Problems getting VHF 12 and 13 in Middleburgh, NY, possibly caused by missing antenna elements? - AVS Forum
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post #1 of 20 Old 08-25-2013, 05:51 AM - Thread Starter
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I have a summer home in Middleburgh, NY that has a Radio Shack VU-210XR that I installed in 1995, along with an Archer rotor. It has seen some pretty harsh winters, and it has lost some VHF elements, but its still in ok shape.
Recently I noticed that WNYT Ch 13 (VHF12) has been sporadically dropping out. And also I notice WNYA Ch 51 (VHF13) I can no longer get. I am wondering now if the elements that I lost are directly related to those channels. I have no problems receiving the other channels.

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post #2 of 20 Old 08-25-2013, 07:29 AM
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Basically, a Log-Periodic Dipole Array (LPDA) is a bunch of small yagi antennas....a resonant element (cut for a particular frequency), a slightly longer element (acting as a reflector, or "back" element) and one or more shorter-than-resonant director elements (to guide the signal toward the active one).
A LPDA combines several of these, at different frequencies, in to one unit......the shortest VHF elements (at the front) are directors for the highest frequencies, the longest element (at the rear) is a reflector for the lowest frequencies. All the other elements function as reflectors, driven elements or directors at different frequencies.

Looks like the missing ones on your antenna are right up in the upper VHF range. Also, having one side missing on a dipole (which is what those are, individually) would pretty much make it non-functional at that frequency.

The V-shaped thing at the front is a yagi for UHF channels, and is a single dipole active/driven element, a reflector array, and about 20irector elements. Being UHF, the bandwidth is only about an octave, so it functions across the whole band.

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post #3 of 20 Old 08-25-2013, 07:43 AM - Thread Starter
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Thanks, I was just wondering if there is a way to identify if the elements I am missing do correspond to VHF 12 and 13, and if they do is there anyway to repair it cheaply.
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post #4 of 20 Old 08-25-2013, 07:56 AM
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They'll definitely have an effect on those channels.
My grandfather, who was a skilled railroad pipe-fitter and tinsmith, bought a huge antenna and rotator from an insurance auction, and rebuilt them both. You could look around for a broken antenna, and use the longer elements for replacements, cutting them to match the length of the existing one on the opposite side of the boom. Most are crimped in place and have a rivet that you'd have to drill out and replace.
You can often purchase thin-wall aluminum tubing at a home improvement store, too.

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post #5 of 20 Old 08-25-2013, 09:43 AM
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When Radio Shack discontinued that model of antenna, they were selling them for ten dollars each. I was tempted to drive around to all the regional stores that had them in stock and scoff them up, just so I'd have them in my inventory if anyone ever needed a cheap relatively high gain, low-band antenna for residential attic use. I'm in the commercial antenna business, so I can't use them in that application because they are not durable enough.

That kind of a combo antenna, using a log-periodic design for VHF (2-13) has its VHF elements tapered in length from front to back, so if you decide to make any elements to replace the broken ones, the correct length of each can be interpolated or extrapolated from the line that includes the end points of the elements that you still have, which you can determine with a string or straight edge.

While the highest channels (12 and 13) tend to be received most efficiently by the shortest elements (probably a little over a foot long), the rear elements all receive those signals somewhat. Frankly, you would do much better at a reasonable price if you just replaced the antenna with either a 7-51 antenna or a 7-13 antenna mounted on the same mast. They are inexpensive, more durable than what you have, and will have more gain on channels 12 and 13.
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post #6 of 20 Old 08-25-2013, 12:42 PM - Thread Starter
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Thanks for the suggestions. Next time I go up I'll try adding the missing elements. I have some 1/4" copper tubing, I'll see how that works.
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post #7 of 20 Old 08-25-2013, 01:39 PM
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VHF Radio-Shack VU-190XR LPDA "VEE" - Without and With MISSING ELEMENTS:

I just uploaded 4nec2 analysis of VU-190XR LPDA "VEE" Antenna to illustrate effect of FOUR MISSING ELEMENTS:
http://imageevent.com/holl_ands/zigzaglpa/vhflpda/vhfrsvu190xrlpdanouhf
Note that the Ken Nist's EZNEC Model for the VU-190XR (converted to 4nec2) found a big GAIN HOLE on 213 MHz (Ch13). And with Four Missing Elements the HOLE became HUGE.

