HDTV Forum

This is perhaps too detailed, but I have a mystery on my hands and I will try to give you the facts. I just installed a Channel Master 7777 pre amp and my signal strength went down. Any help appreciated as I am stumped. As they say "the devil is in the details"

I have posted on several threads on my new OTA antenna for my DirecTV HR 10-250 for local digital channels. I need to pick up one VHF channel 8 from 66 miles away(and 1000 ft higher than me) while getting 4 UHF channels (18, 26, 38, 44) from 8 miles away-line of sight and all within a 40 deg azimuth. I replaced an RCA UHF corner reflector(from Best Buy) that was working OK on the UHF with a Channel Master 3677 Ultra Hi-Crossfire VHF/UHF combo on the same sturdy tri-pod based mast on our patio roof which puts the antenna about 15 ft. up. We have 60 mph winds here so I have not gone higher

When I got it installed I was pleased that all 5 channels locked in and maybe I should have stopped there. I found channels 26,38 and 44 were above 80 on the strength meter, but channels 8 and 18 were in the 64-70 range, just above the 60 where drop outs begin to get serious. On rotation, I found about a 10 degree angle where readings did not go higher. So I re-connected my 2 yr old Channel Master 7777 and signals on everything dropped. Thinking it was defective I bought another one.

Much to my surprise the new 7777 behaved the same way. Channel 8 dropped to 5-15 and channel 18 dropped to 30-40- ( the others dropped just a little) I tried rotating the mast and found no improvement. Also replaced the cable on the antenna to the preamp- no change.

Thinking of tuner overload, I installed a 10db F connector attenuator between the indoor preamp power box (to TV outlet) and the input to the HR10-250. All the readings came back up to about the same as with no pre-amp at all. I have a set of these attenuators so I tried 3,6,10 and 20db drop. All worked except the 3 db which improved channel 8 a little up to the 40 range, at 10 db it was up to 68-70. But no combination is better than no pre-amp at all.

My guess: I think I am dealing with trying to pick up digital stations that are still at very low power in a sea of high power analog signals, and the HR 10-250 can't handle the adjacent channel interference. Ken helped me with this a couple of years ago (thanks again) on the channel 18 case where analog channel 19 is much more powerful. In the case of channel 8 ( KFFX-DT), their high powered analog channel is on channel 11-KFFX Pendleton OR. I thought a 3 channel separation would be enough. They are our FOX affiliate. However I should add channel 26 has their high power analog on channel 25 and they come in the best with a signal above 90 in all tests.

Am I correct the problem is a power ratio problem and a pre amp just boosts both the low power digital channel AND the high power analog overloading the tuner and the AGC is lowering the gain? I understand FCC is going to require full digital power in July 2005 for the top 100, markets, but July 2006 for me.

Just wait until 2006?

Don, are you sure that you have the VHF option inserted in your 7777? If, in fact, your channel 8 is an actual channel rather than virtual, you need to make sure the switch is thrown the right way. The switch is in the antenna side enclosure.

Yes, I opened it when I had the trouble, to verify the switch was in" VHF/UHF combined" and used that input on the outside. Even cycled them a couple of times to be sure they were "in". The FM trap is "in". This model has a VHF only switch and input but I have not tried it.

Yes the channel is actually 8 and is remapped to channel 11-1 to match the analog channel 11 logos. I measure the signal strength on channel 8. The HR 10-250 will do an auto local digital channel search on 2-65{?}. it goes "boing" when it finds one and puts it in the channel list. It found channel 8.

I also just connected the antenna with the pre- amp on and no attenuator on the antenna 2 input of my Toshiba HD rear proj. monitor using the built in NTSC tuner. Got a beautiful channel 11 picture(analog) coming from the same tower 66 mi away- no ghosts. I read somewhere to check this for multi-path. I carefully checked white letters on black during a couple of ads. All other local UHF analog also good, but I had forgotten how poor analog is on sharpness. Love HDTV!

For any others with OTA antenna problems you should be aware of this statistic from Gary Shapiro of CEA at the 2004 HDTV Summit in April:

"two-thirds of the 1,175 local stations currently broadcasting a digital signal are operating at reduced power, which shrinks their signal coverage area."

http://www.crutchfieldadvisor.com/ISEO- ... 1&page=all

It is possible to find the "authorized power" of most stations, but very difficult to find what they are actually doing. Thus problems may not be with your OTA antenna system, but just low, and possibly changing, broadcast power.

Don: You definitely have an overload problem. Channel 11 is not causing it. Channels 25 and 26 might be. If a close FM station is strong enough, you might need two FM traps.

Try this experiment: Open up the 7777 and switch it to

Ken,

Thanks very much for the ideas. It will be a while before my son-in-law, who can do the high ladder work, will be back. I will report back as soon as we can make the test. (I have been grounded with an inner ear balance problem for a few years)

There are NO other VHF channels within 160 miles except 11, but FM is a possibility. Plus I am within 8 miles of 5 analog UHF stations in the multi 100Kw power range.

