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Biquad antenna

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Sceadwian

Banned
About a year ago someone posted an excel drawing that had all the ratio measurements needed to construct a biquad antenna for any frequency. I've lost the doc, anyone know where I might be able to find this information out again?
 

MikeMl

Well-Known Member
Most Helpful Member

Sceadwian

Banned
Which has a fixed reference for 2.4ghz Where's the equation to determine the element length for other frequencies?
 

stevez

Active Member
I don't know what you mean but "quad" antennas are popular for VHF and UHF - lots of proven designs in the handbooks. IM me if you like - maybe I can share what little I have. I did build what was called a quagi - a yagi antenna with quad director and driven element - it worked well.
 

Sceadwian

Banned
I've only ever heard them called bi-quads, Because they're two (bi) quadrilaterals (squares) next to each other.
 

Boncuk

New Member
you might take a look here:

Biquad Antenna Construction

This page contains the construction drawings. I'm sure there will be the required forumulas as well.

Boncuk
 

Sceadwian

Banned
Boncuk, that's the same link Mike posted. I've as yet not found the equation to determine the element length for an arbitrary frequency.
 

Boncuk

New Member
Boncuk, that's the same link Mike posted. I've as yet not found the equation to determine the element length for an arbitrary frequency.
Sorry for that. I'll try another way in a German forum. I read an article there about the quad antenna.

If I have a good find I'll post it, or better make a drawing including all formulas. :)

Boncuk
 

Boncuk

New Member
Hi Sceadwian,

I found an article with a very good explanation of a "Hybrid-Double-Quad-Array".

Unfortunately the article is written in German, but the instructions are done in English.

The instructions refer to a 1.2 and a 2.4GHz antenna. If your design has to use a different frequency use the formulas given in the article.

If you find the article useful I will translate the first page for you to English. (at least I'll try to :) .)

The first page also contains the email address of the author. I'm pretty sure he will answer your questions.

Quad antennas are normally lamda/4 * shortening factor (electromagnetic waves don't travel at the speed of light in insulated conductors.) I used a shortening factor of 0.98 with excellent results for my antenna constructions making long yagi antennas in the 2m band.

Kind regards

Hans
 

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stevez

Active Member
For many (but not all ) antennas you can scale the dimension linearly. As an example - if a given distance is 0.05 wavelengths then scale up or down to 0.05 wavelegths at the frequency of interest. That applies to open air construction. If feedlines or transmission line transformers are used then scaling this way works but you have to give attention to the velocity factor of the transmission line.
 

Sceadwian

Banned
I feel a little silly, the 2nd page has the equations on it, and it's absurdly simple. Each side of the biquad is simply 1/4 wavelength. Even without translating that doc has all the info I need, thanks tons for looking for me Boncuk.
 

microtexan

New Member
I feel a little silly, the 2nd page has the equations on it, and it's absurdly simple. Each side of the biquad is simply 1/4 wavelength. Even without translating that doc has all the info I need, thanks tons for looking for me Boncuk.
Yeah, Boncuk, great article!:)
 

Boncuk

New Member
Hi Sceadwian and microtexan,

it is always a pleasure for me if I can help.

Sceadwian, if you plan to build such an antenna, I strongly recommend bending the squares sharply and precisely and keep distances between "dipoles" and reflectors also precisely.

I read at another forum that the antenna gain decreases considerably if the parts don't exacly match each other (referred to frequency).

Boncuk
 

G7VGG

New Member
You can scale any antenna made for one particular frequency or band to work on a completely different frequency or band by using very simple calculations. The double quad article posted above was for an antenna designed for 2.4Ghz (the 12.5cm band). The dimensions given are not spot on for 2400Ghz, they for a frequency a bit higher in the band as the calculated length of the sides of each quad loop should be longer than he used for exactly 2400Ghz. Not only that but the designer seems to have been unaware that actual antenna dimensions are always shorter than a calculated antenna in "freespace", usually by a factor of about 0.95x.
You did not mention what frequency you want to convert the design to, but lets assume its for the 70cm amateur band 430-438mhz. The length of the sides of a quad loop, irrespective of the frequency, are always 1/4 wave long at the frequency of choice multiplied by 0.95. So to find the length of the sides of a quad loop made for the middle of the 70cm band, for example, simply divide 300 by the frequency in Mhz (435Mhz), then divide that figure by 4 and multiply it by 0.95. This gives a figure of 16.37cm (163.7mm)...For convenience we can round this up 164mm. So that is the length of each side of a quad loop made for that design frequency. The article states that the reflector should extend at least 1/4 wave larger on each side than the Bi-Quad driven element...So for 435mhz that would make it 164mm x2 (328mm) taller and wider and than the driven element.
The spacing between the driven element and the reflector is about 1/8 wave...That's 164mm divided by 2 (about 82mm for 435mhz).
The bandwidth that the driven element (the Bi-Quad in this case) will cover (where it has a low SWR either side of the design frequency) is determined by the ratio of the diameter of the wire or tube used to actually construct the driven element to the frequency the driven element loop is designed for. The golden rule for conductors used to make driven elements in antennas is the bigger the diameter the conductor has, the greater the bandwidth it will cover.
Obviously, wire has a very small diameter to frequency ratio for low frequencies like HF but as the frequency goes up the ratio changes until at UHF frequencies or higher, a driven element made of wire can be appreciably large in diameter in relation to the frequency and therefore it can still have a relatively large bandwidth. In the UK, the 70cm band covers at least 10Mhz...In order to cover a band that wide, wire is normally replaced with small diameter Copper tube. The 23cm band is a lot wider than the 70cm band (1240-1300Mhz...60Mhz wide!) and in order to maximise the bandwidth of 23cm antennas Copper tubing of around 15mm OD is often used. The advantage of using the latter is that broadband Quad driven elements for 23cm can be easily constructed with off the shelf Copper plumbing parts, like 90 degree elbows etc. Alf, G7VGG.
 

JimB

Super Moderator
Most Helpful Member
Alf, welcome to the forum.
Lots of good information you have given in your post, but as Ian says, this is a very old thread and the original participants are no longer here.
JimB
(Also known as GM3ZMA)
 
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