VHF-high in the L.A. foothills: strange reflection?

[John Stimson]

Duplicate post.

[John Stimson]

Early last year, based on a tvfool report for my location, I chose a CM-2016 antenna to receive the TV stations on Mt Wilson (approx 300 degrees azimuth, 10.5 miles distant). Everything was peachy; I could receive KABC, KNBC, KCBS, KCET, KTTV, KOCE, KDOC, KTLA, and a plethora of foreign language channels. After a few months, KNBC became marginal then disappeared. I also had trouble with KTTV.

Just the other day, I added a 4 foot section to my antenna mast to raise the antenna, in the hope of getting KTTV and KNBC back. I did get those stations, but lost KABC.

By accident, I discovered that I got a very strong signal from KABC with the antenna mast lying on the roof so that the antenna was pointing up at about 20 degrees from vertical. With the antenna mast erect and the antenna boom parallel to the ground, the signal strength of KABC is reported as zero in the channel diagnostic screen on my TV.

Does anyone have a guess as to why I lost ABC when I lengthened the mast? Or why I get good signal aiming the antenna almost straight up? Or what I could do to get it back?

Possibilities: The resistance from the antenna boom down to the ground wire is about 1.5 Ohms. The mast is a rusty 2-piece unit with the top section that slides onto the bottom section. I could probably decrease the resistance of that path by cleaning all the corrosion off the contacting pieces. Is ground resistance of the magnitude of 1 Ohm significant for this application?

Being channel 7 and at the low end of the designed operating band of the antenna, reception is going to be marginal and I might get better results with an antenna that covers all of VHF, perhaps even an indoor antenna?

Would angling the antenna boom upward help, since the broadcast antennas are on the top of a mountain? If so, is there readily available hardware to change the elevation angle?

Original installation (estimated 18 feet above ground)

Raised by 4 feet

[mtownsend]

By clicking on any of the transmitters in your report, you can see a cross-section view of the terrain between you and the transmitter. The transmitter is shown on the left, and your location is on the right, like in the following view of KABC:

KABC and the rest of the Mount Wilson transmitters are bending over the tops of the mountains to reach you, so yes, the strongest signal can be found by pointing your antenna at the nearest mountain ridge rather than into the side of the mountain.

However, I think the bigger factor is your current antenna. The CM-2016 is not a very powerful antenna. It only has 2 long elements at the back, which means it's not very strong on VHF stations (ch 2-13) like KABC. I think that if you had a better antenna, you would not have to worry so much about raising your antenna. The change in height makes very little difference in your predicted signal strengths, yet a change in antenna performance might improve your net signal strength by several dB. Your focus should be on getting a better antenna.

[John Candle]

The Winegard DS-2000 , J pole mount has U bolts so as to clamp the foot mounting to the top of a mast and then a antenna to the J pole. The J pole can be adjusted to different angles so as to point the antenna at the top of mountain to receive the defracted signal. http://www.winegarddirect.com , http://www.amazom.com

[John Candle]

See what you can do with the CM 2016. You can hold the antenna in your hand and walk the roof looking for a good hot spot. A better antenna will be a Winegard HD7696P antenna.

[Tower Guy]

Originally Posted by John Candle

A better antenna will be a Winegard HD7696P antenna.

I agree. The CM 2016 has little or no front to back ratio and is susceptible to multipath.

[John Candle]

For more mounting ideas , the Winegard DS-2016 has holes drilled in both ends of the J pole so the J pole can be mounted either end in the mounting foot.

[John Stimson]

Thanks everyone -- I did not realize that the CM-2016 is omnidirectional in the VHF-high range. ChannelMaster doesn't make it easy to find their spec sheets. It looks like the CM-2018 and 2020 aren't all that directional in VHF-high either. I will check out the Winegard antennas.

[John Stimson]

Looking at the Winegard 7694p compared to the 7696P, it looks like there is a roughly 2dB difference in signal gain, with similar front to back ratios. Do I want all the gain I can get, or would those two antennas be equivalent for my situation? The local antenna shop has the 7694 in stock, and I would have to order the 7696.

[John Candle]

Try the HD7694P.

[John Stimson]

After wandering the roof with the CM-2016, it seems that the original location is the best place for UHF reception. I lowered the antenna to the original height and get all of the UHF channels I want as well as ABC strongly enough to not glitch.

There was a spot elsewhere on the roof where I got several stations very nicely with the antenna down close to the roof, but could not receive some of the weaker stations. Hoisting the antenna above about 4 feet caused the signal strength to drop off on all channels.

Anyway, the only stations I can't receive right now are on actual channel 9, 11, and 13. Along with the weak signal for channel 7, those are all VHF-high stations. Apparently the CM-2016 is not so good for VHF-high reception, as you have told me. I'll be ordering a Winegard -- $58 for a 7694P or $84 for a 7696P...I may go for the 7696P just to be sure.

[John Stimson]

I replaced the CM-2016 with a Winegard HD7694P today, and checked the relative signal levels.

There was a massive improvement on real channel 7 (SNR increased 11dB), I now get real channel 9 with excellent SNR where I couldn't get it at all before, and I can barely get real channel 13. Still can't get real channel 11. SNR stayed the same or slightly improved in the UHF range as well, with most of the improvement showing up in real channels 28-36.

I could try wandering the roof again with this antenna, but the current location did seem to be the best overall with the CM-2016. Worth trying?

[No static at all]

I would certainly experiment with various heights & walking the roof.

I would also try an FM trap to see if that helps with the problem VHF stations. Your local Radio Shack carries them & you can return it if it doesn't help.

Let's see what others have to say.

[John Candle]

Try tipping the front of the antenna up to look at the ridge of the mountain. The J pole mounting is a way to mount the antenna in a nose up position.

[John Stimson]

The J-pole mount came in a separate shipment. The elevation of the highest crest in the direct line to the Mt. Wilson antennas is about 7 degrees. I set the J-pole to provide 7 degrees of elevation and mounted it to the top of the existing 6-foot mast with the antenna mounted to the top of the J-pole. Not much difference. I lost channel 13, which was weak. Several other channels lost around 2dB SNR, and one already strong station gained 3dB. So I took the J-pole out. Given that the J-pole is painted, I doubt that the antenna boom was electrically connected to the mast, as it was when it was mounted directly to the mast. Would that matter?

I won't be able to wander around the roof looking for hot spots until this weekend when I will have an assistant to watch the signal meter on the TV.

[GroundUrMast]

Originally Posted by John Stimson

... Given that the J-pole is painted, I doubt that the antenna boom was electrically connected to the mast, as it was when it was mounted directly to the mast. Would that matter? ...

Very unlikely... the antenna is horizontal, the mast is vertical, there is very little coupling or interaction between the working elements (which are insulated from the boom) and the mast.

Ditto the suggestion to experiment with changes to mounting location and height.