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W8FGW
07-07-2008, 01:52 PM
I am trying to find plans for a two meter Yagi antenna with the following specs and I have not found anything!


* 2 Meter Yagi Antenna 8' or Less
* I don't care how many elements are used... whatever is needed.
* 9db of gain or more.
* Boom construction Aluminum 8 feet in length or less
* Driven Element (Folded Dipole)
* Feed Point: Coaxial balun
* Vertically Polarized Setup
* Not for SSB use.


I have found many variations of these specs but none seem to show exactly how they made the antenna. For instance I have found many portable units with wood construction in the 6 to 8 foot boom lengths. I wanted to use this as a permanent antenna on my roof so I really want to make it out of aluminum. (These are the supplies that I have.) I also have seen many computer designed layouts but I am really looking for plans that have already been implemented so I know that it will have a good SWR. I am very new to antenna building so I really want lots of detail. I have recently checked out older versions of the ARRL handbook (2005) and Antenna Book (2003). They seem to have very small yagis (tape measure build) and very long high performance antennas. Nothing in the middle. I was thinking of buying the latest ARRL Antenna Handbook but I wanted to know if they have actual plans for an 8 foot long yagi. I did find some ARRL handbooks from the 70's that have these designs but they dont seem to go into a lot of details about how it was feed and what kind of performance they were getting.

Aside from all of the specs above I have tried to put a yagi together based off of a few different peoples designs. I have not had any luck with the SWR. One question that I have is when making the driven element... I am using a folded dipole design. Should the entire driven element be insulated from the boom? The boom is made of aluminum.

I also wanted to know how I should place the balun after I solder it... Can I coil it around itself and zip tie it to the boom or should I let it hang down? Should I run my coax from the back of the antenna or should I let it hang down and attach it to the insulated mast? As you can see this is some of the detail that I am really looking for.

If anyone has any ideas where I could find some detailed plans like this I would really appreciate it. I am willing to purchase plans or buy any book.

Thanks,

W8FGW :)

WB3BEL
07-07-2008, 02:20 PM
There is a lot of information about 2 meter antennas on the web. So you should be able to find something to use as a starting point. Maybe it wont be a complete detailed set of plans, but probably you will find enough to make a decent antenna.

I am wondering why you insist on folded dipole Driven element? This restricts the designs that you can use.

For example there is a pretty good design of a wideband six element yagi here:

http://www.dk7zb.fox28.de/2m-short/6-ele.htm

This is not a complete set of plans, but if you look around at other antennas on this site, you will see how to build this.

I am also wondering what kind of problem you had with your previous attempts. How high was the VSWR? At what frequency? Did it have good VSWR at some frequency? A folded dipole can be insulated from the boom or can be connected at the exact center of the middle of closed part of the loop opposite the feedpoint. (Not grounded at the feedpoints).

To make a VHF yagi you have to use materials that match the design. If you change the element diameters you must compensate. If you change the method of mounting (Insulated/NonInsulated) Bracket style etc you must compensate. If you change the boom diameter you may need to compensate.

So if you already have the materials on hand, and want to use them, you have to know how to modify the existing design to do these compensations.

I'd be glad to help you out. Let us know what went wrong the first time and you will likely be successful on the next go around.

As far as feedline routing, there are lots of options. Off the back of the boom is cleanest, but is kind of a mechanical hassle. As far as the balun, you can likely mount to the boom and zip tie it. I would not leave it swinging in the breeze as it will likely fail from wind induced vibrations.

73, Harry WB3BEL

WA9SVD
07-07-2008, 02:43 PM
Just aa a comment, since I haven't researched your "requirements" in detail...

1. You specify a given boom length. Regardless of the element NUMBER, gain WILL be more dependant upon boom length than element number. Element number (and placement) will affect feedpoint (to some extent) impedance, and beamwidth (radiation pattern, including F/B ratio) more than actual gain for a given boom length.

2. Be wary of designs that make unrealistic claims, or try to compare gainfigures that mix dBi and dBd gain figures. Remember, a gain figure based on a "dbi" figure will always have a 2.15 dB advantage over a figure based on a semi-real world dipole, real world fiigures can be much more complex.

