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Old 9-May-2019, 4:20 PM   #1
newmguy
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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)
http://www.netmedia.com/modulators/mm73.html
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 newmguy; 9-May-2019 at 8:37 PM.
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Old 9-May-2019, 5:03 PM   #2
ADTech
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The problem isn't the splitters, it's most likely that the modulator isn't very "clean" and is transmitting signals that are below the channels that you've selected. That splatter is interfering with the desired UHF OTA signals. FWIW, those two local stations will be leaving those operating channels sometime in the next year or two as a result of the repack.

Are you sure that your RF modulator can do VHF? The spec sheet indicates UHF only frequencies are offered. If it can do VHF, which channels? Do you have any VHF stations in your area? It would be simple to combine a VHF signal to an existing antenna feed if you don't have any low-VHF using an HLSJ or any VHF channel using a UVSJ.
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Last edited by ADTech; 9-May-2019 at 5:06 PM.
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Old 9-May-2019, 5:17 PM   #3
newmguy
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Thanks for your response.

Quote:
Originally Posted by ADTech View Post
FWIW, those two local stations will be leaving those operating channels sometime in the next year or two as a result of the repack.
yes I am aware of this but here in Canada things seem to take longer than anywhere else.

Quote:
Originally Posted by ADTech View Post
Are you sure that your RF modulator can do VHF?
yes in the past I have used cable TV mode on the modulator. So I was thinking of trying to set the channel outputs to the VHF low channels (like 2, 4, 6)
because I have super strong channels on 8 & 10 (10 will be moving to 9 in the near future)

I can try this and post my results.


EDIT: the modulator does work on VHF but only on channels 70 -94, 100-125.

Last edited by newmguy; 9-May-2019 at 5:22 PM.
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Old 9-May-2019, 5:26 PM   #4
newmguy
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I installed a 20 db attenuator on the modulator cable going into the splitter and one of the channels came in, but a bit choppy (I guess it's not really watchable). The modulator output is watchable but not really the best quailty.
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Old 9-May-2019, 6:25 PM   #5
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Are your TV channels on 20 and up? If so this will allow the use of channels 14, 16, and 18.

https://www.solidsignal.com/m/product.aspx?p=nf-469

This model would allow use of channels 65, 67, and 69.

https://www.solidsignal.com/m/product.aspx?p=lpf-750

You will also need the 20 dB attenuator on the output of the modulator.

Last edited by Tower Guy; 9-May-2019 at 6:32 PM.
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Old 9-May-2019, 7:13 PM   #6
newmguy
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Quote:
Originally Posted by Tower Guy View Post
Are your TV channels on 20 and up? If so this will allow the use of channels 14, 16, and 18.

https://www.solidsignal.com/m/product.aspx?p=nf-469
no there are 3 channels, 14, 15, and 19

Quote:
Originally Posted by Tower Guy View Post
This model would allow use of channels 65, 67, and 69.

https://www.solidsignal.com/m/product.aspx?p=lpf-750

You will also need the 20 dB attenuator on the output of the modulator.
So where would this install?

Between the cable coming from my antenna to the splitter input?

or

From the modulator output to the splitter input?
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Old 9-May-2019, 7:17 PM   #7
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Quote:
EDIT: the modulator does work on VHF but only on channels 70 -94, 100-125.
Those would be UHF frequencies of 500-650 MHz and 650-800 MHz respectively. They're not VHF. Here's a chart that translates CATV channels to frequencies: http://eaglecomtronics.com/Cable_Tel...ency_Chart.pdf

I don't think that either the LPF or the NF will do the needed job. UHF channels 60-64 are far enough removed from UHF channels 43-49 that the modulated analog signals, if clean, shouldn't interfere with the OTA signals.

