[mrvanwinkles]

Drove through Smith's Falls last time, going from Ottawa to Kingston. Noticed this "Flat Parabolic" (?) half way up the tower - center of picture.

Just had to take a picture with cell phone.

The antenna in the center of the picture looked like it had a 2-dimentional flat parabolic reflector and a yagi-style antenna as the receiving elements at the "focus" point.

Anyone know what this antenna is designed for?

Sorry if picture is small and not very good. I might be able to post a better picture later. But the "Flat Parabolic" (?) antenna was high up and I used the maximum zoom available to me on my cell phone.

It was across the street from the old Tim Hortons, main strip, downtown Smith's Falls, Ontario, Canada. Buildings near base of tower are a Fire Hall and I think a Bell Canada building.

When I looked at it from the ground, it looked like they had something like a YAGI style antenna as the receiving elements at the focus of the parabolic reflector.

Could this type of antenna ... or a version of it ... be used for OTA TV reception? UHF ?

 

[stampeder]

Anyone know what this antenna is designed for?

I saw one just like that in Powell River, BC pointing across the water to Texada Island, and I assumed it was a telecom link. I don't have any further info, though. I wonder where that Smith's Falls antenna points?

 

[mrvanwinkles]

Re: Unknown Flat Parabolic (Rectangular / Sectional Parabolic)

Re: Unknown Flat Parabolic (Rectangular / Sectional Parabolic)

I did some searching and researching on the internet and apparently those "flat parabolic" antenna's pictured in my previous post are also known as Rectangular Parabolics (as opposed to Circular Parabolics). Also referred to as Sectional Parabolics.

Like many parabolics, they seem to be used alot at frequencies higher than UHF. > 1 GHZ ex. 2.4 GHZ . They seem to be used for WLAN links. ie. Wireless Computer Networking links and networks.
ie. Data transmission at frequencies higher than UHF
Also for Radar, and I'm sure for satelite.

Internet research did find a few references of this type of antenna being used for UHF reception though ... perhaps in the past.

ex. I found references to the following antennas, which may or may not be rectangular or sectional parabolics for UHF reception:

Jerrold JUP4
Finco P7
Winegard U-630
Lance Industries Inc, Models 18 and 21 and 24

There's alot of interesting info and technical stuff on the internet about Parabolics. Gain, beamwidth, how to illuminate if you're transmitting. Formulas. Deep and Shallow dishes. Side lobes. Diameter to Focus ratio etc etc. And lots of interesting info and alot of reading in Patent descriptions.

I saw one formula, and it seems the GAIN of a parabolic antenna is inversely proportional to the square of the wavelength of the frequency of use. So as you go down in frequency (say to UHF) the GAIN decreases quite a bit.

Regardless ... there's some good gain values mentioned. 18 dB, 20 dB, 24dB. So there might be some hope in experimenting with a flat, rectangular, or sectional parabolic designs - as well as a circular parabolic - for UHF OTA.

The flat, rectangular, or sectional parabolics might be easier to build from scratch. Bend two sides of a square frame into a proper parabolic curves and then run straight reflector rods or wires horizontally across the frame.
Distance between reflector rods or wires - I have read - must be no greater than 10% of the wavelength of the lowest frequency use or your reflector does not work so well.

In addition, I read that focal distance to the receiving elements is critical to gain. So placement of receiving elements at the focus is critical.

Shape of the parabolic curve is important too - based on wavelength.
But at UHF, wavelength is larger, so probably reflector accuracy is more forgiving.

Fascinating ... maybe someone could do some computer simulations to see what the possibilitites / performance might be at UHF frequencies.

 

[stampeder]

Scala Paraflector

Here's another antenna fitting that description, with some great photos:

Scala Paraflector with 4220 feed

 

[Dalek Prime]

Wow! Where'd the guy get the lawn chair! Seriously, that's a pretty awesome reflector. Both of them. Great workmanship! I'm rather in awe, and jealous a bit. Though, not of the lawn.

 

[mrvanwinkles]

Corrections to my previous post. oops

Distance between reflector rods or wires - I have read - must be no greater than 10% of the wavelength of the lowest frequency use or your reflector does not work so well.

