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Scanner / Receiver Antennas For discussion of any type of receiving antenna used by a scanner or receiver base, mobile or handheld.

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Old 12-02-2011, 4:55 PM
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Question Parabolic dish vs. Yagi

Could I use a parabolic dish the same way I could a yagi or are there differences in how the two perform. It makes sense to me that I could use a parabolic dish with a higher gain and a smaller footprint than a conventional yagi, but was wondering why it does not seem to be a common practice. Any suggestions?

P.S. The parabolic dish I had in mind has the ability to be vertically polarized.
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Old 12-02-2011, 5:11 PM
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What frequency do you want to use it on?
The lower the frequency the larger the reflctor. It won't work unless it is many wavelengths in diameter. That is why you don't see them used below about 900 MHz.
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Old 12-02-2011, 5:12 PM
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Cost is the main reason. Size is another.
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Old 12-02-2011, 5:48 PM
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We would need to know the frequency you want to operate on, and the dish size. A 2-3 foot dish could get you about the same amount of gain as, maybe, a 12 element yagi, but it would have a lot more weight and wind load.

You won't get higher gain with a "smaller footprint" with a dish. It's just not how they work. You need LOTS of gain, have no limits on weight and wind load, or expense, then a dish will get you there.
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Old 12-02-2011, 9:34 PM
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go with yagi for directional reception. just make sure you get the right one for the freq you want to monitor.
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Old 12-03-2011, 12:35 AM
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Hi everyone, thanks for getting back to me! The frequency range I had in mind is the 770MHz range. One dish I was looking at had 21dBi of gain, but was 55lbs, so I guess a yagi would be better unless someone has evidence otherwise. Antenna Manufacturer | ZDA Communications, the website I was looking at, does have smaller grid dishes that have as much as 15dBi of gain and weigh under 8lbs.The grid dishes have a significantly lower wind load and weigh less then conventional solid parabolic dishes beings that the reflective dish is made of a wire grid. The manufacturer also reccomend this dish for applications such as 700MHz Public Safety systems, which is what I would use it for. Is this a possibility?

Here is the dish datasheet:
http://www.zdacomm.com/images/PDF/ZDAGP750.pdf

Thanks for your input everyone, and if anyone else comes up with more ideas, please post!
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Old 12-03-2011, 1:25 AM
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What's happening that you think you need so much antenna gain? Usually, antennas like that are used to resolve a specific problem.
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Old 12-03-2011, 1:30 AM
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I just happen to have terrible reception on some towers downstate and would like to try and receive some radio systems that are very far away (60+ miles). The area of Newark that I am in is known for terrible reception as it dips down and has lots of trees and other obstructions.

Also I think a dish looks cool, but that's not the real reason I'm interested in it.

If you don't think I need the amount of gain I mentioned, how about if I chose the smallest dish with 10dBi gain. The dish looks about 12-14in wide and weighs 4lbs? Or is using a dish technically overkill?
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Old 12-03-2011, 1:42 AM
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Quote:
Originally Posted by myspacebarisbroken View Post
I just happen to have terrible reception on some towers downstate and would like to try and receive some radio systems that are very far away (60+ miles). The area of Newark that I am in is known for terrible reception as it dips down and has lots of trees and other obstructions.

Also I think a dish looks cool, but that's not the real reason I'm interested in it.

If you don't think I need the amount of gain I mentioned, how about if I chose the smallest dish with 10dBi gain. The dish looks about 12-14in wide and weighs 4lbs? Or is using a dish technically overkill?
If you're trying to hear systems 60 miles away, all that gain might help. But keep in mind that a lot of systems are designed specifically not to cover too far outside their service areas. There's only so much you can do if you're really that far outside the designed coverage area.

The extra gain from a larger antenna can help if trees and things are attenuating signals. Also, be a bit leery of the claims of some antenna manufacturers. 21db gain at 700 MHz is going to require a dish on the order of 6 feet in diameter, and if someone claims that sort of gain with something much smaller than that, they're lying.
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Old 12-03-2011, 1:54 AM
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Quote:
Originally Posted by myspacebarisbroken View Post
I just happen to have terrible reception on some towers downstate and would like to try and receive some radio systems that are very far away (60+ miles). The area of Newark that I am in is known for terrible reception as it dips down and has lots of trees and other obstructions.

