Ham Radio - My durable and portable 2 meter Yagi antenna.

Hello, for the accuracy of measuring gain I would have made a simple 1/4 gp for the 0db reference. I am not a fan of the slim Jim, and although your rv is OK with the SJ mounted on a plastic pole it's not useable when mobile. I realise that's not your intension, but bear with me... There is a far better design (same height) for a 1/2 wave over a 1/4 stub called a topfkrise antenna easily made from copper pipe and not only very robust, I still am using one I made 40 years ago!!! But unlike a Slimjim it can be mounted against metal without any change to its resonant freq or swr. So it could be permentally mounted to the side of the rv or on the door mirrors of a truck. In fact for field use it can even be hand held, dropped into your backpack and still keep its 1:1 swr...
So back to your gain figures.. With a std dipole at 2.8 dbi, we can usually calculate gain by adding 1db for every element added.. Eg a 9 el gonna can be calculated as a dipole (2.8dbi) with 7 (7db) directors and one reflector (1db) making a total of 10.8 dbi gain. Very very close to the real gain of a 9el gonna antenna.
Using this technique, your antenna should be 2.8 plus 2db for the director and reflector.
This would give a more realistic gain of 4.8 dbi. Or 2dbd.
*1, I would also add that when using a 1/4 wave gp, one ought to not cheat by bending down the 4 radials to get a 1:1 swr on the usual wire and pl259 1/4 wave design as this is all ready heading towards becoming a dipole.
Better to use more radials or a metal mesh or tin plate to reproduce the real charistics of a test reference 0bdi antenna.
Even if the test antenna has a slight swr (as its not 50 ohms) the error is only a few %.
Same with a dipole, its not 50 ohms, but 75 ohms, and requires some form of impedance adapter to match the misguided chosen standard of 50 ohms (or 52 ohms).

Note *1, this is why I call DBI decibels italiano, as all the cb antennas produced in the 70's in Italy had inflated dB ratings, and were never as higher gain as claimed.
If you really want to increase the performance of this antenna, just use 75 ohm coax, (that's less lossy) and an impedance matcher at the rig to lower the impedance back to 52 ohms.
Your power test would then be able to show the increase in performance.
Another way of increasing gain is to drive two of the three elements. As per the HB9CV design, and add a director in front of the active elements.
This design also provides an easier way of tuning the antenna to match your 52 ohm feed. And again it's feed point at the center of mass. This design increases forward gain and has a better front to back ratio. You can still make this with plastic tubing,
I hope this information helps and perhaps will promote some to do some experiments and build a real 0db reference antenna for all their future projects.
F5vhz.

 

Nope. There are many models of Yagi type antennas. Towards so called optimized models we only can say every time boom length is doubled the gain increases theoretically by 3dB and practically slightly less (usually some 2.5dB) due obvious imperfection of real antenna. For non-optimized models doubling the boom length will increase the gain less and depending on model may have no practical sense. We cannot say each element overall or each director apart adds 1dB as this is not true. Sure one can find some particular model where one additional director increases the gain exactly by 1dB but one more director won't increase by same amount anyway.

 

On small antennas, beams up to 16 el it holds pretty accurate.
Take for example..
A 16 el vhf beam with a given forward gain of 17.55dbi gain, their data not mine.
Now take off the active element and you have 15 elements (15dbi) plus the dipole 2.8dbi this equals a total of 17.8dbi
Ok an error of 0.25db...big deal, you will find my method is pretty accurate.
Go back to the 9el tonna it has a gain of 10.8 dbi, remove one element and you have 9.8dbi..
Now your statement of doubling the boom length doesn't hold water as you say its +3dbi..that would make the 16 el 12.8 dbi... But in reality I'm correct with my 17.8dbi.. Your calc is 5db out!!
Why? Well putting two 9 or 8 el antennas up side by side gives 3db gain, minus patch lead losses... Where as the 16el beam is more efficient with its much narrower beamwidth.. Hence the better gain.
You won't prove me wrong, I've been doing this professionally for 20 years, and for over 40 years as a ham..
However making a vhf antenna longer than 20 elements, (used in eme) is not practicable. Stacking is the easy way to balance out loads but with just 3db more gain its not very cost effective.

