Cold Weather losing UHF Stations

[jerrymc]

I seem to recall someone here that had this issue.

Channels are good during the day. We had a couple evenings when temps dropped in the 40s. All UHF Stations dropped completely. As soon as sun arose and got a bit warmer, stations returned.

We had no issues with U-VHF Stations.

Could this be a bad CM7777 Preamp?
Or is this simply an environmental issue based on our location?

Thank you for any light on this subject.

[jerrymc]

I guess no one has ever heard of this?

Today it's about 65F and sprinkling rain.
I have bypassed the CM7777 pre-amp, and have lost all UHF signals but KNBC-LA.

I only have a dist. amp in the attic now; No pre-amp at the antenna outside.

I suppose I will need 2 antennas just to get both UHF and H-VHF stations when the weather gets below 70F ...
even though I have a rotor motor..

[jerrymc]

Sorry, we only have the KVCR's and KNBC's now. (it's about 60F and drizzling rain)

Everything else is gone...

Again, no pre-amp, but only a dist. amp in attic. I even swapped out the dist. amp for a Motorola one with the same results...

I just can't figure out if I should junk this YUGE antenna and get 2 separate ones...

[Jake V]

Here's a link to your earlier thread: http://forum.tvfool.com/showthread.php?t=16082

It's best to keep all your questions in a single thread so we can see what's been discussed earlier.

Here's a link to your TV Fool Report: http://www.tvfool.com/?option=com_wr...e2cb74ea324603

And your set up: HD8200XL with CM7777 pre-amp aimed at Mt. Wilson (Los Angeles) in Etiwanda area.
CM3410 dist. amp in attic for a 5 room hookup.

Located in foothills behind Mt. Baldy, and experiencing signal loss and pixelation depending on time of day.
Using RCA rotator motor now to tweak.

---

And you have mountain peaks between you and Mount Wilson. Time for some study. Be patient. Others will post.

[jerrymc]

Thanks Jake.

I figured this was not cell interference since practically all stations dropped when the temps dropped, hence new thread.

I bypassed the CM7777 pre-amp just to be sure it wasn't faulty when temps drop, but the symptom remains.

Interesting to note I've lost most H-VHF stations as well, or at least they are unwatchable...

The sun is out this morning, so I'll check it this afternoon if it warms up outside.

[rickbb]

I'm guessing lose or corroded connections.

Metal shrinks when cold making a lose or corroded connection worse. Sun comes out, heats it up swelling the connection making it tighter.

Could also be something inside the 300 to 75 ohm transformer doing the same thing, if you have an external one.

[jerrymc]

Originally Posted by rickbb

I'm guessing lose or corroded connections.

Metal shrinks when cold making a lose or corroded connection worse. Sun comes out, heats it up swelling the connection making it tighter.

Could also be something inside the 300 to 75 ohm transformer doing the same thing, if you have an external one.

The cable from antenna is a short homemade quad-shield that I personally made. The connecting coax is pre-made dual-shield going directly to attic running approx. 30ft before into the dist.amp.

Two weeks ago, I ran a coax directly from the antenna to one TV.
(No pre-amp, dist. amp, splitters, etc...)
Early evening I had a few (H-VHF) stations completely dropping out , but all UHF seemed OK. The only thing I can think is that the temps would drop from 90+F to 60s.

The balun was included with the antenna; could these actually go bad or be faulty under cold temps?

Thank you for your input!

[Nascarken]

Yes that' could be varrey TRUE I have read some thing like THAT happens TACK an ohm meter and test it out it should be 75,ohm

[RF Steve]

The 75 ohm specification on an impedance matching transformer is 75 ohms impedance. Not 75 ohms resistance, and cannot be measured with a simple ohm meter which is only capable of measuring DC resistance.
On most antenna systems you will find nothing that measures 75, or 300 ohms DC resistance.
An ohm meter can still be a useful tool for testing coax end to end for continuity, and shorts.

[Nascarken]

Originally Posted by Nascarken

Yes that' could be varrey TRUE I have read some thing like THAT happens TACK an ohm meter and test it out it should be 75,ohm

Sorry about that You want to test the continuity on the coil first do the -SIDE first and then test +SIDE ok steven

[jerrymc]

OK...I'm pretty sure I found that it was in fact the balun 300 ohm crappy leads.
I barely unscrewed the first lead and it simply broke right off.

