Help Needed: Outdoor TV Antenna Connection

[rp2813]

Last year we had some roof work done and I told the guys to take down the large TV antenna that hadn't been connected in years and was unsightly.  I hacked off and saved the UHF section from it in case I ever wanted to use it for OTA digital reception.  It's a couple of feet long, a sort of traditional "Yagi" style flat section with about a dozen short aluminum rods, and two reflectors at rear with similar configuration that fold out at an angle above and below the main mounting body, kind of like the first image below.

 

Well, the time to give this salvaged rig a try has come, as the stormy weather is making reception with bow tie/rabbit ears annoying.  Now that I've examined the section I saved, I see no way that the two wingnut terminals where leads would be connected would in turn provide a connection to the remaining elements.  The rods that connected these wingnut terminals to various parts of the entire array had to be cut as part of the salvage process, and with the VHF section gone, there don't appear to be any points left to connect them to.  I can't see how the connection point as it stands now would employ any elements other than the two short aluminum tubes extending from either side of its plastic block.

 

Is there any way to rectify this?  I've checked on line and have seen some examples of building an indoor UHF antenna out of 8 V-shaped coat hanger sections (see image below), which is nothing like what I'm dealing with, but the one feature they do have is a pair of connecting rods that crisscross, each of which shares a fastener with 4 the V-shaped components.  From what I've read, this configuration is important, so I'm wondering how I'd apply that to what I'm working with. 

 

Or should I just forget about it and buy something?

 

 

[earthling177]

Ralph:

I think that your antenna is complete, at least according to what Wikipedia has to say.

The main article (Antenna ) says:

The dipole antennas composing it are all considered "active elements" since they are all electrically connected together (and to the transmission line). On the other hand, a superficially similar dipole array, the Yagi-Uda Antenna (or simply "Yagi"), has only one dipole element with an electrical connection; the other so-called parasitic elements interact with the electromagnetic field in order to realize a fairly directional antenna but one which is limited to a rather narrow bandwidth. The Yagi antenna has similar looking parasitic dipole elements but which act differently due to their somewhat different lengths. There may be a number of so-called "directors" in front of the active element in the direction of propagation, and usually a single (but possibly more) "reflector" on the opposite side of the active element.

And the Yagi article (Yagi) says:

A Yagi–Uda antenna, commonly known as a Yagi antenna, is a directional antenna consisting of multiple parallel elements in a line,[1] usually half-wave dipoles made of metal rods.[2] Yagi–Uda antennas consist of a single driven element connected to the transmitter or receiver with a transmission line, and additional "parasitic elements" which are not connected to the transmitter or receiver: a so-called reflector and one or more directors.[2][3][4] It was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Japan,[5] and (with a lesser role played by his colleague) Hidetsugu Yagi.

Anyway, that seems to also match my recollection of such antenna when we did not have cable eons ago.

Cheers,
   -- Paulo.

 

[rp2813]

Thanks Paulo,

 

I guess the "active elements" of my antenna went to the scrap heap.  I might be able to recreate one "half-wave dipole" pair at the rear of the section I have left. 

 

All this time I thought the short "director" elements at the front end were for UHF, so that's why I saved that portion.  Now I'm understanding that it's the wide-span dipoles that pull in the signals, whether VHF or UHF.  

 

Since digital signals in my area are far more easily received than analog used to be, adding that one pair might be enough to improve reception over what I'm getting with the small indoor bow tie I'm currently using.  It's worth a try.

 

 

 

[man114]

I'd personally suggest buying the Antenna Direct XG91. It's a super long range UHF Yagi.

I don't need an antenna first and foremost. I have cable. I use a siding mounted pole mount and have this thing on my garage. I use a Channel Master amp on it.

My primary reason for buying it was to watch hockey a few years ago.

They have an awesome warranty and have replaced it once as it was damaged in a storm.

I live in a fairly rural area. I have it pointed at the CN tower which is probable 100 miles away. It does quite well.

