Shielding the back side of a yagi antenna

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bleech

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May 12, 2020
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Waukesha, WI
I am right in between two markets (Madison and Milwaukee WI) . There is an LP station in Milwaukee that I want to get reception of (WTSJ), but it shares the same frequency as a full power Madison station (WKOW). Normally, the Madison station overpowers the Milwaukee station. However, I have found that if I stand right behind (West side) my Yagi antenna while it is facing east, I can get reception of the Milwaukee station. I assume this is because my body is blocking the signal from the west. I have also found this to be the case if I put the antenna to the east of a tree with a thick canopy. Since neither of these situations will work permanently, is there something light weight I could attach to the antenna to block the interfering signal from the back side?
 
navychop

navychop

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If you’ve already got some chicken wire, maybe a double layer, overlapped unevenly, and stood off with a non metallic rig, MIGHT work. Or hardware cloth might be better.

This is a SWAG, so let’s hope someone more knowledgeable than me chimes in.
 
primestar31

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I am right in between two markets (Madison and Milwaukee WI) . There is an LP station in Milwaukee that I want to get reception of (WTSJ), but it shares the same frequency as a full power Madison station (WKOW). Normally, the Madison station overpowers the Milwaukee station. However, I have found that if I stand right behind (West side) my Yagi antenna while it is facing east, I can get reception of the Milwaukee station. I assume this is because my body is blocking the signal from the west. I have also found this to be the case if I put the antenna to the east of a tree with a thick canopy. Since neither of these situations will work permanently, is there something light weight I could attach to the antenna to block the interfering signal from the back side?
Ok, looks like WKOW is RF26 which is UHF, and a small sized RF waveform. You can TRY wire-tying a square piece of very small mesh metal screen (like old screen door stuff) to the back of the antenna, and see if it works for you. You might have to use some square tube aluminum outriggers screwed to the antenna to hold the mesh screen in place. This will increase wind load though.

Alternatively, you can also experiment with how HIGH or LOW the antenna is. In other words, for this case, try lowering the antenna on the mast, to the point where it still picks up WTSJ, but lowers the received dB's of WKOW.
 
TheKrell

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Most yagi's I've seen have reflectors on the rear to increase SS from the front and reduce it from the back. Maybe there's a manufacturer who has taken this to an extreme, and have a null in the sensitivity pattern toward the rear. The only thing like that I have personally encountered was when the MHz Network was still broadcasting from a tower 1 mile away from me. They were 90 degrees off to one side compared to the rest of my stations. So I aligned my antenna as closely as I could to 90 degrees, since the nulls on a yagi without any reflector should be +- 90. Alas, all yagi's I have seen recently have gnarly diffraction elements that are designed to broaden that pattern and remove the nulls. So I added a trap for the relevant channel frequency, and that seemed to work pretty well. Now the MHz Network has gone to streaming, so I no longer have to worry about them.
 
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bleech

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Thanks for the quick replies and suggestions. I will try them out when weather and time permits. The antenna I am using is the Stellar Labs 30-2370. As for the height , I am pretty much limited to 0 (on the roof) to 4 (top of mast) foot height, because the antenna is eave mounted. I have tried different heights in this range and it did not seem to help.
 
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primestar31

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Thanks for the quick replies and suggestions. I will try them out when weather and time permits. The antenna I am using is the Stellar Labs 30-2370. As for the height , I am pretty much limited to 0 (on the roof) to 4 (top of mast) foot height, because the antenna is eave mounted. I have tried different heights in this range and it did not seem to help.
That's a 91XG clone. Wire-tie the screen I mentioned slightly off-set (use something like plexiglas or nylon washers to hold it off) the backside of the corner reflectors.
 
primestar31

primestar31

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Most yagi's I've seen have reflectors on the rear to increase SS from the front and reduce it from the back. Maybe there's a manufacturer who has taken this to an extreme, and have a null in the sensitivity pattern toward the rear. The only thing like that I have personally encountered was when the MHz Network was still broadcasting from a tower 1 mile away from me. They were 90 degrees off to one side compared to the rest of my stations. So I aligned my antenna as closely as I could to 90 degrees, since the nulls on a yagi without any reflector should be +- 90. Alas, all yagi's I have seen recently have gnarly diffraction elements that are designed to broaden that pattern and remove the nulls. So I added a trap for the relevant channel frequency, and that seemed to work pretty well. Now the MHz Network has gone to streaming, so I no longer have to worry about them.
Here's the problem: when Obama was president, he had his FCC change the laws to allow the same frequencies to be used closer than the old 200 mile apart. So, now we have co-channels as close as 70 MILES or so to each other!

I have this problem myself, with RF17. In my case, lowering my antenna, and moving it over 2 feet on the separate well mast pipe cured it.
 
