TV antenna questions

oldsloguy

Help!
I would appreciate thoughts on TV antenna’s and antenna principles. After the digital change over I’ve lost several channels. A couple of channels come in sometimes pretty good, but with continuous minor annoying breakups. I would like to understand a few principles before I attack in ignorance.

My understanding is that all digital transmission is via UHF, which is the smaller portion usually on the tail of a standard antenna. Does the VHF portion serve any purpose now other than maybe AM radio reception? And can it be removed it as it causes interference with other structures limiting antenna rotation?

How much help would raising the antenna provide? How does the signal strength usually vary with height? The antenna is currently about 20 ft. or so above the ground, but only about even with the peak of the second story roof, which is in the direction the broadcast is coming from and also metal tile.

Thanks for any help anyone can provide.

negitron

AM radio does not use the VHF spectrum, it uses the MF band. That said, UHF is more line-of-sight than VHF so if your transmitting station is over the horizon, you want to get your antenna up higher. Gain is another important antenna characteristic and in this case more is usually better. Gain will be specified in dB; the larger the number, the higher the gain. Gain essentially means the antenna can pull in more signal; each 3 dB increase represents a doubling of signal level. There's no need to modify an existing TV antenna by removing the VHF elements; the corner-reflector Yagi design typically used for the UHF portion is highly directional. But, you can make more better use of your available antenna real estate by tossing it entirely and getting either a standard high-gain UHF TV antenna or one designed specifically for digital. In either case, unless you live out in the sticks, you don't need amplification and a good antenna should run you less than $75 or so.

oldsloguy

Thanks, really appreciate it!! Is there anyway I can tell what the dB rating of my current antenna is by its dimensions so that I'll know where I'm at relative to what I might buy? Also, if I'm getting intermittent reception now, how many signal doublings or 3dB gain increases would you guess that I would need to bring-in a solid signal?

oldsloguy

One other question. If I were to purchase a second antenna would it be useful to mount it on the same pole and wire the two in parallel? Does this increase the signal strength or would the two interfere with each other?

Thanks Again

berkeman

In case you hadn't already checked this out for info:

http://www.fcc.gov/cgb/consumerfacts/dtvantennas.html

.

dlgoff

You might find this interesting. Just type in your zip code then continue and find which signals are available in your area. I found that I can receive digital signals better on the back side of my combination UHF/VHF antenna than I could with the old analog signals.
http://antennaweb.org/aw/Address.aspx

oldsloguy

Thanks guys, that was great help! The Sites allowed me to align the antenna with the towers for the stations I wanted and presto. I hadn’t realized that different local area stations are on different towers.

Really appreciate the help!

I hadn't realized at how sensitive the directional alignment was. An 8 to 10 degree turn made a huge difference. What is the physics cause of this severe directionality?

negitron

This animation shows a Yagi antenna in action:

http://www.nottingham.ac.uk/ggiemr/research/Yagi.htm

The thin rectangle is called the driven element and is the element connected to the feedline. The longer line below it is called a reflector. There may be more than one, although one or two is typical. The shorter lines above the driven element are called directors. There are usually several; the more directors, the greater the overall gain and the higher the degree of directionality. The reflectors and directors operate on the principle of parasitic radiation--the absorb signal and re-emit it either in phase or out-of-phase depending on placement and dimensions. Simplistically, these parasitic emissions combine out-of-phase to cancel signals in unwanted directions or in-phase signals to reinforce them in the desired direction.

If you have several stations broadcasting from different directions, you may wish to look into having a rotator installed. This will allow you to change the direction of the antenna from your couch whenever you change channels.

oldsloguy

One last question, just of general interest. Is it possible to wire two antenna in parallel and increase the signal strength?

vk6kro

You can do this for a single frequency antenna but not for a TV antenna which must deal with many frequencies. (Each channel is a different frequency range).

The problem is that the antennas are not in the same position and so you must feed them with feedline but feed each in phase with the other.
Different lengths of feedline would thus be required for each frequency or you would have some channels where the two antennas would cancel each other out.

oldsloguy

Thanks, just something I've always wondered about. With the DTV antennas being so directional, I thought I might cheat and put a couple in parallel pointing at different towers.

Appreciate your letting me knnow!!

chroot

Don't try to connect multiple antennas, it won't work. Go purchase a decent quality antenna meant for HD reception, install it (alone) according to the manufacturer's directions, and come back if it does not meet your needs.

As fun as some RF experiments might be, I don't think you'll be able to do better than the engineers who designed the store-bought antennas -- and least not with the kind of things you're playing with.

- Warren

berkeman

[ hijack ] So here in the Bay Area, we lost our CBS Channel 5 digital over-the-air signal about a week ago due to moving an antenna higher up a tower or something. Re-scanning all the channels does not pick up the new channel, but how do you re-scan the DTV and rotate the antenna at the same time? Do digital TVs have the option to scan for one particular channel as you rotate the independent antenna? So far I've only found a scan-all TV option, which takes several minutes.

And if I re-scan at different antenna positions, do I lose the channels that were previously found if I move the antenna to an angle that drops a previously-found station? [ /hijack ]

vk6kro

If you already had two identical TV antennas there is nothing to stop you trying to put them in parallel. However, the chances of success are not good.
You stand to gain maybe 3 or 4 dB but possibly lose 20 dB if you don't get it perfect.

If you have two signals coming from different directions, you can either:
1) Get two antennas and point one in each direction and switch the feedlines
2) Get one antenna and rotate it between directions as required.
3) Get cable TV :)

oldsloguy

I’m also in the Bay Area, but over the hill in Tri-Valley area. That sounds exactly like the problem I was having when I started this thread asking for help. I was getting 7 and 11 very clearly, but either broken or no signal for all others including 5. I thought that since I was getting 7 and 11 very clearly that my orientation was OK, but it wasn’t.

The website that you listed in your earlier post had embedded within it the key, which was:

http://www.fcc.gov/mb/engineering/maps/

On the above website, enter your address and a list of stations and a map will appear. If you click on the stations in the list you will get details of signal strength, and which tower they broadcast from. I was surprised that there were three towers I could get signals from, W, S and N(5). Also when you click on the station it shows a map showing you the location of the tower. I compassed it on a map and rotated the antenna. The change was only 8 to 10 degrees but the difference was remarkable. Poof, all channels appeared.

I also realized that there were multiple towers, which prompted my question regarding parallel wiring of antennas that was answered by vk6kro and chroot above.

Again, thanks all for the help and let us know if worked for you.

dlgoff

I live on a hill between two large cities and rotate a fairly directional antenna. When I bought my new DTV, I did the scan and wanted to do it again in the other direction. But like you realized it would delete the previous scan. Turns out that the signals coming in on the back side of the antenna are strong enough to be caught in the scan. The TV then has an option to mark a channel as "weak" or "hard to get". I guess it is some sort of automatic gain at work. I haven't had to use my rotor since.

vk6kro

If you did go with two antennas, and got good signals with each, you could consider a signal combiner. Like these:
http://www.solidsignal.com/cat_displ...al%20Combiners

They are a bit lossy, so the signals end up slightly weaker than before, but they avoid the catastrophic cancellation effects you can get if you just put the antennas in parallel.