OTA recommendations

[rickbb]

Originally Posted by rabbit73

I had thought that a Antennas Direct DB4e with the reflector removed might be worth a test. You would have to add a dipole for VHF channel 13.

I will try to find some antenna patterns; they are getting hard to find.

If the OP is handy with tools he might try to look into Mclapps 4 bay 10.5 x 9 design. I use this with no reflector and get channels from both front and back as well as VHF 11 from about 50 miles from the tower quite well. As a worst case he will have a 4 bay test antenna for less than $30.

[rabbit73]

Originally Posted by rickbb

If the OP is handy with tools he might try to look into Mclapps 4 bay 10.5 x 9 design. I use this with no reflector and get channels from both front and back as well as VHF 11 from about 50 miles from the tower quite well. As a worst case he will have a 4 bay test antenna for less than $30.

Yes, that is a good idea for a DIY antenna.



http://www.m4antenna.eastmasonvillew...com/index.html







Looks like the whisker length and vertical spacing of the bays should be increased for the new 14 - 36 UHF band.

[OTAFAN]

The links above and here to the Mclapp UHF antenna are not secure, but neither is TV Fool presently. Here's everything but the kitchen sink, including a YouTube video:

https://sites.google.com/site/maycre...setup/mclapp-4

https://www.youtube.com/watch?v=4dNP4vuQ9Z0

Outstanding work as usual, rabbit73!

All the best.....

[rabbit73]

Originally Posted by verder

I saw the antenna plans. I've actually considered putting together a Gray-Hoverman antenna. That might be a good Labor Day project!

You might need to do the GH with NARODS for PBS.

I'm considering getting either a cheap HDHomeRun or perhaps a Hauppauge TV tuner. These include signal meter, signal quality, and symbol quality information that are provided by GUIs / phone apps. Curious if others out there have gone this route and what signal information they've been able to use to their advantage.

I compared the two. Both work, but I prefer the HDHR.













Here is a calibration chart for the HDHR. 100% SS is 0 dBmV = -48.8 dBm. Actually it's calibrated in the factory for 60 dBuV = 0 dBmV. Antenna installers and cable guys in other countries use dBuV instead of dBmV.


Tuners can handle much stronger signals.

I'm currently in contact with Tablo support to see why I'm getting pixelation and failed recordings. I asked about their device providing real-time diag info. They said their engineers are investigating supplying this info, but there is nothing definite at this point.

I wonder if the Tablo tuner is as good as a TV tuner, especially with multipath caused by the trees. If you made a side by side comparison, you might need a low gain preamp before splitting; 100 ft of coax acts like a 5 dB attenuator for UHF. If you mount a preamp at the antenna, it should be a medium gain preamp resistant to overload.

[rabbit73]

This is a simulation of the DB4e antenna pattern with the reflector removed. Most other 4-Bay antennas would be similar:



Since the antenna is now bi-directional, the gain will be reduced by about 3 dB.

If you combine two CS2 antennas side by side, you get a CS4. The CS4 has more gain, but the beamwidth is more narrow; there is no free lunch.



Antennas Direct used to publish technical documents that showed antenna patterns. I couldn't find one for the C4MAX, so I had to create this simulation:



Since the C4MAX doesn't have a reflector, its pattern is bi-directional and the gain will be about 3 dB less than indicated for the C4.

[rabbit73]

Originally Posted by verder

I now have the antenna connected to a Tablo Quad (4-tuner) DVR. The problem I'm having right now is that I occasionally get dropped recordings for
channels
4-1
11-1
15-1
28-1

It doesn't happen all the time. I'm questioning if it is the Tablo or the antenna.

Channels checked:

4-1 (KRDK) COZI, Real channel 24, 324.8 deg True, Signal Margin 39 dB
6-1 (WDAY) ABC, Real channel 21, 306.1 deg True, Signal Margin 68 dB
11-1 (KVLY) NBC/CBS, Real channel 36, 331.6 deg True, Signal Margin 53 dB
13-1 (KFME) PBS, Real channel 13, 307.5 deg True, Signal Margin 59 dB
15-1 (KVRR) Fox, Real channel 19, 111.7 deg True, Signal Margin 56 dB
28-1 (K28MA) CBS/CW, Real channel 28, 331.6 deg True, Signal Margin 40 dB
30-1 (KXJB) CBS/CW, Real channel 30, 101.7 deg True, Signal Margin 60 dB

[rabbit73]

Originally Posted by verder

Yesterday I assembled an MClapp antenna out of spare 12-2 copper wire I had laying around. I also made a Gray-Hoverman w/NARODs (no reflector version) using 8 gauge bare copper wire I picked up at the Big Orange Box Store.

I haven't tried either one at elevation -- just in my backyard at ground level to see what SpectrumSpy reported. Nothing to write home about but it's more or less what I expected. The siding from my house + the siding from the shed + the trees + ground elevation all come together I'm sure to attenuate things.