16 Elements 2m The World's Best G/T -2 dB Low Noise Yagi Antenna PA144-16-12DGP
16 el and 17.55 dbi

 

Nope again. It has no sense unless we discuss particular model. Some 16el (assuming 2m band) will be some thirty feet while some other some forty feet. Longest model expected to have more gain by some 1dB than shortest: not a big deal for everyday activity but pretty noticeable for EME. Meanwhile shortest model expected to have more bandwidth.

Sure we won't take it in account because this fraction is obviously less than difference between models.

Why Tonna? Please get me right, I really respect Tonna. But why not DK7ZB? Why not YU7EF? Their models are different. With same number of elements they will have different length, F/B, bandwidth and gain. Hams used to count elements as an antenna characteristic but actually it has no sense. More accurate is boom length in wavelength units.

You seem missed 'optimized models'. My calc is MMANA but let's see what other people say.
This is 3λ (20 feet) 28 Ohm model from DK7ZB. It has 10 elements.
This it pretty same but 6λ (40 feet) model. Same 28 Ohm but due doubled length it accommodates 17 elements.
With your favorite calculator you may check author's specs. Hope you'll find 7 more elements add less than 3dB forward gain, not 7dB.

Antenna has no amplification. Any kind of forward gain is only possible with suppressing the radiation to undesired directions. In fact there is no fundamental difference between stack and one long antenna. Practical difference there is, sure.

I'm newbie in the hobby so I cannot use my 'weight' as argument in dispute. In such disputes I mostly rely on MMANA.

Sure. Single VHF antennas longer than 20~25 feet exist but pretty rare: in most cases it'd be more efficient to build a stack.

 

so to use your reference antenna here we have a boom of 3.4 and 7.3m
The first at 3.4 will be a 7 element ant 6x1 +1x 2.8dbi gain approx 8.8 dbi not the claimed 12+ dbi gain but the bigger 7.3m ant will have 15 elements. 14 x 1 + 2.8 dbi 16.8dbi against the claimed 15.9dbi that's just 0.9db of my guide calculator..
As for the other Dk7zb I could only find wattage levels not actual gain quotes..
You need to realise that what I've said is correct and a lot of the gains quoted by manufacturers is false. Just look at similar designs and the different gains quoted..
My reference if you read it, is a guide.. Not a computer simulation.
I rest my case...
You can make an antenna worse by not having the correct element lengths or spacing, but you can't make it suddenly 6db better than everyone else's copy of the same basic design.. Don't believe all the hype, as I said before. In the 70's dbi meant decibels Italian... And they were bigger db's than all the others... Get real.

 

YU7EF's antennas are well known for great sidelobe suppression. EME'ers should love them.
So let's look at EF0208c (3.4m i.e. approx 11ft 2in) versus little more than twice longer EF0212 (7.3m i.e. almost 24ft). They claimed to have 12.9dBi and 15.9dBi forward gain respectively. Exact difference is 3.0dB; boom length ratio 1:2.15 (slightly more than exact 1:2). These figures are tending to proof my statement: doubling the antenna length theoretically increases forward gain by 3dB and practically approx by 2.5dB.

Charts are even better.

True. That's why we must understand how antennas work. We aren't users but qualified (supposed to be qualified) personnel at our stations. So we able and actually should check every spec we doubt.

On what based the guide?

Exactly.

I know what do you mean but let's be honest: Italians manufactured bunch of great equipment; some brands were world famous. I owned few Italian made VHF/UHF antennas and they were almost perfect. One of them is still on my car and after a decade I see no any degradation; i'll better move it on a new car than buy modern junk. My former neighbor abandoned his Italian 27MHz antenna on the roof and the antenna stayed some two decades w/o service until was probably stolen (definitively not fallen by itself).

To be real one must build an antenna and measure it. There are two ways to do this: to build real model and measure in an anechoic chamber or virtualize it. Each method has its own advantages and disadvantages. With understanding of limitation both methods will show true specs.

 

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