Theory of BiQuad Antenna Design (now a digital TV channel question)

[monchosoad]

Hi everyone, I'm Designing a BiQuad antenna for WiFi applications. The design and optimization are relatively simple but i need some references of the theory behind this antenna.
I'm reading some theory of Bi-Conic Antennas but it's a bit difficult to extrapolate the idea between both.

Thanks in advance.

 

[Nicolas guzman]

How is it that a biquad i made for tv uhf band 586 mhz calculated receives better signal on vhf low tv channels 2 threw 6 rather than i yagi or dipole for vhf low on digital tv.it seems like all the tv hd channel would be in uhf.i did this experiment in analog and it does matter the length of the dipole but in digital tv i don't get it,un less tv channels 2 threw 6 on digital would be in the uhf band.does anybody has ant idea about this?

 

[Baluncore]

The digital channels may be on UHF or VHF. There is no international standard.
You will need to look up the channel assignments for your location.

 

[Nicolas guzman]

So in analog tv channels, what ever channel you tuned had to be a certain frequency asigned,which i was awear of,but in digital i thought that in the convertable box,the channel you tuned it would have to be a certain frequency were ever you go,but i guess its not like that.so digital channels could be Manipulated at any frequency by the tv station,thats my big question so ill have no more dought,i apreciated,thanks.

 

[Nicolas guzman]

Here is a much clearer question.channels 2 to 6 in analog is on vhf low but on digital,channels 2 to 6 could it be on uhf or vhf high?i checked a listing of frequencies on line several ways and it shows that same one as analog standard but in my experiments it doesn't make sense.

 

[Baluncore]

Each digital TV stream uses the bandwidth of an old analogue channel, but it can carry several of the old analogue TV channels.

In Australia, the same analogue TV channel number was allocated to the same TV company in many cities. That company was then named after the channel number. Since the transition from analogue TV to digital TV required parallel operation for a number of years, the Company names could not remain attached to the original allocated channel. I think you may now be confusing traditional channel names with the channel allocated for the digital TV stream. That channel information is available on the web if you look.

You seem to be confused and to be guessing how it might work. You are then asking for confirmation.
Without knowing your country, region or city it is very hard to guess at the answers to your questions.

 

[jim hardy]

When US went digital
stations kept their channel number
but it might be assigned to a different frequency.

Lots of vhf stations moved to uhf frequencies
here in US every station has its channel number by which it's known to the public and appears in all its logos
and another "RF Channel" number denoting what frequency it actually occupies.

EG my local channel 6 PBS moved from RF channel 6 to RF channel 40
so now i can use a much smaller antenna for that one.
But my other PBS channel, 19, only moved one slot up to RF channel 20.

see https://en.wikipedia.org/wiki/Television_channel_frequencies

My first "converter box" displayed both numbers, the actual RF channel and the number by which it is known, plus its actual frequency in mhz .

I guess our FCC took to heart that line from old Beatles' song "Rocky Raccoon"...
Her name was McGill, she called herself "Lil", but everyone knew her as Nancy.

 

[Jeff Rosenbury]

Antennas work by accelerating electrons. Accelerating electrons emit photons.

Since the EM field is a gauge field, phase matters. Thus an "antenna" a full wavelength long has an average phase of 0º and doesn't emit many photons. For every electron moving one way, there's another going the opposite way. But an antenna that is ½ wavelength long has all the current adding part of the time. So 180º phase radiates well.

There is a principle called reciprocity which states that antennas which emit well also receive well. I'm not fully clear on why.

Something called image current also exists. An image current is caused in a conductor when electrons (or other charges) move near it. The negative charge of the electron draws a corresponding positive charge which exactly matches the electron's movement.

This image current can be used to adjust the electrical length of an antenna. For example a quarter wave dipole creates another quarter wave in the ground plane like a mirror. This adds up to a half wave which makes a good antenna.

Finally, an antenna attaches to a transmission line. It acts as an impedance transformer between the transmission line and free space. Transmission lines can be any impedance, but 50Ω is typical. Air has an impedance of 377Ω. So the antenna typically needs to have an input impedance of 50Ω. Mismatching impedance causes energy to reflect back to the power supply, causing problems.

Understanding these principles, let's look at the Biquad:

The antenna is a pair of squares of wire above a ground plane. Each side of the square is ¼ λ. The squares are arranged like a bow tie. Typically they are fed from the center of the tie with an unbalanced coax cable. The center conductor is attached to ½ of the center, and the ground is attached to the other, the post, and the ground plane.

Let's look at the current distribution. The current is fed as a sine wave. Let the feed point be 0º. So at t=0, there is no current at the feed point. The current is maximum at the end of the first leg, and zero again at the end of the bow. It then goes negative and back to 0 at the ground feed. This happens in both squares of the bow tie shape.

Since the square legs are orthogonal. Thus only the legs in the same direction add phases to signals at one polarity. (The overall wave will combine phases from the two directions, but we will only look at one.)

A point far from the antenna will see the positive current (which is made of accelerating electrons) and the exact opposite current. But these opposing currents are 90º out of phase. In addition it will see an opposing image current off the ground plane, again 90º (nominally anyway) out of phase. This adds up to two identical ½ dipoles in phase for each of the two squares. That's a total of 4 ½ wave dipoles.

