On 31 May 2004 15:28:40 GMT, (Fractenna) wrote:

|I'm just about getting around to making my 5 or 6 element marine band yagi,
|covering 156 - 162 Mhz RECEIVE ONLY. Do I have to employ a folded dipole or
|can I use just a simple hertz dipole as the "driven" element? I'm trying to
|get away from using a folded dipole if I can. Any wideband designs out there
|that use a simple hertz dipole that I can scale? I'm looking, but if you
|have some links aready. TIA.
|
|
|A folded dipole as the driven element will have the best match over your
|frequency range..

Unless of course it doesn't.
|
|What you do have to worry about is the feedpoint resistance. Some Yagi-Uda
|designs are expressly designed for (a driven) folded dipole, which in isolation
|have a much higher feed resistance than 50 ohms. When you put parasitics in the
|near field, in certain gain-optimized Yagi-Uda designs, that feed resistance
|falls closer to 50 ohms.
|
|Typically, a non-matched regular dipole as driven has a feedpoint resistance of
|much less than 50 ohms in a high gain Yagi-Uda app.

Unless it doesn't. For the example I offered earlier:


Freq R X VSWR Gain F/B dB

156 51.50 1.97 1.050 9.95 14.34
157 50.12 2.77 1.057 9.99 16.43
158 49.34 5.22 1.112 10.04 19.20
159 50.38 8.31 1.180 10.08 23.41
160 54.16 9.95 1.230 10.10 30.84
161 59.78 5.68 1.229 10.07 28.01
162 57.99 -8.24 1.237 9.96 21.35

Lucky for those of us when it does, Wes.

Nice model. Seems to meet a certain need. It is possible to get better F/B and
F/S though. Not with your approach, however.

73,
Chip N1IR

On 01 Jun 2004 03:12:14 GMT, (Fractenna) wrote:

|Lucky for those of us when it does, Wes.
|
|Nice model. Seems to meet a certain need. It is possible to get better F/B and
|F/S though. Not with your approach, however.

I'm not taking credit. This is a minor adjustment of a Cebik design.
Seems to fit the OP's rqmts, however: no folded-dopole feed, 50 ohm
matched BW of 156-162 MHz and 5-6 elements.

Regards,

Wes

I'm not taking credit. This is a minor adjustment of a Cebik design.
Seems to fit the OP's rqmts, however: no folded-dopole feed, 50 ohm
matched BW of 156-162 MHz and 5-6 elements.

Regards,

Wes


This looks like a modofied approach to an NBS Yagi-Uda.

Modern Y-U design allows for a higher feedpoint SWR. Matching loss is offset by
higher gain and better consistency of F/B. If no matching, then a shaped,
folded dipole is used. Typically a F/B better than 22 dB (sim) is acheived with
a 10% bandwidth.

The design you discuss sounds more than adequate for this need though, in fact
most needs.

73,
Chip N1IR

Fractenna wrote:
I'm not taking credit. This is a minor adjustment of a Cebik design.
Seems to fit the OP's rqmts, however: no folded-dopole feed, 50 ohm
matched BW of 156-162 MHz and 5-6 elements.

Regards,

Wes


This looks like a modofied approach to an NBS Yagi-Uda.

The only special feature of the NBS designs (other than the prestigious
mailing address) was a self-imposed restriction to equal spacings
between directors. That turned out to be an evolutionary dead-end, and
you won't find it in any modern optimized yagi design.

The design proposed by Wes doesn't have that feature - fortunately - and
is actually one of the OWA (Optimized Wideband Array) family. One of the
special features of that family of yagis is a very close spacing between
the driven element and the first director, which raises the feedpoint
impedance to 50 ohms and allows a simple split-dipole driven element.

Even this feature is not original to the OWA family. I don't know when
(or indeed how often) it might have been invented, but it was first
systematically used by DL6WU some 25 years ago. With additional help
from computer analysis and optimization, the OWA family continued the
development.