Of course, the VU-210XR uses two additional VHF elements, probably with a higher value for LPDA design parameter Tau, meaning that ALL of the element lengths and spacings are different from the VU-190XR. However, a significant Gain Loss on the higher Band frequencies is very typical for LPDA Antennas since the active region only involves a few elements on the front of the antenna, whereas on lower frequencies the number of elements contributing to radiation increases.


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post #8 of 20 Old 08-25-2013, 02:36 PM - Thread Starter
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Interesting test results. Thanks. I guess out of the box, the antenna has issues getting ch 13.
Worst case if I need to, I can turn the antenna NW and get WKTV NBC affiliate out of Utica. Surprisingly I get that channel with no issues 52 miles away.
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post #9 of 20 Old 08-25-2013, 03:58 PM
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Updated above post, now with 4nec2 analysis for FOUR versions:

a) VU-190XR per Ken Nist's Antenna Model. Note that this has a GAIN HOLE on 213 MHz (Ch13).

Suspecting that the missing Corner Reflector might have an effect, I constructed an alternative model (not shown), copying (JUST) the Corner Reflector from his VU-75 model and shifting position on the X-Axis to the same distance in front of the forward element as is found in his VU-75 model...a sensitivity analysis revealed that moving the X-Axis location back and forth several inches only caused a few tenths of a dB Gain difference, and made very little difference whether it was included in the model or NOT.

b) Item a), with Shorting Stub added across Rear Elements (perhaps it's already in the Boom???). This ELIMINATED the GAIN HOLE on 213 MHz, restoring the expected smooth, decreasing Gain Curve.

c) Item a), but with FOUR MISSING ELEMENTS to assess impact of severe antenna damage. This exhibited SEVERE GAIN LOSS across entire VHF Band, esp. Ch11-13.

d) Item c), FOUR MISSING ELEMENTS, but with Shorting Stub added. This also exhibited SEVERE Gain Loss across entire VHF BAND, although not quite as severe as without Shorting Stub.


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post #10 of 20 Old 08-25-2013, 04:20 PM
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Middleburgh, NY is near Albany, but you have placed yourself on Long Island in your member information. Where are you? From the middle of Middleburgh, NY's zip code, you have two-edge impeded signal path from channel 12. That is not good, but more importantly, such reception will vary considerably within your zip code. You may have more problems than gain or flatness to deal with. You can gain or lose reception with the seasonal variations of foliage density and sap.

The info Hollands furnished makes your antenna seem putrid at the high end of the VHF band. I don't see how it can even be worth experimenting with.
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post #11 of 20 Old 08-25-2013, 06:47 PM - Thread Starter
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Quote:
Originally Posted by AntAltMike View Post

Middleburgh, NY is near Albany, but you have placed yourself on Long Island in your member information. Where are you?

Long Island is my primary location, but my family's summer home is in Middleburgh.

Before I start thinking about replacing the antenna, I'm at least going to give it a shot and replace the elements.
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post #12 of 20 Old 08-26-2013, 03:24 AM - Thread Starter
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Quote:
Originally Posted by holl_ands View Post

Updated above post, now with 4nec2 analysis for FOUR versions:

b) Item a), with Shorting Stub added across Rear Elements (perhaps it's already in the Boom???). This ELIMINATED the GAIN HOLE on 213 MHz, restoring the expected smooth, decreasing Gain Curve.

Are you suggesting I should try a shorting stub on rear elements of my antenna?
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post #13 of 20 Old 08-26-2013, 04:09 AM
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Why torture yourself? The Radio Shack VU-210 antenna is an albatross that became obsolescent in most markets when nearly all of the low-band VHF channels went away. About 3/4s of its size exists for reception of channels 2-6. Any old ten element, 7-13 antenna will outperform anything you could ever get a VU-210 to do on VHF highband and it will last for ever and they cost less than $50. You might lose even more elements on a VU-210 during the next ice storm
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post #14 of 20 Old 08-26-2013, 05:49 AM
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If you don't need VHF lowband reception, how about having holl_ands see what kind of high band performance his model calculates that you'd get by just trimming the whole VHF section to about the same taper as any common 7-13 antenna? If you did that, you'd wind up with lots of left over element stock to diddle with.
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post #15 of 20 Old 08-26-2013, 06:11 AM
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I just got an e-mail from Solid Signal. Their HD7694P is on sale today for $43.69 (plus $7.95 shipping) and their HD7697 is on sale today for $85.49 (plus $12.95 shipping) and you can then take another 5% off if you use promo code WG5P during checkout.

