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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Wireless and Communications => Topic started by: Jason Lavoie on November 02, 2015, 09:07:54 am

Title: LPDA with a reflector?
Post by: Jason Lavoie on November 02, 2015, 09:07:54 am
Never mind the fact that the antenna is almost touching the truss and is certainly not performing as well as it should be
But is this amateur-hour or a useful trick to put an LPDA on a dish/reflector?
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Jason
Title: Re: LPDA with a reflector?
Post by: Lyle Williams on November 03, 2015, 05:39:50 am
Amateur hour.  The active area of the ldpa can't be at the dish focal point for anything but a very small range of frequencies.  With found parts, that is unlikely to be at the frequency you want.  Better to stick with the ldpa if you want moderate gain and wideband response, or cut your own dipole for the dish if you need high gain at a fixed frequency.
Title: Re: LPDA with a reflector?
Post by: Scott Helmke on November 03, 2015, 09:57:13 am
Are you sure that it's a LDPA at UHF frequencies? Could be something else entirely.
Title: Re: LPDA with a reflector?
Post by: Jason Lavoie on November 03, 2015, 11:43:38 am
Are you sure that it's a LDPA at UHF frequencies? Could be something else entirely.

It is in a large store, and is aimed towards an adjacent building which is their head office, so a data link makes sense.

And after thinking about it I agree with Lyle. Frequencies aside, the dish is only useful at its focal point

maybe it is a plastic cover for an LDPA over top of something else?

Jason
Title: Re: LPDA with a reflector?
Post by: Lyle Williams on November 03, 2015, 02:43:17 pm
... or the thing shaped like a ldpa is just a support for the driven element.  That would provide room for inbuilt electronics.
Title: Re: LPDA with a reflector?
Post by: Pete Erskine on November 03, 2015, 02:55:48 pm
It is in a large store, and is aimed towards an adjacent building which is their head office, so a data link makes sense.

And after thinking about it I agree with Lyle. Frequencies aside, the dish is only useful at its focal point

maybe it is a plastic cover for an LDPA over top of something else?

Jason
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LPDA are used with a reflector but sometimes point towards the reflector for added narrow band. 
This is true especially in the wifi band but this unit seems a little larger.  Without seeing inside the radome it is hard to tell.

UHF antennas often have rear reflector on a LPDA.




Title: Re: LPDA with a reflector?
Post by: Henry Cohen on November 03, 2015, 07:15:45 pm
This is a Yagi-Uda antenna enclosed in a radome, and as with all yagi's there's a reflector behind the driven element. Some manufacturers use a large grid style reflector (rather than just a another rod set) for increasing directivity. I've actually seen this antenna product in a catalog, though I can't find it right now, and for some reason I suspect it's 900MHz ISM band; I'll continue to look for the OEM.
Title: Re: LPDA with a reflector?
Post by: Lyle Williams on November 03, 2015, 11:26:08 pm
I don't recall seeing a lpda with a reflector.  Lots of vhf ldpa with a uhf yagi+reflector on roofs for tv though.