Modern Y-U design allows for a higher feedpoint SWR. Matching loss is offset by
higher gain and better consistency of F/B. If no matching, then a shaped,
folded dipole is used. Typically a F/B better than 22 dB (sim) is acheived with
a 10% bandwidth.

We're now in the excellent position of having several alternative ways
to design a yagi to meet each user's specific combination of
requirements. That approach is certainly one of them.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

I can easily be mistaken, but I don't believe the NBS Yagi designs were
ever promoted as being optimal in any way. Their purpose, I believe, was
to provide a set of easy-to-duplicate designs whose gains were well
documented (although I understand an error was made in measuring the
originals) and which anyone could construct as reference antennas with
known gain.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:

The only special feature of the NBS designs (other than the prestigious
mailing address) was a self-imposed restriction to equal spacings
between directors. That turned out to be an evolutionary dead-end, and
you won't find it in any modern optimized yagi design.

The design proposed by Wes doesn't have that feature - fortunately - and
is actually one of the OWA (Optimized Wideband Array) family. One of the
special features of that family of yagis is a very close spacing between
the driven element and the first director, which raises the feedpoint
impedance to 50 ohms and allows a simple split-dipole driven element.

Even this feature is not original to the OWA family. I don't know when
(or indeed how often) it might have been invented, but it was first
systematically used by DL6WU some 25 years ago. With additional help
from computer analysis and optimization, the OWA family continued the
development.

Modern Y-U design allows for a higher feedpoint SWR. Matching loss is
offset by
higher gain and better consistency of F/B. If no matching, then a shaped,
folded dipole is used. Typically a F/B better than 22 dB (sim) is
acheived with
a 10% bandwidth.

We're now in the excellent position of having several alternative ways
to design a yagi to meet each user's specific combination of
requirements. That approach is certainly one of them.

Roy Lewallen wrote:
I can easily be mistaken, but I don't believe the NBS Yagi designs were
ever promoted as being optimal in any way. Their purpose, I believe,
was to provide a set of easy-to-duplicate designs whose gains were well
documented (although I understand an error was made in measuring the
originals) and which anyone could construct as reference antennas with
known gain.


They were in fact described as "optimum designs" in the original paper,
but it's clear that claim was made subject to a number of reservations.
Most of the hype came from antenna manufacturers who subsequently picked
up the designs from the published reports, and misused the "NBS" name to
boost the credibility of their own products.

With hindsight, the NBS designs were really quite good, and their
forward gain is still competitive with more modern designs of the same
boom length. But modern yagi designs are generally better, because there
have been a further 35-40 years of development and optimization. In
particular, the last 10 years have benefited from the availability of
computer modeling techniques - you can now do more good work in a few
evenings than the NBS program achieved in as many years.

As a result of this development, you can now have a better combination
of features - for example, cleaner patterns, wider bandwidth and/or
greater tolerance to dimensional errors, and easier impedance matching -
and keep the good forward gain as well.

Many modern designs have been developed as complete 'families', with
simple design rules that let you add or remove elements (changing the
boom length accordingly) to create new yagis, each of which will be
close to optimum for its boom length. The NBS yagis don't have that
'family' feature - each one is an individual design, and attempts to
adapt them have generally not been successful. This lack of adaptability
is a direct consequence of the original decision to use the same spacing
between all elements; that's why I described it as an "evolutionary
dead-end".

It costs exactly the same to build a good, modern yagi as it does to
build an older, inferior design - the only differences are in *where*
you apply the hacksaw and the drill. Therefore there's not much point in
building an inferior design... which is what the NBS yagis have now
become.

For many years, the greatest value of the NBS yagis was that they had
accurate gain measurements, so they could be used as reliable benchmarks
in antenna gain shootouts. (The known error in the gain measurements
applies to the 2-element yagi only.) Even that use has now been
overtaken by computer modeling.

In summary, the NBS yagis deserve respect for their major contribution
to the art and science of yagi design, but they are now mainly of
historical interest.

For examples of modern yagis, and tips on construction, see the 'VHF/UHF
Long Yagi Workshop' on my website.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek