Ok, I know someone will have the answer for this and it has been bugging me. Why is it that UHF Antennas, mostly the ones you would purchase at stores such as RADIOSHACK always seem to be circular where as the VHF has the bunny ears? Why dont we just use one for both?

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This message was edited by hogger on 05/30/05 08:40 ET.

Simply speaking, it's a matter of wavelength. The higher the frequency, the smaller the antenna (physically). There are ways to make an antenna behave as if it were a different length than it really is.

Still doesnt answer my question as to why UHF is a circular ring while VHF tends to use bunny ears...

I haven't signed up (yet) or even browsed the site, but there's no better authority on RF than the ARRL (American Radio Relay League).

still not sure

The rabbit ear has one lead lead of each ear tied to one screw on the 300 ohm lead. The bow tie "UHF" antenna that is circular has both ends of the circle tied to each screw in effect sort of like a short circuit vs the VHF rabbit ears open circuit - go figure ----

Than a 300 ohm to 75 ohm converter takes these two screw teminals and combines them into the one input 75 ohm lead with a grounded sheath around the outside of the cable. I think you could actually not connect the sheath and you would still see a signal on the other end but I have not actually tried this.

And alebowgm I hear you about the two configs and don't have an easy answer and I have read a lot of these antenna explanation sites.

I will check the site above and see if it offers a definitive answer.

-Don B

One possible design for an antenna is make make a circle of conducting material where the circumference of the circle is the same dimension as one full wavelength. After I ran across this idea several years ago, I amused myself by making an FM antenna. It was about 40 to 45 inches in diameter, an unwieldy thing at best.

I found that it was directional, but not very much. If I turned it to a direction where it would not receive a station, that station died at a very specific azimuth angle, but if I then turned it 90 degrees from there to maximize the signal, a very wide range of azimuth appeared to work equally well. In other words, it had very sharp null but hardly directional peak behavior.

The FM band is very narrow, so I was not able to investigate whether this antenna would also pick up signals quite a bit off of the target frequency.

If a circular antenna has poor azimuth peaking behavior AND poor frequency peaking behavior, it could be used to pick up signals in a broad range of frequencies and directions. As long as there is plenty of signal in the area, poor performance at one frequency could translate as decent performance across the band.

Calculate the circumference of the little UHF antenna, then figure out the frequency where that would be one wavelength. I'll be it is pretty much in the middle of the UHF band.

You couldn't do that so well with VHF because it has a larger range of frequencies. When you think of range of frequencies, you have to think in terms of percentages. AM has a range of a little more than a megaHertz, but the proportion of top to bottom frequency is almost three to one, which is a HUGE range; FM, from 88 to 108, has a range of 1.22 to one, a tiny range. UHF has a large range, but VHF has a larger range, when looked at this way.

The large range (again, percentage-wise) in VHF has a lot to do with the gap between channel 6 and channel 7; regardless, it would make a circular antenna hard to work with. It might end up being a couple of feet in diameter, which would be quite unwieldy for home TV use.

Rabbit ears are shorter because they are based on a quarter wavelength; they can be turned, and lengthened and shortened, to peak for a particular station. It would be possible for rabbit ears to need to be adjusted for EACH station, but since they are usually used where there is plenty of signal, their real shortcomings are overwhelmed by signal, and you can't see how bad they are. But when rabbit ears are bad, we just look at them and mutter, "of course, anyone can see this is a piece of crap antenna, gotta get an outdoor one." And not think twice about them.

Thanks for the opportunity to think a second time about rabbit ears.

Ernie,

What would happen if alebowgm left one side of the UHF circular antenna disconnected ? Would he still receive UHF signals ? Would he in effect see 1/2 wavelength UHF reception instead of full wavelength?

What if alebowgm connected the tips of the vhf ears together with a jumper wire ? Would he still get VHF reception ? I guess this would be hard to do as the ears would have to be placed close together messing up VHF reception or left apart with a long jumper in between messing up the tuned wavelength.

So what does one see on the electronics side of a 300 ohm VHF/UHF tuner ? Do the twin leads get combined before they go into the detection circuitry or are the two leads handled independently by the detection circuitry ?

Sorry for the dumb questions,

-Don B

Well let me tell you what I have done.

Basically, the place my girlfriend is staying at right now doesn't have cable in her bedroom (and why, I honestly have NO IDEA, as it has more electrical and telephone outlets in that room compared to anywhere else in the house). Anyways, not knowing this, I brought over an antique 18'' TV set that has both the VHF and UHF knobs and a Cable Box. After realizing there was no cable, I came home and found a set of bunny ears that has a coaxial output. I then connect that to a Radioshack Amplifler (cant find model over at TheSourceCC website, if I find it I will post), run that into a Radioshack 5 MHZ - 2450 Splitter (to cover both Analog and Digital Spectrum), with one end running into the 75 OHM Coax input (For VHF reception) and the other end into a 300 OHM Converter which fits into the screws on the back of the TV for UHF reception. There is enough reception in the Bathurst / Steeles area of Thornhill to easily recieve CBC (English), CFTO-CTV, CHCH (directional) and I can once in a while pickup both CHEX and NewVR. On the UHF side, by adding the antenna stations which were coming in with strong audio before and bad video such as Global, CityTV, CITS, CBC (French) now are at least viewable. In the late evenings, I can even pull in WB, Omni 1/2, PBS and I have seen slight makeoutable picture on both UPN and FOX. Nothing from ABC/CBS/NBC (where are all on the VHF).

