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DIY antenna designs for attic?

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Turkeytop
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Re: DIY antenna designs for attic?

Post by Turkeytop » Wed Apr 08, 2020 3:14 pm

How long is the dipole? Did it work out OK?


I started out with nothing and I still have most of it.

km1125
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Re: DIY antenna designs for attic?

Post by km1125 » Thu Apr 09, 2020 7:42 pm

I was originally going to build two, one for just WNHE and a shorter one for VHF-high to pick up Fox2 and Channel 9

I built an 84" long one and tested it next to the TV. I picked up all three with great signal, so I just used that.



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Turkeytop
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Re: DIY antenna designs for attic?

Post by Turkeytop » Thu Apr 09, 2020 7:54 pm

How did you calculate that length?


I started out with nothing and I still have most of it.

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SolarMax
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Re: DIY antenna designs for attic?

Post by SolarMax » Fri Apr 10, 2020 10:00 am

km1125 wrote:
Wed Apr 08, 2020 2:10 pm
plan was to move all the antennas to the house attic which would add another 8 or 9 ft of height and cut out about 20' of RG-6 downlead.
Characteristic feed point impedance of a folded dipole is 300 Ohms, so trust you've got a 300 Ohm to 75 Ohm matching transformer ahead of that RG-6.



km1125
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Re: DIY antenna designs for attic?

Post by km1125 » Fri Apr 10, 2020 10:07 am

A bit of a swag.

492/63 to get the 1/2 wavelength in ft and then dropped about 5% for vop to account for the wood that I was mounting it on.

Yes, I did use a 300:75 matching transformer that I had laying around.



k8jd
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Re: DIY antenna designs for attic?

Post by k8jd » Fri Apr 10, 2020 12:39 pm

Who Knows ?
I am not sure if the FCC is trying to get the stations on consecutive channels in an area, or spreading them out in freq.. Just went to FCC page and listed Detroit stations and they are listing two or three different channels for the same station. all say license, not application or Construction permit. what's REALLY going on ?
They go from Ch3, 60 MHz to Ch 45, 656 MHz (lower end).
Last edited by k8jd on Fri Apr 10, 2020 12:42 pm, edited 1 time in total.



k8jd
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Re: DIY antenna designs for attic?

Post by k8jd » Fri Apr 10, 2020 3:47 pm

km1125 wrote:
Thu Apr 09, 2020 7:42 pm
I was originally going to build two, one for just WNHE and a shorter one for VHF-high to pick up Fox2 and Channel 9

I built an 84" long one and tested it next to the TV. I picked up all three with great signal, so I just used that.
84 inches, ? , if you feed with coax as a halfwave dipole would resonate on 66.1 MHz, good for channel 3.
Take the same length as 2 halfwaves in phase , feed with 300 Ohm twinlead, would peak in the Aeronautical VHF band at 132 MHz !
You can assume that in a receiving situation the antennas will receive almost anything if it's strong enough. Transmitting is not so forgiving :D

I still am not sure what channels are in use after the current phase of re-packing that the FCC mandates.
I need to hook up an antenna to one of my TVs and do a sweep to see what is out there. Got cable and it made me lazy :D



k8jd
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Re: DIY antenna designs for attic?

Post by k8jd » Fri Apr 10, 2020 3:51 pm

km1125 wrote:
Fri Apr 10, 2020 10:07 am
A bit of a swag.

492/63 to get the 1/2 wavelength in ft and then dropped about 5% for vop to account for the wood that I was mounting it on.

Yes, I did use a 300:75 matching transformer that I had laying around.
I always have used 468/F=L for calculating dipole length, has the End Effect 5% factored in, BUT my dipoles are in the HF part of the spectrum, the number is a bit different for V/UHF freqs.



CK-722
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Re: DIY antenna designs for attic?

Post by CK-722 » Fri Apr 10, 2020 6:48 pm

It was allowed with digital stations to operate on adjacent channels if they are collocated or nearly collocated. Out farther, there is a ratio of undesired to desired signals. Now, these adjacencies may create problems even if collocated, in particular intermodulation, which was the basis of the 2nd, 3rd, 4th, and 5th adjacent channel restrictions under analog rules. The requirement was 20 miles. There was a treatise about the basis for these "taboos" in another thread here on the board. You may not even know about what is causing a reception problem like this, like was obvious in analog. You just won't receive the signal, and you won't know why, it just won't lock on. Probably the worst case would be an LPTV sandwiched between near adjacent full power signals. You would have to have expensive equipment probably to figure out these problems. I've seen VHF and UHF antenna books which use the 468 constant. However, the old books used an adjustment for the size and material of the boom. I think that was in an old ARRL book on VHF and UHF from the 1960s. Nowadays, home brew design is aided by computer modeling, and factors hardly even thought about in the 1960s are factored in.

UHF Taboo Paper from 1987.

https://transition.fcc.gov/Bureaus/Engi ... s/TM87.pdf

All of those factors like 492, 468, 300, etc., English and Metric Units related to the speed of light, and velocity ratio (like 95% dipole or 80% transmission line velocity) of a particular design. My favorite formula is Wavelength in Feet=983.5/Frequency in MHz. Simple example, what is the wavelength of WCFL/WMVP on 1000 kHz? 1000 kHz=1 MHz. 983.5/1 MHz=983.5 feet. 983.5 is often rounded to 984, as the velocity of light is actually about 299,792,500 meters/second, which is rounded to 300,000,000, or 300 for 1 MHz.
Last edited by CK-722 on Fri Apr 10, 2020 7:21 pm, edited 1 time in total.


