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Discussion Starter #1
I am getting pretty good OTA reception with the antenna/reflector I mounted on my roof, here is a breakdown of my system:

1. RCA ANT 751 mounted about 7' above my chimney, about 25' AGL
2. 4' wide by 3' high parabolic reflector I made out of aluminum bars (I bent the two ends myself) with construction screen as a reflector material
3. RCA TVPRAMP1R connected to mast
4. 25' (approx) of Audiovox blue RG6 coax running from ANT to power unit of pre-amp, goes immediately into a Radio Shack 3.0 GHz 2 way splitter
5. From the splitter, 10' of Audiovox blue RG6 coax to one television and 35' of Audiovox blue RG6 coax to the other television (runs under the house)

Here is my TV Fool report:
http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id=f1f012f8fe0296

I am having trouble with KPIX, KPPX, and KTSF (all UHF, I believe)
From reading many posts, and without a background in electronics, I believe the only thing that would help is more antenna. I want to stick with a rear mounted system so that it will work with the reflector I built (based on how many more channels I got after building it, I think it is the single largest contributor to my reception), and I think I can infer from context that it is possible to add element to a factory made antenna to increase gain (many people have added VHF element to UHF antennas, but nobody that I could find has added more UHF to UHF)... but I see references to dipoles/spacing/ etc. from smart guys and it makes me hesitant to just buy another antenna, cut off the UHF element and bolt it to my RCA ANT 751 (which does work pretty well). I have submitted a question to RCA about modifying the antenna, but have waited a week with no response.

Can anyone tell me if purchasing another antenna (or the same antenna?), cutting off the elements and attaching them in some symmetric fashion to my antenna would be a benefit? I'm not worried about the efficiency of cost; I just want a system that works better long term.

Thanks!!
 

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I would have to do some modeling to determine what the Reflector is REALLY doing....I suspect that it is probably in a useful location to help Hi-VHF Channels....but I'm skeptical whether it's helping or HURTING UHF Reception since it's WAY TOO FAR away from the UHF Elements...and might result in the Reflection being OUT OF PHASE at some UHF Frequencies.

The Parabolic Shape isn't actually working as a REAL Parabolic Reflector would work. with the "Feed" Antenna pointing TOWARDS the Parabolic Reflector so that is receives the concentrated signal "NEAR" the Focal Point...reiterating the need for the "Feed" Antenna being much smaller than the Parabolic Reflector:
http://imageevent.com/holl_ands/parabolic

I calculated Gain vs Diameter for a true Parabolic Dish, presuming an Optimum "Feed" Antenna and reasonable Antenna Efficiency (how much Area of the Dish is actually being used). This is for a 3D Dish.....your 2D Parabolic Trough Reflector would have about 3 dB LESS Gain:
http://www.digitalhome.ca/forum/showthread.php?p=2444866
A 3D Dish of at least 50-in Diameter would be needed to simply MATCH the Hi-VHF Gain of the ANT-751....and 100-in before it exceeds the Y10-7-13.

Your Reflector acts more like HALFWAY between a FLAT Reflector and a CORNER Reflector on a Yagi, as shown in fol. W-G HD-9032 picture:
http://www.winegard.com/kbase/uploads/HD-9032.pdf
Note that the Active Bowtie Elements fit just in front of the Apex on the Corner Reflector.....the ANT-751's Active Bowtie is WAY TOO FAR FORWARD.

BTW: A Corner Reflector adds only about 3 dB to the overall Gain of a long Yagi.

So AT BEST your Reflector adds 3 dB to the 6-7 dBi Hi-VHF or UHF Raw Gain of the ANT-751....and Worst Case the Reflection cancels out this Gain.

You would be much better served using a purpose built, High Gain UHF Antenna...with perhaps the ANT-751 used ONLY for Hi-VHF, connected to the VHF Port on the RCA Preamp (you MIGHT need to upgrade to Y-10-7-13 to reliably receive NBC).

