How can I design of a Corner Reflector antenna?

[imranbd]

Hello every body!
Last weak I decided to make a corner reflector antenna for reciving TV signal.Now I need design and construction theory about it with total idea.I also need circuit diagram.Please share with me if anybody knows.
sorry for my bad English. Thanks

 

[jim hardy]

a search in Google or Altavista ought to get you to hobbyist sites.

"ARRL Antenna Handbook" is a tremendous source of practical information with explanations of underlying theory

 

[imranbd]

I have searched on google but I didn't get circuit and blog diagram.I just got Introduction,definition,Image etc.Please can you give any Link?

 

[imranbd]

a search in Google or Altavista ought to get you to hobbyist sites.

"ARRL Antenna Handbook" is a tremendous source of practical information with explanations of underlying theory

I have searched on google but I didn't get circuit and blog diagram.I just got Introduction,definition,Image etc.Please can you give any Link?

 

[vk6kro]

A simple corner reflector is not a good antenna for TV unless you only have one channel. This is because you need a wide bandwidth for many channels and the corner reflector with a dipole radiator really only works over a narrow frequency range.

You just make a dipole at the required frequency and then place it near the corner of two pieces of metal joined at an angle of about 90 degrees.

You then adjust the distance between the dipole and the reflector until you get the best signal.

One approach would be to take an existing TV antenna and try a corner reflector at the back of it to see if you get an improvement.

The main advantage would be an improvement in front to back ratio. If you were getting "ghosting" from reflected signals, a corner reflector might help.

Here is one design using reflector rods instead of a flat metal reflector: Doing this cuts down wind resistance. (Data from the ARRL Antenna Book):

Frequency 220 MHz
Reflector rod lengths: 30 inches
Reflector spacing (distance between reflector rods) 3 inches
Side length 52 inches (so there are 17 rods on each side of the reflector)
Angle between two sets of rods: 90 degrees
Spacing from dipole to the corner of the reflector: 25 inches.

A dipole for this frequency would be about 25.75 inches long

Like this:

 

[imranbd]

I need a Circuit Diagram and Block Diagram.Another question ..Corner reflector antenna will work perfectly For which device?

Thanks

 

[vk6kro]

A dipole is connected as in the above diagram.

The feedline would connect to your TV set.

A corner reflector is used for single band communication although for about the same cost you could arrange for the reflectors to be in a parabolic shape and hence get more gain. It would then work like a dish antenna but only curving in two dimensions.

 

[berkeman]

A dipole is connected as in the above diagram.

The feedline would connect to your TV set.

A corner reflector is used for single band communication although for about the same cost you could arrange for the reflectors to be in a parabolic shape and hence get more gain. It would then work like a dish antenna but only curving in two dimensions.

Does the rectangle in the middle represent a Balun?

 

[jim hardy]

i've had success for US digital TV using a "folded dipole" which is easily fabricated from copper wire (i used #10 solid) and PVC pipe(i used 1 inch). It has an advantage of being somewhat broadband, working tolerably well over nearly an octave.
You can use the folded dipole as active element and surround it with any type reflector you wish.

First i surveyed the channels in my area and tabulated their frequencies, then made the antenna approximately the proper length for most of them. Luckily that length was about 3X length for a few more channels. Reason it was lucky is a halfwave antenna works okay if it is any odd number of half wavelengths long.

there's an exhaustive treatment of multi-element antennas in a document "NBS Technical Note 688", which is a PDF at NIST's site and a google search on that title took me directly there.

Be aware that in US when we went digital the previous relation between channel number and frequency was abandoned. You have to look up what frequency your station uses now. Most of them left the old VHF band and moved into old UHF frequency slots but kept their old channel numbers. It is very confusing to novices like me.

an antenna is just a wire i don't see how one could have much of a circuit diagram for one.
ARRL Antenna Handbook is filled with practical photos and drawings of how to mechanically fabricate many types of antennas. In that respect it is way better than a college textbook which will be mostly on theory. It is still in print and i advise buying a copy from ARRL or local used bookstore. That NBS brief is handy also in that respect.