3. Radiation patterns are often shown with a clear "near and far" field horizon; obstructions (buildings, even trees, etc. of a conductive nature) can upset the radiation pattern in the far (and fare, far, DX) field.

W0LPQ
07-07-2008, 04:01 PM
Another comment .. since you specify vertical ... you ARE aware that you don't want to mount the antenna by itself on the mast vertically..! That puts the elements in line with the mast which is a bad thing.

Horizontal is a non issue, but vertical is.

W8FGW
07-07-2008, 05:44 PM
The mast is made of wood.... does that make a difference?

thanks

K2WH
07-07-2008, 05:53 PM
Use a simple gamma matching system. This allows all the elements to be bolted directly to the boom, no insulators are needed at all.

K2WH

WB3FXW
07-07-2008, 06:24 PM
I have the ARRL Antenna Book. It comes with a CD of programs ... one of particular interest to you is "YW" ... meaning Yagi for Windows. Using this program I have designed a 6 element 2 meter beam that is 7.7 feet in boom length.

The design criteria you have posted is very similar to what I have developed using YW.

The specs of the beam are (144 MHz, 146 MHZ and 148 MHz):

Gain - 17.2, 17.5, 17.8 all dBi
F/R - 20.7, 20.9, 21.8 all dB
SWR - 1.1, 1.0, 1.2 to 1

The driven element is a dipole length aluminum tube, 38" long and it is matched with a gamma match that YW calculates for you. You can get assembly tips from the Anntenna Book text under the Yagi section. I encourage you to get the book. BTW, a gamma match will provide a great SWR match over a large bandwidth ... easily covering the entire 2 meter band. The CD also has a scaling program which will allow you to take a proven antenna from another frequency range and rescale the components to the band you wish to construct the antenna for. That's exactly where I started when developing my antenna. HTH

73,

Buzz

WB3fXW

WB3BEL
07-07-2008, 06:43 PM
I doubt the gain is 17+ dBi must be a typo.

The wood mast will be okay if varnished otherwise fiberglass would be better.

I personally don't like gamma match and prefer direct coax feed. Less to go wrong and fewer places for water to get in.

KC7YPJ
07-07-2008, 07:01 PM
http://www.hamuniverse.com/2ssbyagi.html

that will get you started, yes it exceeds your boom length by a little over 8".
yes it is optimized for the ssb portion of the band, a simple matter of trimming to correct.

The article has a link to the free software used to design it.
if the 8'8" length is unacceptable you can redisgn it as a 5 element rather than 6.

if you have a little extra cash pick up a fiberglass shovel (or any other long handled yard tool) handle and use that for your mast.

W8FGW
07-07-2008, 07:12 PM
Thanks everyone for the replies. I think that I have a good idea of where to start and the different options I have. One question I have is where I should position my mast if I want to use it vertically. Can it be located in the center of the boom or on one end behind the reflector? Does this vary with the design?

Thanks,

w8fgw

WB7DMX
07-07-2008, 07:24 PM
all the 2-meter antennas I have ever seen and used, have the elements mounted to one side of the boom and the boom can then be mounted to the mast so that the elements are in the vertical position because the elements will not be in line with the mast.

I think it would be very difficult to build a beam antenna for two meter and when mounted to the mast with the elements vertical to be in line with the mast, in fact it would be almost impossible to do.

WB3BEL
07-07-2008, 07:25 PM
If the mast is non conductive like fiberglass or dry wood, then it can be placed pretty much anywhere. Although I'd leave a few inches of clearance to the nearest element. Mounting it near the center of the boom will make the antenna structurally stronger for an antenna this long.

I'd really recommend to do a direct coax feed. If you want a DC ground, this is pretty easy to obtain by designing the antenna for 30 or so ohm impedance and making the driven element a bit shorter and then using a hairpin match.

