Channel Detection Scan (channels 2-6)

Want to write your own code to work with a HDHomeRun or work with the HDHomeRun DVR? We are happy to help with concepts, APIs, best practices.
NOYB
Posts: 202
Joined: Thu Dec 12, 2013 2:12 am

Re: Channel Detection Scan (channels 2-6)

Post by NOYB »

Don't believe every online calculator you see.
That calculator also thinks a ch. 10 (195 MHz) bowtie would have to be about 4 feet wide (2 feet each side). When I know for a fact that a 13 inch wide (6 inches each bow, 3 inches high, and 1 inch feed point gap) works just fine. Gets 100% indoors at 17 miles LOS. I suspect it will go much lower. That is a bowtie from the ch. 83 era. that came with a TV back in 1979. That is a very wide bandwidth. A bowtie for any frequency should not need to be any wider than a dipole and will have significantly wider bandwidth. A lot of online calculators are geared for designing transmission antennas for/by HAM operators to keep SWR acceptable and not burn out the transmitter. Receiver antennas can deviate from that considerably and still work very well.

Sure a combiner can be used. But overkill if a single antenna can do the job. For many people within 20 miles LOS it probably can.

jasonl
Expert
Posts: 15637
Joined: Sun Oct 28, 2007 9:23 pm
x 33

Re: Channel Detection Scan (channels 2-6)

Post by jasonl »

The problem is that since wavelength is calculated by dividing by the frequency, the lower the frequency goes (especially below 300MHz), the more the wavelength changes from one channel to the next. Bowties are effective for UHF because the wavelength doesn't change very much across the band. The wavelength of channel 14 at 477MHz is 24.7" and the wavelength of channel 51 at 695MHz is 17". Not a huge difference and you get an extremely wide bandwidth on a bowtie as a result. The wavelength of channel 2 at 57MHz is 207" and the wavelength of channel 6 at 85MHz is 139", so a bowtie designed for that has much less bandwidth. VHF antennas are almost always log periodic or some variant of it (log-yagi, etc.), and that's because it's the only way to get enough bandwidth to get relatively consistent gain and a low SWR across the band. And SWR matters, since a higher SWR means reflections on the transmission line which can cause unpredictable reception. It's great that yours works on 10, but that same setup might fall apart on a different high VHF channel, and it would have no relevance whatsoever to low VHF.

NOYB
Posts: 202
Joined: Thu Dec 12, 2013 2:12 am

Re: Channel Detection Scan (channels 2-6)

Post by NOYB »

Go back and look at the conditions I cited. Within 20 miles LOS. That frequently results in favorable reception situation. Very modest antennas can and do work very well in those situations. Even for VHF low band. Piece of wire, single rabbit ear, 13" x 3" bowtie. A dedicated antenna for VHF low band very well may not be needed.

Rabbit ears worked for VHF low for many decades. And they still work. And so do bowties. Probably better now than ever because of digital (ghosting, etc.).

There are 4 VHF high channels here and they all work fine. Ch. 10 was just the example cited. It should not have been taken as though it was the only one and that others would fall apart. None of them fall apart 8, 10, 11 (low power), and 12. With signal levels/quality of 100/98, 100/100, 70/64 (low power), and 95/98 respectively.
That covers most of the VHF high band and none of them fall apart and all of them are in line with UHF channel reception performance. Yes it works for me, and it will work for others too.

I'm not saying it is some miraculous 50 mile indoor antenna. But for those in good reception situation like I've cited it may be all that is needed for the entire TV spectrum. Nicer than having to deal with an extra antenna if it's not really needed.

Typical receiver antennas don't just fall off a cliff at their bandwidth spec. It's a roll off and they can and do work well beyond that depending on the reception situation. ex: UHF bowtie, and other style antennas, can continue working well below UHF band in favorable reception situations.

As with many indoor antennas. Of course it won't work for some people. None of them are the best solution for everyone. I've read stories of others using the modest little indoor bowtie loop with success too. One particular person went trough buying and returning several antennas, ranging up to over $100 if recall, before finally hooking up the modest little bowtie. As with others of us he found the solution to be simple and cheep. It's pretty pathetic when $100+ indoor antennas get beat out by a 1979 freebie that came with a TV. Their big liability is that they don't have "HD TV", or "Digital TV" silkscreened on them. Ditched my store bought indoor antenna too. Was never satisfied with it's performance.

As glad as I am that there are no VHF low channels here. It would be kind of interesting to see how this modest little bowtie would perform. Or build one for VHF low. Then maybe I could eat crow or you could take a glass half full view. ;)

SWR important. Sure is. But not nearly as much as it is for transmitters. For reception we can deviate from ideal SWR greatly and still have functional antenna in favorable reception situations, like within 20 miles LOS. But for transmitters it is critical to keep SWR in a very narrow range.

ppasteur
Posts: 80
Joined: Thu Sep 05, 2019 3:54 pm
x 1

Re: Channel Detection Scan (channels 2-6)

Post by ppasteur »

I think, judging by the history of the posts, NYOB just likes to argue. So, for arguments sake, all that can be said is that his bowtie works in his situation. Anecdotal data based on a very small sample. I can say that in some situations I personally have seen situations where just connecting the center conductor only of a twenty foot run of coax will get a decent picture. But, for many (likely most) people in the real world, it does not work. Nor would the small bowtie.
Physics defines the length of an antenna that is optimum for a given frequency. The calculators are based on the physics and electrical theories that are well established and have been since the advent of radio communications. If one wants to construct a "good" antenna for a given frequency, it is a good thing to follow the well established formulas. They can unconditionally be depended upon to be accurate.
Of course, if there happens to be an old bow tie antenna laying around and one wants to simply play, give it a shot. Maybe they will be lucky. Maybe you are line of sight to a high power transmitter that also happen to have a favorable radiation pattern in your direction. Heck, maybe you can just stick a metal coat hanger up in the air three feet and it will work. Just don't count on it.

If you are serious about it and want the best chance of getting the station that you want to received, design and build, or buy an antenna that is designed properly for the frequency that you need.

NOYB
Posts: 202
Joined: Thu Dec 12, 2013 2:12 am

Re: Channel Detection Scan (channels 2-6)

Post by NOYB »

Antenna physics didn't begin with HDTV. The modest bowtie TV antenna many people and I use conforms to physics. That's why they were designed and built. That's why they work. The modest bowtie that has about 30" of 300 ohm twin lead was common place back in the days of analog TV transmission and were designed according to antenna physics. I didn't design it. I use it because it works better than the indoor antenna I bought. Others have testified to the same experience.

There are a lot of people in the 20 mile LOS window. Many of them will have a favorable reception situation. A modest bowtie is very likely to work for them too and all the antenna they need.

Takes two to argue. Maybe you just like to argue, so injected yourself here to engage in an argument. I wouldn't have anything to argue about if people wouldn't put forth unfounded assertions. If you read this entire thread you should have come across one persons assertion that had merit. Therefor I did not challenge it. Those who put put forth merit-less assertions are the ones who want to argue or perhaps are disingenuous.

Post Reply