Winegard claims 9.9dB gain on channel 11 and 11.5 dB gain on channel 13 over "reference dipole" for the smaller antenna, and 10.8 dB gain on channel 11 and a whopping 14.6dB gain over reference dipole for the larger one.
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post #16 of 20 Old 08-26-2013, 07:04 AM
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Unless you just want to do some experimenting, the easiest thing to do is just replace the antenna.
If you have no low-band (2-6) DTV stations in your area, you can go with a Hi-VHF+UHF antenna, like mentioned above.....I'd recommend throwing in a small FM Radio antenna and combiner for FM stations, which are just above the 2-6 channels, at 88-108 MHz. A small FM Yagi is good for directional use, or an omni antenna if your FM stations come from more than one direction.

You'd be surprised what a difference an outside antenna makes on FM radio.smile.gif

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post #17 of 20 Old 08-26-2013, 07:52 AM - Thread Starter
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OK, thanks. Did not realize the VU-210 had issues.

I still need a VHF LO for WRGB Ch6 though.
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post #18 of 20 Old 08-26-2013, 11:46 AM
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Quote:
Originally Posted by mikepier View Post

Are you suggesting I should try a shorting stub on rear elements of my antenna?

Based on the modeling results, I have a STRONG suspicion that the Rear elements are ALREADY shorted together on the VU-190 (and possibly others in the R-S series), but Ken Nist overlooked this detail when he prepared the original EZNEC Model file.

Since I suspect the Short may be built INTO the Boom interconnection point, it would help if people could put an ohmmeter between the rear elements and check for a short. HOWEVER, since the zig-zag feedline is ALSO shorted out by the Folded Dipole Active Element at the other end, the feedline between them would have to be temporarily disconnected to make the measurement.

BTW: With the next increment of CHANNEL REPACKING looming on the horizon, I predict increased use of Ch2-6 as channels above Ch35 are taken over for Cellphone use.


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post #19 of 20 Old 08-28-2013, 05:45 AM
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Quote:
Originally Posted by holl_ands View Post

...Since I suspect the Short may be built INTO the Boom interconnection point, it would help if people could put an ohmmeter between the rear elements and check for a short. HOWEVER, since the zig-zag feedline is ALSO shorted out by the Folded Dipole Active Element at the other end, the feedline between them would have to be temporarily disconnected to make the measurement...

If the two rear elements are shorted, the shorting conductor will be visible. It will be a narrow, flat metal strip, probably U-shaped, held in by the same rivets that the elements pivot on, and located between each element and the plastic element support.
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post #20 of 20 Old 09-07-2013, 05:41 PM
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VHF Radio-Shack VU-190XR LPDA "VEE" - RECUT to compensate for MISSING ELEMENTS:

I uploaded additional 4nec2 analysis results for a FIFTH version, presuming FOUR Missing Elements and determining the optimum lengths for the remaining elements using nikiml's Python Optimization Scripts:
http://imageevent.com/holl_ands/zigzaglpa/vhflpda/vhfrsvu190xrlpdanouhf
Remaining Element PAIRS were shortened the SAME amount & NO Shorting Stub was needed.

[Note that the undamaged Element Lengths do NOT follow a constant Taper Rule, with the
Tau=L(i)/L(i+1) parameter being VERY irregular along the LPDA...see my other LPDA analyzes.]

As was addressed in Cebik's "VEE" LPDA Analysis paper, in a Ch2-13 Antenna the elements
resonate at Lamda/2 for Lo-Band VHF Channels and 3*Lamda/2 for Hi-Band VHF Channels,
which results in significantly higher Raw Gain for the Hi-Band than a smaller Lamda/2 sized
antenna [another example of the "Bigger is Better" rule.....]:
http://w4rnl.net46.net/download/v1.pdf

So it was no surprise that SOME of the Elements were close to their original full length....and
others were 1/4 to 1/2 length. However, the Optimization also resulted in the two most forward
Elements being at the MINIMUM allowable length (I stipulated 1-in to avoid segmentation problems
in the optimization process)...so those particular X-Axis placements must be causing a problem,
rather than contributing to the overall Gain of the Antenna:

Optimized performance was much better after the Elements were cut, and although Azimuth Pattern
was irregular, overall performance was quite Good. Details are found in the posted 4nec2 File.

Hi-VHF Raw Gain = 9.6 dBi +/- 0.3 dB, F/B & F/R Ratio Min = 16.2 dB (GOOD) and SWR Under 1.3.




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