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Old 2-Mar-2013, 10:04 PM   #1
Alan909
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"stacking two 91xg's question"???

Question 1. If one 91xg antenna gives me 10dBd on channel 22(66) FOX, then how much more dBd gain would I have "total" by stacking 2 91xg's for channel 22(66)FOX???

Question 2. Should I stack verticle or horizonal for most gain...

P.S. Using 25ft RG-6 coax straight to Reciever...no splitters...not grounded yet but soon will be...no trees or buildings to interfere but have a few HUGE hills between me and transmitter...

http://www.tvfool.com/?option=com_wr...1dda5c23b61028
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Old 3-Mar-2013, 4:40 AM   #2
GroundUrMast
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Your report indicates an estimated NM of -16.4. Click on the call sign of WFXP and you see that the hill is rather close to the receive end of the path. These factors don't bode well for reliable reception even if you could find an antenna with gain in excess of 20 dBd. This qualifies as needing 'extreme measures'.

Here's an attempt at answering your questions;

If you can invent a losses combiner, you would hope to get twice the power. Expressed in decibels, that would be 3.01 dB additional gain (rounded to the closest 100th of a dB). Many folks have tried using a common two-way splitter in reverse... but it has at least 3.5 dB insertion loss, thus a net loss of gain.

Stacking horizontally versus vertically will not produce any more or less gain on average. In practice, chance may prove one better than the other... from moment to moment as multipath can affect one antenna differently than the other. Each antenna may not receive the same amount of power as the other, and a slight change in antenna location can produce a significant change in reception conditions.

You could use a low loss network made of two sections of 106Ω coax (you may need to build your own coax, I don't know of an RG-X type to specify). Each section of 106Ω coax would need to be 1/4 wave length long, which is 0.144 (electrical length in meters) at 521 MHz, the center frequency of CH-22. Presuming the velocity factor of the coax is less than 1.0, you would need to adjust length to match the coax used... multiply the electrical length by the velocity factor to obtain the physical length. The ends of the matching sections opposite the antenna ends would be joined together with the RG-6 down-lead.

If this seems a bit much... I think it is, at least for most folks. In the real-world instances that have used this type of combiner, the net gain is about 2 dB due to the losses that are lower but still present.

If you really want to try building something... consider a rhombic: http://forum.tvfool.com/showthread.php?t=2865 This is a wire antenna, very big, roughly 30' to 40' long. But the up side is that if you can cut wire connect it together, you may have the skill set needed to build an antenna of this type.
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Old 5-Mar-2013, 8:46 PM   #3
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Thanks for the info GUM...brings me back to memory lane of old tube Ham Radio days 30 years ago and building antennas...Might as well start studying for my Ham License again and kill 2 birds with one stone as I live in the country and have plenty of space for an antenna farm...
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Old 5-Mar-2013, 9:01 PM   #4
Stereocraig
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Quote:
Originally Posted by Alan909 View Post
Thanks for the info GUM...brings me back to memory lane of old tube Ham Radio days 30 years ago and building antennas...Might as well start studying for my Ham License again and kill 2 birds with one stone as I live in the country and have plenty of space for an antenna farm...
Alan,
Have you checked out some of the antenna arrays on the CM 4251 Tribute page?

Simply gorgeous!

Sorry, forgot to paste.
http://www.rocketroberts.com/cm4251/cm4251.htm

Last edited by Stereocraig; 5-Mar-2013 at 9:24 PM. Reason: forgot link
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Old 5-Mar-2013, 9:07 PM   #5
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No I have not but will a.s.a.p. ... gotta get my HoneyDo list done first, lol...thanks for the info...
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Old 18-Mar-2013, 7:29 PM   #6
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If you can invent a losses combiner, you would hope to get twice the power. Expressed in decibels, that would be 3.01 dB additional gain (rounded to the closest 100th of a dB). Many folks have tried using a common two-way splitter in reverse... but it has at least 3.5 dB insertion loss, thus a net loss of gain.

Now this is interesting >>> 2-way, 4-way, and 8-way gangs are common. Successful gangs of other numbers are very uncommon.

For a 4-way gang, there is a trick for eliminating the combiner, but it only works for balanced line such as 300-ohm twinlead:

The “two in series, two in parallel” connection maintains 300 ohms. If well constructed, there is no loss whatsoever. A variation on this is to connect all four antennas in parallel and use a 1-to-1 balun.
http://www.hdtvprimer.com/ANTENNAS/ganging.html
Attached Images
File Type: gif GangOfFour.gif (4.1 KB, 615 views)

Last edited by Alan909; 18-Mar-2013 at 7:35 PM.
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Old 18-Mar-2013, 8:00 PM   #7
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Custom twin-lead is easier to build than coax... at least that's my opinion. Still, either can be built by the experimenter. Wikipedia articles provide sufficient math and related theory to get one started. And a quarter wave matching transformer can be built from either twin-lead or coax. Given the integrated strip-line balun built into the 91XG, a coax based matching system would be the logical option. (I've built 'ladder-line' style twin-lead using bare copper wire and spacers made from plastic pail lid material. One could use aluminum or copper tube and wire to build short sections of coax... the dimensions are very important.

http://en.wikipedia.org/wiki/Quarter...ce_transformer
http://en.wikipedia.org/wiki/Characteristic_impedance
http://en.wikipedia.org/wiki/Twin-lead
http://en.wikipedia.org/wiki/Coaxial_cable

The 'cost' of a quarter-wave transformer is that it will have a relatively narrow operating bandwidth compared to a wire-wound broadband transformer. A quarter wave matching system designed to operate in the UHF band is going to be very inefficient in the VHF band... but your proposed application would not be affected by this given the 91XG is also limited to the UHF band already. (I toss this in for anyone hoping to try this with a pair of all-channel antennas.)

