Optimizing an old RS 15-1702 rooftop antenna for HI-VHF? - AVS Forum
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post #1 of 11 Old 07-09-2013, 12:01 PM - Thread Starter
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Chicago south suburbs, according to antennaweb 25 miles to xmitters downtown..

 

Comcast + rooftop OTA through TIVOs.  OTA is cleaner than Comcast HD locals (mpeg compression), plus OTA works those couple times a year after a big storm takes down cable for days as happened recently.

 

Anyways: had been using a winegard 4400 UHF that works well, but does nothing for WBBM (CBS 2, RF-12 VHF 8kw).  Found in the attic an old RS 15-1702 (small V-U-FM) still in the box.  Puzzling where it came from, previous homeowner for sure...bought but never installed.

 

There's nothing in VHF-low band in Chicago I'm interested in, just CBS @ VHF 12.  Took measurements of the 8 VHF log elements on the ant, appears to work half wave in VHF-L band, full wave on VHF-H.  Simply chopping the long elements to retune to full wave RF 11-13 should work?  Chopping to half wave would likely pick up too much UHF/interfere with the Yagi on the front of the antenna.  Elements would be chopped to 26",27",27",28".  Any reason this would not work?

 

Spacing of the elements in question is 11.25" (4 crossconnected dipole halves or 8 elements)

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post #2 of 11 Old 07-15-2013, 09:51 AM
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I tried googling RS 15-1702, but could not find anything.
If it has 8 VHF elements, does it look like this?

http://www.hdtvprimer.com/antennas/VU-90XR.html

That antenna should get RF11-13, so you should not need to modify anything on the antenna.
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post #3 of 11 Old 07-15-2013, 03:45 PM - Thread Starter
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The antenna you referenced (VU-90) is nearly identical.  Minor difference on UHF portion (#of reflectors/directors).  Im aware it's current design will grap High-Vhf.  I'm looking to modify the lengths of the VHF elements to maximize gain/ tighten beamwidth at channel 12 (obviously at the expense of the lower channels I don't need anyway).   This should also help with wind loading as these older RS antennas aren't known for being the most durable.   LPDA theory maintains the elements need to be progressively longer (inactive elements toward the rear act as reflectors to the shorter elements in front at higher freqs).   In the meantime I found a similar current CM antenna where the VHF portion has been tuned to hi-VHF.  It appears in comparing 2 similar CM antennas (one for 7-69, one for 2-69) spacing is close/identical primarily the element lengths are modified.

 

Optimizing for ch12 may seem knit picky.  WBBM with 8 kw @ 25 miles leaves a bit to be desired in a EMI/noisy metropolitan environment.  I've heard accounts from a couple neighbors with small rooftop antennas that have trouble with dropout on WBBM despite adequate strength.  Co-channel WINM/Angola IN is conveniently 90 degrees from my point to downtown Chicago.  Nothing else on CH 12 for hundreds of miles, so bi-directional pattern likely to result shouldn't present significant issues.

 

CM 2018 (7-69) product link

http://www.channelmaster.com/HD_television_antenna_Channel_Master_CM_2018_HDTV_Antenna_s/51.htm

CM 3016 (2-69) link (pdf)http://www.channelmaster.com/HD_television_antenna_Channel_Master_CM_3016_HDTV_Antenna_s/49.htm

 

Been doing more research.  Believe I'm settling on lengths @ 26, 27,28,35.  35's would be inactive @ ch12 essentially reflectors and may be cut from the TL.   From theory/equations I've been eyeing, it should be more effective at CH 12 than the original design, though not truly "optimized".  The length of the reflectors isn't set in stone.  I may want to subtract the distance (gap) down the middle which would take about .75 from all lengths.

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post #4 of 11 Old 07-16-2013, 03:45 PM
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Can you post your tv fool results?
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post #5 of 11 Old 07-16-2013, 08:16 PM - Thread Starter
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http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d46ae8e255416d8

 

The report would lead one to believe WBBM should be easily picked with set top rabbit ears -- LOL.  The UHF repeater on CH26 isn't up yet, but it'll be LP @ 30 miles and downtown in the path (have my doubts).

 

Where I used to live in Indiana ( http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d46ae16a5f3511a) I could 23.9/7 pick everything everything green, yellow and all but 2 red with one of those ancient double bow tie set top antennas.

That antenna here gets maybe the 5 strongest channels.  The issue isn't that the signal isn't there, the set locks on, the problem is interference which is intermittent in nature.  If there were nothing but a big cornfield between here and downtown there wouldn't be a problem.

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post #6 of 11 Old 07-17-2013, 03:42 AM
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I don't think rabbit ears would pick up WBBM at 25 miles out. VHF stations are more difficult to receive using indoor antennas than UHF channels.

Based on your tvfool report, I don't think you need to modify anything on your RS antenna until you try it first. I think it will work fine. Just make sure you point it North. You probably know this already, but the front of the antenna is the right side of the pic above in the link I posted.

Also you mentioned you have some existing TIVO's. If you are using a splitter, or splitters, bypass them for now when you test, and see what signal you get. As you start to add the splitters back, if you notice dropouts, then you need an amp to compensate for the splitter losses.


