VHF vs. UHF at long distance
 

My house happens to be on high ground on a line between Lexington, KY and Cincinnati, OH so that the TV towers are 180 degrees apart & I can get signal from the front and back of a rooftop Winegard GS1100 Sensar antenna (www.winegarddirect.com/pdf/spec_SENSAR.pdf) with a Pico Macom LA2150 amp (http://200.78.236.213/specs/LA-2150.pdf) at the antenna.

Here is my TV Fool report: http://www.tvfool.com/?option=com_wr...c77a4300c8c715

The Lexington stations around magnetic South are fine, and I reliably receive WCPO & WLWT (magnetic North give or take a degree, and 60 miles away). I'm also getting WKON (317 degrees). I am on the edge of receiving WPTO and WXIX (Dishnetwork ViP722 tuner shows signal in the 60s, but drops in and out).

I'd like to be able to see WKRC [channel 12], but I've never gotten a hint of it. Is there someting about VHF that makes it harder to receive than UHF?

Thanks in advance,

Shorty

There is more RF noise on the VHF bands, especially the low VHF band (channels 2 through 6).

However, in your situation, there are two stations transmitting on channel 12, WKRC and WYMT. The GS1100 has little if any ability reject the signal from WYMT when it is aimed to receive WKRC. It has a front to back ratio of virtually 0 dB.

A Winegard YA-1713 or an Antennacraft Y10713 would have much better forward gain and rejection of signals from the rear. Your TVF report predicts a noise margin of 5.1 dB for WKRC, add the 10.0 dB forward gain of the YA1713 and you are at 15dB NM... that's pretty good if there was no co-channel interference. The predicted receive power for WKRC is -85.8 dBm vs. -103.2 dBm for WYMT, a difference of 17.4 dB. Given the published F/B ratio of the YA1713, I expect the YA1713 should provide you with a difference of 36 dB between the desired signal and the interfering signal.

If you really want to see WKRC... the YA-1713 should do the job. For situations were even more rejection of signals from the rear is needed, stagger stacking is a useful technique.

Agreed. But WKRC would be the only channel 12 that you will see unless you buy two identical antennas and stagger stack them by 14.26" http://www.anarc.org/wtfda/stagger.pdf or 43" apart horizontally stacked. http://www.hdtvprimer.com/ANTENNAS/ganging.html

Thanks for the insight.

Is there a rule-of-thumb number for how many dB difference in signal you need to avoid interference problems?

- Shorty

The exact value is 15.91 db, but that assumes that there is no noise at all. With both noise and interference figure roughly 18 db.

Is that 18 dB above the strongest co-channel interference?

.

No, all sources of noise add together... Co-channel interference is continuous and spans the entire spectrum of the broadcast channel, making it more problematic. But random noise from any other source (including amplifiers and receiver front-ends) will contribute to the problem.

If you build in a cushion, you are more likely to be happy with the resulting service... If you build to the bare minimum, expect unreliable reception.

It depends on the NM value of the stations.

A NM of more than 30 db means that the difference between stations can be as little as 16 db. A NM that is a minus number would require an NM difference of almost 19 db.

Of course propagation changes constantly, especially when the signals are 1 or 2 edge as they often are. So in a practical sense, the goal should be 10 db extra margin for reliable reception, or roughly 28 db.

Antenna height ?
 

On a related note, how reliable are the TV Fools signal numbers at different antenna heights?

For example, here are NM (dB) numbers at 5 antenna heights (same location) for a couple of stations about 60 miles away:

______________10'___15'____20'___25'___30' AGL
WKRC(#12 )____3.5___3.5___3.8___4.3___4.6
WLWT(#35)____2.3___2.0___2.1___2.4___2.8

It looks like I can get more improvement from changing antennas (gain) or pre-amp (noise figure) than I can from putting up a taller mast.

Any comments?

Also, from looking at which stations I receive and which ones are on the ragged edge, can I infer approximately how much gain I have in my current setup?

Thanks in advance,

Shorty

I trust the math... However, TVF says: I botched the math in my first post... making things look worse than they really are. The bottom line is, catch a usable signal with the right antenna mounted at the necessary height, then try to do the least damage to that hard won signal... amplify only when necessary, using the right amplifier. A Winegard YA-1713 mounted as high as is practical, aimed at 2° Mag. should see WKRC. If you need amplification to overcome distribution losses, an Antennacraft 10G201 has the desirable qualities of overload resistance, low noise and an FM trap.

Mixing WKRC into your existing system could be done with a channel 12 Jointenna, if you can find one. Warren Electronics may still stock some. Otherwise, an A/B switch may be the best option.

Your description of your situation in your OP is consistent with the predictions in your TVF report. Your existing antenna performance confirms the quality of the TVF predictions, but gives little insight regarding possible reception of WKRC. Rather than trying to infer gain from your reception, I think it is more accurate to look at the antenna. For VHF, it's obviously a dipole, which is going to have a nominal gain of 0 dB and a F/B ratio of 0 dB. It's not as obvious how the UHF section is configured, and the documentation offers no meaningful numbers, just an polar plot lacking any scale in the gain axis. (Mileage claims are the invention of the marketing department, not the engineering department, YMWV.) The specifications for your amplifier are quite clear, no inferring required.