Also I think a dish looks cool, but that's not the real reason I'm interested in it.

If you don't think I need the amount of gain I mentioned, how about if I chose the smallest dish with 10dBi gain. The dish looks about 12-14in wide and weighs 4lbs? Or is using a dish technically overkill?
If what you say is true, a super large gain antenna may not help you much. A gain antenna is only helpful if there's some signal (over the noise level) for it to capture. If you're in a low spot with many obstructions to block the signal and/or trees or other things to absorb the signal there may be nothing much left from that far away but noise. With that large of a distance, you'll also be fighting the curvature of the earth, at least to some extent, causing the height issues and signal loss to be even greater.

What you may need to work on is height, not antenna gain. That would put you above the obstructions and up where there may be enough signal to capture. You'll not only need to get above the trees, but over the obstructions as much as you can. That height will probably require a very good quality coax (think Andrew Heliax like LDF4-50A - Heliax Coax - LDF4-50A or better) to reduce the associated coax loss.

Your situation may be similar to you standing outside of a soundproof studio watching a band play inside of that studio. The soundproofing (like your low location) keeps the band's sound from being heard by you, even if they turn their amps up quite loud (you using a high-gain antenna). Now if somebody happens to open the studio doors (you getting your antenna where there's enough signal level), you can hear fine.

A high gain dish is used to provide not only gain, but even more often a narrower beamwidth. A yagi may give you a 20 degree beamwidth while a dish may only have a 5 degree beamwidth. This is often necessary since microwave links are generally point-to-point and the frequencies are reused for signal paths going different directions. With a large dish, you would need to point the antenna very accurately (and hope that the wind doesn't blow it off course) while with a yagi, you need much less accuracy when pointing the antenna.

Last edited by n5ims; 12-03-2011 at 2:01 AM..
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Old 12-03-2011, 2:17 AM
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Ok, with that in mind, do you think a small 10dBi dish with a 25 degree beamwidth would be sufficient? I know someone who is farther north than I am, but is able to receive sites very far south with an 6dBi omni so I don't think noise level is too much of a problem. His elevation is a couple hundred feet higher than mine, but do you think the 10dBi gain would compensate?
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Old 12-03-2011, 12:05 PM
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Quote:
Originally Posted by myspacebarisbroken View Post
Ok, with that in mind, do you think a small 10dBi dish with a 25 degree beamwidth would be sufficient?
There are yagis available with higher gain than that. If you're fixated on a dish, get the largest dish you can hang up. If you're fixated on gain, get a bigger dish. If you're looking at cost effectiveness vs performance vs ease and safety of installation, you'd probably end up with a yagi.

We can't set your priorities, but for only 10dbi gain, a dish is not your best choice.
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Old 12-03-2011, 12:18 PM
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n5ims make a very valid and all to often overlooked point right at the start of his comments regards signal and noise - more specifically: your SNR, or signal/noise ratio.

In nearly every case I have dealt with over the years, in which a weak or low level signal has had to be resolved and demodulated with off-the-shelf consumer grade scanning/receiving hardware, I have achieved at least as good a result - if not better - by leaving overall antenna gain as is and concentrating on SNR improvements.

If there is ANY resovable signal to start with, then the chances are you will achieve more improving your signal to noise ratio i.e. for a given bandwidth (range of freqs - in your case around 770Mhz if I recall correctly), filtering out all the rf stuff around and either side the bandwidth of interest, before it gets to your scanner/receiver frontend and saturates & overloads it, is going to give you a better "received result" than increasingyour antenna Gain.

From what you have said I'd be putting some type of bandwidth/bandpass filter between the antenna and the scanner/receiver antenna input socket, and leaving Gain just as it is.

With respect to SNR performance (i.e. Gain aside) - dishes, as a rule provide a better SNR figure than Yagi's, and solid dishes provide a better SNR than grid type dishes.

Good luck
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Old 12-03-2011, 2:45 PM
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Well, I guess I should try and make a homemade "test" yagi with the same gain as the dish and see if reception is even feasible, if not, I guess I won't even bother buying the dish.

Thanks for the info guys!
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