get the
best balance between gain, pattern, bandwidth and element currents.'''

Is that true? And if so, I would like to have the theoretical
explanation behind this.

Thanks and 73 de Pierre

"ve2pid" wrote in news:1174098010.745176.35010
@e65g2000hsc.googlegroups.com:

Hi to all,

I read somewhere that, in the case of Yagis, ''in the range of 25-35
Ohm you get the
best balance between gain, pattern, bandwidth and element currents.'''

Is that true? And if so, I would like to have the theoretical
explanation behind this.

I don't understand why there is such a relationship, if there is.

Yagis are often designed for a feedpoint impedance that is relatively
easily transformed to 50 ohms for the main transmission line.

A quarter wave transformer from 28 ohms to 50 ohms is relatively easily
made from two parallel 75 ohm lines. Perhaps that is the attraction to
the impedance range you mention.

Owen

Owen Duffy wrote:
"ve2pid" wrote in news:1174098010.745176.35010
:

Hi to all,

I read somewhere that, in the case of Yagis, ''in the range of 25-35
Ohm you get the
best balance between gain, pattern, bandwidth and element currents.'''

Is that true? And if so, I would like to have the theoretical
explanation behind this.

I don't understand why there is such a relationship, if there is.

I don't think there is such a relationship either. The feedpoint
impedance is part of the results from the design process, along with the
gain, pattern and beamwidth; but it does not determine any of those
other properties.

The so-called "best balance" between all these properties will depend
entirely on the personal priorities of the designer or the user.
(Antenna optimization software demands very clear instructions about
this, and forces you to think very hard about what you really mean by
"best".)

Many good designs do have a feedpoint impedance in that 25-35 ohms
region; but so do some real lemons, too, so it isn't a reliable
indicator.

As Roy says, yagis with much lower feedpoint impedances tend to have
high internal (I^2*R) losses, which increase rapidly as the impedance
falls and element currents rise. Those can be classified as poor
designs, simply because there are plenty of better alternatives.

Also, it is possible with many designs to increase the feedpoint
impedance towards 50 ohms by adding a director at very close spacing
(about 0.05 wl). That director has relatively little effect on other
performance parameters, so it can be added fairly late in the design
process as a means of matching. (After construction, that close-spaced
director also allows final adjustment of the matching, by bending the
ends towards or away from the driven element.)


Yagis are often designed for a feedpoint impedance that is relatively
easily transformed to 50 ohms for the main transmission line.

A quarter wave transformer from 28 ohms to 50 ohms is relatively easily
made from two parallel 75 ohm lines. Perhaps that is the attraction to
the impedance range you mention.

That is a very reasonable strategy: if the impedance comes somewhere
close to a convenient value for matching, then optimize it to exactly
that value.

For example, DK7ZB has developed a range of yagi designs optimized for
28 ohms. There are links to these and several other designs from:
http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Roy Lewallen" wrote

Sorry, there's no theoretical basis for declaring what the "best balance"
of those parameters is. So there's no theoretical basis for deciding what
the feedpoint impedance will be for the "best balance".

But. . .

If the Yagi impedance is very low, it indicates very strong coupling
between elements and high element currents. This indicates a sharply tuned
antenna which might have high gain if the losses are minimized, but also
narrow bandwidth. This is a common result of trying to squeeze too much
gain from too small an antenna. To understand why, try googling "super
gain" or "supergain" antennas or look this topic up in an antenna text.

If the Yagi impedance is high -- close to that of a dipole -- it means
that there's very little coupling from the driven element to the parasitic
elements. Consequently, the parasitic elements won't have much current
with which to produce fields, and they won't do much. The antenna won't
have much gain relative to a dipole, and its pattern won't be much
different from a dipole.

So while a Yagi having an impedance outside very roughly the 25 - 35 ohm
range can still perform well in one or more respects, you should look
carefully at it to see what tradeoffs have been made.


When we design antennas, we try to optimize the design for desired gain,
F/B, bandwidth. The impedance is secondary consideration, we can match it to
the feedline, but any transformation, matching adds losses. All the
parameters are interdependent and we can always try to aim for the best
desired compromise. In a typical Yagi, as Roy indicated we end up with range
of impedances that are appropriate for particular design.

