Is there a great difference between a claimed 110 mile antenna and a 100 mile antenna in the world of DTV?
Using a satellite dish to make a HD parabolic antenna? Worth it?
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You speak as if greater wind drag, huge turning circle and high rotational mass are desirable in a TV antenna.
If the specifications (other than size and weight) aren't more favorable, what's the point? The Crossfire was 18" longer, presented almost 50% more wind load, weighed more than twice as much yet had comparable numbers for VHF gain and inferior numbers for UHF gain versus the Advantage 100.
I took an old C-band parabolic and repurposed it for terrestrial TV. Doesn't that sound like fun?
So, the first thing you have to realize is that the 10' C-band antennas are pretty solid, heavy and with a fair degree of wind resistance. As the reflecting surface is typically expanded metal with 2 mm perforations, this thing is going to present quite a wind load. Even after removing all the C-band electronics and unnecessary aiming motor, the parabolic and its mounting structure is probably 150 lbs...so this is not going up on a mast.
Photos can't seem to be posted here, sorry.
What it does get mounted on is 3" galvanized steel, and for obvious reasons that pipe needs to be at least 5' out of the ground. In my case, I went for 7 feet out of the ground...but that means 5' in the ground, surrounded by at least 300 lbs of concrete. Unless you have a really big augur, it's hard to do much more than that. Luckily, the ground drops off significantly from where the pole is, so it's effectively maybe 12' AGL.
Once the dish is mounted, configuring the antenna element is pretty straightforward. I live in an area with almost no signals in any direction, so I don't need a reflector behind the antenna element: it's just a matter of making the element (bow-tie) the right size and in the focus of the dish.
According to theory, the bow-tie is of infinite size...and I'd like to get down to to channel 7 so the wavelength is almost 2 meters. But the focus of the dish is probably less than a meter across, so there's not going to be any advantage to an bow-tie even that big. My first pass is with bow-tie wires 16" long. (I will experiment with solid bow tie and other variations in a week or so.) Through experimentation and measurement, I find the best distance for the bow-tie element and we're off to the races.
My main signal challenges are about 100 miles away, and my baseline for comparison metrics is a pair of yagis, with the bottom one about 12' AGL actual...but due to drop off of ground level effectively 17' AGL. According to the models, I shouldn't be getting much of anything...but with the stacked yagis and a nice preamp most of the time I get about 75 virtual channels (from about 20 transmitters). The new dish with a preamp gets nearly all the channels the same way, and the first pass of spectrum analysis (using the Nuts about Nets USB-based system on a PC) shows nearly identical signal strength. The dish is more directional than the Yagis (no surprise) but not as super-directional as I was worried about.
So now: how do I make the bow-tie outperform the Yagis? They are signficantly farther off the ground, which gives them a big gain advantage (I'm guessing 6 dB)...but the "collecting surface" of the dish should give the bow-tie even more than that.
The answers come in the next post...
So, the first thing you have to realize is that the 10' C-band antennas are pretty solid, heavy and with a fair degree of wind resistance. As the reflecting surface is typically expanded metal with 2 mm perforations, this thing is going to present quite a wind load. Even after removing all the C-band electronics and unnecessary aiming motor, the parabolic and its mounting structure is probably 150 lbs...so this is not going up on a mast.
Photos can't seem to be posted here, sorry.
What it does get mounted on is 3" galvanized steel, and for obvious reasons that pipe needs to be at least 5' out of the ground. In my case, I went for 7 feet out of the ground...but that means 5' in the ground, surrounded by at least 300 lbs of concrete. Unless you have a really big augur, it's hard to do much more than that. Luckily, the ground drops off significantly from where the pole is, so it's effectively maybe 12' AGL.
Once the dish is mounted, configuring the antenna element is pretty straightforward. I live in an area with almost no signals in any direction, so I don't need a reflector behind the antenna element: it's just a matter of making the element (bow-tie) the right size and in the focus of the dish.
According to theory, the bow-tie is of infinite size...and I'd like to get down to to channel 7 so the wavelength is almost 2 meters. But the focus of the dish is probably less than a meter across, so there's not going to be any advantage to an bow-tie even that big. My first pass is with bow-tie wires 16" long. (I will experiment with solid bow tie and other variations in a week or so.) Through experimentation and measurement, I find the best distance for the bow-tie element and we're off to the races.
My main signal challenges are about 100 miles away, and my baseline for comparison metrics is a pair of yagis, with the bottom one about 12' AGL actual...but due to drop off of ground level effectively 17' AGL. According to the models, I shouldn't be getting much of anything...but with the stacked yagis and a nice preamp most of the time I get about 75 virtual channels (from about 20 transmitters). The new dish with a preamp gets nearly all the channels the same way, and the first pass of spectrum analysis (using the Nuts about Nets USB-based system on a PC) shows nearly identical signal strength. The dish is more directional than the Yagis (no surprise) but not as super-directional as I was worried about.