Today's fun will be to see if either are suitable for attic installation. I'm using a simple wood "backbone" for assembly so neither would survive the elements. I'm also curious about how well (or horrible) an attic installation is. The thing about the attic is that somewhat near my network center where my Tablo is, so I'd be able to deal with a much shorter run of cable (not that my current run is all that excessive.) From there, I'll probably look into mounting things on PVC tubing and mount it near the location of the 4Max.

Last night, I also happened on discussion about the Antop AT-400BV. I think that would be an interesting antenna to try out. If it's a bust I could always return it.

Finally, I'm working with Tablo support on Tablo to see why my failed recordings are happening. I'm curious to find out what they say.

Sounds like you have some interesting experiments planned.

[rabbit73]

Originally Posted by verder

One additional note -- I wonder if the ClearStream 2Max would be another option in terms of beamwidth. From what I understand, it's wider than the 4Max. Maybe I bought "too much" antenna when I picked the 4Max? Then again, if I have potential issues with multipath interference, widening the beamwidth might induce other issues that the 4Max avoids? :P

The 2MAX does have a wider beamwidth than the 4MAX, and less gain on axis. It also probably has less gain than a 4-Bay.

[rabbit73]

Originally Posted by verder

It's not much, but one experiment I performed has been to find the pilot frequency for each channel using my Airspy R2 + SDR#. The Airspy has a wide enough bandwidth so I can see the "whole" signal at once. I used this to see what gain setpoints I needed to place that pilot frequency peak at -40dB to -45dB. The Airspy gain setpoints ranged from 3 to 13:

KFME: 6
KVRR: 7
WDAY: 3
KRDK: 8
K28MA: 13
KXJB: 8
KVLY: 10

I realize this doesn't show any sort of bit error info or whether I've got multipath issues going on, but I'm at least able to "see" things.

Thanks for help so far and relaying my posts to the thread!!!

Using the pilot is certainly helpful in finding a channel, but the strength of the pilot is not a reliable measurement for the strength of the channel. Multipath reflections can affect the strength of the pilot without making a comparable change in the strength of the channel.

Analog TV signals use a peak measurement of the video carrier, but the strength (actually power) of a digital TV channel is the average power across the whole channel.



If you look at your scan, you might be able to see that there isn't always a good correlation between the average power of the channel and the strength of its pilot, like KXJB vs KVLY, or KVRR vs KRDK.



I think a better indicator of the signal quality of a channel is the SNR of the channel implied by the scan when the gain of the of SDR# or Spectrum Spy software is properly adjusted for max SNR. Bring the gain up to the point just before the adjacent noise floor starts to rise.

Video of adjusting gain for CH 31
I should have shown some of waterfall so that the freq scale would show

https://www.dropbox.com/s/hzmz4mvte4...50-57.mp4?dl=0

https://www.dropbox.com/s/hzmz4mvte4...50-57.mp4?dl=1

CH 16

https://www.dropbox.com/s/0jlgno1gtj...02-31.mp4?dl=0

https://www.dropbox.com/s/0jlgno1gtj...02-31.mp4?dl=1

The difference between the adjacent noise floor and the average level of the top of the signal that you see on the screen is a fairly good estimate of the SNR. However, it might not be exactly the same as the SNR given by the TV diagnostics menu. It would be difficult for the TV to measure the level of the noise floor when the channel is in use. What the TV is giving is actually the MER (Modulation Error Ratio) which is derived from the constellation diagram. The MER includes the affects of impediments like multipath errors.
https://blog.solidsignal.com/tutoria...h-snr-ber-mer/

[rabbit73]

Originally Posted by verder

Not all tuners are equal
(If you are becoming tired of copying these PMs to the main thread, let me know and I can try bringing this conversation over to the AVS forums if you know of a good spot for them.)

It doesn't take a lot of time to post your messages, but it does take a lot of my time to think of useful answers. However, I find your reception problem interesting and I always learn something new when helping a poster.

I am willing to stick with it a little longer; it's kind of peaceful here. When I run out of ideas, it's time for you to go to AVS for some new ideas.

Today's experiments consisted of some simple bench top testing and some garage rafter testing. The bench top testing was literally bench top. I've got a workbench in my garage and just placed the Gray-Hoverman build facing due south. I did that to perform some sanity checking with the Airspy R2 on my build. Turns out that I've apparently got some issues with the R2 where I see spikes every 8MHz no matter what center frequency I'm at. These appear at a mid-gain setting. This happens with no antenna, with antenna, USB cable swapped, computer swapped, within SDR# and within SpectrumSpy. I've got a support request in to Airspy US to see what may be the problem... could be between the chair and the keyboard (ha!), but I don't think I'm doing anything different from before and the behavior is certainly different.

