Re: Simple dipole compared to a folded dipole in a 2.4 GHz Wifi Yagi Antenna

Folded dipole has about fourfold impedance of simple dipole. All other points have been already discussed in your previous Yagi thread. Please consider that 73 ohm impedance doesn't apply for Yagi, it's for simple dipole without directors. In so far the quote is misleading in the Yagi context.

If your Yagi has small director spacing, folded dipole may come nearer to 50 ohm impedance than straight dipole, see previous discussion.

I recently purchased some antennas from a design house who document some precision requirements. They say that dipole dimensions are very critical, and they have 0.02mm (!) repeatability for their yagi dipoles in the 1-2 GHz frequency range.

Looking at your photos, I don't think you have a chance to realize a yagi with many elements with useful precision. Better look for a "robust" design with more bandwidth.

Thank you FvM,

It appears, however, that you (and Volker) believe that I am still trying to build the 20 element Yagi antenna described in my first posts. I am, however, attempting to build a different Yagi antenna that has only has 10 elements (8 directors), using this VK5DJ online calculator. I also made a loop balun and a folded dipole. See attached photos.

I found, however, that it was difficult to make the folded dipole and loop balun precisely, and I have not permanently attached the folded dipole to the Yagi antenna yet. And I am interested in possibly replacing the folded dipole with a simple dipole and using a resonant stub as a balun. But I cannot determine the dimensions of the parts of the resonant stub. Can you tell me how to determine the dimensions "a" and "b" of the resonant stub at this link for a simple dipole connected to a coax cable?

I do not see anything in the earlier posts (above) answering the difference in gain between a simple dipole and a folded dipole in a Yagi antenna. Also, how can I determine the impedance of a simple dipole in a Yagi other than by measurement?

Thank you,

A.

volker,

thanks for your reply, but I am no longer trying to salvage my old 20 element Yagi design, but I am trying a new 10 element design, hoping that it will need less precision to work (see previous post with photos of new 10 element design). I think you advised fewer elements in an earlier post. I was hoping I could make a simple Yagi that would be operable (as described on the internet at pages such as those referred to in my first posts), without measuring electrical wave forms, making sophisticated baluns, etc, but perhaps that is not realistic. Perhaps I must get more deeply into the theory of baluns, as described here , which is a link that I think you referred to earlier. And perhaps I have to make measurements with instruments I do not own and perhaps I need to use materials other than copper wire (which is described as a suitable building component at some pages on the internet).

I am presently using what I believe is RG174 coax cable (about 0.85 meters in length), because that is what I had been able to obtain. But perhaps I will not be successful with this simple approach, even with a 10 element yagi. And perhaps I should just try a cantenna as vfone advised in a post in another thread here .

What do you or others think?

Thanks,

A.

I would use a much narrower boom, the capacitive effects of so much wood covering half the elements would seriously de-tune it. I have used brass rod as the boom before with good results. 6mm dia rods can be drilled and the reflector/directors pushed through it and soldered for stability. For the balun, it does look like RG174 but remember that bare ends connecting to the dipole are effectively part of it and there are currents in the balun itself that due to orientation can subtract from the signal, if you can use a rigid or semi-rigid coax instead it would give more predictable results.

Brian.

The answer to the post #20 question, folded versus straight dipole depends on the Yagi design and balun type. The lambda/2 balun presented in post #23 fits a 200 ohm folded dipole. As previously discussed, the driving element impedance depends strongly on the director distance, what's the outcome of your design tool?

Brian (and FvM),

Thanks for your replies. Regarding boom width, I took that into account. The VK5DJ calculator allows the designer to enter a boom width. I think I used about 26mm for a width. I have attached the VK5DJ design parameters of the design I am using to this post. The width of the boom is reflected in the fact the "Insert to" lengths (distance from boom edge to tips of directors) is about 11 to 12 mm, which is small compared to total director length (about 50 mm). Also I was thinking that the dielectric constant of dry wood is low (about 1.4), which is closer to air than most materials.