Ooops .

Correction #1:

I think that should be "...10% of the wavelength of the highest frequency of use ... "

[ wavelength gets smaller as frequency increases, so reflector wire or rod spacing has to be close enough with respect to the smallest wavelength of interest ... and that is the wavelength of the highest frequency you wish to receive, not the lowest. ]

Correction #2:

Jerrold JUP-4

[ I saw a dash in the model number, where I found that reference ... that might be import for anyone trying to search on the internet ]

 

[mrvanwinkles]

Flat Parabolic for UHF - possible design.

Stampeder,

Thank you for that link, it seems someone has tried the rectangular parabolic for UHF, and had some results.

Ya, something like that is what I had in mind for construction.

But what I had in mind for the receiving element was a dipole, like with a YAGI, because that is what I think I saw on the antenna in the photo I took from Smith's Falls, Ontario.

Maybe rather than rods, one could pull 10 GA or 12 GA copper wire across the frame horizontally as the reflectors.

Size and curve of parabola would need to be calculated to give good results, I think - for UHF frequencies.

I think you would need to calculate a starting point (size and shape and curve) for the design ... and then try to simulate with computer modeling to see how it performs. If promising ...then try and build one and test in real world. That's usually how it goes.

The dipole and Yagi receive elements, I think, would suit this type of reflector better ... because the focus point is a line in front of the parabola reflector. And the YAGI reflector at the back reflects the signal coming from the front ... front to back ratio of the YAGI.
(the YAGI would be turned around / reversed to point in to the center of the parabola reflector, and the receive dipole or folded dipole ... at or near the focus.)

YAGI gives gain from the front too, for the signal reflected from the parabolic reflector, and provides a vertical beamwidth and horizontal beamwidth too, to catch (mostly) (only) the signal reflected from the parabolic reflector.

In addition, I think the YAGI blocks less from the front ... making the thing more efficient (than a screen and bowties) ... an issue mentioned in the posts that follow your link.

So the design of the receiving YAGI and the Parabolic Reflector are inter-connected.

This would be a YAGI designed for UHF frequencies - of course - so it would not be that big.

... I THINK all this makes sense from what I've gleaned so far about parabolic design in what I have read so far.

Please anyone ... correct me if I am going in the wrong direction.

 

[300ohm]

Yeah, with a nice cushion, that reflector would be a very nice lounge chair.

 

[Damon Hill]

That's actually a Scala commercial-grade UHF antenna, or rather the reflector. Something you usually see for transmitter links. Looks pretty darn rugged too.

 

[Dalek Prime]

Yeah, with a nice cushion, that reflector would be a very nice lounge chair.

Not so sure of the armchair down below. Update: I need glasses. I looked again, and mistook some boxes or whatever for an old armchair on the lawn. How embarrassing.

Yeah, looks totally rugged. I have gear envy.

 

[300ohm]

The good part is he can do real time tweaking on the porch, its up a nice height.

The bad part is he has a metal pole in front of the antenna thats holding up his porch. And maybe some power lines in front to boot.

 

[Dalek Prime]

Oh yeah, just noticed that. What a kicker, eh? aka. "Perfection with a monkey wrench thrown in."

 

[JamesK]

The antenna in the center of the picture looked like it had a 2-dimentional flat parabolic reflector

That looks like a "corner reflector".

Like all other antenna types, it can be used for any radio service, by varying dimensions with frequency. Many years ago, corner reflectors were often used for UHF TV.

 

[stampeder]

He is specifically asking about antennas that use reflectors that are a section of a parabola, not about the basic theory of a corner reflector.

 

[mrvanwinkles]

Scala PARAFLECTOR antenna

Found it on the internet ... Thanks!

Link to PDF with specs, drawings and graphs:

http://www.kathrein-scala.com/catalog/PR-TV.pdf

Kathrein Scala Division
PR-TV series
PARAFLECTOR antenna
15.5 - 17 dBd gain
470 to 862 Mhz

"High Gain, half parabolic design"
"Designed for professional receive and low power transmit applications..."