You can easily figure out hairpin or beta match dimensions using the freeware program Hamcalc available at:
http://www.cq-amateur-radio.com/HamCalcem.html

Another good source of yagi design information is at:
http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm

This shows how do scale designs based on differences in element diameter, or boom mounting style or diameter among other things.

Also it might be good reading to go to W4RNL (SK) website for some yagi theory and background. It requires registration but is very useful.
http://www.cebik.com/content/index.html

All of these sites will likely be more useful for VHF/UHF antenna design than the ARRL handbook.

VK3NEA
07-07-2008, 08:07 PM
Try this calculator it will give you everything you want to know
for a beam that has a folded driven element and balun

Click on Download -Antenna calculator for DL6WU yagis As well as calculating the dimensions it also calculates balun details, antenna stacking spacing, SWR from power/voltage readings, analyses an existing antenna in cooperation with Yagi Optimiser (available elsewhere), estimates gain for existing antennas etc. courtesy of John VK5DJ


http://vkham.com/Software/downloads.html#Antennas

Alan.

VK3NEA
07-08-2008, 07:14 AM
By using the calculator with 5 directors
gives a boom length of 8' 4-3/4" and a suggested gain of 9.8 dBd
so your right saying about 9 db give or take..


VK5DJ's YAGI CALCULATOR

Yagi design frequency =144.10 MHz
Wavelength =2080 mm
Elements contacting a metal boom 25.00 mm in diam.
Director/reflector diam =8.00 mm
Radiator diam =8.00 mm

ELEMENT LENGTHS AND SPACING
Reflector
1023 mm long at boom position = 30 mm

Radiator
Single dipole 983 mm tip to tip at boom posn =446 mm
Folded dipole 1002 mm tip to tip at boom posn =446 mm

Dir Length Spaced Boom position Gain Gain
(no.) ( mm ) ( mm ) ( mm ) (dBd) (dBi)
1 929 156 602 4.9 7.1
2 920 374 977 6.6 8.7
3 911 447 1424 7.9 10.0
4 903 520 1944 8.9 11.1
5 896 583 2527 9.8 12.0

Director spacings are measured from the previous element
Tolerance for element lengths is +/- 6 mm

Boom position is the mounting point for each element as measured
from the rear of the boom and includes the 30 mm overhang.The total boom length is 2557 mm including two overhangs

The beam's estimated 3dB beamwidth is 51 deg

Here are some construction details for a folded dipole

Measurements are taken from the inside of bends
Folded dipole length measured tip to tip = 1002mm
Total rod length =2047mm
Centre of rod=1023mm
Distance HI=GF=469mm
Distance HA=GE=508mm
Distance HB=GD=547mm
Distance HC=GC=1023mm
Gap at HG=15mm
Bend diameter BI=DF=50mm

If the dipole is considered as a flat plane (see ARRL Antenna Handbook) then its resonant frequency is 142.2MHz and K is 0.951

YOU ASKED..
<I also wanted to know how I should place the balun after I solder it... Can I coil it around itself and zip tie it to the boom or should I let it hang down? Should I run my coax from the back of the antenna or should I let it hang down and attach it to the insulated mast? As you can see this is some of the detail that I am really looking for.>

Just zip tie the balun to the boom no need to coil it.
you can mount the beam on a metal outrigger or horizontal support. If it is the only antenna on the mast or at the top you will be better of with a timber or fiberglass mast section.
Your coax can run along the boom and down the mast with no effect to your radiation pattern.

I would set the design frequency in the center of the 2 m band and not at the bottom
the advantage of the folded dipole is that it is broad banded
Hope this helps.
Alan.

K9STH
07-08-2008, 03:02 PM
FXW:

The program that you are using obviously must be the same one that certain antenna manufacturers use. You just cannot get over 17 dBi gain from that short of a boom. Like NEA points out, just under 10 dBd (or around 12 dBi) is much more like it.


DMX:

Unless you mount a vertically polarized yagi either a "ways" off the side of the mast or else mount it so that the entire antenna is in front of the mast, the mast will definitely skew the pattern if the mast is conductive. It doesn't matter if the elements are mounted to one side of the boom, the distance between the mast and the elements is going to only be the diameter of the boom and this is just not enough spacing to keep the boom from interfering with the radiation pattern.