The Winegard you have now is a UHF only antenna, and that is why you cannot receive WBBM. Just curious, can you receive WLS VHF-7?
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post #7 of 11 Old 07-17-2013, 09:16 PM
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Quote:
Originally Posted by mrbrian200 View Post

http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d46ae8e255416d8

The report would lead one to believe WBBM should be easily picked with set top rabbit ears -- LOL.  The UHF repeater on CH26 isn't up yet, but it'll be LP @ 30 miles and downtown in the path (have my doubts).

My experience might give you some hope of receiving WBBM with rabbit ears. I am located 30+ miles from Mt Wilson where the transmitters are located and I do not have LOS, just 1 edge reception of all the high power signals. Yet I can receive the VHF-Hi stations very reliably with a HDTVa inside my house. You do have to locate the rabbit ears near the wall or window facing your local transmitter or you won't get good reception.
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post #8 of 11 Old 07-19-2013, 08:43 AM - Thread Starter
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I got it up on the roof this morning.  From my prior research chopping the VHF elements would (best case) result in better signal for WBBM or (worst case) be about the same.  I have easy access to it once its up so I marked the lengths I wanted (26,27,28,34") with a sharpie but did not cut.   Mounted.  I do not know how the signal meter on a TIVO series 2 correlates to dbm, antenna @ original lengths showed WBBM hovering around 80.  Chopped the elements it jumped to the low 90s.  All the full power UHF locals are pegged at 100.  Fed down about 80' quad-RG6 and a Winegard CC-7870 splitter for 2 TVs.  It's been running on the TV 2 hours I haven't noticed any interference induced breakup or drops, so I'd say I'm good.

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post #9 of 11 Old 07-19-2013, 09:18 AM
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Quote:
Originally Posted by mikepier View Post

I don't think rabbit ears would pick up WBBM at 25 miles out. VHF stations are more difficult to receive using indoor antennas than UHF channels.

Based on your tvfool report, I don't think you need to modify anything on your RS antenna until you try it first. I think it will work fine. Just make sure you point it North. You probably know this already, but the front of the antenna is the right side of the pic above in the link I posted.

Also you mentioned you have some existing TIVO's. If you are using a splitter, or splitters, bypass them for now when you test, and see what signal you get. As you start to add the splitters back, if you notice dropouts, then you need an amp to compensate for the splitter losses.


The Winegard you have now is a UHF only antenna, and that is why you cannot receive WBBM. Just curious, can you receive WLS VHF-7?

WLS-TV shut off their VHF signal back in March. They're operating only on RF 44 now. So WBBM-TV is the only full power VHF in Chicago, & WOCK-CD being the only Class A digital station on VHF-Lo. From Gary, IN, I always had better luck with WLS-TV on RF 7 than on RF 44. WLS-TV still has not fully worked out the bugs on RF 44, & I still have problems with their signal. Their signal in Gary, IN also interferes with WCPX on RF 43 (one station I never had problems a month after the analog shut off date, until WLS-TV started broadcasting on RF 44 in October 2009. What's odd is that the people who complained that they couldn't get WLS-TV on RF 7, but could on RF 44, & those who got them on RF 7, but not RF 44. I got both, but RF 7 had almost no interference, while RF 44 got first adjacent from RF 43 (not sure about RF 45 WSNS, but I never had problems with that RF channel).
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post #10 of 11 Old 07-19-2013, 09:41 AM - Thread Starter
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Quote:
Originally Posted by mikepier View Post


The Winegard you have now is a UHF only antenna, and that is why you cannot receive WBBM. Just curious, can you receive WLS VHF-7?

No sign of anything at VHF-7 with either the 4400 or this antenna. WLS @ UHF-44 is/was good with either antenna.  I didn't check RF-7 before I chopped it.  Only thing I'm seeing on VHF is WBBM-DT and WKQX-LP (analog) which is interesting as the onscreen banner reads "Xfininty - Q87.7".  The TIVO doesn't accommodate manual OTA channel probing so I plugged the line directly to the TV.

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post #11 of 11 Old 07-20-2013, 12:00 AM
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Quote:
Originally Posted by mrbrian200 View Post

No sign of anything at VHF-7 with either the 4400 or this antenna. WLS @ UHF-44 is/was good with either antenna.  I didn't check RF-7 before I chopped it.  Only thing I'm seeing on VHF is WBBM-DT and WKQX-LP (analog) which is interesting as the onscreen banner reads "Xfininty - Q87.7".  The TIVO doesn't accommodate manual OTA channel probing so I plugged the line directly to the TV.

That's because I already posted that WLS-TV shut off their VHF signal back in March, & only broadcast on RF 44. Since you didn't install your VHF/UHF combo antenna before WLS-TV shut off the VHF signal, you're not gonna get anything on RF 7. That is until the FCC decides whether to grant WCHU-LD a CP to move to RF 7 from RF 33. It was granted before, but WLS-TV refused to give up the channel, until the FCC decided not to continue extending WLS-TV's STA for RF 7. WCHU's application for RF 7 is for 3kw on the John Hancock. So if the FCC grants it, you should be able to get their station from the suburbs. I know I'll get the station if they go to RF 7. I got WBBM-TV's RF 3 signal from Gary, IN when they were only at 2.8kw (of course, this was before June 12th, 2009).
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