combining an RF signal with my OTA antenna - Canadian TV, Computing and Home Theatre Forums

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post #1 of 20 (permalink) Old 2019-05-09, 01:10 PM Thread Starter
 
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combining an RF signal with my OTA antenna

Hello everyone,

My house is a 2 storey home where there is only an RG6 coax cable going to the top level of my home.

I use a netmedia triple play modulator (MM73)
TriplePlay? Modulator - MM73
to combine composite signals from three devices to one coax cable outputting on UHF analog channels 60. 62 and 64.

I am trying to combine my OTA antenna with the output of the MM73 using a regular splitter/combiner.
I am using a good quality splitter, (I've tried a few, that isn't the problem).

The splitters I tested are Antronix CMC2002H OR BGI SGHMQK-2, both give the same results.

The OTA signal is great as this is just for a 2nd TV set for the guest room.
When I connect the MM73 output to the splitter/combiner, the upper UHF channels go to zero signal strength.

These channels are 43 (647 MHz) to 49 (683 MHz). There are two local channels in that range
that are usually 100% signal strength and 100% quality. They are channels 44 (653 MHz) and 47 (671 MHz).

If I leave the splitter connected and just disconnect the MM73 output from the splitter it works great.
As soon as the MM73 is connected, the channels mentioned above go to zero signal.

There are no amplified splitters, preamps or anything else in the line other than what you see in the picture.

The MM73 modulator can transmit on UHF or VHF and is configurable. I tried only using one output and transmitting
on channel 30 UHF but the result is the exact same.

I even tried a 7db attenuator on the MM73 line going into the combiner, same results unless the MM73 is disconnected.

I hope someone can give me some ideas on what to try. I really don't want to have to fish a coax cable up and crawl
around in the attic when there must be a simpler solution.

Any help is greatly appreciated.

Last edited by 57; 2019-05-09 at 03:07 PM. Reason: Link fixed
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post #2 of 20 (permalink) Old 2019-05-09, 02:34 PM
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Quote:
Originally Posted by ckeays View Post
I am trying to combine my OTA antenna with the output of the MM73 using a regular splitter/combiner.
I am using a good quality splitter, (I've tried a few, that isn't the problem).
Hello, ckeays

manuals

http://www.netmedia.com/modulators/mm73.html

http://www.netmedia.com/Instructions...structions.pdf

http://www.netmedia.com/vs2005/man-mm73.pdf

Your TV antenna is transmitting 60, 62, and 64. There is probably only 23 dB port to port isolation in the combiner.
https://www.antronix.com/pdf/DS-1010...000-Series.pdf
Insert an LTE filter between the antenna and the combiner.
https://www.channelmaster.com/TV_Ant..._p/cm-3201.htm

Try connecting just the modulator to the TV through the combiner without the OTA signals and increase the attenuation. The goal is to make the modulator signals look good on the TV before adding the OTA signals.

If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 2019-05-09 at 05:25 PM.
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post #3 of 20 (permalink) Old 2019-05-09, 02:54 PM Thread Starter
 
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Hi thanks for your response. I wanted to purchase some of these filters but their shipping to Canada is double the price of the unit. I am wondering if it would also help with my weak OTA signals.

I did try a 20 dB attenuator on the modulator cable to the splitter and it brought in one of the channels (a bit choppy, not really watchable)

The two channels I am missing are strong signals I can get with an indoor antenna, so I figure the modulator must be interfering somehow.
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post #4 of 20 (permalink) Old 2019-05-09, 05:34 PM
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^^^^
You have to be extremely careful in doing what you want. It is entirely possible the modulator will leak through the splitter/combiner to the antenna and interfere with some service. If I'm not mistaken, those channels are in the 700 MHz cell phone band.

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post #5 of 20 (permalink) Old 2019-05-09, 05:40 PM Thread Starter
 
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JamesK look at the pdf of the product:
http://www.netmedia.com/Instructions...structions.pdf
The connection diagram shows exactly what i am doing,

HOW could this be wrong or illegal or ?
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post #6 of 20 (permalink) Old 2019-05-09, 06:34 PM
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Quote:
They are channels 44 (653 MHz) and 47 (671 MHz).
There is a strong chance these channels will be reassigned to lower frequencies soon. Don't know your location so it's difficult to know what will happen and when.

