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Bosse

Yb2normal's Sturdy Ground Plane Antenna

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Hi

I think I have discovered, by digging a bit, and by overclocking my slow brain slightly, why this antenna is radio-technically so great, compared to the dipole-antennas that comes with many wireless video trwansmitters.

Beacuse we generally, by my knowledge, have to use those genious and effective coaxial-cables, wich are "unbalanced", we will have to use a unbalanced load connected to them, if we want to use the equipment to the extent of its potential.

Balanced transmission cables and loads (antennas), would show same amplitude level but with phase inverted, at both leads, at any location along its extension, unlike the coaxial-cables, working as they are supposed to. Since most antenna designs are "balanced", and the coaxial-cables are un-balanced, there is a need for a so called "Balun", wich works as an interface between the two. If this transforming is not performed, one should expect poor performance and in case of a transmitter, interference signals transmitted to other equipement, located near the transmission cable, by radiation from the cable shield's outer surface.

Yb2normal's Ground Plane Antenna, is (not to mention it's funky and easy to realize design), like other traditional ground plane antennas, un-balanced. That makes a good match with the un-balanced coaxial-cable.

Then there is another thing to concider, impedance matching.

Baluns are often misunderstood as being impedance transformers primarly. That is not the truth (according to my findings). They can however be designed so they work as impedance transformers as well. Baluns that are also working as impedance transformes are as I know of, referred to as for example 4:1 baluns, in contrast to a 1:1 balun, wich is purely a balun. The ratio should reflect the impedance transforming ratio. Pure Impedance transformers, are not baluns by definition.

Dipole antennas, wich are balanced, are theroretically supposed to have a nominal impedance of 70 Ω. I guess that is one of the reasons 75 Ω is one of the preferred typical impedances of transmission cables.

I have yet to dig up, wich nominal impedance one should expect for a number of antenna designs.

I do not doubt there are a lot of people reading this forum that knows about this stuff, much much better than myself. However, since I could not find the information I wanted in previous similair discussions, I looked around and now I write about that to get some feedback and possibly helping someone even more ignorant about this matter than myself.

Some feedback would be welcome.

Conclusion:

Yb2normal's Sturdy Ground Plane Antenna works great because it unbalanced, just like the coaxial-cable wich connects it to the transmitter.

Best Regards,

Bosse

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Yb2normal's Sturdy Ground Plane Antenna works great because it is unbalanced

Much of my personal success has also been attributed to the fact that I am unbalanced :)

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<Laugs>

Being unbalanced could be a gift, just like in the case of a coax.

Best Ragrads,

Bosse ;)

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Sorry for those weird spelling errors at the end, I have this troll in my combined building room and computer room, anything can happen. ;)

Best Regards,

Bosse

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I understood that the groundplane antenna design was "balanced." I think it works well because it offers a decent impedance match and the radiation pattern provides a bit of gain (over a typical dipole).

The issue with the unbalanced coaxial line is that it is susceptiable to VSWR issues along its length. This would not be an issue with a fully balanced antenna system.

In our case, the fix is to experiment with the coax length until the best efficiency is found. If RF power measuring equipment is not available then trial and error range testing can be used. Keep in mind that at microwave frequencies, tiny millimeter changes can mean a lot.

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I found out the other day, that the angle between the elements of a dipole, governs the impedance. 180° gives 73 Ω and 90° gives 36 Ω. The ground elements of a grund plane antenna, are bent to an angle of 135° relative to the active elements (or -45° relative to horizontal plane) , wich gives an impedance close to the wanted 50 Ω.

Conclusion: Yb2normal's Ground Plane Antenna is likely to have an impedance close to 50 Ω, wich is good.

Best Regards,

Bosse

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You might wonder why I spend time in investigating why this antenna works so well. I think it is very important to to investigate reasons of good results, thats all.

Discussing reasons of less positive results can be interesting too, but this is so much more interesting.

Best regards,

Bosse

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What is a good length for the coax cable split location to the connector? Has anyone experimented with this length and found a good value for it?

Edited by Andrew

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Only by looking at a ground plane antenna, I get the feeling it is un-balanced. It has at least four ground plane radials and only one radiator.

I never try to be smart. I just like to discuss and share what is important to my and yours interests.

Best Regards,

Bosse

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The length should be odd multiples of one wave-length (approx 20.5 mm). You should concider whole transmision cable as being the whole length (including connectors), that is whole lengt of shielded transmission cable including all connectors, even those in your bought equipement.

If the load on the reciver or transmitter is perfectly matched, this is not theoretically important however.

Best Regards,

Bosse

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I mean it should be odd multiples of one quarter of one wave-length.

Best Regards,

Bosse

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I mean, theoretically, if you use Yb2normal's ground plane antenna design, you can forget about excact transmission cable length, just keep it short to reduce loss.

Best regards,

Bosse

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I remember a CB aerial called a Boomerang that was a dipole with the lower element angle being adjustable to get the SWR right. If I remember right it worked well, maybe it is worth a try at 2.4ghz ?

Bosse, do you know the impedence of a dipole with reflector ? I notice that as elements are added a folded dipole is often used.

Terry

UK

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I mean, theoretically, if you use Yb2normal's ground plane antenna design, you can forget about excact transmission cable length, just keep it short to reduce loss.

I didn't find it to be the case. Ground plane antenna output will still vary with coax length. Also the cable length needs to be adjusted in half wavelengths (40mm for teflon cable) not quarter wavelength. Quarter wavelength will take you from worst to best cable length.

Regards,

Val.

The difference between practice and theory is bigger in practice than in theory.

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That's why I was smart enough to add "theoretically" :P

To be safe, it is always best to use theoretically matched cable lengths. If you have access to instruments, you can tweak your setup from that point and get the best results.