It sounds like RF Interference from a switchmode power adapter (SMPS) or a battery charger. The USB cable that I ordered with my R2 has two ferrite beads on it. Without a signal, I get some noise spikes when using my RTL-SDR dongle, but I don't let it bother me. The noise spikes are greatly reduced when a signal is present.

Unfortunately, the Gray-Hoverman frame turned out to be too big for my rafters, so I switched over to the Mclapp. I used a 50' dual-shield RG6 coax line to run from my rafters to a splitter on my workbench. The splitter feeds 2x identical Dynex TVs. One was set to the internal digital tuner and the other was connected to a cheap MediaSonic digital tuner via HDMI input. The Dynex TVs have a "Channel Strength" reading and the MediaSonic has a "Quality" reading that I referenced.

It turns out that Channel Strength and Quality don't amount to much. In the rafters with the antenna pointed towards the northwest towers, I'd have "Good" (highest Dynex reading) and would see pixelation / drop-out. With the MediaSonic quality almost always about 50%, I'd see it drop-out down to 0% (signal loss).

Testing involved tuning to each 'main' channel for 2 minutes (that is, I didn't bother checking any of the multiplexed channels from a particular tower). One minute was spent watching the channel, looking for signal loss, pixelation, or audio issues, and another minute was spent bringing up the Strength/Quality reading and looking for the worst case value over that period of time.

I had both TVs going simultaneously. One displayed the MediaSonic decoding and the other the internal TV tuner. (I realize the splitter produces -3.5dB loss, but I assume a 50ft run and nothing else wouldn't impact things too much and an amplifier was not necessary. Tell me if I'm wrong.)

Using a splitter is a good way to make a real-time comparison, but you have a signal loss of about 5 dB for the splitter and coax combined. That's too much loss; your signals are already too weak. Try a moderate amount of amplification before splitting.

Here's where things get... interesting. After about 15 minutes of watching KVRR/CH19 (I was sitting there having supper in my garage and decided to do an extended watch), I saw the MediaSonic drop out a few times. During this time the internal Dynex tuner did not miss a beat. The MediaSonic seemed to have trouble whereas the Dynex didn't. Hm. I watched a bit longer. I saw a couple of occasions where they both dropped out. Okay. That sucks but made sense. It seemed like the MediaSonic was a piece of junk where the Dynex dominated. I then changed the channel to KRDK/CH24. The reverse happened. The MediaSonic dominated and the Dynex internal tuner crapped out -- even so bad that I had to power cycle the television. On top of that, I saw both having issues -- but they were intermittent. It took ~15 minutes of watching for the thing to show up. I never caught these problems in my quick onesie/twosie test.

This means that testing could be a massive time-sink. The dropouts on KVRR/CH19 didn't happen right away. Things were great for about 15 minutes like I said and then they flopped out. To me this means that I could perform the antenna aiming and think that everything works -- only to come back later and see that things die for an inexplicable reason. Needless to say, this is a bit frustrating and deflating.

Most of your signals are marginal, which means they are very close to the digital cliff. It is true that all tuners are not equal, but it only takes a small difference in a signal to make a big difference at the cliff. Don't sweat these small differences; OTA signals are constantly changing in strength. You need more signal margin before dropout. Focus on making all your signals stronger as they come out of the antenna before any amplification.

I do plan to test out the Gray-Hoverman still (I shortened the frame after I finished my testing for the night), but I'm kinda thinking that what I'm getting out of the 4Max is probably as good as it's going to get. Yeah, it drops out every once in a while, but this is probably just a nature of the OTA reception? Without adequate tools to monitor the various signal components (strength / quality / symbol quality / bit error rates / etc.), getting a good signal from my location is a bit of crap-shoot.

The tools that you already have are adequate. You can measure relative signal strength and SNR. The HDHR GUI will tell you how close you are to the critical 50% signal quality point, and tell you when you have 100% symbol quality which means no uncorrected errors.

Use those tools to improve signal strength and signal quality.

[rabbit73]

Originally Posted by verder

Unfortunately, I do not own an HDHomeRun yet so I don't have access to that additional signal information.

Sorry, I got a little ahead of you. I like my HDHR4-2US (the gray one); I avoided the 4-tuner models.

The signal quality reading from your MediaSonic tuner should help. At what percent reading does dropout happen? That point should be similar to the 50% signal quality point of the HDHR.

I had an old ViewSonics amplifier lying around (VSAV10-1000-2WZ w/a 10dB forward gain) and used that with the Gray-Hoverman antenna I built. I had the antenna in the same location as Mclapp that I played around with yesterday. Unfortunately, the reception today was worse than yesterday with the Mclapp. I need to go back to the Mclapp build and try it with the amplifier and see if that improves anything. The amplifier is pretty old (~2001) and has been sitting in the garage as a leftover from the previous owners. I'll look at running that experiment tomorrow. If I pick up a new amplifier, do you have any recommendations about what I should buy? I see box stores with amplifiers that range from 4dB to 20dB gains. Not sure how to correctly gauge what I should be looking for.