I crunched a few numbers and came up with a 'home brew' coax with an impedance of about 103Ω. I used the ID of 3/4" type M copper pipe, 0.811" and the diameter of #24 AWG wire, 0.0201". Presuming air dielectric I ran the numbers using the calculator at http://www.rfcables.org/coax-calculator.html.

The #24 AWG wire is commonly used in CAT-3 through CAT-6 type phone and data cable and the Type M pipe is the common thin wall copper pipe used for plumbing. If you use an RG-6 cable, 1/2 wavelength long (electrical length) to connect each antenna to a custom built 106Ω (nominal) coax section that is 1/4 wave length long (electrical length), you would be in the ball park...

http://en.wikipedia.org/wiki/Copper_tubing
http://en.wikipedia.org/wiki/American_wire_gauge
http://en.wikipedia.org/wiki/Wave_propagation_speed

Last edited by GroundUrMast; 18-Mar-2013 at 9:43 PM. Reason: Added link & my number crunching
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Old 22-Mar-2013, 1:03 AM   #8
Alan909
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Converting Old Style C Band Dish To A Antenna

Quote:
Originally Posted by Stereocraig View Post
Alan,
Have you checked out some of the antenna arrays on the CM 4251 Tribute page?

Simply gorgeous!

Sorry, forgot to paste.
http://www.rocketroberts.com/cm4251/cm4251.htm
Love the CM 4251, wish they still made them...

"Wish they had an experimental projects or DIY section in this forum, could prove interesting"

...anyhow...I was up late one night researching DIY antennas and came across and interesting project of converting an old big style wire mesh C Band Dish into a TV antenna by replacing the center with a dual bowties and works just as well or better than the old CM-4251...

For the life of me, I cannot find that site again...so...My question is>>>"Has anyone heard of doing or trying this type of project and/or have access to any DIY plans for converting the old dish to a TV antenna???"

Last edited by Alan909; 22-Mar-2013 at 1:11 AM.
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Old 22-Mar-2013, 1:26 AM   #9
teleview
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http://www.rocketroberts.com/cm4251/...derman_uhf.jpg.

The UHF antenna is moved closer and further way to find the location of full illumination of the dish for maximum reception.

The picture shows the dish on a non penetrating stand on the ground.

For real use the dish will be up higher and aimed at the transmitter.
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Old 22-Mar-2013, 1:50 AM   #10
GroundUrMast
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Was this the site or at least similar? http://www.digitalhome.ca/forum/show...ight=parabolic

The Enthusiast's Exchange is dedicated to DIYers and experimenters.

Last edited by GroundUrMast; 22-Mar-2013 at 1:53 AM.
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Old 22-Mar-2013, 2:23 AM   #11
Alan909
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Thank you and was similar site and will read entire subject later tonight or tomorrow...Good to know there is a forum space on here about projects, did not know it was in the "Enthusiast's Exchange" ...
Going to Upstate New York to work for a week and will be bringing back an old 7 ft mesh Parabolic dish from a family member for my antenna project...
I downloaded a few "Antenna Simulator Programs" that I am trying to learn in the meantime until I get the exact measurements from my Parabolic...should be an interesting project...and will definitely post my progress on "Enthusiast's Exchange"
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Old 22-Mar-2013, 6:18 PM   #12
ghz24
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4nec2

Check out 4nec2 first it is the Cadillac of free modeling software.
All the models you'll find on that digitalhome .ca site will be 4nec2 files.
Sorry to break it to you but a 7 foot dish still won't get you 20+ DBi @ channel 22.

I can maybe save you some time and effort here is a zip of the parabolic models I used to compare 2 meter dishes and feeds.
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Old 25-Mar-2013, 2:18 AM   #13
Alan909
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Thanks ghz24 as I am a step ahead of you and downloaded thr 4nec2 last week but did not know it was the best to use...now just need to learn how to use it...I just got here in upstate new york today and looking at the 7 ft c band dish and was wondering why you got almost 20 db gain on your 2 meter dish but cannot on a bigger dish???...have to admit, have A LOT of catching up to do with you guys, lol...you posted this below and found it very interesting...
................................................................................ .....................
BTW I've nearly completed modeling a 2 meter dish
I'll post a link to the model and a performance page. But here is a quick
overview across channels 14-52

Code:
Total gain (dBi) Net gain (dBi) SWR
max 19.28 max 19.1431516122 max 2.0347958365
min 17.24 min 16.9662580763 min 1.2860276345

Average Total 18.4936842105 (dBi)

Average Net 18.2217478081 (dBi)

Average SWR 1.6396934694
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Old 25-Mar-2013, 6:24 PM   #14
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ANTENNAMAGUS has both free and pay soft-ware.

http://www.antennamagus.com.
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Old 12-Jul-2014, 8:35 AM   #15
ghz24
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broken link

I admit that antenna magnus looks like a real cool program but it isn't modeling software it's design software .
4nec2 uses text files for models allowing for easy colaboration and peer review.
There are active groups that will tell you if your model is well built or flawed and how to fix it (maybe).

Sorry somehow the link above had a typo
parabola models

Last edited by ghz24; 12-Jul-2014 at 9:36 AM.
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