 

[rp2813]

Eric, thanks for the suggestion but I'm looking for something much smaller.  As stated above, the indoor bow tie is pulling in nearly every station available (the majority of transmitters are 45 miles away or less), but sometimes weather conditions cause pixelation, momentary loss of sound, or loss of signal altogether.  All I need is a boost in reception that will eliminate these problems.

 

[kb0nes]

Ralph, I'd love to see a photo of the cut end of your antenna. Combination VHF/UHF antennas have different active elements with a matching line connecting them together. Perhaps you still have the active UHF dipole in place that could be usable.

Next, have you ever done a TV Fool survey of your location? It would be good to confirm that you don't have any signals you want to receive in the High Band VHF frequencies, a UHF antenna isn't well suited for that. Here in the Minneapolis market we initially started out after the digital conversion all in the UHF range, but since about 4 stations have moved back into the VHF Ch 8-11 range.

Link to the Signal Locater

http://www.tvfool.com/index.php?Itemid=29&option=com_wrapper

Finally I built that 4-bay UHF antenna you showed. I always had an outdoor antenna back from the analog days. A couple years ago I had the exterior of the house redone so I pulled the antenna down. I wanted something to get me by as the work was done so I cobbled that antenna together. In my case with the VHF High stations I needed a bit better coverage below UHF so I added a 31" dipole element across the feed. This is effectively tuned at 181Mhz. I swept the completed antenna on my network analyzer and it looks good across the range I need to cover. The antenna is installed indoors and it does well for me, so well that I haven't re-installed an outdoor antenna.

 

[rp2813]

Phil, I'll try to get a picture posted here tomorrow.

 

I've hatched a dipole plan that might improve things.  Tomorrow may be a good day to try it, since it's supposed to be cloudy with possible showers, which presents perfect conditions to see if it will work.  If not, then I may make myself one of those coat hanger rigs.

 

[gizmo]

After carefully re-reading the original post, it sounds like your antenna is complete. "I can't see how the connection point as it stands now would employ any elements other than the two short aluminum tubes extending from either side of its plastic block."

That sounds like a yagi design. The "two short tubes protruding from the plastic block" are probably the active elements. They aren't always a folded dipole or x, sometimes they are just simple straight elements. If you still have a connection block, just connect it up and see what reception you get.

A Yagi consists of three types of element - dipole (also called driven element), reflector and director. ONLY the dipole is connected to the cable. The reflectors are slightly longer than the dipole and are mounted behind it (though stacked reflectors may taper forward of it); directors are always shorter than the dipole, getting slightly shorter as they move away from the dipole. The directors point towards the transmitter tower.
The directors and reflectors are passive elements - they only serve to feed signal back to the dipole. They are not wired to the dipole.

I suspect your two short tubes are the dipole.

A photo would confirm.

hook it up, suck it and see.

good luck.

Yagi Uda Anteena Designing Steps

An yagi antenna, a narrow band antenna working for FM channel and consisting of a reflector, a driven element and two directors.

www.elprocus.com

 

[wayupnorth]

I still have my ancient analog antenna on my roof and it normally pulls all the local digital stations in fine. Except when it is very windy, like today, certain stations either pixilate or no signal shows up on the screen. My satellite always pulls in all the signals fine, even in a big storm.

 

[rp2813]

Pictures

Here's what I'm working with.  The center section (the directors, I presume) looks to be slightly under 3' long. 

 

The connecting block for the dipole is just inside the V-shaped reflector(s).  From tip to tip it measures just under 12".   Wing nuts are on the bottom.   I had to JB Weld it because it had broken in half when the whole rig was tossed off the roof and onto the driveway.  I don't know what the other orange piece with U-shaped rod was for.   It's riveted so wasn't intended for any connective purpose.  I removed the cut rods from the connector block since they no longer served a purpose.  Most assuredly, they connected to a section of the longer elements behind what remains.  I feel as though that missing connection to the rear section is what's diminishing this remnant's ability to pull in signals.

 

 

 

[gizmo]

photo 2 and 3 show your dipole. It looks like they have wing nuts underneath? Connect to there.

