Homebrew 3 (4?) Element Vee Beam
I do not have ham antenna design software or I could answer these questions on my own...
I find myself in the possible situation of buying a house with much more antenna room than I am used to.
I'd like to put up a 40M 3 element Inverted Vee Beam.
If someone has the software, could you please help me out on these dimention questions?
....that the property is about 500' above average terrain
....I can get the wire's apex up about 65' in trees above the ground
What is the gain of a 3 element vee beam at optimum spacing?
What is the gain of a 3 element vee beam at closest spacing?
Same as above for a 4 element vee beam?
What if it were a 3 element bi-directional (radiating element...director on each side)?
My guess is that it would be halfway decent on 30M with a tuner.
Thank you in advance!
Hot Springs National Park, AR
One of THE largest amateur radio forums on the planet, and nobody has antenna software....
Hot Springs National Park, AR
You have asked a lot of questions that would take quite a bit of work for someone to research. I am sure a lot of people have antenna modelling software and you can too for free!
Just go here and download the latest version:
There is even a model in the distribution similar to what you are asking but assumes a single tower mounted support point.
It is in the directory HFbeams and is 3el-inverted-V.nec If you want to have multiple tree mounted supports then you can set the following symbols using the edit capability of the software: SY refa = 0 and SY dira = 0. Dimensions are in meters you can change the heights and inverted vee angles (One critical parameter you did not define) the spacing between elements, element length etc.
You can edit the file to add a fourth element if you really want to try it.
I'd recommend you go to www.cebik.com and register for access and read what the antenna guru LB Cebik W4RNL (SK) wrote about yagis. You can get an idea about what the tradeoffs are for element spacing. They will carry over to wire beams to a first order approximation. A 3 el short boom yagi may have 1.5 or 2 dB less gain depending on how much shorter it is than a long boom version. A four element beam may have 1 or so dB more gain than the 3 ele. It depends on the design details. I don't know how you want to optimize the antenna for gain or front to back or VSWR or maybe some combination. A wire yagi will have narrower bandwidth than a tubing based yagi so it may depend on whether you prefer SSB or CW or some combination.
Since the antenna will be fixed direction, there will obviously be a tradeoff in gain versus azimuth angle. You may find that a smaller antenna covers a greater azimuth angle range of interest for you with good gain. I have built and used tree supported wire yagi. I don't really like the inverted vee as much as the flat top versions, but understand that getting good tree supports is pretty tough. It is a lot easier the smaller the antenna is. A two element wire moxon is a lot easier than a 3 ele and a 4 element is even more challenging. The trees move with wind and these kinds of antennas are pretty high maintenance.
If you are primarily interested in DX you will want to look at how the gain varies with elevation angle over the ground. This is primarily a function of the height above the nearby ground and can be modelled in HFTA for the elevation angles you want to optimize. Maybe you don't have much control over this. 65 feet is kind of low for 40 meters but it is high enough to be in the cross over region where a good vertical array may have similar or better performance for DX. It depends on your soil conditions etc.
So download the 4NEC2 and play with this antenna model. Send me an email if you have questions.
The 3 el Yagi has been Analyzed to death for the past 50 years.
Just about every antenna book I have ever seen has the gain vs boomlength for a simple Yagi, Not worth the electrons to "Model" a more or less randomly supported antenna to the .001 db, or even 3 db.
Even if you could accurately model such a antenna, you could never hope to hear the difference on the air.
I agree that if the goal is to just get a feeling for the relative gain of various yagi variant antennas you can get a close estimate without modeling.
But if you get to the point that you want to actually build something that differs in dimension or materials (Tubing vs wire or tapered vs non-tapered etc) from the original design then modeling is the way to go. Also if you want to have it work in a different part of the band or maybe scale a design so that it works on a different band. Maybe you want to optimize gain where the other design had another goal like great F/B at very low SWR. Modeling is not that hard. When the model already exists in the package it is really about learning how to use the software. Also there have been so many antenna articles published that are full of errors or exaggerations that before actually constructing a design it is easy to make sure that it will work by validating it using antenna modeling software.
As far as getting a gain estimate that is within a tiny fraction of a dB without modeling, that is not accurate. Even with modeling the last tenth is probably not very meaningful unless comparing like to like.
As far as not being able to tell a few dB gain difference, that also is not completely accurate. I hear it all the time and it bugs me. It might be fair to say that if all you make are strong signal contacts then a dB or so won't be very noticeable. It is also true that for a single contact it may be hard to tell the difference in an A/B test. But in weak signal work like DXing or VHF that dB will sometimes make or break a QSO. It is sometimes surprising how much difference a dB or two will make. Going from a dipole to a two element yagi is only about 3 or 4 dB but the difference is like night and day. Going from a two element to three elements is only about 1.5 to 2 dB and it is not as obvious but it is readily apparent. You may argue that the improved s/n on receive is a big part of that improvement and that is true. But the improvement on the signal strength by itself is a big part of the equation.
There is a reason that top DXers and contesters use big yagi arrays that really only have a few dB better performance. Every dB means that they will make more contacts with the stations that they want to work.
Also all these improvements can be cumulative. A dB here and a dB there and before you know it your station capability is really beginning to shine.
We are talking a 40 m wire antenna here, not the Mars rover link.
I get a huge kick out of people that claim to be able to tell 1 db on hf. It's always the same story, some guy just bragging about how much he paid for his antenna.
I have had the good fortune to work on HF transmitters with a EIRP more than 5 MEGAwatts (50KW and 20 DBD rhombics), one director, more or less, on 40 Meters, is undetectable except in pipedreams.
Lets talk about modeling software then, How did you measure the ground conductivity-oops thats a guess.
How about the vegetation-hey another guess!
Maybe a accurate height above ground, Or are we going +/- 10 ft here?
There is a entire radio station in the near field, Isn't there? How about the wiring (couple thousand feet in the average house)-Got that in the model?
Bottom line, The graph in the 1947 handbook is as accurate in the real world, for a 40M wire beam, as the latest cad package.
Edit: I never bought the story some contesters use as to exactly WHY they need multiple amplifiers on their "Stacks", and there are PLENTY of stations on the air that think a pair of 3-500z's make a good DRIVER, Know what I mean?
Last edited by AI3V; 08-21-2008 at 09:12 PM.
Reason: Comments about contesters
I agree with Harry. The question was multifaceted and is best covered by a "good" web site like:
http://www.eznec.com/ (He has a free version)
or good books.
There are various FREE antenna modeling software packages. Yagimax is another one. Look here: http://www.qrz.com/download/antennas/index.html
There are also simpler programs like: http://bfn.org/~bn589/antenna.html
Some of us study a lifetime and still have more to learn. Antenna design can be quite fun.
The inverted V configured yagi will use the same basic formulas and have about the same gain (but slightly lower) as a conventional horizontal yagi.
A quick (but not exact) rule of thumb for easy construction:
1. Make three inverted V dipoles cut for the frequency of interest.
2. Cut 5% off of one and it becomes your director. (Add a second director if you wish)
3. Add that 5% to another dipole and it becomes your reflector.
4. Space them 0.2 to 0.25 wavelengths apart in a horizontal plane.
5. Feed the center dipole with coax and use alligator clips to find the proper delta feed points each side of center.
Your tree locations may require some spacing adjustment. Just use what you have and enjoy the gain in the direction that you constructed it.
Model it for optimum performance and try different feed methods. Have fun with it.
Terry Graves, K7FE
Chief Editor, QRZ.COM
"Some people call CW a MODE but in
reality it is an autonomous LANGUAGE."