O/T: Folded Dipole

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After more than 20+ years of service, my analog TV died.
Since the switch to digital is coming, got an LCD as a replacement.
Unboxed the new unit, plugged in the power and the existing folded dipole antenna.
Followed the set up instructions, unit works like a champ.
That folded dipole took about 20 minutes time and less than $0.10 of 300 ohm twin lead to build.
So much for overpriced antennas and cable.
Lew
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Do you have a link that shows how to make one of these? I have some extra cable sitting around and our current antenna isn't very good.
-Nathan
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"nhurst" wrote:

No, but it is simple enough to build.
1) Cut a piece of 300 ohm twin lead 60" long. 2) Solder ends together on each end to form a continuous circle. 3) At exactly the mid-point, cut one of the leads in half. 4) Solder a piece of 300 ohm twin lead at the cut side, thus forming the antenna lead wire to TV set.
Stretch wire along a wall and attach to wall with a couple of straight pins, then attach lead to TV.
Enjoy.
Lew .
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The dipole you discribe is cut for VHF low. The new digital frequencies are on UHF. You'll need to make a new antenna.
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"CW" wrote:

Seems to be working just fine, especially on the UHF channels.
Matter of fact, digital UHF is much better than analog UHF was.
Lew
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Lew Hodgett wrote: ...

... If you have enough signal level to have digital, that's generally true.
Otoh, when there isn't enough you have nothing w/ digital where analog was probably still just snowy/ghosty but at least visible.
So far here we're 1 for 2; the other two have delayed 'til the June witching date. When they make the switch, then I'll investigate what it'll take to get 'em all if it's within reason; 'til then I'll just do w/o the PBS. Discussion w/ their engineer wasn't promising that they thought would have a signal (and didn't really seem to give a flip, either, of course).
--
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As long as the signal is strong.
When the signal is weak, the picture doesn't just get "fuzzy."
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Maxwell Lol wrote:

This is the big problem with digital--it doesn't degrade gracefully--it's all or nothing.
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writes:

Where did you hear that? My digital can be terrible right in between perfect and nothing. Terrible pixelization and this goes for my neighbor, BIL, and father.
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On Wed, 22 Apr 2009 07:32:38 -0500, Leon wrote:

Yes, you _can_ have a marginal signal. Its just that the signal strength "knee" is quite sharp. If you have a picture with artifacts present due to marginal signal strength, it won't take much of a decrease in signal strength to get to where no data can be recovered ... and no picture produced.
--
Art Greenberg
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I think it goes with out saying that if as the signal degrades enough you eventually get no picture. That said however most satellite receivers have a built in signal strength meter and you can check individually sources for signal strength. Not until my strength gets down to "about" 60% do I actually loose a picture. Anything in the 60 to 80 % range can cause mild to moderate picture quality loss. Typically 90% and better produces a great picture. It is not uncommon for picture quality to degrade on a daily basis and oddly some channels are worse than other.
And this is exaggerated more with satellite HD programming, signal strength has to be higher for a better picture. I have 2 TV's using the same dish but 2 completely different kind of receivers/DVR's One is HD, one is strictly regular definition. The HD receiver picture quality is much more fussy about the strength being higher to get a good picture. With the regular definition receiver signal strength can drop considerably lower than the HD unit before noticeable picture quality drops. I have a regular TV hooked up to a "off air" rabbit ear antenna and digital tuner for local channels and most of the time the picture quality if perfect. I am about 8 miles from the transmitter towers.
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CW wrote:

There are no "new digital frequencies". There are different channel assignments but the frequencies are the same as for analog TV, except that for digital there are fewer of them since the FCC has chosen to divert one block of frequencies to other purposes.
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CW wrote:

The top 108Mhz of the UHF band has/is being assigned or auctioned off for other purposes, otherwise all old TV frequencies are the same. However, there may be some stations migrating from VHF to the remaining UHF frequencies. As well, some stations may not alight on their final frequency assignment until the June deadline.
Formerly K7OQF from the vacuum tube days.
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On Tue, 21 Apr 2009 17:09:43 -0700, Doug Winterburn wrote:

Low VHF (analog channels 2 through 6) likewise.