H-plane beamwidth: 16 degrees (half power)
E-plane beamwidth: 24 degrees (half power)

VSWR < 1.2 / 1
Front to back ratio: 25 dB
Impeadance: 50 ohms

36 inch / 915 mm wide
68 inch / 1727 mm tall

38 lbs / 17.2 kg

Thank you for identifying the Scala antenna for us !

 

[mrvanwinkles]

Scala also has some interesting antennas for VHF HI and VHF LO on their website. PDF's with nice drawings and specs too.

Check out the Scala website .

 

[JamesK]

He is specifically asking about antennas that use reflectors that are a section of a parabola

He asked about the antenna in the middle of the picture, which, to me, looks like a corner reflector. The parabolic is further up the tower.

 

[stampeder]

As per Post #2 I thought it might be a Paraflector as I drove by it but I forgot all about it until you posted the Smiths Falls tower pic. Next time through Powell River I'll take a photo and go around to the front of the building to see if I can identify the company using it (assuming they're in there). My hunch at the time was that an ISP is using it to provide services to Texada Island.

 

[mrvanwinkles]

Indeed, the 2 antennas on the tower in Smith's Falls, Ontario, look awfully like the Paraflector model from SCALA ... or versions / modifications of it.

But not confirmed yet.

From what I saw in person, with my own eyes from the ground, in Smith's Falls ... there's something a little different at the feedpoint / the focus of the reflectors than what is shown on the SCALA website for the Paraflector model. That's a very important point I think.

What's used as the RX / TX element(s), at the FOCUS of a Parabolic is pretty important - I have read.

Wishing someone with a good digital camera and a big ZOOM lens could post up a better picture of those two antennas in Smith's Falls.

In that case ... the solution is to ZOOM in Optically with a good camera and lens, get close optically (or maybe digitally) and save or create the image file so it can be posted to the FORUM.

And so we can get a closer and clearer look at those antennas.

I feel like a SPY.

 

[mrvanwinkles]

UHF antennas - Digging into the books.

So ... after referring to some UHF Antenna books in the Eng. Library at the University ... some very interesting things have come to light:

From one book:

"It is claimed that parabolic reflectors ... provide little or no improvement over corner reflectors of comparable size, and that the latter are less critical to adjust than the parabolic system."

[ However, this is a very old book ... but the point is understood. ]


From another book [much more recent]:

"For a given aperture, the corner reflector does not equal a parabola in gain, but is simple to construct, broadbanded, and offers gains from about 10 to 15 dB, depending on angle and size."

I read (& photocopied) lots of interesting information on the basic design of both the Parabolic and the Corner Reflector antenna - for UHF use - from a few of these books that I found in the Eng library, EE section, of the University.

Lots of basic tips and practical advice - good advice that comes from others who have already designed, built, and tested these types of antennas.

Some of these books are pretty old. Some dating back to just after the Second World War. It seems there was a lot of interest and development of this stuff for public use ... after the war ... due to things learned and developed with the development of RADAR during WW II.

... and then with the development of TV for public use.

These books are well written and explained with good sketches and drawings.

I think there are possibilities for good "home made" / "home brew" antennas - parabolic and corner reflector - for UHF TV reception.

Tables of values for Gain at UHF frequencies, Actual and Calculated, in these books, shows some promise.

ex. One book shows Actual Gain of 23 dB for a cylindrical parabolic, that is four (4) wavelengths wide, and two (2) wavelengths tall.

UHF Ch 14, approx 470 Mhz, wavelength is approx 64 cm (around 2ft).

So an 8 ft x 4 ft ... cylindrical parabolic reflector.
[and it does not have to be a solid surface reflector ... it can be reflector rods appropriately aligned, and spaced closely enough ... or the right kind of screen or mesh]

Ya ... kinda big ... but still do-able.

You need more reflector surface area - esp at UHF - to get more GAIN.

That has been shown with the Double GH ... compared to the single GH.

As I read in another book ... something like ...
"because UHF antenna designs are smaller in size, due to smaller wavelengths at UHF frequencies, they also capture less signal ... and so ... larger reflectors or multiple combined or multiple stacked antennas are sometimes used for UHF."

Antenna "Arrays", and how best to connect them together, are also mentioned.