The piece behind the reflectors in pic 4 is not needed, it was a connection between the VHF and UHF sections. Remove it, it will get in the way of a mounting u bolt. Do NOT cut the centre beam short - leave the bit protruding behind the reflectors, it gives you a spot to drill out to fit a U-bolt so you can attach it to a mast.

Be aware that this UHF antenna might not match your TV station. The width of the elements dictates what stations the antenna suits. Some are wide band antennas, they are cut to cover the whole UHF spectrum but gain isn't great. Others are cut to suit a small section of the UHF spectrum, or even a single channel. The more specific the cut to a channel, the better gain you will get on that channel but the more the antenna will actively reject other channels. It is quite a science. Having single bar dipoles (rather than a folded dipole or an X) suggests it is a single channel UHF antenna, but that isn't certain. If the directors are much narrower than the dipole, it is probably wide band.

Having said that, there is something to be said for getting more metal in the air, even if it isn't quite the correct antenna. I live in a tourist area with a few holiday homes. A few people who live full time in the city have brought a metropolitan antenna here and stuck it up, despite being cut to suit particular channels that aren't here. Some of them get passable reception any way. This is a terrible reception area, we are a long way from the nearest transmitters, hilly terrain, and for a few years an island about 100 km away had a matching channel so we got co-channel interference on channel 11 in certain weather.(looks like venetian blinds over the screen.) When Australia went digital, most country areas were moved to UHF and the signal here got even worse as UHF doesn't travel as far as VHF. We got subsidised satellite service to replace it which is fantastic.

 

[gizmo]

by the way...

your comment " I feel as though that missing connection to the rear section is what's diminishing this remnant's ability to pull in signals" is wrong. On a UHF yagi, there is nothing behind the reflectors. Everything behind the reflectors (everything no longer there) on your antenna was for the VHF spectrum.

hook it up and see what you get. I hope you get good reception.

 

[kb0nes]

Opps apparently never hit post earlier...

Ralph, The two terminals you see on the dipole in your #3 photo are the connection point for the UHF corner reflector. You could just connect the two points to the leads of an outdoor Balun to adapt to your coax you are likely to run to the TV. Technically the standard Balun is also an impedance transformer which transforms a 300 ohm balanced connection (twinlead) to a 75 ohm unbalanced connection (coax). In this case your feed point of the corner reflector is actually close to 70 ohms so the 300-75 ohm transformer isn't needed. But 75 to 75 ohm Baluns aren't common. It would likely work fine anyhow even with the mismatch.

The U shaped rod is a hairpin which is likely used for impedance matching and it also creates a DC short across the antenna feed to protect against static and voltages being built up across the feedline. It was part of the transmission line that linked the VHF section to the UHF.

If you like to tinker and play by all means hook up the antenna and see how it goes. If you want an antenna that works with less fuss I'd buy a new one. The corrosion and lack of a proper matching section are likely to make this antenna not work like a new one. At least the digital signals are generally easier to receive a Q-5 picture with compared to the old VHF analog signals that had to deal with noise issues.

 

[rp2813]

Thanks for the excellent explanations on what I'm dealing with.  If I'm understanding things correctly, just that little dipole is pulling in the signal and the rest of the assembly is only there to help zero in on it.  That's kind of amazing.

 

I gave the antenna a trial run, and reception was excellent on what had been weak and problematic channels, but the NBC affiliate rendered a "no signal" display.  I tried aiming the antenna about 30 degrees in either direction from what I know was best when this rig was intact.  Still "no signal."  This is a deal-breaker, since I'd rather get NBC than more Asian language dramas and game shows.

 

I never would have believed that a little bow tie could pull signals indoors better than a rooftop unit.

 

Phil, I may take a shot at one of those 4-bay antennas.  That may be all I need.

 

 

[kb0nes]

Ralph, are you positive that the NBC affiliate station isn't a VHF station? If you antenna works great on the others but not on this station it is a possibility. Since the digital change over, the number of the channel has zero relation to the frequency they choose to use. The "virtual" channels are confusing at times!

