XGH (Extended Gray-Hoverman): Design Has No Phasing Line - Canadian TV, Computing and Home Theatre Forums

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post #1 of 107 (permalink) Old 2008-11-25, 06:06 PM Thread Starter
 
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Cool XGH (Extended Gray-Hoverman): Design Has No Phasing Line

Well the new King has done it again ! !

This time it is a stunning design approach to DBGH, without the need for an interconnecting phasing line, which John calls the Extended GH

For forum members that are looking for new R&D design approaches, and wish to contribute...get your modeling tools and have a go.

http://www.jedsoft.org/fun/antennas/dtv/xgh.html
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post #2 of 107 (permalink) Old 2008-11-25, 06:30 PM
 
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isn't using 1/8" wire diameter a little flimsy? wouldn't 1/4" better for stiffness? would 1/8" for element and 1/4" for reflectors have any effect?
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post #3 of 107 (permalink) Old 2008-11-25, 08:25 PM
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Looking at the XGH charts is it correct to think of it as a fairly narrow bandwidth "channel peaked" antenna?

Isn't the SWR quite high down in the lower UHF channels?
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post #4 of 107 (permalink) Old 2008-11-25, 11:41 PM Thread Starter
 
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Originally Posted by stampeder View Post
Looking at the XGH charts is it correct to think of it as a fairly narrow bandwidth "channel peaked" antenna?

Isn't the SWR quite high down in the lower UHF channels?
As presented, this is indeed a narrow-band design with a peak gain 18+ dBi. I can't speak for John, but as you know the SBGH can be designed as narrow or wide band, simply by parameter changes on the collinear reflectors, and this new DBGH geometry, or other variants, may be workable as wide band as well. Additional R&D will be required, and here is a great opportunity for members that are keen to get involved with 4nec2 to contribute to GH improvements.

The intriguing possibilities with this particular design is to have the equivalent of a Yagi, in a much smaller package. If you check out K7MEM's yagi design web site, a 15.85 dBd Yagi (15.85dBd = 18dBi) at 615Mhz has a total of 23 elements with a boom length of 138.5" or almost 12 ft !

For DTV reception, this design is very useful for co-channel interference problems, such as we have here in Ottawa in trying to get wnpi on channel 23. We have co-channel interference from two strong local channels; CBC-French digital on Ch22 and analog TVO on Ch24.
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post #5 of 107 (permalink) Old 2008-11-26, 12:51 AM
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The intriguing possibilities with this particular design is to have the equivalent of a Yagi, in a much smaller package.
Yes, interesting design. And the gain curve looks like a typical high gain yagi too.

My builds/plans (not the latest models) are located here.
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post #6 of 107 (permalink) Old 2008-11-26, 12:19 PM
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Looks good in theory, but very difficult to build anywhere near the theory.

How tall is that monster? There's going to have to be a significant chunk of metal mast in there to support it, and thicker diameter elements would certainly improve robustness. But what does that do to the design?

One can get >20dBi with four small, lightweight yagis (6' long each) in a 3'x3' H-array. Easier to build to spec, and much less of a wind sail.

Heh.. it might be interesting to stage a head-to-head shootout of the GH monster versus the quad yagi array!

Cheers
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post #7 of 107 (permalink) Old 2008-11-26, 11:25 PM
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How tall is that monster?
44 7/16 times 2 equals 88 7/8 inches. And its 25.74 inches wide.

I think you can build it, I have faith in you, heh.

I think a problem maybe just like with a yagi. With trying to eliminate the two adjacent channels, doing it with a model is one thing, but then the actual build is a whole other thing when peaking for just one narrow channel. If those adjacent channels are the problem, have you guys considered ordering a custom made trap ?

Looking at that design, it does look very counter-intuitive. Lengthening the middle legs to almost 11 inches narrows the bandwidth, instead of logically increasing the lower bandwidth as would be expected.

My builds/plans (not the latest models) are located here.
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post #8 of 107 (permalink) Old 2008-11-27, 12:39 AM
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If those adjacent channels are the problem, have you guys considered ordering a custom made trap ?
I may yet build one. But a good F/B ratio is sufficient -- the interfering adjacent channels are about 120 degrees around from the direction of our WNPI-DT.

