Apparent severe multipath issues?

kb2fzq · 1-Apr-2010, 11:15 AM

Hello all,
Here's my TVFool report:

http://www.tvfool.com/?option=com_wr...b7c8bf10412194

I would like to ask a question about a scenario that happened this weekend, I'll set the stage.
A new transmitter has come on line here as of late on RF 18, running 15 KW, with a side mount antenna on the southwest corner of the tower (180° away from me). It is a dupe of RF 12, but would be handy as the VHF RF 12 station very rarely has issues, but is frustrating when it does. Reception wise, I have good days and bad days on RF 18, but I can't explain the following...maybe one of you can.
This last Saturday, all afternoon, clear skys, I watched the golf match on NBC RF 18 at a 30-32% SS no dropouts. Saturday, I was dead on pointed at 185°, not 186°, not 184°, I had to point DIRECTLY at the tower, maybe because of the phased log periodics having a narrow lobe, I don't know, but it was working well, none the less. No reception from 204° at that point.
Sunday for most of the afternoon, same thing, but as the rain approached from the southwest, the signal began to fade at 185°...as the heavy rain began here, drop outs abounding at 185°, I finally said "the heck with this" and turned the beams to the Heilderburgs, (where the local antenna farm is located, 204°, and RF 12) not changing the channel yet to NBC RF 12,
As I hit 204°, boom, the RF 18 signal went to 45-47% SS, and remained there until bedtime, and was there same SS in the morning, heavy rain still falling. That's a 19° beam heading change away from the RF 18 tower....
This is the third rain event that I've noticed this, it happened with our last heavy snow as well. It didn't become clear until Sunday that it was even happening. I actually watched it happen in front of my eyes.
I expect this is all a multipath issue of some kind, rain fade and all that, but what could the signal possibly be bouncing off of at 204°, and why only during rain? And why so strong on UHF only in the rain? VERY weird....the rain is gone now and last evening, RF 18 returned to the usual hit and miss in clearing skys...
I hope one of you might be able to explain this...maybe it will help me change something here to make the 45-47% SS stick permanently on RF 18. I've been contemplating raising the whole array up 2 feet (coax length restrictions) but I'm not really sure if that would even help at all....
Any help would be greatly appreciated...

Tigerbangs · 1-Apr-2010, 11:46 AM

What are you using for an antenna??

Dave Loudin · 1-Apr-2010, 1:36 PM

Couple of other questions:

1) How does RF18 map into your reciever? 13.1? 13.something else? 18.1?

2) Your TVFool plot shows repeaters for WRGB and WNYT at almost due west from you that should be pretty strong. Have you seen these, yet?

To quickly asses potential gain/loss from changing antenna height, use the the "start MAPS" option on TVFool. Note that the reception predictions update dynamically when you change the value from 10, so try different values and see what you get.

kb2fzq · 2-Apr-2010, 10:45 AM

Yes, I guess some more info on the antennas and system might help....sorry about that...
I am using a pair of VHF-hi/UHF Digiwave ANT2075 log periodics, phased at the RF 12 wavelength frequency (as I remember, 32 inches, I believe). The pre-amp is a 30 db AntennaCraft 10G212. Present height to top antenna is 15 feet.
The 2 lower antennas are RF 13 and RF 6 antennas, elements cut to receive there respective frequencies only. These were problem stations initially. The 3-way combiner that is hard line coupled before the pre-amp is a Channel Vision HS-3, the two cut antennas use an HLSJ to merge the Hi/Lo signals, and all interconnecting coax are exactly 2 feet long. The ferrite chokes are an experiment to reduce what appeared to be cell tower interference, 1/4 mile away from me, (that has apparently worked), as the interference/pixelation on a certain channel has stopped.

Regarding RF 45 and RF 39, this is the short comings of TVFool, if a transmitter is on FCC dockets, it's considered up and running. Both of these transmitters are not even on the mountain yet, RF 45 is scheduled for September/October of this year, RF 39, because of station bankruptcy issues, is still in the works.
Let me leave this info, and let you all re-diagnose my issues. I hope this info will help.
Just as added info, all local stations 53 miles out south come in at a 70% SS or better (as well as the 2 stations 91 miles out to my north, WPTZ and WCAX, in Burlington, VT), except, of course, my current issue with RF 18.
Oh, the mapping...18 HD maps as 18-3, 18 WX maps as 13-3 and 18 radar as 13-5 (I think, have to look again). The main WNYT station subs are 13-1, 13-2 and 13-4, respectively. I use an NOC 26" HDTV.

Tigerbangs · 2-Apr-2010, 2:09 PM

I swear that I've seen that antenna array before: don't you post in High Def Forum, too?