Hence why I was asking why I need a round UHF antenna. While I would love to go and install some coax into her residing residence, there may be technaicalities with those whom own the property and as a result, she may be stuck with Analog-OTA for a bit. Moreoever, I may need to go over to RatShack (we need a new nickname for Source, lol), and grab a bunch of antennas and see which work the best. Unfortuantly, being that this is such an antique, I am stuck needing to use the 300 OHM for UHF (I can use 300 or 75 for VHF), so I think the splitter and 75 to 300 adapter will need to stick in place.

I am almost tempted to go out and grab her a ASTC tuner, although I would need to find out that would also do NTSC. I figure at least we will be able to pull in CityHD, CBC (E and F) and CTV (as we get them all now). Obviously, being that we are relient on the NTSC, it is a bit of a bind.

Question: Being stuck with having to get an indoor antenna, and perferibly not having to order one (hence, going to Source, Futureshop, BestBuy, etc) what is the best model to grab in the GTA? We are about 10 - 15 miles from the CN Tower. Also, is there any good ASTC tunners (that will down covert to 480p) that also do NTSC?

He would then have a 75 ohm quarter wave length antenna now tuned two octaves lower, with an unbalanced output connected to a balanced 300 ohm input. Both of those factors would degrade the performance. In the real world, there might be enough signal to overwhelm the poorness of the antenna, as I mentioned above. This is totally inefficient, but it actually might help in some circumstances.



Depending on how much the signal strength overcomes the poorness of the antenna.


No, a single wire sticking in the air is a quarter wavelenght antenna,



All of this is just messing with the hunks of wire, in an untuned and unpredictable way, and it might unbelievably improve things, more than likely not.



They generally go through a 300 - 75 ohm circuit inside the tuner, or at the entry to the tuner, because nobody wants to spend the money to make amps that will deal with balanced inputs in that frequency range. Those would be instrumentation amps, and very costly. You can change the signal to single-ended and feed either transistors or ICs specially made for those frequencies.


They're not dumb. Asking questions like that, combined with actually doing the stuff, can teach you a lot. The reason they might not is that you would best learn by doing this AND having a signal level meter or a spectrum analyzer, so you could see what is happening in detail. My best boss learned about transistors by taking a bunch of resistors and meters and varying one thing at a time, monitoring everything, and seeing what happened. Our lack is being able to monitor the results. Also, TV signal inputs are capable of taking a wide range of voltages, so the TV antenna input won't even give you a good idea of what is happening -- the automatic gain control covers subtle changes.





Your admittedly antique receiver does not care about Digital. Remember that UHF is a range of frequencies, and you can put AM, FM, digital, or WM (watermelon modulation) onto it. There are no Digital frequencies, just frequencies.


This is exactly what I did with my old TV when I put a VHF/UHF antenna on the roof. You need the 300 - 75 ohm transformer to get into the antenna input EFFICIENTLY. Better than a splitter would be a UVSJ, which is short for OHF/VHF Splitter/Joiner; this is functionally a diplexer that diplexes UHF and VHF. Don't get hung up on "diplexer" being a satellite term; it is a name for a device that combines or separates two different RANGES of frequencies, not any particular type of signal. Anyway, the splitter lowers the signal by about 4 dB, but the UVSJ would lower it only about a half dB. But they are hard to find.



What you do with your girlfriend while watching TV is hardly our business!


You don't. You could hang an eight-bay bowtie, about 42" square, on the back of that chair over there. Or you could install cable. Or you could get a yagi-style and sling it under the ceiling light. But if you want a small antenna and the signals are strong enough, the circular one will work pretty well.


How about Sores?



No,you are not. You have already solved that limitation by using the 300 - 75 ohm transformer.



They work, right?

Well, I took apart my Phillips antenna, (cheapo from Wlam*Mart) and i lost a screw or the wire lead for the second connection and it still got the UHF channels, just the same. Oh well

Ernie,

Thanks for the great answers.

By the way it sounds like alebowgm may have cable in another room of the apartment. If so he may be able to use a rabbit device to transmit the signal to the tv in the bedroom. I know that we used one of these devices for my mother until we got coax in her room.

-Don

Wait a minute barlow, how did you do this? Transmit normal cable OTA to be picked up via VHF/UHF?

I know of modulating a specific channel but I have never heard of doing the entire analog spectrum...

Don't tell anyone, but most antennae work equally well receiving and transmitting. Try connecting an antenna to the coax output of a VCR and see if a nearby TV will receive it.

The Rabbit Barlow is speaking of is probably the old wire-connected unit that transmits the TV signal one way and the IR signals the other, on a thin 2-conductor wire. No, it's single-channel.