Cheers
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post #9 of 107 (permalink) Old 2008-11-27, 09:51 AM
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Quote:
Originally Posted by Autofils
For DTV reception, this design is very useful for co-channel interference problems, such as we have here in Ottawa in trying to get wnpi on channel 23. We have co-channel interference from two strong local channels; CBC-French digital on Ch22 and analog TVO on Ch24.
Minor nit: this is known as adjacent channel interference. Whereas co-channel interference refers to the situation where two or more stations are using exactly the same channel.
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post #10 of 107 (permalink) Old 2008-11-27, 10:14 AM
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co-channel interference problems, such as we have here in Ottawa in trying to get wnpi on channel 23. We have co-channel interference from two strong local channels; CBC-French digital on Ch22 and analog TVO on Ch24.
Sorry to pick nits, autofils, but you're talking about adjacent-channel interference.
Co-channel is interference from another signal on the same channel -- for example, channel 22 Burlington is co-channel to CBOFT-DT Ottawa.

As an advanced topic, one can talk about n-adjacent channels, for example, 23 is first adjacent to 22, 24 is second-adjacent to 22, etc.

This terminology also applies to North American FM broadcast channels, which are allocated 200 kHz apart on odd 100kHz centres. For example, your goal of 89.5 (Potsdam) is second-adjacent to local stations on 89.1 and 89.9, for example.

/end RF lesson

Sigh -- never mind, mlord got in while I was typing....
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post #11 of 107 (permalink) Old 2008-11-27, 12:24 PM Thread Starter
 
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Is a 20 dBi Extended GH Narrow Band antenna possible?

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Sorry to pick nits, autofils, but you're talking about adjacent-channel interference.
Co-channel is interference from another signal on the same channel -- for example, channel 22 Burlington is co-channel to CBOFT-DT Ottawa.

As an advanced topic, one can talk about n-adjacent channels, for example, 23 is first adjacent to 22, 24 is second-adjacent to 22, etc.

This terminology also applies to North American FM broadcast channels, which are allocated 200 kHz apart on odd 100kHz centres. For example, your goal of 89.5 (Potsdam) is second-adjacent to local stations on 89.1 and 89.9, for example.

/end RF lesson

Sigh -- never mind, mlord got in while I was typing....
Looks like the "nit picker" race was on
Thanks for correcting my wording, I wouldn't want to mislead anyone.
Indeed, it is adjacent channel interference....what was I thinking ...!

Re 89.5 FM (potsdam): Thanks for the RF lesson.
I've modeled a GH4_VHF-Hi/FM combo that looks like it's worth a try. Total height is 53 inches.

20dBi Extended GH NarrowBand - Possible ??
I ran the extended GH8(jed) model by John Davis, and it shows peak gain at 614mHz with the following parameter values:
RawGain: 18.64 dBi
SWR: 1.295.
(Net gain is calculated at 18.56dBi)

We know from past modeling history for the SBGH, that the gain delta between a GH4 and GH10 is in the range of 2 to 2.5db. If you view this extended GH8(jed) model as "two GH4 type bays", it might have similar delta gain increases for more reflector pairs.
If so, a 20+dBi extended GH with total height of about 89" might be possible.

Who will put the "R" into R&D ?
Since this is an R&D thread, I will pose two questions for anyone interested in doing further research on the extended GH(jed) Narrow-band design ?

1. Can the gain be increased by 2dB or more by adding more reflector pairs?
2. Can the SWR over a channel's six Mhz bandwidth be further reduced?


From a construction point of view, each array element would have to be made with two sections, joined at the feed-point. Other than that, it should not be that difficult to build....just a few more right-angled array bends, but I'll leave that up to the "build-king" [mlord].
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post #12 of 107 (permalink) Old 2008-11-27, 12:36 PM
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Lightbulb A rapid prototyping jig for GH variants

Quote:
Originally Posted by Autofils
From a construction point of view, each array element would have to be made with two sections, joined at the feed-point. Other than that, it should not be that difficult to build....just a few more right-angled array bends, but I'll leave that up to the "build-king" [mlord].
Heh.. this one is too large to safely use on my tower, so I probably won't build one.