Tower Guy · 3-Apr-2010, 10:44 AM

Quote:

Originally Posted by kb2fzq

I would like to ask a question about a scenario that happened this weekend.

Have you considered interference/overload, perhaps from an FM station, that varies depending on your antenna aiming?

Your preamp has significant gain and has no filtering nor preselection. I like the Winegard AP-8700 better. It's really two amplifiers in one package, one for VHF and one for UHF. Each preamp has a filter to minimize overload from out of band signals.

kb2fzq · 4-Apr-2010, 10:59 AM

Quote:

Originally Posted by Tower Guy

Have you considered interference/overload, perhaps from an FM station, that varies depending on your antenna aiming?

Your preamp has significant gain and has no filtering nor preselection. I like the Winegard AP-8700 better. It's really two amplifiers in one package, one for VHF and one for UHF. Each preamp has a filter to minimize overload from out of band signals.

Yeah, I've considered that and a million other possibilies, The FM trap changes nothing, I've dropped the gain on the amp down to 10 db, I just lost the signal completely doing that, the thing that bugs me is the SS changes in the rain, I would think I'd loose signal strength, not gain it in another direction in a rain storm.
The only thing that might be causing this is a very, VERY large oak tree in my direct path at 204° about 75 feet away from the antennas, this tree not causing any problems winter OR summer on all the rest of the channels, but may be a giant reflector on RF 18 when it gets wet...that would be the only possible cause I can fathom. Beyond that tree, there is no large structures or mountains in the way to the transmitters, the path goes over the Hudson Valley, it's all down hill from me. And I'm very likely aiming under the lower branches anyway with the top antenna only 15 feet up, it's a big tree, so I don't know.....
What do you think T.G.?

For those that want to take a direct look, here's lat/long:

lat 43.325412

lon -73.570225

dbsanfte · 4-Apr-2010, 8:37 PM

There's nothing that could be causing your multipath interference, eh? Nothing that could possibly be a reflector?

What about those 3 antennas below in your setup? Their reflecting elements are indiscriminate.

(For instance... the chandelier in my living room reflects RF 22 so well that I keep my indoor UHF antenna pointed towards it to catch a problem channel.)

Take that specific antenna off the boom and try it mounted somewhere else, I bet your problems will clear up.

kb2fzq · 5-Apr-2010, 7:47 AM

Quote:

Originally Posted by dbsanfte

There's nothing that could be causing your multipath interference, eh? Nothing that could possibly be a reflector?

What about those 3 antennas below in your setup? Their reflecting elements are indiscriminate.

(For instance... the chandelier in my living room reflects RF 22 so well that I keep my indoor UHF antenna pointed towards it to catch a problem channel.)

Take that specific antenna off the boom and try it mounted somewhere else, I bet your problems will clear up.

Yes, I could do that, but that's the rub, do I start dismantling the array to attempt to receive this one problem station (that is a duplicate of RF 12 anyway), while jepordizing the roughly 35 other channels that are fantastic? I don't think I want to go there...there's been too much work and fine tuning done on that setup to disturb it now...if RF 18 can't coexists as things are, I will just have to give up the ghost chase. I made some minor changes yesterday and may have secured a marginal, but stable RF 18, only time will tell....

kb2fzq · 9-Apr-2010, 10:39 AM

The question was why WNYT RF 18 only comes in strong here in the rain and snow (and now add heavy fog to the conditions where the signal is UP)
It has come to my attention that WMFP RF 18 in Boston (Lawrence), Ma. is running 1000 Kw of power and their directional antennas have a strong lobe in my direction. Being WNYT RF 18 is on the fringe here with their 15 KW, even a slight atmospheric enhancement allows WMFP to interfere with WNYT RF 18. Pointing the antennas at Boston last nite showed strong broken activity on the signal strength meter in that direction, as there was an obvious enhancement of SS on the other Albany, N.Y. stations. At that time, WNYT RF 18 was unreceivable.
The rain and snow (and now fog) apparently act as a blocking agent, reducing the WMFP signal enough to allow WNYT RF 18 to be received at 45-47%. This morning, as the heavy rain moves east, the WNYT RF 18 signal is, as expected, 45-47% SS.
Your comments are welcome....

Dave Loudin · 9-Apr-2010, 3:28 PM

Aha! I bet you're onto the answer. Combine tropo enhancements with antenna gain factors against the two stations (non-intuitive pointing leads to best combination of gains to favor WNYT) and I think you've got it.

kb2fzq · 10-Apr-2010, 10:07 AM

Quote:

Originally Posted by Dave Loudin

Aha! I bet you're onto the answer. Combine tropo enhancements with antenna gain factors against the two stations (non-intuitive pointing leads to best combination of gains to favor WNYT) and I think you've got it.