Quote:
HOW could this be wrong or illegal or ?
Causing interference to licensed RF communications services is illegal. It can result in fines or worse. The TV channels occupying the 700MHz band have been assigned to and used by wireless telephone carriers for some time and the same is happening to the 600MHz band. They will take a dim view of anyone interfering with their services. Channels below 40 will still be used for TV but it's also illegal to interfere with TV signals. Those frequencies may be used privately as long as they don't interfere. Connecting the signal to an antenna is not allowed for most frequencies and is strictly regulated when it is allowed.

I agree that an RF low pass (LTE) filter should be inserted in the line from the antenna. That should cover the 700MHz band for the current situation but would need to be replaced by a 600MHz band model once the TV channels move to a lower frequency. That will most likely happen in the next year or two. It's also possible those stations will shut down.

It sounds like the RF modulator is generating out of band signals. That's not uncommon with cheap modulators. I see several options to solving the internal interference being caused by the modulator. One is to purchase a better modulator (or 3 modulators) that do not generate out of band signals. An LTE filter will still be required if they are used above channel 39. The other is to purchase a custom band pass filter for the channels being used by the current modulator. (That will likely cost $100+.) Another is to use HDMI, component or composite video inputs (plus audio) on the TV. The last option is the best choice since it will provide a better picture and eliminate any chance of RF interference. If the TV does not have enough inputs, it may be necessary to obtain an external switch.
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post #7 of 20 (permalink) Old 2019-05-09, 06:54 PM
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Quote:
look at the pdf of the product
There are a number of issues with those instructions. It's copyrighted in 2000-2003 which dates it considerably. It also shows the use of a S-VIDEO an interface that was deprecated over a decade ago and has not been commonly available for at least as long. It shows combining the signal with a "CABLE/ANTENNA FEED." Interfering with cable signals is also an issue. In addition, most modern cable systems use close to 100% of available frequencies so combining an RF modulator with a cable system will not work well. It may be possible with a combination of band pass filters designed in a way to prevent interference and loss of signals but that is not mentioned. Depicting use of the product by combining it with antenna or cable feeds is misguided and misleading. The guide shows CRT TVs in its depiction so that gives a good indication as to its suitability to modern TV setups. Modern TVs and other AV equipment neither require RF modulators nor do they work well with them. CRT TVs have been obsolete for a decade and so have NTSC modulators.
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post #8 of 20 (permalink) Old 2019-05-09, 08:37 PM
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Quote:
Originally Posted by ExDilbert View Post
Don't know your location so it's difficult to know what will happen and when.
IIRC, ckeays is in Newmarket, ON
old report, just about ready to drop off the TVFool server.
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Quote:
Causing interference to licensed RF communications services is illegal.
Correct; without the filter his setup would be an unlicensed transmitter. Some modulators are sold with a warning sheet about the modulator being connected to an antenna.

If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 2019-05-09 at 10:44 PM.
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post #9 of 20 (permalink) Old 2019-05-09, 09:18 PM
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You could try using two of these 75 ohm directional couplers. One on each downlead heading in to your combiner.
That should help to isolate the two combiner ports a little better, since a directional coupler passes significant RF with negligible insertion loss in one direction. Thus the name, directional coupler. Realistically could probably get away with just one on the output of the modulator.

One on the output of modulator to prevent signals from the antenna from mixing around inside the modulator spewing back out as intermod distortion, and one on the antenna downlead to keep the modulator from radiating out the antenna of the opposite port if you were suspecting that. You would then put 75 ohm terminators on the unused port of the directional coupler that is labelled 'tap' since ya wouldn't be using that in this application.

https://www.amazon.com/Directional-C.../dp/B00VK7D7N4

edit: But I suppose you could also optionally use those Taps to drop some signal to some other device(s) too, combining them with another two way splitter and u could feed another device. Don't know how well that'd work without seeing some spec on the specific directional coupler used. One step at a time though as Rabbit73 suggests.

DB8E/VHF Yagi rotor FM Bandstop ap-8700 preamp 8way split LG lcd.
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post #10 of 20 (permalink) Old 2019-05-10, 07:46 AM
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Quote:
HOW could this be wrong or illegal or ?
That PDF shows it working with other equipment and sources. It does not show an antenna as you're using. There will be some leakage from your equipment through the splitter and to the antenna, where it will radiate and interfere with other servces. That is most definitely illegal. Just being in a PDF does not make it legal.

BTW, you may have heard a recent news item where someone's homemade device was interfering with key fobs, etc.. I'm sure that guy didn't think what he was doing was illegal.

A Mystery Frequency Disrupted Car Fobs in an Ohio City, and Now Residents Know Why



Quote:
They are channels 44 (653 MHz) and 47 (671 MHz).
I thought he had channels 60, 62 & 64 listed in the sketch.