If you use almost perfect components, cable length will affect performance just like the books says. (just keep lengths short and forget about the millimetres)

If you start wrong, from a theoreticl pow, and you can not check your performance with help of instruments, you might find it hard to find the right formula of success.

Best Regards,

Bosse

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Last thing I want to do is to make you sour but the "Odd multiples of 1/4 wave-lenghts" rule is what I have read.

I could be wrong. (that happens regularly) I would be grateful to be enlightened.

Best Regards.

Bosse

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Terry,

About Di-poles with reflectors.

I am sorry I can't tell. However, I do have a extremely strong feeling the reflector, or director, if used, does not change impedance.

It should be 73 Ω according to my feelings :P

I guess you have looked at one of those round antennas with a di-pole as driven element and one round reflector as well as a round director ?:P

I think those looks good myself.

However, right now (just like the wind changes), I look more at wave guides (horn designs), for my next receiver antenna.

Best Regards,

Bosse

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It is quite possible a transmitter does not have an output impedance of for example 50 Ω, enventhough the manufacturer says so. Same goes for coax-cables.

1/4 wave gp antennas might not have an impedance near 50 Ω just because 1/2 wave ones do.

This would force us to carefully adjust cable lenghts unless we use Baluns, wich would require us to know the excact impedance anyway.

I wish I could afford a DIP-meter, Field-Strenght meter and a Network Analyzer. I would gladely make exepriments and tell you then.

This is old well documented stuff. However, since it is a bit complicated, it requires much effort to get a hold on for laymen like myself.

Best Regards,

Bosse

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Cyber-Flyer.

I think I know the reason of your objection now hehe.

I guess you say "work with half wave-lengths" I say (or my source says), "work with ODD multiples of 1/4 wave lengths"

Well, it is excactly the same, with one exception: the first multiple, or the first maximum you will find in the cable.

Essentially I guess we agree !

Best Regards,

Bosse

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"work with ODD multiples of 1/4 wave lengths"

Bosse,

Yes, we are on the same page. 40-41 mm in teflon cable is about 1/2 wavelength, so it always takes you from one odd number to another. Beware of odd number rule though - it's not always going to work because both antenna and output stage of amplifier can have impedances different from 50 ohm. In fact you get exactly opposite results with the same antenna if one's amplifier has 45 Ohm output impedance and another's is 55 ohm - therefore I was always reluctant to recommend any specific length of the cable to other people. It definitely pays to tune up each specific system imho.

Regards,

Val.

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I look more at wave guides (horn designs), for my next receiver antenna.

Funny you should bring up horn designs bosse, I have given them some though for use on my auto tracking system, have you any idea how narrow you can get the beam ?

I'm thinking it may be easier to use a parabolic reflector to get a tall but narrow beam, what do you think ?

Terry

UK

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Hello Cyber-Flyer and everone else !

Terry,

I assume, you have already found out that you can use a wave-guide also known as a horn, not only as an antenna by itself, but also as the active element in a dish or parabolic antenna, wich is also the truth for some other antenna designs. (they are called parabolic but how many of them are parabolic and not spherical ?).

The horn itself has a quite narrrow reception aperture, or beam-witdth as people like to call it. I have no numbers right now but you will easily find out if you look around on the net, I think it is less that +/- 10°. The dish will not widen the the angle, it will restrict it and enhance the gain maximum. The larger you make the dish, the narrower the aperture will be (mostly if the dish is parabolic that is).

Now I'm guessing a bit more than usual:

By knowing the maximum distance to the transmitter, the sensitivity of your receiver and the field-strengt of the source signal, you can optimize the size of the dish. I do not think you will benefit from a 25 meter dish if the transmitter is only 1000 meters away, to make a funny example.

You can make a dish with a customized aperture shape to some degree. Radar antennas and radar reflectors used in aviation typically have an elliptical outline to obtain what I guess you are looking for.

Do not forget that a dish will put more strain on your antenna stand and your servo motors. If you want it to move snappy, then the stand must be heavy or be fixed to something heavy, and the servo motors must be powerful enough.

Best Regards,

Bosse

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As I assume Cyber-Flyer means,

It is always best to make empirical studies and adjust the setup accordingly.

Empirical approach is the final blow on a problem of this kind !!!

However, strictly empirical way of looking at a problems, does not give as much feedback as the theoretical way does. Both are completely essential.

Best Regards,

Bosse

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Because different receivers and transmitters uses different brands of connectors, and cable connectors wich you might use, comes in different brands, it is not useful to express recommended cable-lenghts in inches or millimetres, reason is the dimensions of the connectors are not completely standardized and exact dimensions are essential, when making a matched transmission cable for use with equipment wich is not perfected.

However, for my "old type" lawmate receiver, I have made a goof proof patch and I used Radiall connectors and RG-174 cable from Bedea.

Components used are ( in ELFA numbers), you can translate it to yours, I don't have the time to do that for you right now, sorry for that:

46-123-13

46-122-06

55-909-14

Link to ELFA (for reference) is: http://www.elfa.se/en/

The measurement of the finished cable is 170,0 mm, when I put a moderate strain on the cable. Measurement is from nut edge to nut edge.

Measurement is NOT proved to be optimal, it just gives reasonable good results.

This is to make it possible to get a reference point for some of those who do not like "odd multiples of 1/4 wave-lengths inside the cable with 0,66 x lightspeed factor disscussions"

A side note:

Recently, I wrote about the loss you should expect, using connectors. I wrote someting about 0,7 dB loss per connector and I wrote that was not too bad. Well, I screwed up, I should have written 0,07 dB.

0,07 dB loss per junction is not a huge amount of loss, concidering you are able to replace the antenna with another one, easy as pie.

Best Regards,

Bosse

Edited by Bosse

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