Yeah,Aha! It was right in my face, I just had to open my eyes.
Now, how does one deal with this issue? Filters are out. I dropped the gain on the 10G212 gradually, hoping the local station might have enough Umpf to overcome WMFP...nope. WMFP is just a hair shy of locking here, the meter rocks back and forth
And with summer coming fast and furious, the enhancements will be stronger and more numerous. In a heavy duct, I may just be able to receive WMFP, which is cool on one hand, but a PITA on the other hand. Or not, as the signal from Boston has got to be pinging off of all kinds of mountains in the the path. MULTI-PATH CITY!!!
Any suggestions as to a direction I might go to correct this? (Yeah, I know, I'm a dreamer, ain't I)

Tower Guy · 10-Apr-2010, 12:08 PM

Quote:

Originally Posted by kb2fzq

Any suggestions as to a direction I might go to correct this? (Yeah, I know, I'm a dreamer, ain't I)

You can offset your two antennas to the left and right of the mast. The offset distance would be calculated to make the distance 1/2 wavelength at the angle toward WMFP when the antenna is aimed at WNYT-18. The antennas would still be in phase on the main beam.

There's also a channel 18 in St. Johnsbury, VT. How sure are you that the interference is caused by WMFP?

What's your goal with reception of 18? Are you having problems with channel 12? (The ERP on channel 12 will be increasing slightly.)

kb2fzq · 11-Apr-2010, 11:27 AM

Quote:

Originally Posted by Tower Guy

You can offset your two antennas to the left and right of the mast. The offset distance would be calculated to make the distance 1/2 wavelength at the angle toward WMFP when the antenna is aimed at WNYT-18. The antennas would still be in phase on the main beam.

There's also a channel 18 in St. Johnsbury, VT. How sure are you that the interference is caused by WMFP?

What's your goal with reception of 18? Are you having problems with channel 12? (The ERP on channel 12 will be increasing slightly.)

OK, since a spec analyzer is not in hand, I am only going by my signal meter. I have pointed towards St. Johnsburg, and looked for some signal on 18, WVTB is only running 61 KW, if I remember, and I see no noticable influence on the meter, on 18, in that direction. Towards WMFP (121°), as I said above, the meter is all over the place, there is a very strong signal that appears to be unorginized, the TV won't lock the signal, but it knows it's there on 18-1. I can only assume 1000 KW "might" sneek thru to me on a duct. With rain, WNYT 18 is good, again assuming WMFP's signal is attenuated. And tropo maps obviously show numerous mountain ranges in the path.
And again, I don't really want to mess with the other local and DX stations that are great. Why WNYT 18? Same reason I built a 3 element yagi to get WRGB 6 trouble free....I didn't need it, but it was there and I wanted it. It's a Ham thing, you might understand. RF 12 is rarely a problem, but when it is a problem, it's usually at the wrong time, so a local NBC backup would be convienient.
Rich Klien suggested moving the antennas up or down...but if I'm right, I don't see that helping much. I'm a little confused about your suggestion, maybe you can elaborate a little more.....

Tower Guy · 12-Apr-2010, 5:16 PM

Quote:

Originally Posted by kb2fzq

I'm a little confused about your suggestion, maybe you can elaborate a little more.....

WNYT-18 is at 183 degrees true, WMFP is at 121. The difference is 62 degrees.

A half wavelength on channel 18 is 11.88 inches.

If you build two right angle extensions and position the top antenna more toward WMFP and the bottom antenna further from WMFP, there is a magic distance when WNYT will be in phase while WMFP is out of phase. That distance is 1/sine62 * 11.88 inches = 13.45 inches. Therefore boom two offsets of 6.7 inches, one left and one right of the mast, and you'd drop the signal strength of WMFP by 15-20 db while not changing the signal strength of the main beam on any frequency.

kb2fzq · 13-Apr-2010, 10:49 AM

Quote:

Originally Posted by Tower Guy

WNYT-18 is at 183 degrees true, WMFP is at 121. The difference is 62 degrees.

A half wavelength on channel 18 is 11.88 inches.

If you build two right angle extensions and position the top antenna more toward WMFP and the bottom antenna further from WMFP, there is a magic distance when WNYT will be in phase while WMFP is out of phase. That distance is 1/sine62 * 11.88 inches = 13.45 inches. Therefore boom two offsets of 6.7 inches, one left and one right of the mast, and you'd drop the signal strength of WMFP by 15-20 db while not changing the signal strength of the main beam on any frequency.