I took the balun and tested continuity between the metal fork lead and wire and it was iffy; IE: I had to press pretty hard with my VOM leads to get any reading.

So, I decided to cut and replace the end leads with the round type ones and tin and solder the wires inside, rubbed some dielectric gel on all metal, and wrapped them nicely with elect. tape.

My UHF stations are all back and it's 66F outside tonight; granted only a couple weak stations are out, but that's normal for me behind this damn mountain.

I'll wait for tomorrow's weather as we're expecting rain this weekend and possibly another thunderstorm...

THANK YOU, THANK YOU, THANK YOU, FOR EVERYONE'S HELP!

[rabbit73]

Originally Posted by Nascarken

Yes that' could be varrey TRUE I have read some thing like THAT happens TACK an ohm meter and test it out it should be 75,ohm

You were correct, Nascarken. That was a good idea to suspect the balun.

Sometimes an ohmmeter can help, sometimes not. Not all baluns are wired the same way.

Originally Posted by holl_ands

Modern Baluns typically use PAIRS of wires wound either in air or on Ferrite Cores as Transmission Delay Lines. PCB Baluns do the same thing via sinuous circuit board etches and "Coax Baluns" via different lengths of Coax. These Delay Lines have different delays so that when summed together they "Transform" from one impedance to another (unfortunately......it's all in the math).

OUTDOOR TV Baluns usually have DC Continuity between the 300-ohm wire(s) and 75-ohm outer Coax connector in order to discharge the static electricity that builds up on the (ACTIVE ONLY) Antenna Elements due to wind passing across them (like shuffling your feet across a carpet)....which will ALSO protect the Balun against voltage arc-over, thereby avoiding user complaints. Whether there is DC Continuity between the two 300-ohm twinleads depends on choices by the design engineer such as how to use those pesky frequency compensation capacitors...or not...and the chosen interconnection between the Delay Lines.

Since the ACTIVE Elements are NOT connected to the grounded Mast, the leakage path through the Balun is the ONLY way to discharge static electricity buildup on those element. The PASSIVE Elements can (Optionally) be connected to the grounded mast, such as in typical Yagi designs, but if they "float" it's usually unlikely they will spark over due to the distances involved....probably...YMMV....

I as far I as can determine, the Philips and Channel Master OUTDOOR Baluns are known to be high performers....and indeed have DC Continuity between 300-ohm wires and 75-ohm outer Coax connector.

http://www.avsforum.com/forum/25-hdt...l#post23942435

[rabbit73]

Originally Posted by jerrymc

OK...I'm pretty sure I found that it was in fact the balun 300 ohm crappy leads.
I barely unscrewed the first lead and it simply broke right off.

Good detective work, jerrymc!

[jerrymc]

Originally Posted by jerrymc

OK...I'm pretty sure I found that it was in fact the balun 300 ohm crappy leads.
I barely unscrewed the first lead and it simply broke right off.

I took the balun and tested continuity between the metal fork lead and wire and it was iffy; IE: I had to press pretty hard with my VOM leads to get any reading.

So, I decided to cut and replace the end leads with the round type ones and tin and solder the wires inside, rubbed some dielectric gel on all metal, and wrapped them nicely with elect. tape.

My UHF stations are all back and it's 66F outside tonight; granted only a couple weak stations are out, but that's normal for me behind this damn mountain.

I'll wait for tomorrow's weather as we're expecting rain this weekend and possibly another thunderstorm...

THANK YOU, THANK YOU, THANK YOU, FOR EVERYONE'S HELP!

Well, I spoke too soon about the balun being bad...

When the temps dipped to below 50F, almost all channels would disappear again!

I contacted Channel Master, even though my 90 day warranty was over, and their suggestion was "signal overload."
I'm no engineer, but how in the hell can their be a good signal at 60F+ temps as opposed to signal overload at 40F?
One other thing I noted, in the evening KVCR (a local station) would also drop out COMPLETELY.

So, I ordered another $22 RCA preamp TVPRAMP1Z from Walmart.
Replaced the CM-7777 with the TVPRAMP1Z on Tuesday. VIOLA!
Not only did I get all my channels back, I now receive about 15 more!

Right now it's 36F outside and ALL CHANNELS ARE PERFECT!

While the CM-7777 was still installed, during a cool day, I aimed my thermometer at it from the upstairs bedroom and noticed the left side of the box was 10F colder than the right side, even though the sun was not shining on the box. (Maybe the warmth power on the left side?)