I'm 23 miles line of site to my transmitter complex here in MN. The 4 bay antenna worked great except for the 3 VHF High stations. If you have VHF stations the 4-bay may not do it for you.

One other thing, if you build the antenna, avoid using coat hangers , steel just isn't that great a conductor for RF. It will work but why build with inferior materials? I used copper clad welding rod but solid copper house wiring would work well too, although it is hard to make it straight and pretty.

 

[rp2813]

Phil, I've been wondering about whether the NBC channel could be VHF.  In the analog days it was channel 11 but these days I don't know what it really is.  It has always been an issue to receive 11's signal because the transmitter was south of town instead of north like all the other local stations.  When it became the NBC affiliate, in order to serve the entire greater Bay Area, they moved their transmitter and changed media markets from Monterey to San Francisco, but they didn't move to where the bulk of local stations transmit from.  Still, it's in the general area and shouldn't require a different aim than for the other channels.  I'm pretty sure their tower is on the same hill as channel 2, which comes through fine most of the time.

 

Do you think I could use some 12 gauge romex wire for the 4-bay antenna?   I've also contemplated harvesting dipoles from a set of rabbit ears and attaching them to the hairpin block by drilling holes for a couple of screws, and then connecting the rabbit ear dipole to the UHF dipole with some wire.  I know spacing is key, so am not sure if fastening the rabbit ear dipole on the hairpin block would be the proper placement.

 

Last night, with mostly cloudy skies, I tweaked the bow tie arrangement I have the TV hooked up to, and all stations I care about came through.  It's unsightly, but I might not be able to do much better.  See attached picture, and yes, looping the twin lead around that one side of the bow tie makes a huge difference.  The TV is a 19" Magnavox with built-in DVD and VHS players.  It still delivers a sharp, crisp picture with perfect color.

 

[kb0nes]

Ralph, I just ran a TVfool channel listing for San Francisco. I used the zip code 94080. I'd really recommend running one for your exact address (or at least your Zipcode) so you really have an idea of what is where for you. I attached an image of the report. I assume you are talking about KNTV 11.1 which is on real channel 12 which would be about 205Mhz in the High VHF range. The San Fran area has more VHF channels then here in MN, typical of a denser population. I'd sure be designing in some VHF capability.

Using the wire from Romex would be fine or solid THHN single strand would work too. One thing that helps when using solid copper is to loop the ends around something like a broom stick and attach it to something solid and yank on it good. It helps straighten and work harden the wire so it holds shape better.

UHF signals are prone to some absorption from moisture in the air, if it is foggy or humid or raining there may well be some degradation. If you are using the antenna indoors beware of what is in your walls. Stucco is often placed on a metal lath and it has high attenuation. Some asphalt shingles have conductive particles in them and that can cause a lot of loss, especially when wet. My house was aluminum siding and now that I have fiber cement the signal levels to my indoor antenna improved

The other issue with UHF is that the peaks and nulls are much closer together. Moving the antenna just a foot or two could make all the difference in receiving a given station.

 

[rp2813]

Thanks Phil.  I did learn that KNTV uses a VHF frequency.   I think my rabbit ear dipole plan is worth a try. 

 

Also, back when KNTV first began broadcasting in the '50s, because their transmitter was in the opposite direction from where area antennas were aimed, they distributed free small rooftop dipoles that could be spliced into the existing antenna system and aimed toward their transmitter.  I still have one, so the other option is to hook it onto the UHF antenna's terminals with some twin lead.   That would eliminate the need to create a fastening system for the rabbit ear dipoles.

 

I'll post a picture of the antenna tomorrow for your review.

 

[rp2813]

Here's the KNTV antenna.  If it helps with reception on that station, it will once again serve its original purpose some 60 years after it was issued.  Due to one terminal screw having snapped off many years ago, the connection block can't be used as a splitter anymore, but the pair of terminals marked "SET" is all I need. 

 

It's bordering on a Rube Goldberg solution, but if it works, that's all I care about.  I'll test today and report back with results.