My recommendation would be to 1) move the analog to 65-69 and then, if necessary, 2) install a HIGH pass filter (~750 MHz) on the modulator's output to suppress the frequencies below that output prior to combining. Unfortunately, that would likely be a custom device. I would still use the suggested attenuation to cool off the whole signal from the modulator.
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Last edited by ADTech; 9-May-2019 at 8:12 PM.
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Old 9-May-2019, 8:45 PM   #8
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The 750 MHz Low Pass Filter goes between the antenna and the combiner(splitter). The attenuator goes between the modulator and the combiner (splitter). The attenuator may need to be a bit more or less than 20 dB. Note that the modulator has a rated output of +30 dbmv. This equates to a NM in the 70 dB range for digital signals or roughly 35 dB for analog signals.

Here’s my thinking:

I agree with ADTech that the modulator is putting out signals that interfere with the off air channels. The attenuator will fix that.......BUT the OTA antenna is picking up LTE signals above 700 MHz, so they interfere with the modulator. The LPF-750 will filter out the LTE signals on channels 65-69.
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Old 9-May-2019, 9:08 PM   #9
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This LTE filter will do the same thing as the LPF-750 and is cheaper. https://www.channelmaster.com/TV_Ant..._p/cm-3201.htm
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Old 9-May-2019, 10:24 PM   #10
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Thanks Tower Guy. I broke down and ordered one.
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Old 10-May-2019, 2:20 AM   #11
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Quote:
Originally Posted by Tower Guy View Post
I agree with ADTech that the modulator is putting out signals that interfere with the off air channels. The attenuator will fix that.......BUT the OTA antenna is picking up LTE signals above 700 MHz, so they interfere with the modulator.
correct
Quote:
Originally Posted by newmguy View Post
Thanks Tower Guy. I broke down and ordered one.
Looks promising. Please let us know how it works out.

See also this thread:
combining an RF signal with my OTA antenna
https://www.digitalhome.ca/forum/81-...a-antenna.html
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Last edited by rabbit73; 10-May-2019 at 2:24 AM.
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Old 5-Jun-2019, 12:23 PM   #12
newmguy
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finally got the LTE filter after almost a month. Installed it and the 20db attenuator as per post #8 above.

There is absolutely no difference.
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Old 5-Jun-2019, 3:01 PM   #13
rabbit73
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Quote:
Originally Posted by newmguy View Post
finally got the LTE filter after almost a month. Installed it and the 20db attenuator as per post #8 above.

There is absolutely no difference.
Thanks for the report. Looks like this will not be an easy fix.

Did you try what I suggested on the Canadian forum?
https://www.digitalhome.ca/forum/81-...ml#post3079969

Quote:
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.
Keep increasing the attenuation at the output of the modulator until just before the analog channels start to show snow. Then connect the OTA channels to the combiner.

If that doesn't work, you will need to do what you don't want to do: run another coax line.

I'm a little confused by your diagram; it only shows one TV. Is the OTA antenna only for the 2nd TV, or for both TVs?

Is the modulator output just for the 2nd TV, or both TVs?
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Last edited by rabbit73; 5-Jun-2019 at 4:55 PM.
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Old 6-Jun-2019, 1:25 AM   #14
Tower Guy
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Quote:
Originally Posted by newmguy View Post
finally got the LTE filter after almost a month. Installed it and the 20db attenuator as per post #8 above.

There is absolutely no difference.
Have you also switched your modulator to real channels 65, 67, and 69?
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Old 6-Jun-2019, 1:29 PM   #15
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Revisit post #7.
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Old 7-Jun-2019, 2:33 AM   #16
newmguy
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Quote:
Originally Posted by rabbit73 View Post
Thanks for the report. Looks like this will not be an easy fix.
Keep increasing the attenuation at the output of the modulator until just before the analog channels start to show snow. Then connect the OTA channels to the combiner.
I had a long response all typed out and this forum timed out, so i lost it all!
SO this response will be very short because its late.

yes i tried this with 37db attenuation. Modulator channels were snowy but this didn't help. I am trying to get CITY TV, channel 44, 653 MHz. Without the modulator it comes in. I even get Fox and ABC but not CITY-TV with the modulator connected.

Quote:
Originally Posted by rabbit73 View Post
I'm a little confused by your diagram; it only shows one TV. Is the OTA antenna only for the 2nd TV, or for both TVs?