 

[vk6kro]

Does the rectangle in the middle represent a Balun?

No, it looks like a mechanical support. Just a joining point, like the ones at the ends of the dipole.

The problem with using a dipole at multiples of the fundamental frequency is that the radiation pattern becomes multidirectional and a lot of the reception can be in directions that are not where the antenna is pointed.

So, you may get a strong signal from the side of a nearby building, but almost nothing from the direction at right angles to the dipole.

The corner reflector will limit reception from the rear, but the reception from the front will be a mess except at the resonant frequency.

A 1.5 wavelength dipole has 6 lobes with only a small one at right angles to the dipole on each side in the horizontal plane.

 

[berkeman]

No, it looks like a mechanical support. Just a joining point, like the ones at the ends of the dipole.

But won't you get reflections from just joining the shield to one of the dipole elements? Going from the unbalanced coax to the balanced dipole would seem to require a balun to get a reasonable SWR...

 

[imranbd]

thanks everybody for important information.which metal can I use as reflector?

 

[sophiecentaur]

thanks everybody for important information.which metal can I use as reflector?

Any metal will do. Cheap alloy is a good, in practice as anything else for TV reception.
If you want a decent antenna pattern, you need a balanced feed for your dipole so that the currents are equal in each half of the dipole. It is fairly easy to make a Pawsey Stub balun (loads of links on Google) which will eliminate currents flowing along the outer of the down lead and producing unwanted additions to the radiation pattern.

 

[davenn]

But won't you get reflections from just joining the shield to one of the dipole elements? Going from the unbalanced coax to the balanced dipole would seem to require a balun to get a reasonable SWR...

actually in practice it works very well, even on transmit
For a long time I fed my 40m inverted V dipole directly with 50 Ohm coax. At the resonant spot on the band the reflected power was minimal, a Watt or two with 100W going in.
I eventually used a coil of ~ 6 turns of the coax right at the feedpoint as a Balun to stop the RF currents on the shield of the coax and the problems it causes when it radiates like that.
With a plain dipole being 75 Ohms and using 75 Ohm coax, even tho its balanced to unbalancd it doesn't really cause a problem.
For those purests amongst us ;) OK use a 1:1 BALUN for peace of mind :)
But putting that dipole in front of a corner reflector or other reflector is going to markedly change the feedpoint inpedance anyway

Dave

 

[sophiecentaur]

The thing is, dave, that, without pretty sophisticated measuring equipment, we don't actually know how well our home equipment is working. It's all pretty subjective (with the exception of VSWR measurements). Antennae and loudspeakers can be really pretty ropey before we are aware of their shortcomings. The problems of unbalanced feeds are less of a worry with some of the high gain Yagi antennae because there are all those other elements which do, in fact, have balanced currents in them so the pattern tends to be somewhat 'tamed'. And then there's always the problem of the gasworks or the nearby block of flats . . . . . .

 

[vk6kro]

But won't you get reflections from just joining the shield to one of the dipole elements? Going from the unbalanced coax to the balanced dipole would seem to require a balun to get a reasonable SWR...

The main effect of not having a balun is that the outside of the coax becomes part of the antenna.
Signals and noise can get picked up by the outside of the coax and they become part of the received signal.
The SWR would be largely unaffected because the 50 ohm coax is a reasonable match for the dipole.
A dipole with a huge reflector like this could have a feedpoint impedance of about 10 ohms, though, so the unbalanced feed would be the least of your problems.However I balked at mentioning baluns considering other aspects of the thread.

From this OP's point of view he could use something like this:

These are like this internally:

This would give a 4:1 step-down effect but the large reflector would already give a low feedpoint impedance.
So, a folded dipole would be better, and even that would give a feedpoint impedance of about 100 ohms with this reflector. Stepped down 4:1 this gives about 25 ohms.

So, I suspect that the balun would add more complexity than benefits.

The main point is, though, that this is not a viable project without reasonable test equipment.
Wide band TV antennas are designed with great care and then experimented with, to actually get them to work.

Edit...
One possibility would be to add a number of ferrite toroid rings on the outside of the coax near the antenna feedpoint. With suitable ferrite, this may be a simple enough solution.