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post #11 of 20 (permalink) Old 2019-05-10, 08:34 AM
 
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Quote:
I thought he had channels 60, 62 & 64 listed in the sketch.
Those are the output channels of the RF modulator. Channels 44 & 47 are in use as Toronto OTA stations that the modulator is interfering with.
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post #12 of 20 (permalink) Old 2019-05-10, 11:46 AM
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^^^^
That could be either those modulators putting out signals where they shouldn't or the TV is having problems with whatever the modulators are putting out. If the modulator signal strength is too strong, it could overwhelm the TV tuner. It's also possible inter-modulation is generating signals on those frequencies. Regardless, using those modulators on a system that includes an antenna is a very bad idea.

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post #13 of 20 (permalink) Old 2019-05-10, 07:51 PM
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Your BEST approach would be to add the aforementioned CM3201 LTE Filter on the Coax in-between OTA Antenna and existing 2-Way Splitter. Splitter/Combiner is shown in fol. MM diagram:
http://www.netmedia.com/Instructions...structions.pdf

Charles Rhodes (TV Technology Mag) measured Attenuation of the CM3201 LTE Filter, finding 60 db on 740 MHz (Ch59), 44 dB on 730 MHz (Ch57), 31.5 dB on 720 MHz (Ch55), 20 dB on 710 MHz (Ch54), and only 8.8 db on 700 MHz (Ch52)....with attenuation of most OTA Channels by 2-3 dB, rising as approach Ch51 (current OTA Max, changing to Ch36 in near future: my SOCAL location changed last month):
https://www.tvtechnology.com/opinion...yth-or-reality

So your choice of modulator Channels 62/64/66 would ensure at least 60 dB of Attenuation going towards the OTA Antenna....and only 2-3 dB on desired OTA Channels. This is a LOT MORE than you will get by adding some additional 2-Way Splitters [aka "Directional Couplers"] which typically have Port-to-Port Isolation of only 20-30 dB.

Specs for MM73 state that Output Level is [an extremely strong] +30 dBmV. After RF Splitter Loss and LTE Filter Loss, max signal level going into OTA Antenna would be about -27 dBmV...which is NOT "adequate" to prevent interference to YOUR and your next door neighbor's Phone Reception, given Antenna Factor Loss and MAYBE some additional Loss due to Gain Roll-off in Antenna on Ch62-66. So you should also Attenuate the Modulator's Output down to say +0 to +10 dBmV by inserting a 20 dB Attenuator plus fol. Variable Coax Attenuator on it's Output and reduce level to minimum actually needed to overcome Loss in House Coax Wiring:
https://www.amazon.com/Holland-Elect.../dp/B005EGPDL6 [Fixed 20 dB]
https://www.solidsignal.com/pview.asp?p=TVA-20 [Variable up to 20 dB]

CATV systems must keep leakage under 20 uV/m....preferably 10 uV/m:
https://www.scte.org/TechnicalColumn...20It%20(1).pdf
On Ch60 (750 MHz), 20 uV/m leakage corresponds to Antenna Input Signal Level of -57.5 dBmV [someone should recheck these calculations]:
https://www.scte.org/TechnicalColumn...a%20factor.pdf

Antenna Simulations, Overload Calculations, etc: http://imageevent.com/holl_ands

Last edited by holl_ands; 2019-05-10 at 08:08 PM.
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post #14 of 20 (permalink) Old 2019-06-10, 10:39 AM
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Quote:
Originally Posted by ckeays View Post
I am trying to combine my OTA antenna with the output of the MM73 using a regular splitter/combiner.

When I connect the MM73 output to the splitter/combiner, the upper UHF channels go to zero signal strength.

These channels are 43 (647 MHz) to 49 (683 MHz). There are two local channels in that range
that are usually 100% signal strength and 100% quality. They are channels 44 (653 MHz) and 47 (671 MHz).

If I leave the splitter connected and just disconnect the MM73 output from the splitter it works great.
As soon as the MM73 is connected, the channels mentioned above go to zero signal.
Using the equipment that I have available, I was able to add analog channel 65 to my OTA channels:



Obviously, the TV must be able to receive analog channels as well as digital channels.






If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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post #15 of 20 (permalink) Old 2019-06-10, 10:51 AM
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^^^^
How do you know the modulator is not leaking through the splitter and radiating from the antenna? Consumer grade splitters do not have enough isolation between ports to avoid it.

I haven't lost my mind. It's around here...somewhere...
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