Yes, I have mounted a single vertically polarized yagi directly to a metal boom. Yes, the antenna has worked. However, the actual gain and directivity of the antenna are no where near optimum. That is one of the reasons (plus the additional gain) that my 2-meter vertically polarized yagis are mounted on a horizontal section of tubing that goes on either side of the mast on my main tower.

Glen, K9STH

WB3FXW
07-08-2008, 04:27 PM
Hi, Glen,

The gain figures came directly from the ARRL program YW ... not something I made up. If you can't trust the ARRL, who can you believe. I have built antennas from YW output and they have performed well. However, I have not done any independent verification of the output specs. Even without regard to the gain figures, YW does model excellent antennas. BTW, the 6 element beam is supported in the middle of the boom on mast of PVC pipe ... no metal ... the boom is aluminum, however. If you can tell me how to post a screen print here, I will provide the output of YW for you to see.


73,

Buzz

WB3FXW

WB3FXW
07-08-2008, 04:57 PM
Glen,

I think I figured out how to get the YW output to the forum. It is attached as a file. HTH.

73,

Buzz

WB3FXW

WB3BEL
07-08-2008, 05:42 PM
Buzz,
Your attachment explains it. Your calculations have ground gain where everyone else was referring to free space. So the antenna in free space is about ~ 12 dBi or a bit less maybe and when mounted 24 feet above the ground will have 17 dBi at a peak angle of 4 degrees elevation.

73,
Harry WB3BEL

WB3FXW
07-08-2008, 05:56 PM
Harry,

I should have mentioned the "modeled in place" aspects of the antenna. I never model in free space, only where the antenna is going to be installed. BTW, the antenna is a great performer.

73,

Buzz

WB3FXW

K9KJM
07-09-2008, 08:27 AM
Build a "Quagi" Very easy to build, And they WORK! Simple to match for the FM part of the band instead of the SSB portion. Just make the driven element slightly shorter.

To use a metal boom instead of wood, As the plans state, Just mount the elements to one side of the boom.

And as pointed out by Overbeck, Baluns do NOT work well at VHF/UHF!!!!!!

As already stated, A vertical antenna will NOT work well with a mast in it's center! The real answer is to build a pair of short antennas and feed them in phase mounted on a horizontal mast boom for extra gain, Getting the mast out of the pattern.

http://commfaculty.fullerton.edu/woverbeck/quagi.htm

N8EKT
07-09-2008, 02:29 PM
A 4 element Quad can be END mounted thus eliminating the pattern distortion of center mounts and will easily produce over 9 Dbd.

Boom length will be less than 5 feet.
And direct feed with 50 ohm feeline means to matching system loss.

WA9SVD
07-09-2008, 02:38 PM
I doubt the gain is 17+ dBi must be a typo.

The wood mast will be okay if varnished otherwise fiberglass would be better.

I personally don't like gamma match and prefer direct coax feed. Less to go wrong and fewer places for water to get in.

With all due respect, I also question the 17+ dBi gain. That's approximately 15 dB referenced to a dipole; and according to the NBS designs (which are typical, but are within 0.5 dB or so of other designs) a Yagi with a boom length of 4.2 wavelengths (at 144.200) MHz would be over 28 feet long, yet provide only 14.2 dBd gain.

Going to an 8' (actually, about 8' 2", or 1.2 wavelengths) boom length, typical gain will only be 10.2 dBd.
There's really no magic that can change that gain figure by more than a few tenths of a dB.
As the boom length gets longer, the increase in gain increases more slowly.

But it appears the 1.2 wavelength boom antenna (just over 8') will meet the originator's needs. The problem is the driven element and matching system to be used. The folded dipole is a balanced antenna, so a suitable balun will be needed if coax is to be the feedline. But proper placement of the Driven element and matching system can greatly influence the feedpoint impedance, so it's not safe to assume a folded dipole in a Yagi configuration will have a 300 Ohm nominal impedance; it will probably be lower; perhaps much lower.

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