I wonder whether anyone can formulate a convincing answer to the following
question.

Let us assume you wish to design a 3-element Yagi antenna for good performances,
but with NO REGARD AT ALL TO ITS IMPEDANCE (i.e. assume that you anyway intend
to have a matchbox at the antenna terminals, so that the antenna impedance is of
no concern at all for you).

At this point you run a good antenna design program, giving it no constrain in
terms of antenna impedance. The program will give you the "optimal" solution in
terms of element length and spacing, typically also showing the antenna
impedance at the feedpoint, that we here assume to be e.g. 30 ohm.

At this point you change your mind and, instead of using a 30-to-50 ohm
matchbox, you decide to match the antenna to the cable using the well known
hairpin technique, by which the radiator length is reduced somewhat (to create
some capacitive reactance in series with the antenna resistance) and an inductor
is put in parallel to the antenna (to resonate the residual capacitance after
resistance transformation).

Doing so the radiator length will result to be shorter that the length
calculated by the computer program, and one could then presume that the actual
antenna radiation characteristics (gain & front-to-back performance, radiation
pattern) do not correspond to those calculated by the program any longer.

Even though the changes may be small, I would anyway be interested to understand
whether, in principle, the hairpin technique results in a change of the antenna
radiation characteristics.

If the change is confirmed, we would inevitably also come to the conclusion that
the antenna radiation characteristics depend on the technique which is adopted
to match the antenna to the cable (e.g. matchbox or hairpin). This would sound
real odd!

Thanks and 73

Tony I0JX

On Sun, 4 Feb 2007 15:39:49 +0100, "Antonio Vernucci"
wrote:

I wonder whether anyone can formulate a convincing answer to the following
question.

Let us assume you wish to design a 3-element Yagi antenna for good performances,
but with NO REGARD AT ALL TO ITS IMPEDANCE (i.e. assume that you anyway intend
to have a matchbox at the antenna terminals, so that the antenna impedance is of
no concern at all for you).

Hi Tony,

That is a good start for discussion, but you abandon this care-free
attitude almost immediately:

...that we here assume to be e.g. 30 ohm.

Chances are it will be much less, possibly 5 Ohms. So, you see, you
have already introduced a constraint, and you continue adding mo

At this point you change your mind and, instead of using a 30-to-50 ohm
matchbox, you decide to match the antenna to the cable using the well known
hairpin technique, ...

Which leads to another constraint:

Doing so the radiator length will result to be shorter ...

As you are using a modeler from the beginning, you should also use
these constraints from the beginning.

Perhaps you are thinking of this as a two step process starting with
no constraints and then adding them. That works too and it requires
you to perform successive refinement. You can not escape this anyway.

If the change is confirmed, we would inevitably also come to the conclusion that
the antenna radiation characteristics depend on the technique which is adopted
to match the antenna to the cable (e.g. matchbox or hairpin). This would sound
real odd!

The change could be small. The question is how much change makes it a
"real odd" result?

73's
Richard Clark, KB7QHC

"Antonio Vernucci" wrote in message
...
I wonder whether anyone can formulate a convincing answer to the following
question.

Let us assume you wish to design a 3-element Yagi antenna for good
performances, but with NO REGARD AT ALL TO ITS IMPEDANCE (i.e. assume that
you anyway intend to have a matchbox at the antenna terminals, so that the
antenna impedance is of no concern at all for you).

At this point you run a good antenna design program, giving it no
constrain in terms of antenna impedance. The program will give you the
"optimal" solution in terms of element length and spacing, typically also
showing the antenna impedance at the feedpoint, that we here assume to be
e.g. 30 ohm.

At this point you change your mind and, instead of using a 30-to-50 ohm
matchbox, you decide to match the antenna to the cable using the well
known hairpin technique, by which the radiator length is reduced somewhat
(to create some capacitive reactance in series with the antenna
resistance) and an inductor is put in parallel to the antenna (to resonate
the residual capacitance after resistance transformation).

Doing so the radiator length will result to be shorter that the length
calculated by the computer program, and one could then presume that the
actual antenna radiation characteristics (gain & front-to-back
performance, radiation pattern) do not correspond to those calculated by
the program any longer.

Even though the changes may be small, I would anyway be interested to
understand whether, in principle, the hairpin technique results in a
change of the antenna radiation characteristics.