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mikekohl

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Thanks for the quick replies and suggestions. I will try them out when weather and time permits. The antenna I am using is the Stellar Labs 30-2370. As for the height , I am pretty much limited to 0 (on the roof) to 4 (top of mast) foot height, because the antenna is eave mounted. I have tried different heights in this range and it did not seem to help.
Find a piece of roofing tin (sold in 3 foot widths of varying lengths).
Start with perhaps a 3 foot x 6 foot section and with a friend's help move it around the back side of your antenna (Madison side). See if you can reduce the signal enough to get Milwaukee. The obvious solution is to be on an east facing wall blocking Madison, and a clear shot at Milwaukee, but when working on rooftops, if tin can be enough to make a permanent barrier, I would erect some framework with lumber to create that needed vertical wall. If you want a fairly secure anchor for such a structure, I do have some 50 x 50 inch NPRM frames that could be had at a rock bottom price, and you could weight with cement blocks on the roof, once you find a place and a design that accomplishes signal shielding on 26.
 
Trip

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Here's the problem: when Obama was president, he had his FCC change the laws to allow the same frequencies to be used closer than the old 200 mile apart. So, now we have co-channels as close as 70 MILES or so to each other!

Obama had nothing to do with it. The current DTV spacing rules were decided back in the 80s and 90s for full power stations, and in the early 00s for LPTVs.

- Trip
 
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Tower Guy

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There are two good tricks to reduce interference from a cochannel station. Both require two identical antennas

One is called stagger stacking. The two antennas are stacked one above the other, but one yagi has new holes drilled in the booms to place that yagi 1/4 wavelength closer the desired station. Next, the antennas are added together with the feed line going to the antenna closer to the station that you want to receive longer than the other feed line by 1/4 wavelength electrically. (The physical length of a 1/4 wavelength of RG-6 is 82% of that of a quarter wavelength In free space.) A quarter wavelength in free space on channel 26 is 2952/545, or 5.416”. The extra length of RG 6 would be 4.44”. The two feedlines would be summed in a two way splitter used backwards. The input port of the splitter goes to your TV set or preamp.

More info here http://anarc.org/wtfda/stagger.pdf

The second way to null interference is to stack the two antennas horizontally with a critical spacing between them that places a null in the direction of the interfering station. To calculate the critical spacing, it would be helpful if you posted your rabbitears reception report.

More info here

I have actually used both of these techniques simultaneously using four antennas to pick up a station where I wanted to null the interfering station by more than 35 db.
 
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bleech

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May 12, 2020
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Waukesha, WI
Thanks for posting these more elaborate techniques but hopefully the simpler (and less $$$) approaches will work. I may also try moving the antenna below the roof line on the east side of the house and see what happens. Here is my rabbit ears report: RabbitEars.Info

My goal is to get the BUZZR game show network. Up until a few months ago i was able to get a strong signal with this antenna from WFLD in Chicago. However, as luck would have it, someone started a new LP station (WDMW-LD) with the same frequency as WFLD and a transmitter that, according to the FCC report, is located in a residential subdivision a few miles from my house, in the same direction I have to point the antenna to pick up Chicago!
 
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arlo

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$$$ considerations usually require a bit of experimentation and more $$$.
I've had a lot of these personal missions.
First thing is a "yagi" was mentioned. A 4 element yagi? Or a full-blown high quality one?
Front to back ratio is based on element count. 1 or 2 reflectors does allow a large rear beamwidth.
You could add reflectors behind the driven element and give it a go. If you get the design data to calculate lengths and spacing.
So. Do we build a grounded screen and place it behind the reflector elements and then play around to get the best performance? Could be.
Stacking could work, and stagger stacking surely could work if you don't mind building phasing arrays and doing the math.
But don't laugh. Years ago I went to a guys house and he had a bed spring hanging from a tree on a pulley and rope to peak the Buffalo game. That was around 90-100 bird-miles from Buffalo out here in the sticks.
I was shaking my head until the springs went up and the snow on the old Zenith went away.
 
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mikekohl

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Thanks for posting these more elaborate techniques but hopefully the simpler (and less $$$) approaches will work. I may also try moving the antenna below the roof line on the east side of the house and see what happens. Here is my rabbit ears report: RabbitEars.Info

My goal is to get the BUZZR game show network. Up until a few months ago i was able to get a strong signal with this antenna from WFLD in Chicago. However, as luck would have it, someone started a new LP station (WDMW-LD) with the same frequency as WFLD and a transmitter that, according to the FCC report, is located in a residential subdivision a few miles from my house, in the same direction I have to point the antenna to pick up Chicago!
Are you in Waukesha, or further west--towards Madison? The low power channel 23 station from Madison (transmitting from Odana Road) now has Buzzr on their dot 2 subchannel.
Probably reliable to about 30 miles eastward without making special installation.
Buzzr is likely also available via Roku on the Internet if you dig deeply.
 