I realized that Yagi has low impedance and I generally do not like any
matching and introducing unnecessary loses. The way for maximum gain, clean
pattern, great F/B, 50 ohm feedpoint impedance and no matching gizmos to me
was to go Quad and Quad/Yagi element combinations. Quads have higher
impedance and by adding elements, the impedance would drop to around 50
ohms.
My design goals were to in order of priorities: close to 50 ohms impedance,
best possible clean pattern and F/B, broad bandwidth and maximum gain. I
prefer better pattern over max gain. In order to get max gain one can tweak
the design for about +- 1 dB, while differences in major vs. minor lobes can
be in order of 10s dBs, which means much better S/N ratio and capability to
dig weak signals.
The results was series of designs from 3 el Quad, through 5 el. Razor (3Q, 2
Y), to 7 (8, 10 ) element Razors with log cell driven element and quad and
yagi parasitic elements, while achieving 50 ohm feedpoint.
I would not claim that 50 ohm was the indicative of best performance design
and should be considered "rule" for design, but that I managed to optimize
the arrays for best performance and minimum loses while achieving 50 ohms.
Later, when I wanted to further improve the designs or check them in
software modeling (the original designs were done on 2m antenna test range)
and started with 3 el Quad comparison and optimization, the results were off
and I did not get the chance to go back and follow the process in soft and
hard modeling and see where the discrepancies are.
Pictures of my 15m 7 el. stacked Log Razors are at
http://www.k3bu.us/razor_beams.htm
showing the 7 el. design having Yagi Reflector, Quad Reflector, dual Quad
driven log cell, Quad Director and two Yagi directors. Impedance was 50 ohms
and SWR 1:1.1 at the band edges. In real life, the Razors were head and
shoulders above the Yagi variety and helped me to cream bunch of world
records from VE3BMV.
So I guess the lesson is, one can achieve desired compromise and use any of
the design parameters as priority and work around, but there are limitations
as what would be the results. Back to Yagi, as Roy outlined, You could have
50 ohm dipole like Yagi (lousy F/B and gain, but "good" impedance and match)
or great pattern and gain at the price of lower impedance and some lossy
matching, which still outweighs the former.

73 Yuri, K3BU

"Yuri Blanarovich" wrote in
:

....
I realized that Yagi has low impedance and I generally do not like any
matching and introducing unnecessary loses. The way for maximum gain,
clean pattern, great F/B, 50 ohm feedpoint impedance and no matching
gizmos to me was to go Quad and Quad/Yagi element combinations. Quads
have higher impedance and by adding elements, the impedance would drop
to around 50 ohms.

Yuri, that is your approach, but it is not the only one.

Others of us quantify the expected transformation losses, and add them into
the gain equation to deal with the effects, making a design selection on a
rational basis rather that just excluding a whole bunch of solutions
because of a prejudice about matching loss.

Owen

On Mar 17, 1:22 am, Ian White GM3SEK wrote:
Owen Duffy wrote:
"ve2pid" wrote in news:1174098010.745176.35010
:

Hi to all,

I read somewhere that, in the case of Yagis, ''in the range of 25-35
Ohm you get the
best balance between gain, pattern, bandwidth and element currents.'''

Is that true? And if so, I would like to have the theoretical
explanation behind this.

I don't understand why there is such a relationship, if there is.

I don't think there is such a relationship either. The feedpoint
impedance is part of the results from the design process, along with the
gain, pattern and beamwidth; but it does not determine any of those
other properties.

The so-called "best balance" between all these properties will depend
entirely on the personal priorities of the designer or the user.
(Antenna optimization software demands very clear instructions about
this, and forces you to think very hard about what you really mean by
"best".)

Hmmm. Interesting comments considering below.


Many good designs do have a feedpoint impedance in that 25-35 ohms
region; but so do some real lemons, too, so it isn't a reliable
indicator.

As Roy says, yagis with much lower feedpoint impedances tend to have
high internal (I^2*R) losses, which increase rapidly as the impedance
falls and element currents rise. Those can be classified as poor
designs, simply because there are plenty of better alternatives.

Also, it is possible with many designs to increase the feedpoint
impedance towards 50 ohms by adding a director at very close spacing
(about 0.05 wl). That director has relatively little effect on other
performance parameters, so it can be added fairly late in the design
process as a means of matching. (After construction, that close-spaced
director also allows final adjustment of the matching, by bending the
ends towards or away from the driven element.)

Yagis are often designed for a feedpoint impedance that is relatively
easily transformed to 50 ohms for the main transmission line.