If the change is confirmed, we would inevitably also come to the
conclusion that the antenna radiation characteristics depend on the
technique which is adopted to match the antenna to the cable (e.g.
matchbox or hairpin). This would sound real odd!

Thanks and 73

Tony I0JX


Tony,
by using hairpin matching, you take out portion of the highest current on
the element and fold it into the hairpin where it is taken out of antenna
"participation" for the price of match.

Better solution is to feed the element off center (K7GCO idea), find the 50
ohm point, insert the insulator and feed it with balun. This way you
preserve the full size of the driven element and its coupling to parasites
and higher antenna efficiency.

Better way is to use quad elements where the stacked elements, higher
impedance and full size elements produce most gain per boom length (up to 5
elements)

73 Yuri, K3BU

On Feb 4, 2:35 pm, "Yuri Blanarovich" wrote:
[snip]

Tony,
by using hairpin matching, you take out portion of the highest current on
the element and fold it into the hairpin where it is taken out of antenna
"participation" for the price of match.

You're kidding right?

Disclosu I use a hairpin (beta) match on my HB Yagi.

"Wes" wrote in message
ups.com...
On Feb 4, 2:35 pm, "Yuri Blanarovich" wrote:
[snip]

Tony,
by using hairpin matching, you take out portion of the highest current on
the element and fold it into the hairpin where it is taken out of antenna
"participation" for the price of match.

You're kidding right?

Disclosu I use a hairpin (beta) match on my HB Yagi.


In that case I must be :-)

Can you elaborate why would I be kidding?

As far as I know:

If you use hair pin inserted in the middle of the element, you get the
shorter physical length of the element - smaller high current carrying
length.
Half wave resonant element has maximum current in the center, by folding
portion of that element into a hair pin we take that portion "out of the
picture". Just like a loading coil at the base of the vertical - current
drop along the coil.

Hairpin is usually folded back on the boom, 90 deg. to radiator, with any
current left, not participating in the plane of the elements.

Hairpin loading stubs were proven to be worse loading elements than good
quality coils.

You would be better off by inserting coil at the feedpoint instead of (Beta
match) hairpin, or use Delta match, or offset feeding at the 50 ohm pointas
mentioned before.

We might be splitting hairs here, but I hate to lose even fraction of dB if
there is a better way.

I know Beta match works, I used it in some antennas (don't like Gamma
matches), even made a QSO with a ligthbulb as an antenna. It's all relative.
Contesters like to chase every fraction of a dB lost, soon they can add up
to some noticeable real dBs.

YMMV

73 Yuri, K3BU

On Feb 5, 7:59 am, "Yuri Blanarovich" wrote:
"Wes" wrote in message

ups.com...

On Feb 4, 2:35 pm, "Yuri Blanarovich" wrote:
[snip]

Tony,
by using hairpin matching, you take out portion of the highest current on
the element and fold it into the hairpin where it is taken out of antenna
"participation" for the price of match.

You're kidding right?

Disclosu I use a hairpin (beta) match on my HB Yagi.

In that case I must be :-)

Can you elaborate why would I be kidding?

As far as I know:

If you use hair pin inserted in the middle of the element, you get the
shorter physical length of the element - smaller high current carrying
length.

I believe that your first problem is that you are considering the
hairpin (Beta) to be part of the radiator instead of considering the
actual case; it's part of the matching network.


Half wave resonant element has maximum current in the center, by folding
portion of that element into a hair pin we take that portion "out of the
picture". Just like a loading coil at the base of the vertical - current
drop along the coil.

op cit. I'm not "folding a portion of the element." If you subscribe
to the idea that part of the antenna can be "folded into" the feeder
and that by selecting the right feeder length you can "lengthen the
antenna" (as shown in a lot of old ARRL literature) then I have a new
limited space antenna for you. It's a one foot long radiator with a
variable length feeder (a la Cecil) that "makes up" the missing
antenna length.


Hairpin is usually folded back on the boom, 90 deg. to radiator, with any
current left, not participating in the plane of the elements.

If I placed a discrete (lumped element) L-network at the feedpoint the
current in it would not be "participating" either.


Hairpin loading stubs were proven to be worse loading elements than good
quality coils.

When, where, by whom, etc? Oh, BTW, did I mention "THE HAIRPIN IS NOT
A LOADING ELEMENT. It's the inductance in an LC L-network.


You would be better off by inserting coil at the feedpoint instead of (Beta
match) hairpin, or use Delta match, or offset feeding at the 50 ohm pointas
mentioned before.