I didn't see KPPX in your TVFool list....perhaps you meant ION on KKPX???? With a very weak NM=3.3 dB, THAT is going to require the BIGGEST UHF Antenna you can afford....and CW Network on KBCW isn't far behind. So I'm going to recommend CM4228HD (with HHH Harness Mod), 91XG, or DB-8e. If you are interested in a DIY Project, even more UHF Gain (and moderate Hi-VHF Gain) is provided by FF4....and a bit less by M4 (10x9.5)....for which you can order a prefabricated 4-Bay Sub-Assembly from mclapp's website to which you add your own Double-Angle Reflector:
http://imageevent.com/holl_ands/multibay/4bayrefl
 

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Discussion Starter #3
I didn't see KPPX in your TVFool list....perhaps you meant ION on KKPX???? With a very weak NM=3.3 dB, THAT is going to require the BIGGEST UHF Antenna you can afford....and CW Network on KBCW isn't far behind. So I'm going to recommend CM4228HD (with HHH Harness Mod), 91XG, or DB-8e. If you are interested in a DIY Project, even more UHF Gain (and moderate Hi-VHF Gain) is provided by FF4....and a bit less by M4 (10x9.5)....for which you can order a prefabricated 4-Bay Sub-Assembly from mclapp's website to which you add your own Double-Angle Reflector:
http://imageevent.com/holl_ands/multibay/4bayrefl
Thanks for the great insights!!!

I did mean Ion, just a mistype. I don't need this to be a DIY, building the reflector was a recommendation from someone that I don't believe has a strong a background in this. It did seem to increase the channels I picked up after scanning with and without the reflector, but water under the bridge at this point. I would gladly replace the system with something engineered that will work better. Of the three antennas you recommended, is the CM4228HD the best one? I hate to ask an even more remedial question, but how do I order the HHH attachment? The only search results for Hollands Horiz. Harness that I am finding is a technical model on image event; it doesn't come up on a search at amazon, sears, etc...

Do new CM4228HD come already on that type of harness, therefore only older or used ones would have to be avoided? Thanks again!
 

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Above I listed the three antennas in increasing PRICE and ALSO PERFORMANCE order (lower to higher): CM4228HD (with HHH), 91XG, DB-8e.

CM4228HD (and esp. older CM4228) used an imperfect Horizontal Interconnect Harness that had serious Gain Loss problems on the lower channels:
http://www.hdtvprimer.com/antennas/temporarypage.html
An OTA comparison between the original and Ken Nist's modified Horizontal Harness can be found here...mostly improving the HIGHER channels:
http://www.antennahacks.com/Hacks/NistHarness.htm
My comments re Ken Nist's analysis and Horizontal Harness mod are found here:
http://www.digitalhome.ca/forum/showthread.php?p=2387554

When I analyzed a more accurate 4nec2 model for the CM4228HD, I found that the Gain Loss problem on the lower channels was even more severe than reported by Ken Nist on HDTVPrimer:
http://imageevent.com/holl_ands/multibay/8bayrefl/cm4228hd

I ran an OPTIMIZATION to find the dimensions for the HHH (Holl_ands Horizontal Harness)...which FIXED the Loss on lower Channels and improved overall Raw Gain by about 1 dB:
http://imageevent.com/holl_ands/multibay/8bayrefl/newcm4228hdwithhollandshorizharness

CM4228HD Horizontal Harness is easy to remove and replace with HHH. I assumed AWG9 Aluminum wire size (which I found on the Internet), rather than Copper to avoid dissimilar metal problems [AWG10 Copper could also be used, but should have Aluminum Eyelets crimped on the ends:
http://www.whimsie.com/aluminum%20craft%20wire.html#anchor939097

Since CM4228HD uses a PCB (Printed Circuit Board) Balun that severely attenuates Hi-VHF signals, I would replace it with the usual cylindrical 300:75-ohm Transformer Balun if you wanted to try the CM4228HD for Hi-VHF channels....and is MUCH EASIER than trying to adapt the old PCB Balun. No, I don't sell a "KIT"....it really is very simple, just form TWO pieces of wire (29-in long each) bent into the requisite "U" shapes with "Loops" on the ends to fit over the screw terminals:
http://photos.imageevent.com/holl_a...horizharness//HHH Dimensions for CM4228HD.jpg

Long before I did the HHH Optimization run, BOUVAL constructed a similar, but different Horizontal Harness Mod (apparently without benefit of an Optimization process???):
http://www.digitalhome.ca/forum/showthread.php?p=1428968
http://www.digitalhome.ca/forum/showthread.php?p=1428968
http://www.digitalhome.ca/forum/showthread.php?p=2182369
 

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Please include a location in the title of all antenna help threads. See the sticky and my edit.
 

