On 05/31/05 12:33 ET, barlow said...
Ernie,
What would happen if alebowgm left one side of
the UHF circular antenna disconnected ?
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.
Would
he still receive UHF signals ?
Depending on how much the signal strength overcomes the poorness of the antenna.
Would he in effect
see 1/2 wavelength UHF reception instead of full
wavelength?
No, a single wire sticking in the air is a quarter wavelenght antenna,
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.
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.
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 ?
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.
Sorry for the dumb questions,
-Don B
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.
On 05/31/05 18:51 ET, alebowgm said...
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),
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.
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.
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.
PBS and I have seen slight makeoutable picture
What you do with your girlfriend while watching TV is hardly our business!
Hence why I was asking why I need a round UHF
antenna.
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.
(we need a new nickname for Source, lol)
How about Sores?
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
No,you are not. You have already solved that limitation by using the 300 - 75 ohm transformer.
I think the splitter and 75 to 300 adapter will
need to stick in place.
They work, right?