That is a cable drop amp. If it's working OK, it should be helpful. It should be placed close to the antenna to compensate for the coax and splitter loss. The 7 dB noise figure is a little high. I use a CM 3410 or a CM7777HD Amplify for that kind of indoor testing. For outside close to the antenna I would use a regular preamp like an Antennas Direct Juice or a CM 7778 V3. Avoid the RCA TVPRAMP1R; the quality control is poor. I bought two new ones in sealed boxes; both failed.

It's important to have a preamp close to the antenna for the best system noise figure. The first device in a system primarily determines the system noise figure as shown by the Friis Formula:
https://en.wikipedia.org/wiki/Friis_formulas_for_noise

Do I understand you correctly; you have your test antenna inside?

That would mean you have
tree loss,
building loss,
cable loss,
and splitter loss
before the signals get to your tuner.

That's not too different than mine; I have an indoor antenna behind a tree and cable loss (25 ft), but my antenna looks out a window to reduce building loss. My signal report is similar, except there is just one direction:

https://www.rabbitears.info/searchma...tudy_id=145947

Today's experiments consisted of some simple bench top testing and some garage rafter testing. The bench top testing was literally bench top. I've got a workbench in my garage and just placed the Gray-Hoverman build facing due south. I did that to perform some sanity checking with the Airspy R2 on my build. Turns out that I've apparently got some issues with the R2 where I see spikes every 8MHz no matter what center frequency I'm at. These appear at a mid-gain setting. This happens with no antenna, with antenna, USB cable swapped, computer swapped, within SDR# and within SpectrumSpy. I've got a support request in to Airspy US to see what may be the problem... could be between the chair and the keyboard (ha!), but I don't think I'm doing anything different from before and the behavior is certainly different.

Regarding the Airspy issues: if I go to "Free" mode in SDR# and adjust the LNA Gain between a value of around 6-16 or so, I see the 'ghost' signal appear. Values outside of that range do not cause it to turn up. Jacking with the IF and Mixer gains don't cause any problems. Is it possible that the TV signals I was measuring the other day with SpectrumSpy did something to the LNA?

The signal would have to be extremely strong to damage your Airspy SDR. That's unfortunate that you are having trouble with your Airspy R2; they are expensive.

I don't completely understand your problem. Can you show us a photo?

Does your Airspy receive FM signals OK?

Have you tried a new download of SDR#?

Are you using an SMA to F adapter or an SMA to F adapter cable on the R2? I leave an adapter cable on all 3 of my SDRs to protect their SMA input connectors. If I just used a small metal SMA to F adapter for the RG6 to connect to the SDR it might weaken the SMA connector.

[rabbit73]

Originally Posted by verder

Here are a few screen caps: https://imgur.com/a/mIFfUUQ

Gain @ 8 -- no ghost
Gain @ 9 -- ghost signal appears to the right of an FM signal
Gain @ 9, shifted center frequency -- ghost slides along with the shifting.

Thank you for the images and the detailed explanation.

On SpectrumSpy it appears as a ghost signal that appears spaced at 8MHz intervals.

I assume Airspy support is out of the office today due to the holiday. Hopefully I hear back from them this week. I guess the device has a one year warranty so I may not be totally hosed:

Airspy Warranty
Airspy warranty is limited to 1 year from the date of delivery for purchased units that fail under normal operating conditions. Airspy units will be repaired at a price of $50.00 plus $10.00 shipping/handling. Normal operating conditions do not include excessive mechanical stresses due to mishandling, dropping, or bending; mechanical or electrical modifications; Electro-Static Discharge (ESD); excessive RF signals; excessive thermal stresses due to improper ventilation or cooling, submersion in any fluid; and any unauthorized repairs.
All warranty claims will be determined by Airspy or their representatives.

It appears to be an artifact that only shows at certain gain settings. When an SDR is designed, there are many stages. The goal of the design is to balance the gain of each stage so that they all stay within their normal operating range at all gain settings.

That is why it is so difficult to have an SDR dB scale with absolute instead of relative values. The only SDR that has an absolute dB calibration that I know of is the SDRplay, and that is only under certain conditions.
https://www.youtube.com/watch?v=MRR-x_TjOp4

This my test of the SDRplay calibration:



I wasn't able to duplicate the artifact problem:

https://www.dropbox.com/s/mj126w9mt2...37-37.mp4?dl=0

https://www.dropbox.com/s/mj126w9mt2...37-37.mp4?dl=1

I notice that you have added the SNR meter plugin from GitHub. Did you have the artifact problem before you added the plugin?

Disregarding the curious ghost, does your R2 otherwise function properly?

Your R2 might be defective, or it could simply be a firmware issue.

We are both using the same software version, but since I bought my R2 first, the firmware version might be different. Airspy support would know what firmware version you have if they know what R2 you have.