Why? The hairpin will handle all the power an amateur can supply, it
can (does in my case) form an integral balun and it DC grounds the
element.


We might be splitting hairs here, but I hate to lose even fraction of dB if
there is a better way.

No gain is lost. I can model my Yagi with and without the Beta,
resonating the antenna without it by lengthening the element and the
gain remains the same within 0.01 dB.


I know Beta match works, I used it in some antennas (don't like Gamma
matches), even made a QSO with a ligthbulb as an antenna. It's all relative.
Contesters like to chase every fraction of a dB lost, soon they can add up
to some noticeable real dBs.

I've been a moonbouncer, I know all about fractions of a dB.

"Wes" wrote in message
oups.com...
On Feb 5, 7:59 am, "Yuri Blanarovich" wrote:
"Wes" wrote in message

ups.com...

On Feb 4, 2:35 pm, "Yuri Blanarovich" wrote:
[snip]

Tony,
by using hairpin matching, you take out portion of the highest current
on
the element and fold it into the hairpin where it is taken out of
antenna
"participation" for the price of match.

You're kidding right?

Disclosu I use a hairpin (beta) match on my HB Yagi.

In that case I must be :-)

Can you elaborate why would I be kidding?

As far as I know:

If you use hair pin inserted in the middle of the element, you get the
shorter physical length of the element - smaller high current carrying
length.

I believe that your first problem is that you are considering the
hairpin (Beta) to be part of the radiator instead of considering the
actual case; it's part of the matching network.


Do you have to shorten the (driven) element if you insert the hairpin in the
middle or not (to maintain the resonant frequency)? Telrex 40m Yagis did use
hairpins at the center of the element as a loading and shortening the
element length. To me that is the matching network too, but at the expenses
of shortening the element length - center loading, similar to base loading
the resonant quarter wave vertical.


Half wave resonant element has maximum current in the center, by folding
portion of that element into a hair pin we take that portion "out of the
picture". Just like a loading coil at the base of the vertical - current
drop along the coil.

op cit. I'm not "folding a portion of the element." If you subscribe
to the idea that part of the antenna can be "folded into" the feeder
and that by selecting the right feeder length you can "lengthen the
antenna" (as shown in a lot of old ARRL literature) then I have a new
limited space antenna for you. It's a one foot long radiator with a
variable length feeder (a la Cecil) that "makes up" the missing
antenna length.


You fold the portion of the element, it's called loading, at the base, in
the middle or anywhere along the element length. The folded hairpin has
inductance, just like a coil, and can be replaced with the coil of similar
inductance. The loading element is not the limited space antenna, look at
the currents at its ends and see the difference in curent distribution along
it and how it participates in the overall radiator current distribution and
corresponding area under the cosine curve representative of the efficiency.
We are back to the loading coil "problem" and that's why we pointed out the
efect and its impact on the antenna performance.


Hairpin is usually folded back on the boom, 90 deg. to radiator, with any
current left, not participating in the plane of the elements.

If I placed a discrete (lumped element) L-network at the feedpoint the
current in it would not be "participating" either.


Not much, but help with cleaner pattern.


Hairpin loading stubs were proven to be worse loading elements than good
quality coils.

When, where, by whom, etc? Oh, BTW, did I mention "THE HAIRPIN IS NOT
A LOADING ELEMENT. It's the inductance in an LC L-network.


Here we go again? We are talking about standing wave circuit - antenna
radiator or element. So inductance is not a loading element? As far as I
know inductance (coil, hairpin) or capacitance (top hat, L, T loading) are
used as a loading elements to shorten the physical length of the antenna
element, while maintaining electrical length.
There was an article by W6?? in CQ and other examples when they replaced
hairpin loading on Yagi elements with coils and got significant improvement
in the performance of KLM 3 el 80 Yagi, better gain, much better pattern due
to less interference of the folded back hairpin with the element.
Measurements and modeling before and after showed that.


pointas
mentioned before.

Why? The hairpin will handle all the power an amateur can supply, it
can (does in my case) form an integral balun and it DC grounds the
element.


No argument here, you can do that with coil and gain some edge.

We might be splitting hairs here, but I hate to lose even fraction of dB
if
there is a better way.

No gain is lost. I can model my Yagi with and without the Beta,
resonating the antenna without it by lengthening the element and the
gain remains the same within 0.01 dB.