 

[davenn]

The thing is, dave, that, without pretty sophisticated measuring equipment, we don't actually know how well our home equipment is working. It's all pretty subjective (with the exception of VSWR measurements). Antennae and loudspeakers can be really pretty ropey before we are aware of their shortcomings. The problems of unbalanced feeds are less of a worry with some of the high gain Yagi antennae because there are all those other elements which do, in fact, have balanced currents in them so the pattern tends to be somewhat 'tamed'. And then there's always the problem of the gasworks or the nearby block of flats . . . . . .

Yes I agree, and if I was designing/building antennas for commercial use, particularly in a transceive situation, I would be much more particular about feedpoint matching and BALUN use. But for TVRO its really neither here nor there and the effects of having a BALUN or not can be the least of your worries as stated by the first part of VK6KRO's post...

The main effect of not having a balun is that the outside of the coax becomes part of the antenna.
Signals and noise can get picked up by the outside of the coax and they become part of the received signal.
The SWR would be largely unaffected because the 50 ohm coax is a reasonable match for the dipole.
A dipole with a huge reflector like this could have a feedpoint impedance of about 10 ohms, though, so the unbalanced feed would be the least of your problems.

and that would apply to a lot of situations
With Yagi's yes, it's a whole different ball game and there I do have to pay particular attention to the matching system I use. But even then the matching system is more about impedance matching than for a balanced antenna to an unbalanced feedline.

One possibility would be to add a number of ferrite toroid rings on the outside of the coax near the antenna feedpoint. With suitable ferrite, this may be a simple enough solution

Thats a neat trick and is quite effective even up into the GHz range. I have a commercial 12 element, 2.4GHz Yagi which uses that method for a BALUN right at the feedpoint and they are just covered with some heatshrink

Dave

 

[vk6kro]

I have a fairly current ARRL Handbook and it doesn't even mention the Corner Reflector antenna.

This might be because it is large and cumbersome for VHF frequencies and gives pretty poor gain for the size of it.
About all it has going for it, is good front-to-back ratio.

And unless you can somehow provide multiple radiators for it, it cannot be a wideband antenna.

As a single band antenna, you could probably feed the dipole with a Gamma Match, but that is inherently a narrow band approach and may not even work across a whole TV Channel.

 

[davenn]

cant remember what year my copy of my ARRL Antenna Handbook is. Because its relatively expensive and very little changes from year to year, I will only buy an update ~ every 5 yrs.

I agree with you, that at VHF and lower UHF corner reflectors are very cumbersome, considering you need 1 wavelength sides to get any decent gain out of them. But at 1GHz and up they really shine. With a corner reflector on the 1296MHz, 23cm band it is easy to get 10 dBi gain for a compact antenna.

In another life, when I worked for Telecom New Zealand, we were installing 1500MHz radomed Yagis in rural regions for telephone access. after a couple of winters, these Yagis started failing in mass, they just couldn't handle the freeze/thaw conditions on snow/ice covered mountain/hill tops then the hot summers that followed and the radomes and mounting parts were cracking and breaking.
Telecom switched to 1500MHz corner reflecters which us guys built "in House". I even scaled the PCB dipole and matching section for use on 23cm band .

see the right side of the pic...

the left side antenna is a coaxial colinear for 1296MHz

Dave

 

[vk6kro]

One thing I find annoying about Corner reflectors is that most of the reflector is not even used.

If the reflector has a 90° angle, then the areas immediately above and below the radiator send signal directly forward from the antenna. The signal hits the surface at 45 degrees and reflects off at 45 degrees to be horizontal.

I tried putting reflectors on a 6 meter beam like this (above and below the driven element) and it worked well. All the rest of the corner reflector does nothing for forward gain at all.

I agree about the ARRL Handbook. I got a 2010 edition delivered to San Francisco for $7 from Amazon. I brought it home from there. The whole 3 Kg book is on a single CD in the book.

 

[sophiecentaur]

One thing I find annoying about Corner reflectors is that most of the reflector is not even used.