But that's only the signal polarized in one direction. There's and identical antenna set with orthogonal polarization. So 8 dipoles worth of power are directed outward. That's about 8-9 dB.

The input impedance depends on the geometry, including the feed points and the distance between the ground plane vis a vis the wire diameter. (Remember how the ground plane spacing was nominal? It needs to be adjusted to match impedance.)

This should help you understand what's happening. You can model the actual fields with the right software and or analytical calculus.

When building this, it helps a lot to have a Vector Network Analyzer. Getting everything just right is hard, but you can tweak it on the VNA.

Finally, there are legal issues. The FCC power mask is set as output power from the antenna, so adding directivity to the antenna means you need to reduce the output power correspondingly. While I doubt the FCC goes around checking WiFi specifically, if there's ever a problem with interference, bad things could happen with lawyers and judges.

Good luck!

 

[Nicolas guzman]

Im in the region of United States and mexico and checking the channel frequency for digital tv channel 4 witch falls around 70 mhz and i used a dipole and yagi on that frequency and i get fair signal,but if i use a biquad calculated for uhf 589 mhz on channel 4 ,the signal is much better.it should be the other way around and on the channel list frequency states ch 2 to 6 vhf low,7 to 13 vhf high,and ch 14 to some where 69 uhf on digital u.s and mex.so I am still confusing.

 

[Baluncore]

but if i use a biquad calculated for uhf 589 mhz on channel 4

Depending on what you mean by “channel 4”, there are at least two possibilities. There could be a local UHF digital TV signal available that is stronger than your regional VHF digital TV, or your UHF antenna might be terminated wrongly and the signal is being picked up on the feedline.

Im in the region of United States and mexico

We cannot search the channel allocation data to identify all digital TV in your area unless we know your city. If you cannot identify your city you will have to do that search yourself.

 

[jim hardy]

see if this map is of any help , it''s a US map that works pretty well in Arkansas

http://transition.fcc.gov/mb/engineering/dtvmaps/

 

[Nicolas guzman]

Hi there again,thanks for the info.the channel displaying is nothing but a logo and reference,not rf channel.i scanned a channel no. 4 but the rf channel is on 47-1 and i programed directly. To channel 47-1 and it takes it directly to channel 4-1 automaticly.now i get the point,most scanned tv channels are logo scans,thats what i would think.i have another question on the next paragraph.

 

[Nicolas guzman]

I built yagis and biquads antenas lately,my question doing researches on it and nothing,is when i use any driven or director elements ,its normally cilindrical elements,what if i use squared or rectangular elements,what would be reaction of it.i had problems with that all the time whith yagis working side ways but it is much easier aligning squared elements to a squared aluminum tubular boom.squared or tubular booms shouldn't be a problem.

 

[Jeff Rosenbury]

Element diameter and shape affects the impedance and to a much lesser extent the frequency. For a Yagi I think it might affect the element spacing slightly.

I've had poor luck messing with Yagis. Most antennas change characteristics a little at a time with changing geometry. Yagis often just don't work if you get them wrong.

 

[Baluncore]

With yagis, the element section is not very important so long as diameter is small compared with element spacing.
Neither is the aesthetic squareness of the elements with the boom, 5° or 10° deviation should not be a technical problem.

You can use a plastic block at the crossing of the elements and boom.
Go to a scrap heap and get old commercial antennas. They will have all the bits you need.

Use smaller diameter elements for TV receive antennas, use an insulated boom. The boom can be a PVC tube with holes drilled through for elements.

The problem with yagis is the number of elements. As the number of elements and the gain increases, the bandwidth falls. For VHF digital TV, expect to use only three elements, four elements maximum.

 

[sophiecentaur]

Thus an "antenna" a full wavelength long has an average phase of 0º and doesn't emit many photons.

Now that really is a strange notion and the result of trying to mix ideas from two areas of Physics but not doing it fully. Of course a wire that is one wavelength long will radiate. It just has a different radiation pattern from a short dipole. See this link and many others. The production of photons by accelerating charges must, of course be a possible way of predicting the radiation from a wire but the conventional method is a lot easier, based on Integrating the effect of currents along the wire.

 

[Jeff Rosenbury]

Now that really is a strange notion and the result of trying to mix ideas from two areas of Physics but not doing it fully. Of course a wire that is one wavelength long will radiate. It just has a different radiation pattern from a short dipole. See this link and many others. The production of photons by accelerating charges must, of course be a possible way of predicting the radiation from a wire but the conventional method is a lot easier, based on Integrating the effect of currents along the wire.

Yes it will radiate, just not much. Notice the input impedance is infinite, thus little power will transfer to the antenna.

You are correct that the traditional method works. For odd situations it is better since it allows subwavelength, conformal, traveling wave, etc. antennas. Yet if you want easy power transfer (a good VSWR) quarter and half wavelength elements are a good bet.

BTW, I didn't really understand gauge fields until I realized gauge meant phase and the EM field is a good example of one. So perhaps my understand of gauge fields is off. (Of course it's likely my understanding of EM fields is also off. They are complicated.)

 

[sophiecentaur]

True, if you centre feed it the impedance is very high but there will still be a Radiation Conductance (equivalent of Radiation Resistance. You can feed it elsewhere to make matching more convenient and efficient. There is an advantage in that the beamwidth is narrower.