No gain lost would be 0.0000 dB :-)


I know Beta match works, I used it in some antennas (don't like Gamma
matches), even made a QSO with a ligthbulb as an antenna. It's all
relative.
Contesters like to chase every fraction of a dB lost, soon they can add
up
to some noticeable real dBs.

I've been a moonbouncer, I know all about fractions of a dB.

Then you should appreciate the above.

73 Yuri, K3BU

Wes wrote:
op cit. I'm not "folding a portion of the element." If you subscribe
to the idea that part of the antenna can be "folded into" the feeder
and that by selecting the right feeder length you can "lengthen the
antenna" (as shown in a lot of old ARRL literature) then I have a new
limited space antenna for you. It's a one foot long radiator with a
variable length feeder (a la Cecil) that "makes up" the missing
antenna length.

Why didn't I think of that? The SWR on the ladder line on 40m
would only be 47,000,000:1 and the feedpoint impedance would
be 10 micro-ohms at system resonance.
--
73, Cecil http://www.w5dxp.com

This will be the last word I have on this topic.

I have placed two files he

www.k6mhe.com/n7ws/N7WS_Yagi_Resonant.EZ

and he

www.k6mhe.com/n7ws/N7WS_Yagi_Shortened.EZ

The files are models of my 20-meter three-element Yagi that are as
nearly representative of the physical antenna and its location as I
can make them. The only difference between them is the half-length of
the DE. The actual antenna uses the shortened version with a stub
(Beta) matching/balun arrangement.

Two photos of the details of this are he

www.k6mhe.com/n7ws/YagiFeed-1a.jpg

and he

www.k6mhe.com/n7ws/YagiFeed-2a.jpg

The photos were taken with the antenna mounted on the tower and the
tower folded over in case you're wondering about the orientation.

For the purposes of the discussion I have removed the stub matching
system from the model.

The following transformation and matching exercise can be performed
using a Smith Chart, your favorite computer program or with pencil on
the back of an envelope. I happen to prefer, and highly recommend,
AC6LA's XLZIZL.xls Excel workbook for this stuff.

First let's analyze the full-length, resonant DE version. After
running the analysis we (should) have a feedpoint Z of 26.76 +j0 and a
gain at the selected elevation of 12.91 dBi. The SWR is 1.87:1.

Instead of the integral stub, which Yuri believes is part of the
antenna that is "folded back" along the boom, I will move the matching
system away from this location using an ideal ½ wavelength (34.7 foot)
transmission line with an ideal current balun at the antenna end. I
don't believe anyone would argue that the feedpoint impedance is not
replicated exactly at the input end of this line.

At the input end of the lossless line, the Z is of course, 26.76 +j0.
Because, as will be shown, the stub matching system is nothing more
than an L-network; I will use the same at the input of the half-
wavelength line.

I begin by inserting a series capacitor, C = 448 pF and Q = 1000. At
the input side of this capacitor the Z is now: 26.785 -j25.062. If
using XLZIZL, the loss in this capacitor is shown as 0.004 dB, because
Q is not infinite.

Continuing, I place a shunt inductor, L = 0.6, Q = 200 across the
input of the series capacitor. The resulting input Z = 50.00 +j0.2.
The total network loss is 0.02 dB. This is the baseline.

Returning to the shortened driven element version, after analysis, we
find that the input Z = 24.55 -j25.2 and the gain is unchanged at
12.91 dBi. At the input end of our magical ½ wavelength line, the Z
remains 24.55 -j25.2.

Once again using the L-network system, I find that the series
capacitor is unnecessary and I can proceed by adding a shunt
inductance. Rather than using Yuri's "preferred" discrete inductor,
let's use a "lossy" stub. Instead of using the large diameter,
parallel tube stub of the actual antenna, I'll use a standard
transmission line for the stub. XLZIZL has a number if transmission
lines and their parameters "built in," including the Wireman ladder
lines. The parameters for these are those I derived in my ladder line
paper.

http://www.k6mhe.com/n7ws/Ladder_Line.pdf

Selecting Wireman 553, shorting one end and placing the other in
parallel with the input to the lossless line and doing a little
manipulation and I find that a 14.85" length makes the Z = 50.12
+j0.18. The network loss remains 0.02 dB. So much for this less than
ideal stub vs. Yuri's preferred discrete inductor.