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Here is my TV Fool report:
http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id=f1f012f8fe0296

I am having trouble with KPIX, KPPX, and KTSF (all UHF, I believe)
From reading many posts, and without a background in electronics, I believe the only thing that would help is more antenna. I want to stick with a rear mounted system so that it will work with the reflector I built (based on how many more channels I got after building it, I think it is the single largest contributor to my reception), and I think I can infer from context that it is possible to add element to a factory made antenna to increase gain (many people have added VHF element to UHF antennas, but nobody that I could find has added more UHF to UHF)... but I see references to dipoles/spacing/ etc. from smart guys and it makes me hesitant to just buy another antenna, cut off the UHF element and bolt it to my RCA ANT 751 (which does work pretty well). I have submitted a question to RCA about modifying the antenna, but have waited a week with no response.

Your TV Fool report shows all the stations from SF are weak. You need to forget about modifying an antenna that's intended for strong signal areas and put up an antenna for weak signal areas. The north bay is notorious for weak signals. As a minimum you need an HD7698P or HBU55 up as high as possible. Better still would be a 91XG and a Y-10-7-13 if you can still find one of those.

You can probably find more people in your area over on the San Francisco OTA thread.
 

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Discussion Starter #9
What are you going to do for VHF? The DB8e is not a VHF antenna and is not likely to receive KNTV and KGO.
I was going to set the RCA PRAMP1R to separate incoming signals for VHF/UHF, and keep using the RCA ANT751R for the VHF signal (hooking both antennas to the amplifier). I was assuming that the fact the RCA ANT751R is designed to capture both UHF/VHF wouldn't impede its use for only VHF.... Do you think that logic is sound? Thanks!!
 

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If the RCA antenna was providing acceptable reception of your VHF stations, then your plan should be valid.

However, given the forecasted signal powers of some of the nearby stations, don't be surprised if using the amplifier introduces new problems.
 

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I don't think that Overload Intermodulation Noise in the RCA Preamp will be a problem. He only has TWO Very Strong Stations: Ch23 and Ch3. It takes TWO or more Very Strong Stations in the SAME Band to get problematic INTERMODS....

I also checked www.FMFool.com for his Zipcode and found four Very Strong FM Stations (within about 5 miles) whose Second Harmonics fall on top of Ch9, Ch11 and Ch12.....of which only Ch12 is occupied by NBC (KNTV). FM Filter in RCA Preamp provides some attenuation of FM Signals....but if there are problem receiving Ch12, adding a FULL BAND FM FILTER (aka TRAP) on the coax coming from the Hi-VHF Antenna MIGHT be needed, available from either Antennas-Direct (the better one) or Radio Shack (allows Ch6 to pass....which you probably won't get anyway).

BTW: VHF Input port on the VHF/UHF Combiner will Filter Out all UHF signals...and vice versa.
 

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He only has TWO Very Strong Stations: Ch23 and Ch3. It takes TWO or more Very Strong Stations in the SAME Band to get problematic INTERMODS....
A single strong channel can cause gain compression and amplifier desensitization which will diminish reception of the weakest signals. This may take place in the tuner or in the amp, there are susceptibilities in both receiving stages.

Also, 2nd order products (F1 + F2) can wipe out a 3rd channel.

Example 1: In this situation the 2nd order distortion products of channels 3 and 23 happen to fall into UHF channel 33, occupied by KMTP. This is before the spectral regrowth takes place which will result in distortion products falling into the adjacent channels 32 (KEMO, very strong) and 34 (KFSF, weak). A properly installed HLSJ can attenuate channel 3 (as well as FM) so this can be avoided.

Example 2. A strong channel 3 (60-66 MHz) plus strong FM on 107.9 can cause 2nd order distortion products in the lower half of channel 7. Of course, an FM filter will mitigate this.

Example 3. A strong high-VHF plus a strong UHF in the lower part of that band can cause 2nd order distortion products to fall into upper UHF channels. For example, any channel 7-13 plus a UHF 14, for example, will have 2nd order products that fall into UHF 43 and up.

Example 4: CEA -774-B amplifier distortion performance evaluation criteria calls for a 200 and a 500 MHz signal pair to be injected into the amp and that 2nd order products be checked at 700 MHz and must be below the specified level.