There is a way for me to find out what firmware version I have, but I don't want to use a command prompt on my R2. Since my R2 seems to be working OK, I might mess it up.
AIRSPY R2 & MINI Software Defined Radio Firmware Update Procedure
https://www.youtube.com/watch?v=cMm7vC4gklA

Google search:
airspy R2 firmware

My Airspy R2 just bricked, no idea why...any suggestions for bringing it back?
https://www.reddit.com/r/RTLSDR/comm...o_idea_whyany/
click on VIEW ENTIRE DISCUSSION for this:

melvin2204
RTL-SDR v3, Airspy Mini, R2, HF+ Discovery0 points ·
11 months ago
· edited 11 months ago

It's called 'The Ultimate VHF/UHF RX “Brick”' for a reason. Jokes aside, you might have more luck contacting @lambdaprog on Twitter. He partially runs Airspy.

level 2
byf_43
1 point ·
11 months ago

Haha, yeah I wasn't aware of that less than stellar reputation. I did get an initial email back from Airspy's customer service, but I have a feeling I won't have this up and running by tomorrow.

Do you have any suggestions for another SDR that might be more robust than the Airspy?

Originally Posted by verder

Adapter

Also -- I'm using a SMA <--> F adapter cable. So, no "hard" connection from the Airspy to the coax going on.

Good

[rabbit73]

Originally Posted by verder

This is interesting. Tablo support got back to me and indicated that the signal strength from my antenna may be too much for my Tablo. They recommended I turn my antenna away from the towers. I'm not entirely certain this is the correct course of action.

I asked if I could just put a splitter in-line to drop things if the signal is too strong. I went ahead and put in an Extreme BDS103H splitter with the -3.5dB output connected to the Tablo. I'm still receiving channels after performing a new scan so it didn't kill anything.

If that's not enough, and this truly is the problem, I can switch over to the -7dB port and see if that fixes things. I also have an Extreme BDS102H splitter that I assume I can daisy chain with the BDS103H for a max drop of -10.5dB.

That IS interesting. How did they know how strong your signals are, did you give them a signal report?
https://www.rabbitears.info/searchma...tudy_id=143302





Turning the antenna away from the transmitter direction is one way to make signals weaker, but it tends to pick up multipath reflections which will increase errors. A better way is to use some type of an attenuator if you actually have signals too strong for a tuner. Longer coax or splitters will do fine.

WDAY RF 21, your strongest signal, would be the most likely signal to cause overload. But I never thought it would be a problem because you already have attenuation from the trees and the long coax when the antenna is outside. When the antenna is inside, you have even more attenuation.

A signal report is only a computer simulation which is often not very accurate, especially for 2Edge and Tropo signals. It assumes your antenna is in the clear, and doesn't account for coax loss.

So, it was a good idea to make the tests that you did to eliminate that possibility.

A rabbitears.info report lists a Signal Margin for a channel; it's similar to Noise Margin on a TVFool report.



Add the antenna gain and any preamp gain to the Signal Margin/Noise Margin when you look at the chart.

Interpreting Noise Margin in the TV Fool Report
http://www.aa6g.org/DTV/Reception/tvfool_nm.html

When you add an attenuator, it will make your strongest signal weaker, but it will also make your weakest signals even weaker by the same amount. If your weakest signals are already marginal, they could completely drop out.

When you have very strong signals and very weak signals, there is a great difference between the two. The difference between the strongest signal and the weakest signal is called the Dynamic Range, which is measured from the top of the strongest signal to the top of the weakest signal. To that, you must add 15 or 16 dB to allow for the necessary SNR of the weakest signal above the noise floor. This is called the SFDR (Spurious Free Dynamic Range.

Your goal is to make all the signals strong enough so that even the weakest can be decoded by the tuner, but not strong enough for the strongest signals to cause overload. Overload from two or more strong signals can create spurious signals from IMD (Intermodulation Distortion). The spurious signals will come up from the noise floor and reduce the SNR of the weakest signals.

[rabbit73]

Originally Posted by verder

With a Tablo, the support folks apparently can log in remotely and check signals reported by the device itself. Unfortunately, this signal information is not exposed to end users.

Yes, unfortunate. If it had a GUI that gave that information, more peole would buy it.

I asked if it were possible to give me access to the insides and let me know what I need to target for signal levels. I doubt they'll go for it, but it never hurts to ask.

Indeed.

To give you a feeling for signal levels, here is your report in dBm:
https://www.rabbitears.info/searchma...143302&opkey=C



And these are the specs by ATSC:

ATSC Recommended Practice:
Receiver Performance Guidelines

Document A/74:2010, 7 April 2010

RECEIVER PERFORMANCE GUIDELINES

5.1 Sensitivity

A DTV receiver should achieve a bit error rate in the transport stream of no worse than 3x10E-6 (i.e., the FCC Advisory Committee on Advanced Television Service, ACATS, Threshold of Visibility, TOV) for input RF signal levels directly to the tuner from –83 dBm (-34 dBmV) to –5 dBm (+44 dBmV)for both the VHF and UHF bands.