There you have it. The stub matching method is equal to a discrete L-
network in efficiency, it does not detract from the antenna efficiency
one bit, it can incorporate the balun function without additional
components, it grounds the feedpoint, with a little sealant on the
cable, it is weatherproof and unlike Gammas and Tee-matches, it will
handle full power without being prone to capacitor breakdown. It is
not part of the radiator; it is part of the matching network. Period.

Wes wrote:
This will be the last word I have on this topic.

I have placed two files he

www.k6mhe.com/n7ws/N7WS_Yagi_Resonant.EZ

and he

www.k6mhe.com/n7ws/N7WS_Yagi_Shortened.EZ

The files are models of my 20-meter three-element Yagi that are as
nearly representative of the physical antenna and its location as I
can make them. The only difference between them is the half-length of
the DE. The actual antenna uses the shortened version with a stub
(Beta) matching/balun arrangement.

Two photos of the details of this are he

www.k6mhe.com/n7ws/YagiFeed-1a.jpg

and he

www.k6mhe.com/n7ws/YagiFeed-2a.jpg

The photos were taken with the antenna mounted on the tower and the
tower folded over in case you're wondering about the orientation.

For the purposes of the discussion I have removed the stub matching
system from the model.

The following transformation and matching exercise can be performed
using a Smith Chart, your favorite computer program or with pencil on
the back of an envelope. I happen to prefer, and highly recommend,
AC6LA's XLZIZL.xls Excel workbook for this stuff.

First let's analyze the full-length, resonant DE version. After
running the analysis we (should) have a feedpoint Z of 26.76 +j0 and a
gain at the selected elevation of 12.91 dBi. The SWR is 1.87:1.

Instead of the integral stub, which Yuri believes is part of the
antenna that is "folded back" along the boom, I will move the matching
system away from this location using an ideal ½ wavelength (34.7 foot)
transmission line with an ideal current balun at the antenna end. I
don't believe anyone would argue that the feedpoint impedance is not
replicated exactly at the input end of this line.

At the input end of the lossless line, the Z is of course, 26.76 +j0.
Because, as will be shown, the stub matching system is nothing more
than an L-network; I will use the same at the input of the half-
wavelength line.

I begin by inserting a series capacitor, C = 448 pF and Q = 1000. At
the input side of this capacitor the Z is now: 26.785 -j25.062. If
using XLZIZL, the loss in this capacitor is shown as 0.004 dB, because
Q is not infinite.

Continuing, I place a shunt inductor, L = 0.6, Q = 200 across the
input of the series capacitor. The resulting input Z = 50.00 +j0.2.
The total network loss is 0.02 dB. This is the baseline.

Returning to the shortened driven element version, after analysis, we
find that the input Z = 24.55 -j25.2 and the gain is unchanged at
12.91 dBi. At the input end of our magical ½ wavelength line, the Z
remains 24.55 -j25.2.

Once again using the L-network system, I find that the series
capacitor is unnecessary and I can proceed by adding a shunt
inductance. Rather than using Yuri's "preferred" discrete inductor,
let's use a "lossy" stub. Instead of using the large diameter,
parallel tube stub of the actual antenna, I'll use a standard
transmission line for the stub. XLZIZL has a number if transmission
lines and their parameters "built in," including the Wireman ladder
lines. The parameters for these are those I derived in my ladder line
paper.

http://www.k6mhe.com/n7ws/Ladder_Line.pdf

Selecting Wireman 553, shorting one end and placing the other in
parallel with the input to the lossless line and doing a little
manipulation and I find that a 14.85" length makes the Z = 50.12
+j0.18. The network loss remains 0.02 dB. So much for this less than
ideal stub vs. Yuri's preferred discrete inductor.

There you have it. The stub matching method is equal to a discrete L-
network in efficiency, it does not detract from the antenna efficiency
one bit, it can incorporate the balun function without additional
components, it grounds the feedpoint, with a little sealant on the
cable, it is weatherproof and unlike Gammas and Tee-matches, it will
handle full power without being prone to capacitor breakdown. It is
not part of the radiator; it is part of the matching network. Period.

Hi Wes,

A very interesting design and discussion. Thank you for posting it.

I could be wrong but I think Yuri's main point was that if [fill in
the blank] loading unit radiates at right angles to the beam pattern
(or out of phase with it) it detracts from the gain of the antenna. I
think that's probably a reasonable claim.

73, ac6xg