Reception in San Francisco

tomteriffi · 16-Mar-2015, 11:40 PM

I live on Castro St @ Market in SF. I'm using 4 HD HomeRun tuners connected to WMC box. I'm 1.1 mi. from Sutro Tower and reception here is horrible. I'm using three antennas(2 point in other directions) because if I point my 2 bay bowtie directly at the antenna only the weakest stations are watchable. If I turn it 40' left then some weakers go away but ch5 (strong) comes in. Turn 10' more and 5 goes away and 2 (strong) comes in. Attenuators (antennas direct model 1296f and some german FR 1 FL units) help fine tune some stations once the antenna is positioned but as some stations come in better others fade away. Has anyone else delt with this problem in SF and if so how did you do it? Any help appreciated. Thanks...Tom

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GroundUrMast · 17-Mar-2015, 1:34 AM

We have seen several reports, from various cities, for locations that are very close to extremely strong signal sources as shown in your reception prediction.

The levels shown in this case call for 20 to 40 dB of attenuation between the antenna and tuners input. I've had experience with locations such as transmitter sites, where RF tight enclosures were required to house and shield electronic equipment from the RF coming off the transmitting antenna. I've got several HDHR tuners, but have never faced intense RF levels that caused interference due to direct penetration from the air into the tuner.

What happens when you disconnect the coax from the tuner input? If you still see very high signal strength indicated by the HDHR utility then you may need to look into RF shielding for the tuners, over and above the attenuation of the signal from the antenna(s).
http://forum.tvfool.com/showthread.php?t=820

I would expect best results will occur when the antenna(s) are aimed at the source (the TV transmitter antenna) to receive the highest quality signal possible and then attenuated to a level that does not overpower the tuner(s). The idea is that if you aim off to the side, you're are going to receive more multipath interference which lowers the quality of the signal. A high powered low quality signal is no better than a low powered low quality signal as far a the tuner is concerned.

For others reading this thread, this is an illustration that 'high signal strength' is not synonymous with 'high signal quality'. Poor reception can be the result of too little signal, but excessive signals levels can also result in poor reception due to overloading the tuner. A common, but often erroneous, assumption is that if a signal is 'weak', amplification will cure the problem. Only if the 'weak' signal is of adequate quality could you expect amplification to help, and then, only if there is substantial loss in cable and splitting between the antenna and tuner, or possibly a poor quality tuner.

ADTech · 17-Mar-2015, 2:02 AM

The signals on the ground are far weaker than the TVFool simulator estimates because it doesn't take into account the vertical elevation pattern of the transmitting antenna(s). At 1.1 miles from Sutro, the main signal lobe is probably more than 1500' above his head at Market and Castro. As a result, the TV signals are likely anywhere from 10 to 40 dB below the forecasted levels. FM radio signals , OTOH, are a different story. and are more likely to cause issues than the TV signals from Sutro.

tomteriffi · 17-Mar-2015, 4:17 AM

You both make good points. I've tried pointing the antenna direct at the transmit antenna using the 1296f attenuator. Channels that didn't come in without the attenuator still didn't no matter what atten. setting. Some channels that did come in already improved up to about a third of atten. knob rotation. Past that they degraded again.
I disconnected the cable from one of the tuners and used the HDHR Config GUI to run through the channels to look for signal. Signal strength showed about 25% at the lo end and 35% at the upper end. Should I shield them? I might just give it a try anyway. Can't hurt.
I know I'm getting alot of multipath. I have a single fam house and all around me are taller multi fam homes. There are tall trees between me and the transmitter. I know it's there but I can't see it.
I had not considered elevation of the transmitting antenna. CH 4.1 KRON never comes in well regardless of what I do and it's a fairly strong station.
Just to give some background. I've tried all kinds of antennae. MOHU Leaf connected to TV only gets about 10 stations well. Set top ant.(rabbit ears and loop) is worthless. Put the MOHU on the roof, still no good. Tried one of those 3"ant. that come with USB tuner sticks, even worse. Tried using a single UHF loop like the kind that attach to the back of an old TV, nadda.
I've been into electronics all my life. CompSi Degree and have a decent bench in the garage. I'm used to measuring things. On a scope, a meter, a logic probe. I can't see what's in the air and it s*cks

ADTech · 17-Mar-2015, 12:37 PM

Buildings and trees can take another 10-40 dB off the signal power, depending on individual circumstances.

A spectrum analyzer is the preferred instrument for working with and examining RF signals, but they're not inexpensive. However, there's a used Sencore 1454 on ebay that can be had for under $250 assuming that it works as promised. There's also a 1476 SLM that's available for under $300 but its description leaves me unimpressed. Since you have a solid computer background, an computer-based SDR (software-defined radio) via USB port is extremely inexpensive but slow. Look for some posts from fellow site member "Pete Higgins" on that particular subject.

The 1296F attenuator, now discontinued, works backwards from what is intuitive. Turning the control fully counter-clockwise is minimum signal (maximum attenuation) and fully clockwise is maximum signal (minimum attenuation).

tomteriffi · 19-Mar-2015, 8:34 PM

Thanks for the reply. Sorry for the delay, it's been a bit busy around here.
I hadn't really considered going the spectrum analyzer route. I guess I was hoping for some magic bullet "just do this and you'll be fine" solution. I figured how the 1296F worked a while back. I opened both it and the german model just to see what type of circuit was used. They're both the same except the German model has a 5 turn pot. I will look for Higgins and see what I can learn that will help resolve this issue. I can always "borrow" a password from someone but I just don't want to do that. I will repost as things progress. Thanks for the help.