OK, I think I'm reading that the 2 log periodic antennas would be in a V shape from the mast, with the outer booms tips separated at 13.45 inches? The right angle extentions comment does confuse me.
I will try that, but I've also noticed that the full wavelength of RF 18 (497 mhz) is 34 5/8 inches, the log periodic's boom length is 35 inches, when the beams are pointed at Bald Mountain, the booms would point towards Boston from the side and are almost a full wave resonent length, maybe acting as a pair of phased, sudo-dipoles? As said, I can't lock WMFP, but the TV knows it's seeing some kind of strong RF.
I also have another UHF loop antenna and the original WRGB home brew dipole antenna I will set at 497 mhz length that I plan to test with, just to prove or disprove the sudo-dipole thing and also trying reception from Bald Mt. at a lower height here then the main array is...the loop is portable on a mast and I will move it around the property, sniffing for WNYT 18. I haven't done any field testing in a very long time, this may prove to be fun.

EDIT:
My reference above to 34 5/8" is incorrect....24 5/8" correct, apparently, I can't add correctly, so that theory is out....I continue the hunt.

Tower Guy · 13-Apr-2010, 1:25 PM

Quote:

Originally Posted by kb2fzq

OK, I think I'm reading that the 2 log periodic antennas would be in a V shape from the mast, with the outer booms tips separated at 13.45 inches? The right angle extentions comment does confuse me..

The booms would continue to be parallel and level. Here's more information, but there are no good pictures to help show the scheme.

http://www.kyes.com/antenna/sca/scala.html

kb2fzq · 14-Apr-2010, 10:34 AM

Quote:

Originally Posted by Tower Guy

The booms would continue to be parallel and level. Here's more information, but there are no good pictures to help show the scheme.

http://www.kyes.com/antenna/sca/scala.html

Yes, that kyes/scala report has been on this computer for some time now, I never considered using it as the signals were always great here, until the RF 18 debocle.
Thanks for the reminder, T.G.
I'm off today and tomorrow, and after the tax man gets his dole today, I'll be messing with reception...I'll keep ya posted.

kb2fzq · 15-Apr-2010, 9:10 AM

Yesterday, I found my canabelized VHF/UHF rabbit ears table top antenna, this just has the UHF loop on it. I went up on the roof and set the base of the loop antenna on the RF 6 only antenna, removed the 3-way combiner from the input of the pre-amp, and connected the loop antenna to the pre-amp, tuned up RF 18 and swung the beams back and forth. RF 18 had been expectedly non-receivable all morning with the clear blue skies, but as I turned the rotor...BOOM...there was RF 18 at a 22% SS...the loop catching a signal the main array could not.
This further convinces me I am getting interference from the side of the log periodics, or a reflection off of another antenna from the side.
I will be taking the advice of dbsanfte and removing the bottom 2 antennas to see if there is an issue with reflections.

kb2fzq · 16-Apr-2010, 9:17 AM

Quote:

Originally Posted by Tower Guy

WNYT-18 is at 183 degrees true, WMFP is at 121. The difference is 62 degrees.

A half wavelength on channel 18 is 11.88 inches.

If you build two right angle extensions and position the top antenna more toward WMFP and the bottom antenna further from WMFP, there is a magic distance when WNYT will be in phase while WMFP is out of phase. That distance is 1/sine62 * 11.88 inches = 13.45 inches. Therefore boom two offsets of 6.7 inches, one left and one right of the mast, and you'd drop the signal strength of WMFP by 15-20 db while not changing the signal strength of the main beam on any frequency.

WHY?.....would even think about doubting you, chief? That little magic trick worked slicker then snot!!!! And it's two-fold, I'll explain...
I started by removing the RF 13 only antenna, as it seemed to be the logical offender being so close to the phased pair. Before I did that, all stations were normal, including WNYA RF 13, and RF 18 was gone, as expected.
I dropped the channel 6 only antenna down as far as I could without having to swap the coax out to a longer coax. The result was that all remained the same, except now I've lost WNYA as well as RF 18.
I had the volume up on the TV, tuned to RF 18, front door open. I pulled the lower log antenna out to create the "V" configuration, suddenly, I could hear The Today Show audio coming from the TV!!!
RF 18 was weak, but it was there, solid.

I spent the rest of the day tweeking, looking for the best "V" split, but settled on the above. Does it look squirrely? Yup...but it works.
The other benefit...I don't know if I've mentioned it, but WVNY, RF 13 in Burlington has always desensed WNYA RF 13 in Albany, not strong enough to lock, it just desensed WNYA, hence the addition of the RF 13 only yagi, which worked quite well to stablize WNYA. With this new config, it seems the phasing effect off the back of the beams has reduced the ability for WVNY to interfere with WNYA, RF 13 was on the air and very strong. AMAZING!
As you said, all stations seem to have the same SS after the change, except WCAX and WPTZ may have dropped a few SS units (afterall, we're talking 91 miles out), but being they were so strong to begin with, a slight reduction in SS makes no difference.
I appreciate your help, chief
Thanks.