A pre-amp operated in split-band mode won't generate the interference internally, but it may boost the signals enough so that the distortion occurs in the tuner front end.

While it doesn't apply in this situation, but two strong high VHF channels can cause 3rd order intermods to fall into UHF channels. Article: http://www.tvtechnology.com/digital-tv/0148/inter-band-rfi-should-stations-worry/271017

VHF Input port on the VHF/UHF Combiner will Filter Out all UHF signals...and vice versa.
Well, not "all", but "most". I can see it on the lab gear but it likely would never matter in the wild. ;)

So, it's possible for signals from multiple bands to become an issue.
 

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I was going to set the RCA PRAMP1R to separate incoming signals for VHF/UHF, and keep using the RCA ANT751R for the VHF signal (hooking both antennas to the amplifier). I was assuming that the fact the RCA ANT751R is designed to capture both UHF/VHF wouldn't impede its use for only VHF.... Do you think that logic is sound? Thanks!!

Yes, if you're receiving KGO and KNTV okay on the ANT751 then that will work.
 

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Discussion Starter #14
Wow, I am learning so much from reading all of these responses! This is a great site, and I thank you all for taking the time to help out. I am anxiously awaiting the DB8e; when I get it, I will mount it immediately and let everyone know how well things are coming in. Is there any wisdom regarding how far apart the UHF antenna should be from the RCA ANT 751? I was assuming I would put the UHF on top, as high as possible, and the VHF beneath it. Does it matter what the separation distance is? Thanks...

BTW, it was a ham radio guy at Friedman's that I had a chance conversation with which led me to building a reflector for my antenna... I should have spent some time on threads like this before committing to any hardware or overall design! Lesson learned for next time....
 

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Although I haven't modeled YOUR particular situation, in other Antennas, only 1-2 feet of Metal-to-Metal separation was needed so that GAIN is not affected, and at LEAST 4-ft is needed to minimize impact on Front-to-Back performance (which suppresses unwanted Multipath and Interference coming from the Rear):
http://imageevent.com/holl_ands/stacked/stackuhfhivhf
 

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This is my current setup. The AntennaCraft Y5-7-13 on top of the XG91. There is a 3'-4" separation between antennas, and it works well...



This is what I used to have... the RCA ANT751 below, which I used solely for 2 high-VHF stations. That worked well too...



Good luck with your setup and let us know how it works out for you.
 

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Discussion Starter #17
So I have an update....it took me a couple of weeks after I got the DB 8e before I got a chance to put it up. After pulling down my antenna mast, I removed the home made reflector, mounted the DB 8e on the very top, and mounted the RCANT751 about a foot below the bottom of the DB 8e. I wanted to separate them more than that, but I was concerned about the VHF being overall too low to receive good signal. A mishap occurred while connecting everything; my RCAPRAMP1R VHF input stem broke and was spinning freely as I was trying to disconnect the original coax connected to it. I got the coax disconnected, but clearly I had damaged the unit since no other connection was spinning freely. I took it to a neighbor whom is an electrician, and he felt comfortable taking it apart for a diagnosis. We found that the base of each coax connection is attached at all four corners to a common ground on the circuit card, and the base of the VHF input had sheared at each corner. Another disturbing observation was that the circuit card was discolored, about 1/4 of it looking like it had darkened. My neighbor was able to solder new connections from each corner of the mount to the ground on the circuit cards, and we put it back together.

Needless to say, the overall quality of my reception has gone down slightly. More interference on KGO and KPIX than I had seen before, frequent interruption of the audio signal (dialogue cutting out), KICU and KKPX almost never come in, and many more other channels are watchable but have routine interference.

I am assuming that the main issue is with the amplifier; and perhaps contributing is the relatively short distance between the antennas. Here is my plan to fix it, I would love to hear anyone's opinion:

1. order a new amplifier. Can anyone recommend a better product than the RCAPRAMP1R? I still want a unit that has a separate input of VHF and UHF, one output, and has an FM trap.
2. Extend the mast so that I can increase the height of the DB 8e another 5 feet or so, and raise the RCA 751R back up to it's original height while still keeping 3' between the two antennas. Is it true that the higher the antennas in general, the better the signal? Would a few feet make a difference when almost everything I'm getting is 2edge?
3. Should I worry about a reflector for the RCA 751R? I don't want to use my original because I am concerned about impacting the DB 8e above it, but if I want to put anything behind the VHF now would be the time to do it. I don't know if the interference in KGO is from removing the reflector, the lower height, the close proximity of the DB 8e, or the amplifier damage.