5.2 Multi-Signal Overload

The DTV receiver should accommodate more than one undesired, high-level, NSTC or DTV signal at its input, received from transmission facilities that are in close proximity to one another. For purposes of this guideline, it should be assumed that multiple signals, each approaching –8 dBm (+41 dBmV), will exist at the input of the receiver.

Presumably for Two Strong Input Signals.
Max Signal Input and Output
would need to be DERATED:
3 Signals: -3.6 dB
4 Signals: -5 dB
5 Signals: -6 dB
6 Signals: -7 dB
8 Signals: -8 dB
Ignore signals more than 6 dB below the Strongest.

So, you see that -22 dBm for WDAY even before adding the antenna gain is getting close to -8 dBm, which is why Tablo support was concerned.

Majortom and 905shmich have worked out a way to get more info from the HDHR GUI using Linux, but I'm satisfied with what it gives me already.





The DHC forum software has changed; new link for that post:
https://www.digitalhome.ca/threads/h...4/post-2478858

The signal strength never goes above 0 dBmV, no matter how strong it is, the Signal Quality never goes below 41, and the SNR never goes above 33.

I assume the same thing could be done with the Tablo.

[Tower Guy]

Overload can get very tricky. When calculating the cumulative level of multiple signals you add the peak voltages together (not the RMS) to get the peak voltage, and then calculate the peak power from the combined peak voltages.

For instance, the peak power of two signals of the same amplitude but different frequencies is 6 dB higher than one of the signals. It is this higher peak power that can overload the tuner.

[rabbit73]

Originally Posted by Tower Guy

Overload can get very tricky. When calculating the cumulative level of multiple signals you add the peak voltages together (not the RMS) to get the peak voltage, and then calculate the peak power from the combined peak voltages.

For instance, the peak power of two signals of the same amplitude but different frequencies is 6 dB higher than one of the signals. It is this higher peak power that can overload the tuner.

Good point.

I would hope that the engineers that wrote the ATSC specs took that into consideration, but maybe they didn't.

This part was added by holl_ands, who does modeling and is an RF engineer, so I took it on faith because I'm not smart enough to verify it:

Presumably for Two Strong Input Signals.
Max Signal Input and Output
would need to be DERATED:
3 Signals: -3.6 dB
4 Signals: -5 dB
5 Signals: -6 dB
6 Signals: -7 dB
8 Signals: -8 dB
Ignore signals more than 6 dB below the Strongest.

The rabbitears.info report is only a computer prediction, but we see the reality in the spectrum scan done by verder. Unfortunately, the dBm calibration is not absolute, but we can see the relative powers of the channels. He would need a signal level meter to measure the actual powers of the channels.





The power level of WDAY implied by the scan and shown in the signal report looks like it is close to the limit that could be tolerated by the tuner.

Verder gave me his address in a PM, so I was able to look at the satellite vertical view that shows trees in the signal paths from both directions that would attenuate the signals. I was also able to look at the ground photos that confirmed the trees were higher than the antenna, but he asked me not to show those images in this thread.

That's what led me to the conclusion that overload was probably not a threat unless he added a high gain preamp. I think that the intermittent pixelation that he has on the marginal channels is not from fundamental overload or IMD spurs, but from insufficient SNR of the weak signals and multipath from the signals passing through the trees. Both of those can produce errors that exceed what the FEC can correct.

This is what a signal looks like when it is going through trees on a very windy day:

https://www.dropbox.com/s/ig3va499k2...Path4.mp4?dl=0

https://www.dropbox.com/s/ig3va499k2...Path4.mp4?dl=1

In any event, verder made the correct test by adding attenuation which showed that the pixelation continued, so it probably isn't caused by overload.

A more certain test would require a single-channel custom filter that would attenuate WDAY, but that would be expensive.

[rabbit73]

Originally Posted by verder

I'm going to show my ignorance here because I don't fully understand what you were pointing out with your last post. You say "you see that -22 dBm for WDAY before adding the antenna gain is getting close to -8 dBm". I don't understand (intuitively) how these are close. Given that dBm is a logarithmic scale, aren't these relatively far apart?

I owe you an apology. I have been dumping a lot of technical information on you without knowing your background, but you seemed to be interested in learning more.

I should have shown the math to support my statement.

I'm at a loss (ha!) at how to correlate from the antenna gain to the levels you point out from the ATSC requirements. The 4Max has these specs: "UHF 11 dBi / VHF 2.5 dBi". If WDAY (ideally) has a signal power of -22dBm, then how does the antenna gain affect what is ultimately fed into the tuner? It's not as simple as adding the two values is it? 11dBi + (-22dBm) = -11dBm? That's getting somewhat close (3dB), but I think I'm horribly conflating things at this point.

