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22 hours ago, alenK said:

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.

 

You don't need fancy oxygen free copper or anything of the sort. Also, remember that at high frequencies current density is much higher on the surface of the connector (it's called skin effect) so you can use a thin hollow tube instead of solid wire. It doesn't need to be copper even, aluminum can do as long as you adjust the length properly. That's where a VNA is so handy :)

 

22 hours ago, alenK said:

 

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?

 

It will help you know wether you did a good job. It can drive you nuts as well, because you will notice that stuff getting close to the antenna can alter its performance significantly. I guess manufacturers of wireless microphone transmitters consider the effect of a human body almost touching the antenna.

 

22 hours ago, alenK said:

 

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?

 

Dipoles are essentially single frequency antennas given that the intended impedance is constant. And their performance and impedance is more or less the same. Around 75 ohm for the half wavelength dipole.

 

There are also broadband and multi band antennas. Broadband doesn't necessarily mean much less efficient, the three parameters are size, bandwidth and efficiency. If the antenna is not small (always related to wavelength) you don't need to trade efficiency for bandwidth. The log periodic "axe" antennas are a good example of that.

 

 

22 hours ago, alenK said:

 

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?

 

Yes. 

 

 

22 hours ago, alenK said:

 

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

 

If everything is properly made (that's why I suggested SV1AFN's PCBs) the path from the electronics to the connector should be close to ideal. At high frequencies PCB layout and traces are so important, it's usually said that you need Maxwell's laws to design the circuit rather than using simply the good old Kirchoff's laws.

 

 

22 hours ago, alenK said:

 

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. 🙂

 

It will really make a difference if you are building your own antennas. You will find out that  cutting them for the right length is tricky at high frequencies. 

 

That said: are you intending to test transmission antennas? I am not sure about the legal status of antenna experimentation with wireless microphones. I am an amateur radio operator and we have "legal superpowers" regarding experimentation. But the radio equipment used outside of the amateur bands is subject to approval by the relevant authorities and often both transmitter and antenna are approved in combination. You can't generally replace transmission antennas at will. 

 

You might get in trouble if a poorly matched transmission antenna caused, for example, distortion, and you ended up transmitting spurious distortion products interfering other bands.

 

You might have a look at these two books if you want to delve deeper. Beware, they can contain much more detail than you want, just skip the stuff you don't need/want.

 

The ARRL Antenna book has lots of practical information. It's updated frequently with new designs.

 

And for a not so gentle introduction to antenna theory skipping most of the math, Joseph Carr's "Practical Antenna Handbook" is very good. But sometimes not for the faint of heart :)

 

https://www.amazon.com/Practical-Antenna-Handbook-Joseph-Carr/dp/0071639586

 

 

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Hi Borjam,

 Thanks for the detailed reply.

 

 "Romex" wire is not any sort of fancy wire. It is construction grade wiring used in the building trade. Here in the USA, Romex is a trademark brand name that has become a generic name for a wire packaged to suit electrical distribution within building structures. The name is used sort of like Fridge, or Kleenex. It is a very cheap, and easy to source solid core wire that comes in small gauges such as 14AWG and 12AWG.

 

 I am only interested in building receiving antennas and I am not experimenting with transmission scenarios.

 

 I did some experiments based on log periodic dipole array calculations for a 450MHz to 650MHz range but for the production work I do, with one or two transmitters for documentary and news gathering, the idea of using specially tuned, light weight and compact dipoles seems like a useful option.

 

 I will try some 1.5:1 turns ratios, and then start inquiring with my HAM buddies to see if anyone has a miniVNA.

 

 Thank you.

 

 

 

  

 

 

 

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27 minutes ago, alenK said:

Hi Borjam,

 "Romex" wire is not any sort of fancy wire. It is construction grade wiring used in the building trade. Here in the USA, Romex is a trademark brand name that has become a generic name for a wire packaged to suit electrical distribution within building structures. The name is used sort of like Fridge, or Kleenex. It is a very cheap, and easy to source solid core wire that comes in small gauges such as 14AWG and 12AWG.

 

Oh, ok, sorry, I stand corrected! :) I didn't search for it and I assumed some esoteric cable.

 

27 minutes ago, alenK said:

 

 I am only interested in building receiving antennas and I am not experimenting with transmission scenarios.

 

 I did some experiments based on log periodic dipole array calculations for a 450MHz to 650MHz range but for the production work I do, with one or two transmitters for documentary and news gathering, the idea of using specially tuned, light weight and compact dipoles seems like a useful option.

 

I would recommend you to explore the option of colinear antennas. In very simple terms a colinear is a vertical stack of vertical antennas. It's omnidirectional in the horizontal plane but with a lower radiation angle (ie, it's directional in the vertical plane). That means you have some gain and, as a plus, it will help reject multiple reflections coming from the ceiling and floor. 

 

https://en.wikipedia.org/wiki/Collinear_antenna_array

 

At those frequencies antennas are not large and this is less cumbersome than a log-periodic array.

 

 

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On 9/2/2018 at 2:02 PM, LarryF said:

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.

 

I don't know the impedance of an Idaho potato, but when our car antennas were broken off in the 70s, pushing a potato onto what was left of the antenna worked really well.

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8 hours ago, Glen Trew said:

 

I don't know the impedance of an Idaho potato, but when our car antennas were broken off in the 70s, pushing a potato onto what was left of the antenna worked really well.

The next time I see you, I will have to hear more about this.

Best Regards,

Larry Fisher

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  • 1 year later...

Hi... a coaxial cable common mode RF choke as shown below will help decouple the feed line from the antenna in terms of common mode RF current. This will help prevent the shield of the coaxial cable from picking up unwanted electrical noise and help reduce the impact of the position of the coaxial cable on the pattern of the antenna.
 

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  • 4 weeks later...
On 8/30/2018 at 8:02 PM, 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.

 

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

 

 

Would a design like this one needs an impedance matching circuit between the dipole elements and the Balun to make sure the 50ohm impedance is achieved at the input of the balun? 

@LarryF

@borjam

 

On 8/30/2018 at 8:02 PM, alenK said:

 

 

 

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Hmm I wonder wether the PCB layout might cause more losses than the balun itself would solve. 

 

The real expert on this is @LarryF. The only baluns I have built for UHF frequencies were narrow bandwidth ones based on transmission lines (ie, a piece of coax). I used them successfully for TV reception with a homebrew dipole array.

 

 

 

 

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On 10/19/2019 at 10:02 AM, simmiz said:

 

Would a design like this one needs an impedance matching circuit between the dipole elements and the Balun to make sure the 50ohm impedance is achieved at the input of the balun? 

@LarryF

@borjam

 

 

A 1:1 balun to match the two arms of the dipole to your coax is what is needed here. AT UHF frequencies you can find small PCB mount units at Digikey and others. They are made of twisted wires that are 50 Ohm pairs around a small (1/8") iron powder torroid. The bandwidth of these baluns will cover the entire range of frequencies used for wireless mics. They are low cost also.

Best Regards,

Larry Fisher 

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On 10/22/2019 at 2:58 AM, borjam said:

@simmiz

Now that I remember, this guy in Greece sells PCBs with a proper coplanar transmission line that would help with signal integrity. You could mount a Minicircuits balun on them, they have several parts that would work.

 

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

So funny, I am already in touch with this guy! He is awesome ! :D

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