A quarter wave transformer from 28 ohms to 50 ohms is relatively easily
made from two parallel 75 ohm lines. Perhaps that is the attraction to
the impedance range you mention.

That is a very reasonable strategy: if the impedance comes somewhere
close to a convenient value for matching, then optimize it to exactly
that value.

For example, DK7ZB has developed a range of yagi designs optimized for
28 ohms.

And there he says: "For the VHF-Bands (50-50.5MHz, 144-146MHz,
430-440MHz) a radiation resistance of 25-30Ohm has the best balance
for gain, back- and sidelobes, bandwidth and SWR at tenable losses."

We have come full-circle. [g]

Also, DK7ZB in describing what is clearly an unbalanced connection of
parallel lengths of coax ("Classic" match) says, "2. This line is a
simplified coaxial sleeve balun to avoid sleeve-waves on the braid of
the cable running to the station.", which it is clearly not.


There are links to these and several other designs from:http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm

--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)http://www.ifwtech.co.uk/g3sek

N7WS

"Owen Duffy" wrote in message
...
"Yuri Blanarovich" wrote in
:

...
I realized that Yagi has low impedance and I generally do not like any
matching and introducing unnecessary loses. The way for maximum gain,
clean pattern, great F/B, 50 ohm feedpoint impedance and no matching
gizmos to me was to go Quad and Quad/Yagi element combinations. Quads
have higher impedance and by adding elements, the impedance would drop
to around 50 ohms.

Yuri, that is your approach, but it is not the only one.

Others of us quantify the expected transformation losses, and add them
into
the gain equation to deal with the effects, making a design selection on a
rational basis rather that just excluding a whole bunch of solutions
because of a prejudice about matching loss.

Owen

What is "irrational" with my approach finding the best configuration AND
satisfying my desire for no loss 50 ohm impedance match?
It was not prejudice but "what if I succeed" approach and after over 3
months of fiddling with variety of designs in Canadian winter/spring I
managed to find solutions avoiding matching loss, that I would have to add
another director at X spacing to compensate for.
For example my 3 el. quad, 50 ohm, no matching beat 7 el. KLM Log Yagi with
balun on 2m.
If you can come up with whole bunch of better solutions, I would be glad to
learn about them.

73, Yuri, K3BU

Wes wrote:
A quarter wave transformer from 28 ohms to 50 ohms is relatively easily
made from two parallel 75 ohm lines. Perhaps that is the attraction to
the impedance range you mention.

That is a very reasonable strategy: if the impedance comes somewhere
close to a convenient value for matching, then optimize it to exactly
that value.

For example, DK7ZB has developed a range of yagi designs optimized for
28 ohms.

And there he says: "For the VHF-Bands (50-50.5MHz, 144-146MHz,
430-440MHz) a radiation resistance of 25-30Ohm has the best balance for
gain, back- and sidelobes, bandwidth and SWR at tenable losses."

We have come full-circle. [g]

Why, so we have...

At the risk of going round again, I definitely wouldn't agree with that
statement as written. However, it is valuable to point out that 28 ohms
is one of those "convenient" impedances, which might otherwise have been
overlooked.


Also, DK7ZB in describing what is clearly an unbalanced connection of
parallel lengths of coax ("Classic" match) says, "2. This line is a
simplified coaxial sleeve balun to avoid sleeve-waves on the braid of
the cable running to the station.", which it is clearly not.

Agreed. However, there is an alternative for 50MHz because the
paralleled quarter-wave sections are a convenient length to be coiled up
to make a small, neat choke.

I had a 3-element 50MHz beam which had been thrown together using
existing gamma match parts, but it was picking up a lot of noise and
crud - on surprising on thinking about it, because the coax shield, the
boom and the mast were all connected together and acting as an antenna
for local noise. Since the feedpoint impedance happened to be about 28
ohms, it was very easy to convert it to a fully balanced feed with a
choke, and all the noise problems went away.


There are links to these and several other designs from:http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
I had a 3-element 50MHz beam which had been thrown together using
existing gamma match parts, but it was picking up a lot of noise and
crud - on surprising on thinking about it, because the coax shield, the
boom and the mast were all connected together and acting as an antenna
for local noise.

Sorry, made a t6po. That should read: "NOT surprising, on thinking
about it."


--

73 from Ian GM3SEK