So now: how do I make the bow-tie outperform the Yagis? They are signficantly farther off the ground, which gives them a big gain advantage (I'm guessing 6 dB)...but the "collecting surface" of the dish should give the bow-tie even more than that.
The answers come in the next post...
So with COVID-19 I've had time on my hands at home and did a bunch of comparative measurements so that I could avoid doing all the chores my wife wants. Used Nuts About Nets spectrum analyzer to compare--the only variation in the configuration was the final antenna.
Note that this would have to be called deep-fringe conditions--the nearest real station is 75 miles away and some of the stuff I'm shooting for is 125 miles away. Due to atmospherics, signal strength varies significantly during the day, so I always did my measurements within the same hour and comparable weather conditions.
Baseline antenna was using thin copper "whiskers". They are long because (1) I'm trying to get some VHF channels and (2) from what I read, making them longer doesn't significantly hurt the UHF.
Modified the whiskers to make them solid triangular pieces. Within the variability of the signal/atmospheric effects, no measurable difference.
Put a reflector screen behind the active elements. No appreciable difference.
Modified the whiskers and made "stacked" dipoles. Maybe 1 dB difference.
Took a Channel Master 4228 HD antenna and used that as the active antenna (facing in toward the dish). Depending on its location in the dish focal plane and the frequency, got up to 5 dB of gain. The 4228 HD can be modded to improve its gain by another 3 dB (there are a couple of sites that describe how to do this)--I didn't make those mods, but I'll believe the claims.
But---the advantage of the parabolic--collecting a huge amount of signal--is fighting the disadvantage of "no altitude". And the measurements showed that in my case, it's a losing battle.
I took the Channel Master and raised it about 15 feed higher than the parabolic--and got more signal at every channel from its naked 8-bay bowtie at that altitude. Sigh.
Then I did a comparison of the Channel Master versus a pair of stacked broadband Yagis (AntennaCraft) and got surprising news: at almost every frequency, the Yagis beat the Channel Master. Obviously in VHF (actual channels 7, 8, and 11) there was no contest...but even in high UHF the Yagis were providing more signal, or were within a dB or two of the Channel Master.
So in this deep-fringe reception area, the lessons are pretty clear (even if somewhat counter-intuitive):
Although this has turned out to be a wild-goose chase (I'm back with the Yagis I started with), hopefully my experience will persuade some of you to not go down this path unless you have a really strong tower to put your 10-foot, 150 pound parabolic on.
Note that this would have to be called deep-fringe conditions--the nearest real station is 75 miles away and some of the stuff I'm shooting for is 125 miles away. Due to atmospherics, signal strength varies significantly during the day, so I always did my measurements within the same hour and comparable weather conditions.
Baseline antenna was using thin copper "whiskers". They are long because (1) I'm trying to get some VHF channels and (2) from what I read, making them longer doesn't significantly hurt the UHF.
Modified the whiskers to make them solid triangular pieces. Within the variability of the signal/atmospheric effects, no measurable difference.
Put a reflector screen behind the active elements. No appreciable difference.
Modified the whiskers and made "stacked" dipoles. Maybe 1 dB difference.
Took a Channel Master 4228 HD antenna and used that as the active antenna (facing in toward the dish). Depending on its location in the dish focal plane and the frequency, got up to 5 dB of gain. The 4228 HD can be modded to improve its gain by another 3 dB (there are a couple of sites that describe how to do this)--I didn't make those mods, but I'll believe the claims.
But---the advantage of the parabolic--collecting a huge amount of signal--is fighting the disadvantage of "no altitude". And the measurements showed that in my case, it's a losing battle.
I took the Channel Master and raised it about 15 feed higher than the parabolic--and got more signal at every channel from its naked 8-bay bowtie at that altitude. Sigh.
Then I did a comparison of the Channel Master versus a pair of stacked broadband Yagis (AntennaCraft) and got surprising news: at almost every frequency, the Yagis beat the Channel Master. Obviously in VHF (actual channels 7, 8, and 11) there was no contest...but even in high UHF the Yagis were providing more signal, or were within a dB or two of the Channel Master.
So in this deep-fringe reception area, the lessons are pretty clear (even if somewhat counter-intuitive):
- Altitude trumps everything else (duh)
- If you need VHF channels, a Yagi or two is in your future
- The 10-foot parabolic didn't give that much of a gain advantage at TV frequencies (this was a surprise to me)
- Modifying the active elements of the antenna didn't make as much of a difference as I'd expect.
- I'm glad I ordered the Channel Master via Amazon, as they allowed me to return it
Although this has turned out to be a wild-goose chase (I'm back with the Yagis I started with), hopefully my experience will persuade some of you to not go down this path unless you have a really strong tower to put your 10-foot, 150 pound parabolic on.