But an idea did occur to me for an easy way to prototype these things with simple materials:

Just assemble two large rectangular frames from 300ohm's favourite material: 3/4" PVC plumbing tube & fittings. Line them up, one behind the other, and fix them that way with more fittings, spaced apart by whatever spacing you want for array/reflectors.

Then tape some nylon fencing mesh to the front of each frame -- this is very light black netting available in rolls from Home Depot (and others). Get the stuff with the 2" grid spacing (lower wind resistance). Stretch it tight onto each frame and tape it there permanently (and/or use cable-ties).

Now you've got two RF-transparent mesh mounting planes, onto which thin wire elements can be attached at will.

Bend the zigzags from 14g copper house wire (cheap, easy to get and easy to bend), and tape those onto the front mesh plane.

Cut the rods with wire-snips from more 14g copper house wire, and tape those onto the rear mesh plane.

Hoist it up into the air and go wild. Not good for any kind of extended use, but really great for prototyping. Want to try a design variant? Just remove/rearrange the copper wires to suit.

Cheers
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post #13 of 107 (permalink) Old 2008-11-27, 01:03 PM Thread Starter
 
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Heh.. this one is too large to safely use on my tower, so I probably won't build one.
Have you calculated the loading on your rotor bearings for your quad-Yagi's total weight and turning radius? I assume you consider the extended GH to be worse than the quad yagi ?

If 89 inch height is too much for the extended GH, what is the max antenna height you would consider safe for your tower ? From modeling work that I did on the GH4 VHF_Hi/FM combo, it turns out that the reduction in gain with smaller array half-angles ( <45) is not as great as I originally assumed. The same may apply to extended GH NarrowBand.
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post #14 of 107 (permalink) Old 2008-11-27, 01:20 PM
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Have you calculated the loading on your rotor bearings for your quad-Yagi's total weight and turning radius? I assume you consider the extended GH to be worse than the quad yagi ?
The quad yagis will have a very low centre of gravity (as little as 18" above the top of the tower bearings), and they will hardly weigh anything.

The tower is rated for 3 square feet of wind sail at 3' above the top of the tower (for 85MPH wind, I think), so no issues at all with the yagis. And they don't present a huge profile to the wind anyhow.

But a monster GH is quite different -- very similar to just taking longer yagis and standing them on end. Raises the centre of gravity and presents a much larger face to the wind.

I think the tower could handle a single monster GH, but not a pair of them.

Keeping things within a (roughly) 6' height is probably safest for the tower, as that keeps the center of gravity at around 3' height, within spec of the tower.

Cheers

Last edited by stampeder; 2008-11-27 at 04:07 PM.
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post #15 of 107 (permalink) Old 2008-11-27, 02:51 PM Thread Starter
 
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Originally Posted by mlord View Post

I think the tower could handle a single monster GH, but not a pair of them.

Keeping things within a (roughly) 6' height is probably safest for the tower, as that keeps the center of gravity at around 3' height, within spec of the tower.

Cheers
Thanks for that input.
As I mentioned in the previous posting, it might be possible for a single extended GH Narrow Band with more reflectors, to achieve 20+ dBi for Ch23.

I'm estimating that your existing Winegard Quad PR4400 would have a total height of around 64-68 inches? So perhaps a design objective for max height of 66 inches ( 5.5 ft) would provide a safe margin for your tower.
That would only require a 25% reduction on the current 89 inch height and adds a third question for further single extended GH narrow band research.

So to update the "Who will put the "R" into R&D ?" for anyone interested in doing further research on the extended GH(jed) Narrow-band design; here are the design objectives.

1. Can the gain be increased by 2dB or more by adding more reflector pairs?
2. Can the SWR over a channel's six Mhz bandwidth be further reduced?
3. What gain reduction occurs, when the array half angle is reduced from 45 to a smaller angle that results in max height of 66 inches.

The nec file for the extended GH8 NarrowBand design (half-angle=45) is available on John Davis's web site.
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