So my only guess is either a cold solder on the board or a bad component...?

I recently read a similar story on Amazon's reviews...

"These amps fail in cold temps, I install professionally and right now I have three for three failed. They said they fixed the problem but they did not. I will no longer use these amps." Markus Kemp, Amazon reviewer Oct 23, 2016.

Any thoughts???

[rabbit73]

Thank you for the report about your experience with the CM7777 and the Amazon review.

PCT International bought out Channel Master, fired their best engineers, and closed the North Carolina plant. Then they had the gall to redesign the 7777 and give it the same model number to trade on the excellent reputation of the original.

Glad you figured out what was wrong.

[ADTech]

I'd be most inclined to agree with their support that signal overload was in play. Signals per TVF report are far, far too strong for that amp.

The RCA can handle signals almost 10 dB stronger at its input than can the 7777 at the same level of distortion.

[WIRELESS ENGINEER]

Got to agree on the overload problem
The 7777 is for extreme fringe areas so the 7778 would have been a better choice
30 db gain is far too much gain for most installs and the 8200 is also for extreme fringe areas not line of sight to a mountain top station so it is likely contributing to the overload

[rabbit73]

Overload is very possible as a factor in the reception problem; the math supports that theory.
Channel Master says this in their description of the 7777:

This antenna amplifier is a high gain, low noise preamplifier used to allow weaker signals at the antenna to be amplified to a viewable strength, and is ideal for installations in which all broadcast towers are located at a distance of more than 80 miles.....The Titan 2 High Gain Preamplifier is recommended for professional installers only. Due to the high gain output of this product, it can result in over amplification if not used in the appropriate scenario. Over amplification can cause issues with the television tuner’s ability to receive and display some or all channels.

http://www.channelmaster.com/TV_Ante..._p/cm-7777.htm

OTA signals are constantly changing in strength. I have measured changes of over 10 dB in 30 minutes. Atmospheric changes can cause changes in signal strength, which might account for day/night differences if you are close to overload. The direction of the antenna is certainly a factor.

KVCR has a signal power of -29.6 dBm = 19.4 dBmV
Max input of 7777 is 15 dBmV
You are already at preamp overload before adding antenna gain.
19.4 dBmV + 12 dB Ant = 31.4 dBmV; with antenna aimed at KVCR

If you add the preamp gain, the tuner has a problem:
KVCR -29.6 dBm + 12 dB Ant + 30 dB preamp = +12.4 dBm; tuner overload

A tuner is not designed to handle signals stronger than -5 dBm.

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.

Looking at it from the standpoint of Noise Margin:

KVCR has a NM of 61.2 dB. If you add the antenna gain of 12 dB, that brings you up to 73.2 dB, which is overload territory.



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

[jerrymc]

I should have changed my headline to read "UHF AND H-VHF."

Interesting to note that in colder weather, KABC and KCAL both begin dropping signals. (Even with the RCA preamp)
KABC is the lowest channel frequency out here at 174~180Mhz and KCAL at 186~192Mhz. KTTV and KCOP are good though...

Well, so much for testing for a couple days....we had a MAJOR wind storm Sunday and the pole loosened and spun a few circles, breaking the coax from the RCA input port...

On a note about signal overload; I did remove the dist. amp in the attic, but I lost some signal on some stations. (pixelation)

A few weeks ago I did a couple antenna direct to one TV test. (No preamp, splitters, etc..)
The channels were good during the day, but again, some would drop in the evening, hence I reinstalled the CM7777.

I opened up the CM7777 tonight, but cannot see anything obvious, except for some sloppy solder work...but I need to check the opposite side of the board with a mag glass because my eyes are bad...

Thanks to all. This is great information!

[stoxguy]

Jerry. You are NOT in the twilight zone about losing ota-hd signals during temperature drops. I cut the cord from Cromcrap 3 years ago. I bought a new antenna and it is in my living room facing the bay window. NYC is 20 nautical miles away. I live in central Jersey. During the day I have 40+ channels. Both VHF and UHF. During the summer ALL channels will be received. Now that winter is here, I get most stations during day. Once the temps drop at night, I start to lose signals and I am really getting PO'd adjusting the antenna every day. There is another problem I have read about. The FCC always some UHF and VHF stations to operate at lower power. That happens with me here on channel 11 WPIX NY. My antenna is hooked direct to my TV using rg6 cable. When I can do some more research on this I will post an article. You are not alone with the cold weather phenomenon.