T

Tower Guy

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I just noticed that Newark has a sale on your Stellar labs 30-2370 antenna for $22.20 each, so $$$$ for a two antenna stack should not be an issue.

In your location the angle difference between WKOW and WTSJ is 207.3 degrees. This places WKOW 27.3 degrees off the back of the antenna while the antenna is aimed at WTSJ.

For vertical stacking, the stagger stacking is normally configured for optimum F/B directly off the back of the antenna. That should work OK for you, but because WKOW is 27.3 degrees off the back you can easily move the null to that exact angle by increasing the offset if one of the two antennas by 1/cos 27.3*quarter wavelength at channel 26. In your case that is 6.77”. The extra length of coax remains at 4.44”. The vertical spacing of the two antennas can be in the range of 4’, but a bit less would be OK too. This configuration would be my choice.

For horizontal stacking there are two distances that can be considered, first null at 24”, which is unusually close, and second null at 72”, which is about right for for the 87.2” long boom of the 30-2370. The result would be an antenna with an extremely narrow forward pattern.

Can you slide the 30-2370 mounting bracket down the support boom by 6.77” without re-drilling any holes?? Or one back and one forward totaling 6.77” difference.
 
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bleech

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Waukesha, WI
Are you in Waukesha, or further west--towards Madison? The low power channel 23 station from Madison (transmitting from Odana Road) now has Buzzr on their dot 2 subchannel.
Probably reliable to about 30 miles eastward without making special installation.
Buzzr is likely also available via Roku on the Internet if you dig deeply.
I'm about 10 miles southwest of the city of Waukesha. I checked out 23 from Madison, but can't get reception of it. You guessed it. WTAS-LD is transmitting on frequency 23 from Waukesha, so I'm back to square one. I don't know why Buzzr can't get better affiliates, so many of the diginets on the main stations are junk. I've tried the streaming option but prefer the ability to record on Tivo and watch later.
 
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bleech

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May 12, 2020
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Waukesha, WI
I just noticed that Newark has a sale on your Stellar labs 30-2370 antenna for $22.20 each, so $$$$ for a two antenna stack should not be an issue.

In your location the angle difference between WKOW and WTSJ is 207.3 degrees. This places WKOW 27.3 degrees off the back of the antenna while the antenna is aimed at WTSJ.

For vertical stacking, the stagger stacking is normally configured for optimum F/B directly off the back of the antenna. That should work OK for you, but because WKOW is 27.3 degrees off the back you can easily move the null to that exact angle by increasing the offset if one of the two antennas by 1/cos 27.3*quarter wavelength at channel 26. In your case that is 6.77”. The extra length of coax remains at 4.44”. The vertical spacing of the two antennas can be in the range of 4’, but a bit less would be OK too. This configuration would be my choice.

For horizontal stacking there are two distances that can be considered, first null at 24”, which is unusually close, and second null at 72”, which is about right for for the 87.2” long boom of the 30-2370. The result would be an antenna with an extremely narrow forward pattern.

Can you slide the 30-2370 mounting bracket down the support boom by 6.77” without re-drilling any holes?? Or one back and one forward totaling 6.77” difference.
Thanks, I will definitely keep this in mind. Its not just the extra antenna that's the issue, I would also have to get tools to make my own coax, and I have to do all of this work on the edge of the roof of a two story house, which really isn't my favorite place to be for several reasons.
 
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rabbit73

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Jul 17, 2020
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S.E. VA
Thanks for posting these more elaborate techniques but hopefully the simpler (and less $$$) approaches will work. I may also try moving the antenna below the roof line on the east side of the house and see what happens. Here is my rabbit ears report: RabbitEars.Info
Thank you for the signal report.

You are outside the coverage area for WTSJ:

BleechSatGuysCovWTSJ 1


but it helps that you are LOS for WTSJ:

BleechSatGuysP3WTSJ 1


and there is a hill blocking the WKOW signal:

BleechSatGuysP3WKOW 1


A Yagi with a corner reflector has good rejection of signals from the rear. This for the similar 91XG:

AD 91XG FB FR2 1


The Stellar Labs 30-2370 has a little wider spacing for the reflector rods:

Stellar Labs 30 2370 Closeup3 1


I suggest you try one of the easier ideas first. Maybe 1 x 2 wire fencing on the reflector with the 1" as the vertical spacing or 1/2" x 1/2" hardware cloth. 1/4" hardware cloth would probably cause too much wind load and ice up in the winter.

I like the idea of lowering the antenna so that the roof blocks the WKOW signal from the west.


 
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bleech

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Waukesha, WI
Thanks for that great information! I am also far outside the coverage area of WFLD (Chicago) but was able to pull in a dropout-free signal about 95% of the time until WDMW-LD got in the way. I plan on lowering the antenna first since it doesn't involve any costs, and I've had luck in the past getting good signals with the antenna laying on the ground when new shingles were put on the roof a few years ago.
 

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