If the reflector has a 90° angle, then the areas immediately above and below the radiator send signal directly forward from the antenna. The signal hits the surface at 45 degrees and reflects off at 45 degrees to be horizontal.

I'm not sure I would agree with you there. The reflector can be looked upon as an Aperture (as with a dish reflector) and the beam width relates inversely to the width (aperture) of the reflector. Currents induced all over the whole reflector surface contribute to the pattern. Having said that, the aperture 'in wavelengths' is relatively small compared with your average dish reflector and the main advantage is in 'front to back' ratio. This is where the use of a balun can help as it limits currents in the down lead, which are as likely as not to compromise the rearwards response. If you are not after a good front to back ratio then I can't imagine that the extra bulk / windage of the corner reflector would be justified.

 

[jim hardy]

reason i mentioned the folded dipole is its simplicity of construction from readily available parts. That makes it a safe recommendation to somebody whose skill level we can only guess.

Folded dipole is naturally 300 ohms.
Post 16 had a photo of a 300::50 ohm matching transformer, twinlead to co-ax. They're typically 50 cents at thrift shops.
I put one inside a pvc pipe tee
and solder the twinlead ends to my loop
which drapes around short support pipes on opposite sides of the tee;
bring co-ax down third leg of tee to receiver.
Now i have an antenna that is reasonably close to electrically correct wrt matching
with no math skill required .

i don't recall the exact length i used for the loop but seems it was in the 25 inch range. theoretically correct length for stations in my region was 7 to 30 inches if i recall . The stations were scattered about the map so the "lobes" worked to my advantage on a couple of them.

somebody pointed out the windage problem with a corner reflector

my advice to OP is:
keep it simple.
We can get seduced by an exotic idea - i crave a Rhombic tv antenna for no other reason than "it's a cool idea", and at today's TV frequencies a lot less impractical than it was in 1960. But i doubt i'll build one.
My folded dipole works as well as my neighbor's $100 hardware store antenna. He's made one himself now.

I do admire the expertise of you ham radio guys. One day i'll get that old DX100 down from my attic and start studying for the license...

old jim

 

[sophiecentaur]

Ah yes - the rhombic antenna. A lovely example of a wide band, traveling wave antenna with a good front to back ratio if you just terminate it right.

 

[jim hardy]

Ah yes - the rhombic antenna. A lovely example of a wide band, traveling wave antenna with a good front to back ratio if you just terminate it right.

sexy, isn't it?

 

[sophiecentaur]

Not as sexy as the Beverage antenna. But I've never come across one of them for anything other than mf and lf frequencies. That design was published in a paper by Beverage, Rice and Kellog (!) way back in the thirties.

 

[jim hardy]

Thanks Sophie

Beverage Antenna search turned up interesting articles on long wire antennas.

Thirties was quite a decade... Harlow, Lombard, Davis, Garbo, Astor, Shearer, ...

 

[sophiecentaur]

BBC Monitoring service used Beverage Antennae in the receiving station at Caversham. They were several hundred metres long, strung on poles and looked after themselves pretty well, once the termination at the far end was sorted out. Loads of signal output from them.
Apparently the original long wire was run close to the ground, draped over bushes. Great if you happen to have lots of land available.

 

[vk6kro]

This is the effect you get with a corner reflector. It is almost identical to the situation of two plane mirrors at right angles with a lamp between them:

http://dl.dropbox.com/u/4222062/corner%20reflector.PNG

A viewer at the top right would see 3 images (red dots) if he was close enough, but further away, he would see varying light levels depending on how the 3 images combined at that angle.

The corner reflector is slightly different to the mirror situation because the distances are comparable with the wavelength of the radiation, so the phases of the reflections are important.

You can also see that the corner reflector triples the power from the dipole radiator, at most, so the maximum gain is 4.7 dB. (10 log (3) = 4.77 dB).

You would have to wonder why anyone would build one of these when changing the reflector to a parabolic cylinder would give a lot more gain for almost the same wind resistance.

This image:

from this site:
http://www.antenna-theory.com/antennas/reflectors/cornerReflector.php
Suggests a third reflection directly behind the corner.