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DIY antenna designs for attic?

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djb
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Joined: Tue Feb 15, 2005 11:20 am

Re: DIY antenna designs for attic?

Post by djb » Wed Nov 25, 2020 6:53 am

I built the stealth-hawk antenna, which resides in a tree in my backyard.

Able to pull in all locals and LP-TV stations. Does well on VHF as well.

https://blog.solidsignal.com/tutorials/ ... -antennas/



km1125
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Re: DIY antenna designs for attic?

Post by km1125 » Mon Dec 07, 2020 8:42 pm

Today I took advantage of the WMYD change to make some other antenna changes too.

First, I changed the antenna I use to get RF14 (Lansing 6.1 and 53.1). I was using a home-built Hoverman but since it was just one RF channel that I was looking for, I decided to build a channel-specific antenna. I had an old cheap UHF antenna and I ripped it apart and reconfigured as a Yagi tuned for RF14. Took a couple attempts, as I was using some #6 power cable for the elements and it was jacketed. I was planning on just unbending the folded dipoles that were on it but they kept cracking as I tried.

Here is the original:
Image

And here is the modified version:
Image

I also added some attenuators to the VHF and UHF antennas that I had pointed to the Detroit stations to see if I could knock down RF23 WUDT as it was creating co-channel interference in my attempts to get RF23 WNWO from Toledo. Took a few tries, but finally got it. Ended up losing RF9 CBET in the process but we rarely watch that. Seems like even my VHF antenna was pulling in enough of RF23 WUDT even though it was also filtered through a VHF/UHF diplexer. Now I'm picking up WNWO pretty good. We'll see if I it's still good when I have a few inches of snow on the roof.



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WOHO
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Re: DIY antenna designs for attic?

Post by WOHO » Wed Dec 09, 2020 3:16 pm

km1125, what's your QTH? Nice homemade Yagi- did you use a specific program for he driven rod and reflectors' measurements?



km1125
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Re: DIY antenna designs for attic?

Post by km1125 » Wed Dec 09, 2020 5:00 pm

Western Wayne county. I used one of the online calculators for the length of the elements and spacing but I was getting resonance at a much higher frequency than planned, so I had to scale all the dimensions up a bit to get to the frequency I wanted - 473MHz.



CK-722
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Re: DIY antenna designs for attic?

Post by CK-722 » Wed Dec 09, 2020 11:29 pm

Rule #1 for antenna building. Measure thrice and cut once. If in doubt, cut longer than you expect. The metal boom affects the effective length. Some designers say to use a wooden or PVC boom. Where did you get the tubing? It looks like 3/8" diameter??? That's neat how you have the measurements marked on the boom. Can you move the elements in the design process before fastening them down? Where did you get the mounting fasteners? Tubing and other components? What are the fasteners made of? If at least part metal, are the elements insulated from the boom?


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jimh
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Re: DIY antenna designs for attic?

Post by jimh » Fri Dec 11, 2020 9:52 am

The home-brew yagi shown above as "modified version" appears to have a 300-to-75-Ohm balun at the feed point. Is that correct?

If so, the feed point impedance of a yagi with a split dipole element is usually very low, often 50-Ohms or less.



km1125
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Re: DIY antenna designs for attic?

Post by km1125 » Fri Dec 11, 2020 10:25 am

CK-722 wrote:
Wed Dec 09, 2020 11:29 pm
Rule #1 for antenna building. Measure thrice and cut once. If in doubt, cut longer than you expect. The metal boom affects the effective length. Some designers say to use a wooden or PVC boom. Where did you get the tubing? It looks like 3/8" diameter??? That's neat how you have the measurements marked on the boom. Can you move the elements in the design process before fastening them down? Where did you get the mounting fasteners? Tubing and other components? What are the fasteners made of? If at least part metal, are the elements insulated from the boom?
I did measure several times before I cut the original elements, but I was thinking because I was using a different element material than the program assumed, the calculations would be off by some bit, I just wasn't sure how much or in what direction. So I originally made them exactly to spec, measured what the resonance frequency was, then adjusted to get to where I needed. The elements are all made from a #6 copper power wiring that was used for -48VDC in telcom hubs. Just something I had handy that was very stiff so it would hold its shape. I tried using the original aluminum tubing from the commercial antenna but it was very thin and cracked when I tried to unbend some of the elements.

The supports and boom are from the original commercial wideband antenna pictured above. The boom is aluminum tube and the supports are plastic. I could just slide the supports up and down while I was taking measurements, so that did make it easy. There is a tiny screw now holding them in place.

jimh wrote:
Fri Dec 11, 2020 9:52 am
The home-brew yagi shown above as "modified version" appears to have a 300-to-75-Ohm balun at the feed point. Is that correct?

If so, the feed point impedance of a yagi with a split dipole element is usually very low, often 50-Ohms or less.
Yes, there is a balun there and that confused me too. I tried just feeding it with 75 Ohms but wasn't getting any channel-specific response. When I added the balun I got nice resonance at 548MHz, then recut the elements to get to 473MHz.

I have other dipoles that are definitely 300MHz but that online calc that I used did anticipate the impedance between 50 and 75 (I think it had 67). I had the center of the coax on one side of the dipole and the shield on the other. What am I missing?



jimh
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Re: DIY antenna designs for attic?

Post by jimh » Fri Dec 11, 2020 4:06 pm

km1125 wrote:
Fri Dec 11, 2020 10:25 am
jimh wrote:
Fri Dec 11, 2020 9:52 am
The home-brew yagi shown above as "modified version" appears to have a 300-to-75-Ohm balun at the feed point. Is that correct?

If so, the feed point impedance of a yagi with a split dipole element is usually very low, often 50-Ohms or less.
Yes, there is a balun there and that confused me too. I tried just feeding it with 75 Ohms but wasn't getting any channel-specific response. When I added the balun I got nice resonance at 548MHz, then recut the elements to get to 473MHz... I had the center of the coax on one side of the dipole and the shield on the other. What am I missing?
I can't offer any advice why it works better with a 300-to-75-Ohm balun than it did without. That just seems very strange.

Regarding the behavior of Yagi antennas: Almost any arrangement of element lengths and spacing, as long as the reflector is longer and the directors shorter than the driven element, will produce gain. With modern methods of analysis, it is possible to design yagi antennas with optimized gain AND with very clean radiation patterns.

For receiving a television signal, particularly these 8VSB-modulation ATS 1.0 transmissions, I suspect that having a clean main lobe pattern is probably advantageous. An antenna with a radiation patter of one nice main lobe will tend to be better (that one with many sidelobes) because that clean radiation pattern will have significant ATTENUATION of signals arriving off the main lobe. Signals from the same source often arrive by multiple paths, so attenuating the off-axis response of the antenna is important: it will help reduce multipath interference.

The really best receiving antenna is one with a matching CIRCULAR polarization as the transmitting antenna. However, the TV stations now are tending to use elliptical polarization, with the power split about 70-percent Horizon and 30-percent Vertical--and some are Horizontal only.

Finding a commercial TV antenna with circular polarization would be difficult.



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