The system is working on all 5 digital channels locked OK with a 10 db attenuator. Also I get KFFX on the DirecTV locals package and the digital KFFX-DT is just SDTV anyway, but I need to solve this so I will know if they ever go HDTV. Keeping hope alive for the Super Bowl.

I remember your comment about "if there is a signal out there you can get it" I sort of feel that way about solving this problem. It is there so I want it.

I did e-mail the station yesterday and asked about both power and HDTV.

One question- the UHF section connects to the antenna terminal with two aluminum strip loops were the transformer wing nuts are. Would it be useful just to disconnect this UHF section for a test? Easy to do.

I appreciate your expertise.

Don

Don: If the VHF and UHF are connected in parallel then that might work. But a lot of antennas connect the VHF and UHF in series (with the VHF going through the UHF antenna), and in that case you can

Ken,
Thanks. The VHF has 3 dipoles insulated from the beam connected together with zig-zag strips, the shortest one (near the front) then connects to the common terminal with two rods about 18 inches long forward. The UHF has two U shaped strip loops about 12 inches long bending backward to this same strip which has the balun . Looks parallel to me. This is of interest since if my problem is UHF overload I am going to have to find a way to boost VHF and attenuate UHF.

In any event I will try ALL the tests and should be able to report back in a couple of weeks. My son-in-law's job has an every other Friday off and this is his best time to help.

A related issue -- channel 18 which is also suffering the same reduction with the pre-amp on . This one I know more about. Analog channel 19 is 490,000 watts. Their digital channel 18 is published at 36.4 Kw on the same tower 9 miles away. I wrote them several times, finally to the Corporate President in Seattle( they also own the ABC affiliates in Seattle and Portland and many others across the NW), and finally got a letter from their chief engineer that I was right ( I had passed on your technical input). Nobody had considered adjacent channel interference and they had bought a digital transmitter with a 150 watt(!!!) input- just to get started which "was enough to cover their main service area"( with FCC approval). He didn't know when they could afford more power . This is for a CBS affiliate station. I have since upgraded my DirecTV receivers twice and both had better selectivity so I can pull ch 18 in weakly(64-68 on the meter) with drop outs every 10 minutes or so. I still think they are running this "light bulb" transmitter. It is this token compliance with the digital transition that has me frustrated and my rants about the FCC not being serious in getting digital running for everyone nationwide.

KFFX-DT has only been running a couple of months and may be low power too- they got multiple FCC extensions and I suspect they also are at low power. I am just feeling deja vu all over again on this channel 8 struggle.

Thanks again. Will be back later.
Don

Don- A 3-element LPDA seems pretty small for 66 miles. You might consider getting something bigger and connecting it to the VHF input of the 7777.

For your UHF stations at 8 miles line-of-sight, a paper clip is probably enough antenna. I would suggest getting the dinkiest outdoor antenna you can find and connect it to the UHF input of the 7777. If it is not small enough you might need an attenuator between the antenna and the 7777.

If you feel you can stay with your current antenna, there might be a way to separate its VHF feed from the UHF feed. Then you can attenuate the UHF input without affecting the VHF.

Winegard sells a dual tunable UHF trap, model UT-2700. It can be adjusted to give 15dB attenuation for a single frequency. (I acquired one to test, but my test equipment isn

Ken,
An update. Thanks for your help- FM overload is involved.

I got a FM trap at Radio Shack and inserted it in the line between the 7777 TV out and the receiver. It worked the same as the 10 db attenuator- e.g. signal strength on ch 8 up to 68-71. Would a mast mounted FM trap on the preamp input be better? Who makes one?

A complication. Before I changed anything I found there was no picture or sound on ch8, but the signal strength meter showed the receiver was getting something. The meter would go to 70 for 3 seconds, drop to zero with message "no digital signal" then pop back to 70- cycling like a clock. My guess- their transmitter is on, but they have no picture data inserted. All 4 of the UHF digital stations were normal strength with picture and sound.

I have written to them to ask.

Also I got a reply from their chief engineer:

"Thank you for emailing KFFX with your DTV signal strength. This type of
information really helps us in serving our viewers.

At this time KFFX DT is broadcasting at low power. This will continue foe
several more years. The signal is transmitter from a mountain top above the
Spout Springs ski area southeast of Walla Walla. This could explain the
signal which is lower than the other local stations except KEPR.

As far as broadcasting in HD, this too will take several more years to
accomplish.

Please feel free to ask questions anytime, and thanks for watching KFFX!"


With low power expected for "several more years" picking this up from 66 miles will be a challenge. Also since there will be no FOX HD here for "several more years" I can keep the their analog channel 11 which is in the DirecTV locals. Looks like no real solution until 2006 when FCC has ruled these stations must go to full power or 2007 when DirecTV adds 1000 HD locals. . However FCC has been quite liberal in granting extensions on deadlines- so even 2006 may be optimistic.

No Joy!:x