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sporkme
drop the crantini and move it, sister
Premium,MVM
join:2000-07-01
Morristown, NJ

homemade "hdtv"/uhf antenna ideas?

Hi all,

I assume there's a fair number of folks here that are into this sort of homebrew stuff... Have any of you built any nifty uhf antennas for "digital" TV?

Google just keeps showing me the same 2 or 4 bay "bowtie on a stick" setup. I'm looking for more creative options that I can either hide behind some furniture or mount under the eave outside. All stations come from the same direction (NYC) about 25 miles away, so directional is good.

I got totally scammed on some amplified RCA piece of junk. An FM dipole I had laying around (hey, it has a balun on it already) is working better...

I also have an attic, but it's a rental and I'd rather not deal with fishing something down through the wall...
--
with every mistake we must surely be learning


JTY

join:2004-05-29
Ellensburg, WA

You could try building a Yagi perhaps.



sporkme
drop the crantini and move it, sister
Premium,MVM
join:2000-07-01
Morristown, NJ

said by JTY:

You could try building a Yagi perhaps.
I don't understand the math or physics behind it, but the general consensus I'm seeing is that the multi-bay "bowties" with a reflector grid are supposedly best if you don't have direct LOS to the transmitter(s).

This design is pretty interesting, I'm trying to think of creative ways to slap it together:

»www.digitalhome.ca/ota/superantenna/

b10010011
Whats a Posting tag?

join:2004-09-07
Bellingham, WA
Reviews:
·Comcast Formerl..
reply to sporkme

There is a good discussion on building UHF antennas over at AVS Forum.

»www.avsforum.com/avs-vb/showthre···+antenna

Multi-bay bow ties seem to be the design of choice. I am guessing because the math is easier and the design and over all construction is more forgiving.



SmokChsr
Who let the magic smoke out?
Premium
join:2006-03-17
Saint Augustine, FL
reply to sporkme

One thing to keep in mind, and it seems to be correctly figured out in here, there is NO difference in a "Digital" antenna from a analog antenna. I've had to explain that so many times to people that have a good TV antenna thinking they have to get something totally new to receive HDTV.

Also keep in mind that many (most) of the new "Digital" antennas are UHF only. While it's true that most HD's are now UHF, not all are. So it just depends on what channels the stations are going to be on in your area, to decide if you still need a VHF ant.


b10010011
Whats a Posting tag?

join:2004-09-07
Bellingham, WA
Reviews:
·Comcast Formerl..

said by SmokChsr:

Also keep in mind that many (most) of the new "Digital" antennas are UHF only. While it's true that most HD's are now UHF, not all are. So it just depends on what channels the stations are going to be on in your area, to decide if you still need a VHF ant.
Good point don't forget the The DTV Transition’s Dirty Little Secret »www.cepro.com/article/the_dtv_tr···_secret/

FCC says only 74% of stations will be operating in the UHF band after the DTV cut-off date.


sporkme
drop the crantini and move it, sister
Premium,MVM
join:2000-07-01
Morristown, NJ
reply to b10010011

Thanks for the info... I'm aware of the "dirty secret", and I'm a bit more than upset that our local PBS is not going to be able to go back to their full power VHF allocation on 2/17.

As for the bowtie design, can anyone point to some good reading on the theory behind that one? In general UHF TV antennas must be kind of tough to engineer - the spectrum (used to) go from somewhere in the 400MHz range up to 800MHz. That's a pretty wide swath to try and cover with a single antenna. What polarization do the broadcast UHF stations use? I saw a brand new install out here on "First Mountain", and besides just being absolutely huge, it looked something like a solid corkscrew, suggesting a circular polarization.

I am quite surprised by the relatively small number of homebrew antenna designs out there. I'm basically seeing about 3 of them reposted everywhere, and while the one I linked to above is pretty nifty in it's design, I'm not seeing much creativity in the construction.

One last question as well - most of these designs have a grid behind them. If the grid is omitted, what happens? How many dB does the gain drop, and how does that change the directionality(?) of the antenna?



n1zuk
making really tiny tech things
Premium
join:2001-10-24
Malta
kudos:2

1 recommendation

I'm no "antenna guru", but have enough antenna theory to throw out a couple of observations:

-- The UHF band allocation is pretty wide. But a receiving antenna has less of a concern of matching the resonant frequency of the received signal than a transmitting antenna. It is more about collecting the electrons, and getting them to the receiver. (On transmit, there are matching concerns to protect the equipment's output, and not radiating from your feed line).

-- Speaking of feed line - before CATV became the prevalent method of receiving TV, most antenna systems used 300ohm twin lead. Much less loss between the antenna and the TV. Since CATV and satellite have become the norm, many newer TVs no longer even have a 300ohm input. We may soon see a return of the 300ohm-75ohm transformer on the back of TV sets, to reduce the amount of feed line loss.

-- UHF stations have always been the red-headed stepchild to the bigger VHF stations. Most people accepted the fact that, unless the station was nearby, they wouldn't receive a big signal. Now people want to maximize their OTA choices, to offset not having the 300+ channels that CATV/SAT have to offer.

-- Until now, not much had changed in OTA television reception technology since the 1930s. First people had rabbit ears on their TV. They moved to the outdoor antenna (log periodic, most with UHF as an afterthought in the design). As CATV, then SAT, came to became more popular, the demand for improved designs dropped. As more people will again look at digital OTA as an option, some new designs are starting to appear (most common, the pre-amp panel design). I think more will eventually appear, as people finally lose their analog OTA and look for improvement.

To answer some of your questions: I believe the initial television signal is horizontally polarized, given the traditional VHF log periodic antenna is also horizontal. Of course, as the signal bounces off of obstructions, it develops multipath and changes polarization. that might suggest the "corkscrew" design you saw.

The grid is a reflector. It will increase the gain of the antenna in front of it. Without a particular antenna to evaluate, figure the grid will add 2-3db in the desired direction (and drop the same in the direction behind it). A benefit (with analog anyway, I don't know if it helps at all with digital) is that it also helped reduce multipath reception, especially when mounted to the side of a house. Multipath is what causes "ghosting" of TV images.
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mr weather
Premium
join:2002-02-27
Mississauga, ON
reply to sporkme

I've heard good things about the "Gray-Hoverman" antenna which appears to be loosely based on a multi-bay bowtie construction.

»www.digitalhome.ca/forum/showthr···?t=81982
--
"It's all coming down!!" - Mike Holmes