It is pretty much that simple. I'm used to using the TVFool reports where Noise Margin (NM) 0 dB is for a dipole antenna. The strength of a signal is shown in two ways on a TVFool report: NM or Signal Power. You can add antenna gain to either one. See this FAQ:
https://www.tvfool.com/?option=com_c...ask=view&id=57

Antenna gain is the only quantity that should ever be ADDed to the NM value. Most antennas will specify their gain in dBd or simply dB, and this is the value that should be used. If an antenna's gain is specified in dBi units, then you need to subtract 2.15 in order to get the equivalent value in dBd units.

More recently, the TVFool reports have become less accurate because they use a database with many errors to generate reports. Trip (Trip in VA on the AVS Forum), who is the webmaster for rabbitears.info and now works for the FCC, decided to create the Signal Search Map to generate reports. Trip asked me to be a beta tester, so I was able to make a few comments that I hoped were useful.

The rabbitears reports are now our first choice and I assume I can also add the antenna gain to the dBm value or the Signal Margin value.



Antenna gain 11 dBi - 2.15 = 8.85 dBd (in reference to a dipole)

WDAY -22 dBm + 8.85 dBd = -13.15 dBm, which is weaker than -8 dBm

I don't quite understand what the multi-signal overload text is stating. Is it saying the more strong signals you have, the more you need to derate these strong signals, and the recommended amounts they need to be derated at? 8 strong signals = derate by 8dB, because otherwise the tuner is overloaded?

The ATSC Guideline says that a tuner should be able to tolerate a signal as strong as -5 dBm. Where there are multiple signals, the tuner should be able to tolerate them if they are no stronger than -8 dBm, which is weaker because we are in negative values.

The derating chart after that is not part of the ATSC Guidelines. It came from holl_ands who is a member of the AVS and DHC forums, is an RF engineer, and does antenna computer modeling. The derating chart says that the signals must be even weaker if you have more of them.

Does all of this come together to indicate that maybe it would have been better if I had gone with the ClearStream 2Max instead? It has the specs: "UHF 8.7 dBi / VHF 2.6 dBi". All of this of course assumes that the signals are indeed too strong for the Tablo.

The CS 2MAX does have less gain than the CS 4MAX, so there is less chance of overload if that really is a problem with the Tablo. The advantage of the CS 2MAX for you is its wider beamwidth because your two directions are not 180 degrees apart. I have heard reports that the TiVo and HDHR don't tolerate strong signals as well as the average TV tuner, but I haven't made any tests to confirm that.

I have made overload tests on tuners. The first one I did was from ignorance. I connected two preamps in series because I thought more gain is better; I didn't get any signals at all.

The more informed tests that I made later showed that the strongest signals created IMD (Intermodulation Distortion) products called spurious signals (spurs) that wiped out the weakest signals. As I increased the gain even more, only the strongest signals were left, and finally even they were gone. I was working in the range of -5 to +5 dBm. I didn't want to go any higher because it might damage my tuner.

THREE TYPES OF OVERLOAD

There are three types of preamp or tuner overload, in order of increasing signal strength:

1. The strong signals almost cause enough intermodulation distortion (IMD) to interfere with the reception of weak desired signals, but the spurious signals are at or below the noise floor of the weak signals. This is the point that holl_ands uses in his preamp charts to obtain max SFDR (Spurious Free Dynamic Range). No damage will happen.

As the strongest signals continue to increase in strength, more of the weaker signals are damaged until you reach:

2. The strong signals cause overload to the preamp or tuner that makes it impossible to receive any signals. No damage will happen. The strongest signals are still there, but they can't be decoded because the IMD products have damaged them so that they contain more errors (high BER....bit error ratio/rate) than can be corrected by the FEC (forward error correction).

3. The signals are so strong that the input transistor is toast. You are not likely to encounter OTA signals that strong, unless you live next door to a high power transmitter and you have your high gain antenna aimed at the transmitter's antenna.

As a general rule, tuners can tolerate stronger signals than preamps before overload. The difference in strength is approx. equal to the preamp gain.

[rabbit73]

Originally Posted by verder

I ran the RG6 to a cheap MediaSonic digital tuner I picked up from Amazon that was connected to a TV via HDMI. I used that to get a rough signal quality reading and also would watch one channel on each carrier frequency for a minute or two to look for drop-outs or pixelation.

Originally Posted by verder

You asked in the thread about Tablo vs the TV tuner. I've only really used the TV tuner to do the original setup (finding the sweet spot.) From that point it's been pretty much using the Tablo exclusively. I have not used the MediaSonic TV tuner for an extended period of time.
That said, I'll do some experimentation and some longer viewing with it on the channels I've had issues with to see if there's anything unique with that (drop-outs or what-have-you.) Thankfully the Tablo has been recording what station, day, and times the problems are occurring. I'm starting to record that info in a spreadsheet to see if I can see a pattern or anything emerge.