I've attached a picture of the new setup; my old one (which thus far has worked better!) can be seen above.

As always, thanks!
 

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Needless to say, the overall quality of my reception has gone down slightly. More interference on KGO and KPIX than I had seen before, frequent interruption of the audio signal (dialogue cutting out), KICU and KKPX almost never come in, and many more other channels are watchable but have routine interference.

I am assuming that the main issue is with the amplifier; and perhaps contributing is the relatively short distance between the antennas. Here is my plan to fix it, I would love to hear anyone's opinion:

1. order a new amplifier. Can anyone recommend a better product than the RCAPRAMP1R? I still want a unit that has a separate input of VHF and UHF, one output, and has an FM trap.
2. Extend the mast so that I can increase the height of the DB 8e another 5 feet or so, and raise the RCA 751R back up to it's original height while still keeping 3' between the two antennas. Is it true that the higher the antennas in general, the better the signal? Would a few feet make a difference when almost everything I'm getting is 2edge?
3. Should I worry about a reflector for the RCA 751R? I don't want to use my original because I am concerned about impacting the DB 8e above it, but if I want to put anything behind the VHF now would be the time to do it. I don't know if the interference in KGO is from removing the reflector, the lower height, the close proximity of the DB 8e, or the amplifier damage.
What do you mean by interference on KGO and KPIX? It's unlikely you could know if either of those channels are getting interference just by looking at them. What could be causing interference? You need to explain what you're seeing. A picture would help.

There are no other preamps with separate VHF/UHF inputs. There are two ways to get around this problem with single input preamps. 1) Use a diplexer to combine the antennas. They're only about 0.5 dB insertion loss. 2) Use separate preamps for each antenna and combine them with a diplexer. Put a full band FM trap in front of the VHF preamp.

If the DB8e made no improvement on UHF over the ANT751 then you have some other problem. This is hard to troubleshoot over an internet forum.

It's unlikely that soldering the connector back onto the circuit board of the preamp caused degradation. The preamp either works or it doesn't. I don't know what's going on with the discolored circuit board. I looked at mine and the board looks to be high quality.
 

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Discussion Starter #19
I am probably using the term "interference" incorrectly. What I mean is that the picture is not completely formed, and the audio cuts in and out so badly the programming can't be enjoyed. This would regularly occur on KKPX and KICU when I was using just the ANT 751R with my home made reflector, and across all channels during inclement weather. After installing the DB 8e and all the other details previously mentioned, KKPX and KICU are constantly too weak to detect and every channel seems like it's regularly experiencing whatever this phenomenon is. I've attached some pictures of KGO, a station that used to be very reliable but since my upgrade has been having regular difficulties.

Thanks for looking...
 

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I am probably using the term "interference" incorrectly. What I mean is that the picture is not completely formed, and the audio cuts in and out so badly the programming can't be enjoyed. This would regularly occur on KKPX and KICU when I was using just the ANT 751R with my home made reflector, and across all channels during inclement weather. After installing the DB 8e and all the other details previously mentioned, KKPX and KICU are constantly too weak to detect and every channel seems like it's regularly experiencing whatever this phenomenon is. I've attached some pictures of KGO, a station that used to be very reliable but since my upgrade has been having regular difficulties.

Thanks for looking...

It's called macroblocking. It's due to the signal being very weak teetering on the digital cliff or a multipath problem. Maybe your preamp is bad. You can remove it for a test. The DB8e should easily make up in gain what is lost by having no preamp. Just to be sure, recheck that the correct antenna is on the correct preamp connector although you'll likely receive nothing if they are swapped.

Also, rain doesn't attenuate TV signals. During fair weather is is very common to have temperature inversions that extend the coverage of the stations. Morning and evenings are the best times. The coastal stratus marks the inversion height. As it dissipates during the day so does the extended coverage. Storms wipe out the inversion and so it appears that rain attenuates signals but that's not what's happening. You want to be able to receive your stations when it's raining and not depend on atmospheric enhancements that are unreliable.
 
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