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innate-in-you
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Re: DIY antenna designs for attic?

Post by innate-in-you » Fri Apr 10, 2020 7:15 pm

k8jd wrote:
Fri Apr 10, 2020 3:47 pm
km1125 wrote:
Thu Apr 09, 2020 7:42 pm
I was originally going to build two, one for just WNHE and a shorter one for VHF-high to pick up Fox2 and Channel 9

I built an 84" long one and tested it next to the TV. I picked up all three with great signal, so I just used that.
84 inches, ? , if you feed with coax as a halfwave dipole would resonate on 66.1 MHz, good for channel 3.
Take the same length as 2 halfwaves in phase , feed with 300 Ohm twinlead, would peak in the Aeronautical VHF band at 132 MHz !
You can assume that in a receiving situation the antennas will receive almost anything if it's strong enough. Transmitting is not so forgiving :D

I still am not sure what channels are in use after the current phase of re-packing that the FCC mandates.
I need to hook up an antenna to one of my TVs and do a sweep to see what is out there. Got cable and it made me lazy :D
2 Thru 36.

As for the 84-inch dipole- it will be resonant at about 66MHz, with a broad by-directional pattern (think of the antenna built-in to your pocket AM radio), but its third overtone at 198MHz wound have a quatrofolic pattern.

Note that I've been discussing folded dipole antennas made from twinlead. A simple dipole of two metal rods would perform satisfactorily only near those frequencies that are odd multiples of 66MHz.



CK-722
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Re: DIY antenna designs for attic?

Post by CK-722 » Fri Apr 10, 2020 7:26 pm

Note that I've been discussing folded dipole antennas made from twinlead. A simple dipole of two metal rods would perform satisfactorily only near those frequencies that are odd multiples of 66MHz.
This was the basis for many VHF TV antenna designs, as 54-88 X 3=162-264 MHz, which covers 174-216 MHz.


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Your bullet missed my trial balloon.

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CK-722
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Re: DIY antenna designs for attic?

Post by CK-722 » Fri Apr 10, 2020 8:19 pm

Velocity Factors of various transmission lines.

https://en.wikipedia.org/wiki/Velocity_factor


Is THAT where they got the idea for the 486-SX?

Same (x, y, z), different (t)

Your bullet missed my trial balloon.

RTN Price. Not guaranteed. As of 12:30, 157.71 Down 0.22.

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km1125
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Re: DIY antenna designs for attic?

Post by km1125 » Fri Apr 17, 2020 10:36 am

So, now that I've had some success with fishing a wall to get a coax up to the actual attic, I'm thinking about some type of a reflector for the folded dipoles that I'm going to put up there.

I'm not as interested in gain as I seem to have plenty of signal, but just interested in a little directivity to minimize noise and interference from the backside of the antenna.

Would I be better with just a reflector element or would it be better with some type of a grid? I have some 1/2" screening I was going to use for a different project that I could use. Would 10% larger than the dipole be appropriate?

How far back from the dipole would I want? There will be two folded dipoles... the current one is used for both VHF-Low and VHF-High, as it needs to pick up Ch 3, 7 and 9.

The other folded dipole would be for 11 and 13 so I just needs to be for VHF-High, or centered on Ch12.



CK-722
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Re: DIY antenna designs for attic?

Post by CK-722 » Fri Apr 17, 2020 11:57 am

If you have an FM BC receiver with a fairly detailed signal meter in terms of LEDs, or digital like the Tecsuns, you could experiment with an FM dipole and reflector. I would try chicken wire and PVC pipe for the reflector and experiment. Pull the dipole back and forth to maximize the signal. Say you measured WHNN on 96.1 and it maximized at 27 inches from the reflector. Multiply that by 96.1/63 (MHz center of Channel 3) to determine the appropriate distance for WHNE. I used an oven rack behind a UHF Bowtie to receive WEYI 55 miles away behind the terrain shield to the SE. It was easy with analog TV to optimize it. 10% above the dipole length is a reasonable size to try. It may vary the phase of the reflected signal so that it maximizes at a different distance. In the ARRL Publication FM and Repeaters, which may be out of print but available on eBay, had a lot of plans for reflectors and how to maximize and impedance match them to the transceiver.

Roughly doubling the size of two meter antenna plan, and adjusting the dimensions by 146/63 MHz might be another plan.


Is THAT where they got the idea for the 486-SX?

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CK-722
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Re: DIY antenna designs for attic?

Post by CK-722 » Sat Apr 18, 2020 12:18 pm

You might also look at John Kraus' Antennas Book for ideas. 100 years ago, John Kraus built his first antenna, to receive 8MK/WWJ from his home in Ann Arbor. His amateur callsign was W8JK. Sounds a lot like K8JD. :blink

Dr. John Kraus invented the Corner Reflector, which would be too unwieldy at Low VHF for attic installations, most likely, unless you have a really high open attic.

https://www.google.com/search?q=john+kr ... 80&bih=710

Image


Is THAT where they got the idea for the 486-SX?

Same (x, y, z), different (t)

Your bullet missed my trial balloon.

RTN Price. Not guaranteed. As of 12:30, 157.71 Down 0.22.

Artificial Intelligence is a Child that needs a Parent to guide it through.

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