Originally Posted by verder

I'm considering getting either a cheap HDHomeRun or perhaps a Hauppauge TV tuner. These include signal meter, signal quality, and symbol quality information that are provided by GUIs / phone apps. Curious if others out there have gone this route and what signal information they've been able to use to their advantage.

Originally Posted by verder

Unfortunately, the Gray-Hoverman frame turned out to be too big for my rafters, so I switched over to the Mclapp. I used a 50' dual-shield RG6 coax line to run from my rafters to a splitter on my workbench. The splitter feeds 2x identical Dynex TVs. One was set to the internal digital tuner and the other was connected to a cheap MediaSonic digital tuner via HDMI input. The Dynex TVs have a "Channel Strength" reading and the MediaSonic has a "Quality" reading that I referenced.
It turns out that Channel Strength and Quality don't amount to much. In the rafters with the antenna pointed towards the northwest towers, I'd have "Good" (highest Dynex reading) and would see pixelation / drop-out. With the MediaSonic quality almost always about 50%, I'd see it drop-out down to 0% (signal loss).

Originally Posted by verder

Unfortunately, I do not own an HDHomeRun yet so I don't have access to that additional signal information.

Your comments about the Mediasonic converter box made we wonder about how it would compare with the HDHR at weak-signal dropout.

Tyler the Antenna Man did two reviews about converter boxes, the Mediasonic Homeworx HW-150PVR and the Ematic AT103B:
Mediasonic Homeworx DTV Box with DVR Review - Record OTA Antenna TV
May 14, 2020
https://www.youtube.com/watch?v=6LO7h1YIMv4

Ematic AT103B Digital Converter Box + DVR Recording Tutorial & Review
Dec 21, 2018
https://www.youtube.com/watch?v=a9KUAjjcDww

The GUI for the Mediasonic looks a lot like the GUI for the Channel Master 7003. I have a 7003, so I compared it with my HDHR4-2US using a variable attenuator and a splitter on my best channel (RF 16). I increased the attenuation to bring it down to dropout for the 7003:

                                         7003 > TV
                                          /
Ant > Variable 0-10dB > Fixed 20dB > Splitter 
        Attenuator      Attenuator        \
                                         HDHR > Laptop

The variable attenuator was set at 6 dB, so the margin to dropout for the 7003 was 26 dB for channel 16. The signal quality varied from 39 to 41%. The HDHR tuner was a little more sensitive; margin to dropout was about 28 dB.

Using a converter box and a variable attenuator is almost a useful as the critical 50% signal quality point of the HDHR.

I found this on eBay; had to add BNC to F adapters:



I have some fixed attenuators like this; they can be stacked
1-3dB
1-6dB
1-10dB
2-20dB



https://mjsales.net/products/fam-att...ant=1083705673

https://www.newark.com/webapp/wcs/st...4899&mf=106654

[rabbit73]

When digital TV arrived, I was looking for an inexpensive way to measure the strength of digital TV signals. Signal level meters are made for the task, but they cost more than most people want to spend.

I found this UK site that showed how to use inexpensive attenuators to make comparative signal strength measurements of digital TV signals using the concept of margin to dropout. It's the same link as in my signature:
http://www.me galithia.com/elect/aerialsite/dttpoorman.html

I found two Channel Master slide-switch attenuators, each with a 3dB, 6dB, and 12dB switch. With the proper settings I was able to get 0, 3, 6, 9, 12, 15, 18, or 21 dB for each attenuator; the total for both was 42dB.



The next device I found was the Apex DT502 converter box. It has two scales, one for signal quality and the other for signal strength.



Using my attenuators I was able to get readings for a channel as I brought it down to dropout. Obviously, the greater amount of attenuation needed to bring the channel to dropout, the stronger the signal. Since dropout is at about -85 dBm for most tuners, I was able to calculate the strength of the signal by adding the attenuation to -85 dBm.





Later, if I wanted to compare my calculated strength with a measured strength, I could use my Sadelco 719E signal level meter. Once I knew the strength in dBmV, I could convert it to dBm by subtracting 48.8:



and I was also able to calibrate the DT502 signal strength scale:



You can do the same thing with just your TV tuner. Bring the signal to dropout with an attenuator and add the attenuation needed to -85 dBm; that is the strength of the signal. This is most accurate on UHF. On VHF, the noise level is higher, so dropout will be above -85 dBm.

[tripelo]

Originally Posted by rabbit73

...
When you have very strong signals and very weak signals, there is a great difference between the two. The difference between the strongest signal and the weakest signal is called the Dynamic Range...

This topic probably could be in another thread.

Would like your thoughts on dynamic range of Airspy vs. SDRplay RSP1.

Rabbit73, do you see differences in instantaneous dynamic between the two scanners, for:

- Wideband (several channels displayed)
- Single channel (display maybe 8-10 MHz or so)

No hurry on response. Do not wish to derail current thread.

Thanks