But on the other hand, theory and theories are one thing, experience is another. Maybe other materials and less bare wire would work better. I still have not permanently attached the driven element to the boom. But, so far the antenna is not working very well. I will write a more extensive reply and update soon.

Thanks again.

PrimeEmpiricDesign.pdf

A Yagi-Uda antenna, even poor constructed will in most cases have a clear directivity. At 2.4 GHz is directivity in need of free space. Indoor environment creates to much reflections so that any directivity is more or less random.
Most antenna design found at internet are theoretical antennas, that not will function well in real life. Calculated by people which not are able to verify result due to that it not is within their knowledge how to do such things or lack of even simplest measurement tools.
Gain and other predicted data for an ideal antenna can be hard to realize.
If the design not provides clearly measured result can you assume that given data is wishes, not what to expect.

As for all electronics, you need tools to measure that everything works as expected. Antennas are not excluded, it is opposite, good tools are especially required to achieve expected antenna result.
Antennas can be very hard to tune without tools.

If a flashlight is dead, how to find the problem, if it is the battery, switch or lamp that not is working? Without tools? It is very simple with a voltmeter.
Tuning an antenna is way more complicated then fix a broken flashlight. Doing that without tools is impossible. An alternative is to assume that what you have found at a webpage is a reliable and verified design that you have copied in every detail.

Basic antenna tools are not expensive. Most simple tools for antenna design is a diode-detector to check if a modification of an antenna increases or decreses signal level at a distance of a meter or two. A such tool is very simple to build and cost almost nothing.
VSWR meter is another relative cheap tool and can easily be built if you increase current mechanical and solder skills with an factor 10 or more. Your current soldering looks way off from what can be accepted and coaxial cable did really look like it was opened with a chain saw.
I am sure that you can do much better.
A good an reliable soldering is just as when you glue two surfaces against each other. A little glue/solder as possible makes the strongest joint and smallest losses. A lot of tin and it becomes a mini-antenna of solder which is as weak as your solder.

As a reference, pictures of some of my junk-antennas are attached so that you can get an idea how they are assembled.
It is antennas that have been used as temporary references when I have done some kind of measurement.
Soldering at these antennas are of good quality, wet and soldered at right temperature. Cables are semi rigid with low losses at 2.4GHz.
Iron wire or copper-traces at PCB or CU-tape is what I often use for these temporary antennas. When using PCB must dielectric constant be taken in account which can be a bit tricky but advantage is that it is very mechanical stable designs.
Also notice that coaxial cables have very clean cuts. I use a standard knife or an exacto and use it as a saw, sawing carefully. Sawing only almost through isolation. Or else will it easily make cuts in wires. Even minor scratches from a knife and such wires and they do then more easily become broken.

Thank you for your careful and detailed reply E. Kafeman. I will give the points you made in your reply much consideration. Can you recommend a reliable and inexpensive diode detector and VSWR meter?

AmateurDIYAntenna

You can find a lot of diode detectors by searching.
Here is one example: http://www.dcon.com.br/jd.comment/ar...Gear2_4ghz.htm
VSWR meters: http://vk5ajl.com/projects/swrmeter.php
Buy or copy: https://www.ebay.com/itm/RF-bridge-0...a/332052527822
This is a fun and cheap tool measuring antennas: https://www.youtube.com/watch?v=THc5ZTxQUSg
It does not cover 2.4GHz but can still give interesting insights at lower frequencies of use at higher freq.

The difficulty is that you need not only a directional coupler and detector, but also a signal source with constant carrier.

For testing VSWR of my 2.4GHz antenna designs, I use this instrument http://www.deepace.net/kc901s-2/
Compared to the professional grade instruments, it is very affordable and works well.

Thank you E.Kafeman and Volker and everybody,

I will keep your recommendations and advice in mind.

AmateurDIYAntenna