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DIY Dipole antenna!

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I made the boxes Daniel.

Hi Larry, there is no balun.  It's just the shield going to one arm and the core going to the other.  I've made dipoles before with just a coax cable and they seem to work well. This time i decided to put them in a box and mount them on the side of my bag. 

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If you want to go this route, a simple VNA (real RF guys look elsewhere!) can help.

 

I have a miniVNA Tiny and for the price (and with its huge limitations) it's surprisingly helpful. For instance

it helped me choose some UMTS/LTE/WiFi antennas and the results were consistent with the measurements

I made at the University with a real über expensive Agilent VNA.

 

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10 minutes ago, Derek H said:

And a VNA is what?

 

Vector Network Analyzer. It's an instrument that allows you to perform several measurements for a circuit and/or element.

 

It has some practical purposes when dealing with antennas, filters and transmission lines. 

 

You can check the actual impedance of an element, detect damage in coax lines, check the resonance frequency of an antenna, measure the attenuation of a transmission line or filter, check its band pass, etc. It can help detect damaged connectors and some of the usual RF "witches".

 

It's a very specialized (and expensive!) instrument. But lately some cheap units have been released for the radio amateur market. If a "real" VNA can be in the tens of thousands of dollars, the "toy" but useful ones are in the less than $500 mark and they can be real time savers. In the radio amateur market they are sometimes sold as "antenna analyzers".

 

I own one of the cheap ones (miniVNA Tiny) and despite the ridiculously low price it's surprisingly useful.

 

The attached PDFs show what one can expect. One of them is a test of an HF antenna (a loop) tuned to the 7 MHz band and the other one a test of a sample UMTS/LTE/WiFi antenna we ordered from a Chinese manufacturer. The third one shows a misalignment in the front end filtering of a broadband receiver (should be centered on 140 MHz, the tuning frequency, while it was off by 9 MHz) and the fourth one shows a defective filter which was in theory a band-reject for the VHF broadcast band. Turns out it had been assembled with the wrong capacitors.

 

 

Not bad for a $500 instrument I would say.

 

 

 

VNA_150811_101602.pdf

VNA_170319_232236.pdf

VNA_170924_124857.pdf

VNA_170320_194207.pdf

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10 hours ago, IronFilm said:

How would Lectrosonics SNA600 differ from these?

The SNA-600 matches the 62 Ohm impedance of the dipole arms to 50 Ohms using a microstrip transformer and the balanced elements (arms) are baluned  (should be a word) to the unbalanced 50 Ohm coax. I would expect slightly better performance from the SNA600a compared to the DIY antenna, though sometimes a rusty coat hanger could beat both of them.

Best Regards,

Larry Fisher

 

 

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16 hours ago, allistair said:

Crossing my fingers for a Lectro rusty coat hanger at NAB 2019.

Would be released on April 1

LEF

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I have looked around for baluns for DIY dipole projects sized for use within a range of 470 MHz to 600 MHz, but have never found anything that seemed suitable. It's been a while so my memory is fuzzy. It seemed that some very tiny surface mount items may have had specs that seemed appropriate, but were inconvenient to implement in a home brew project.

 

I would appreciate learning how to utilize two coils of wire to make a suitable device, so that the balanced function of the dipole structure can be taken advantage of.

 

I think it could be fun to commission some small inexpensive project boards that a pair of antenna arms can easily be attached to with solder, or perhaps even substituting the through hole connections with screw terminal type fittings, but I have been stalled each time I try to learn about impedance matching with DIY balun design.

 

balun-project.jpg

 

 

 

 

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6 hours ago, alenK said:

I have looked around for baluns for DIY dipole projects sized for use within a range of 470 MHz to 600 MHz, but have never found anything that seemed suitable. It's been a while so my memory is fuzzy. It seemed that some very tiny surface mount items may have had specs that seemed appropriate, but were inconvenient to implement in a home brew project.

 

 

Do you mean, for example, products from Minicircuits?

 

https://ww2.minicircuits.com/WebStore/Transformers.html

 

For the proper way to mount them on a PCB, this guy can be helpful. I am not affiliated to him but I'm a happy customer. 

 

https://www.sv1afn.com

 

I guess he can be commissioned for some custom work. I would give him a call. For example, if you wish to experiment with SMD components these PCBs can be really useful:

 

https://www.sv1afn.com/rf-experimenter-s-pcb-panel.html

 

They have a proper coplanar line to keep the 50 ohm impedance from the SMA connector to your components. 

 

 

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Thanks for the suggestions. You are correct that some of the SMDs I have found are Mini-Circuits brand, and I agree that it is possible for a DIYer to hand solder a SMD to a PCB, but the process is not what I would call convenient.