Is the modulator output just for the 2nd TV, or both TVs?
yes the clearstream 4V attic antenna is just for the bedroom tv. The modulator output is split into two. One feeds the whole house through a dist amp, and the other is attenuated and then combined with the LTE filtered antenna signal to the bedroom tv.

The output of the two goes to the bedroom tv and is independent of all other tv's in the house.
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Old 7-Jun-2019, 1:45 PM   #17
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Quote:
Originally Posted by newmguy View Post
The modulator output is split into two. One feeds the whole house through a dist amp, and the other is attenuated and then combined with the LTE filtered antenna signal to the bedroom tv.
I would try to avoid the use of a distribution amp on the output of the modulator.
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Old 7-Jun-2019, 2:00 PM   #18
rabbit73
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Quote:
Originally Posted by newmguy View Post
I had a long response all typed out and this forum timed out, so i lost it all!
SO this response will be very short because its late.
I hate it when that happens to me. Now, I sometimes make a copy of the text before posting so I don't lose it all.
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Old 7-Jun-2019, 10:23 PM   #19
rabbit73
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Quote:
Originally Posted by newmguy View Post
yes i tried this with 37db attenuation. Modulator channels were snowy but this didn't help. I am trying to get CITY TV, channel 44, 653 MHz. Without the modulator it comes in. I even get Fox and ABC but not CITY-TV with the modulator connected.

yes the clearstream 4V attic antenna is just for the bedroom tv. The modulator output is split into two. One feeds the whole house through a dist amp, and the other is attenuated and then combined with the LTE filtered antenna signal to the bedroom tv.

The output of the two goes to the bedroom tv and is independent of all other tv's in the house.


I revised your diagram to match your description. Is it correct?

How do the modulator channels look on the other TVs?

You have probably eliminated the overload possibility with the attenuator, but the modulator could be putting out spurious signals in the UHF band, as ADTech has indicated, that would interfere with the reception of your desired UHF channels.

Are you still in Newmarket, ON? Is this report correct for your location?



These are suggestions made by ExDilbert:

https://www.digitalhome.ca/forum/81-...ml#post3079995

Quote:
Originally Posted by ExDilbert
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.
Attached Images
File Type: jpg newmguyTVFdiagram2.jpg (51.5 KB, 275 views)
File Type: jpg newmguyTVReport.jpg (175.8 KB, 263 views)
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Last edited by rabbit73; 8-Jun-2019 at 12:05 AM.
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Old 10-Jun-2019, 2:19 AM   #20
rabbit73
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It Works for Me

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.







The modulator is leaking through the port to port isolation of the splitter used as a combiner. The attenuator, port to port isolation, and LTE filter reduce the modulator signals reaching the antenna.

A preamp below the LTE filter, if needed, will increase the isolation. The attenuator can be increased to bring the modulator signals down to -9 dBmV; the snow begins at about -15 dBmV.

Grounding the coax shield will make the attenuator and LTE filter more effective.

I have the equipment to measure the modulator signals that arrive at the antenna; most user don't. If they are below the Thermal Noise Floor (for a 6 MHz bandwidth TV signal) at -106 dBm (-57 dBmV) , it is probably safe, as far a causing interference is concerned. Without the CM LTE filter, the modulator would be considered an illegal unlicensed transmitter.

These are the measurements for the splitter as combiner that I used:
IN Modulator Port: +1 dBmV
OUT Antenna Port, TV Port not terminated: -6 dBmV
OUT Antenna Port, TV Port terminated w/75 ohms: -23 dBmV

The attenuation of the CM 3201 for channel 65 is 69 dB
That would make the channel 65 modulator signal at the antenna -75 dBmV (-124 dBm) with the TV port unterminated
and
-92 dBmV (-141 dBm) with the TV port terminated.
Attached Images
File Type: jpg Adding a Channel with a Modulator.jpg (69.8 KB, 236 views)
File Type: jpg Blonder Tongue HAVM-1UA_1.jpg (79.1 KB, 228 views)
File Type: jpg How to Use a CM3201 Filter.JPG (62.0 KB, 233 views)
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Last edited by rabbit73; 13-Jun-2019 at 11:02 PM. Reason: Added measurements
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