 

I have been reading about receiver antenna design for a few years, but have yet to get to the point where I understand how to calculate, measure, and or anticipate the impedance bridge ratio necessary to make proper use of a balun when connecting a home made receiving dipole to a 50 Ohm transmission line.

 

 

 

 

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22 hours ago, berniebeaudry said:

Does the coat hanger have to be rusty? :)

Hi Bernie,

All our tests, (one) show that stainless steel coat hangers have weak reception. They are hard to solder also, though they do look good on the set.

LEF

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Looking at the pdf files produced by the MiniVNA testing it seems like the impedance varies widely over a spectrum of frequencies tested.

 

Does that suggest that a dipole structure, as sized in length for some particular frequency, will have some arbitrary impedance at the intended frequency, while at other, perhaps nearby frequencies, the impedance can be significantly different? Can the impedance be accurately calculated, or is a VNA type test the only way to arrive at an accurate figure?

 

It seems that when I read about antennas with an interest in reception, matters that pertain to transmission are treated with more detail, and by the time I wade through those ideas I am left without a clear idea of how useful the information is with regards to reception.

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20 hours ago, alenK said:

Looking at the pdf files produced by the MiniVNA testing it seems like the impedance varies widely over a spectrum of frequencies tested.

 

Yes. Simple antennas resonate well at a given frequency, more elaborate designs can be multi band. Others can be narrow band. The HF loop resonating on 7 MHz for instance has a bandwidth of just 2 or 3 KHz.

 

In general it’s say that there are three properties of which you can choose two, compromising the one left. Bandwidth, efficiency, size. The loop is tiny (a diameter of 1 metre for a wavelength of 40 meters).

 

 

Quote

 

Does that suggest that a dipole structure, as sized in length for some particular frequency, will have some arbitrary impedance at the intended frequency, while at other, perhaps nearby frequencies, the impedance can be significantly different? Can the impedance be accurately calculated, or is a VNA type test the only way to arrive at an accurate figure?

 

Designing antennas is still a bit of a black art. There are mathematical modelling tools like NEC but at the end you need to fine tune the prototype and, in the first place, test it.  And modelling tools such as NEC don’t always produce accurate results. Loops are one of the corner cases for example. Their performance according to the models violates the first principle of Thermodynamics :)

 

Quote

 

It seems that when I read about antennas with an interest in reception, matters that pertain to transmission are treated with more detail, and by the time I wade through those ideas I am left without a clear idea of how useful the information is with regards to reception.

 

In principle a good antenna for transmission will work for reception. Antenna matching is more critical for transmission because a badly matched antenna can affect the performance of a transmitter and, in some cases, damage it or at least make it distort.

 

I’ll dig a couple of books in case you want to read some. Actually you will find that some of your acoustic intuition applies to antenna resonance, just replace the speed of sound by the speed of light ;)

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What Borjam said. Generally a 50 Ohm antenna will only look like a 50 Ohm load (or source) inside its operational bandwidth, and sometimes not even then.

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Thanks for the additional info.

 

My interest in making DIY 1/2 wave dipoles with the little PCBs I illustrated is related to the idea that it would be easy to connect antenna arms that are the relevant 1/4 wave length for some specific frequency within the 450MHz to 650MHz band.

 

For example; I have been making some, minus the balun, out of "romex" solid core copper, and cutting them to length as needed. It is super easy to refresh the unit with a new length of romex in between projects. 

 

If the length is tuned (by choice of length) to resonate near the desired frequency, does that not address efficiency?

 

Bandwidth can presumably be effected by the diameter of the arm, but I have more or less disregarded that by basing the premise on using cheaply sourced romex copper wire.

 

Now I would like to wrap a few coils of wire around a nail and make a tiny balun but I have no idea what the turns ratio should be.

 

Will a miniVNA really tell me the impedance and let me predict the turns ratio?

 

Will using a miniVNA on a dozen examples of dipoles, each sized for some specific frequency found between 450mHz and 650MHz reveal that the turns ratio varies widely, or will such an exercise suggest that when each dipole structure is sized for and evaluated for some specific frequency, there is an average factor that may be suitable for the relatively limited range we work in?

 

In other words, is it likely that a dipole sized to resonate at 500MHz, and another dipole, made of the very same materials, but sized to resonate at 600MHz will have similar efficiency or impedance?

 

Am I over looking small details such as the implication of solder joints? PCB traces etc.?

 

I can see myself buying a miniVNA but I'll be disappointed if I get one, make a bunch of tests, and find out that a 1.5:1 turns ratio will cover most of the builds I am interested in. :-)

 

Thank you.

 

 

 

 

 

 

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