--
Art Greenberg
WA2LLN
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On Wed, 22 Apr 2009 08:57:20 -0500, Art Greenberg wrote:

My bad. I just researched this ... channels 2 through 6 are still allocated to DTV use, but it looks like there will be very, very few stations using those channels for broadcast.
--
Art Greenberg
artg at eclipse dot net
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Cut a piece of 300 ohm twin lead 60" long
was told that this dimension was related to the bandwidth one expected to receive. If so, wouldn't there be a difference in required length for UHF vs VHS, vs FM, say?

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I'll let any Hams on list answer your question; however, 60" covers everthing including FM in my area.
SFWIW, Radio Shack sells a factory made unit which is 60".
Lew
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wrote:

The "accepted" (on FCC tests) simplified equation for a half-wave dipole wire length is: feet = 468/MHz.
A folded dipole simply folds each leg of the dipole back to the center. Effectively, the 5' long folded dipole has 10' of wire. Its resonant frequency is thus 46.8 MHz according to the formula. Interestingly, this works out to about a full wave for the 100 MHz FM band. It might be worth experimenting with slightly shorter lengths, moving the resonant frequency to, say, 75 MHz. The gains will be minimal, if measurable at all (let alone noticeable).
The simplified formula differs from the theoretical value in a vacuum by the velocity factor of the wire, in this case apparently about 95% (from 492/MHz in a vacuum). I wouldn't worry much about it. The antenna's resonant frequency is not nearly so important for receive-only operations as it is for transmitters. A mismatch on a transmitter presents a very high impedance, causing the feedline to also radiate, and plays all kinds of havoc to equipment in the vicinity.
A dipole is also somewhat directional, with about 2 dB of gain in its broadside direction compared to a point radiator. This implies the same 2 dB attenuation in its side lobes, off the ends. Given a choice, I would face the antenna toward the signal and the ends toward the local RF noise. However, if reception is so marginal that this is enough to make or break the chain, consider it broken and get a tuned, multi-element, directional antenna. The same goes fiddling with the wire length.
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On Sun, 26 Apr 2009 13:42:28 -0500, MikeWhy wrote:

That is the "accepted" formula because it yields the proper length for any half-wave long wire antenna, including a folded dipole.

Not correct. The resonant frequency of a half-wave dipole, folded or otherwise, at 60 inches long is 93.6MHz. That is a little below the center of the FM broadcast band (98MHz).
The effect of making the half-wave antenna in this manner is twofold; (1) it raises the feedpoint impedance to 300 ohms, as compared to a single-wire half-wave dipole at about 75 ohms; and (2) it increases the useful bandwidth of the antenna somewhat over that of a single-wire half-wave dipole.
As with any antenna, the actual resonant frequency, feed point impedance and bandwidth will all be somewhat dependent upon the environment around the antenna.

Yes. In free space (vacuum), the wavelength of a radio wave is found by the equation 300/(Frequency in MHz). For example, 50MHz has a wavelength of 6 meters in free space. Converting this to feet, where 1 meter is 3.28 feet, gives 984/(Frequency in MHz). Divide by two for a half wavelength and you get 492/(Frequency in MHz). So you see, the equation used to compute the length of a half-wave antenna takes into account fact that we are computing the length of a physical antenna, rather than free space wavelength. It is resonably accurate so long as the diameter of the conductor is very small compared to the operating wavelength.

Very oversimplified, but probably appropriately so for this discussion. However, I would not go so far as to say that the frequency an antenna is "cut" to isn't important for receiving operations. The antenna will not perform as well if it is mistuned, and that effect can be dramatic, depending upon how far off resonance the antenna is.

I think you're referring to an isotropic radiator? The gain of a dipole (folded or not) is around 3dbi IIRC. This is the ideal gain broadside to (perpendicular to) the antenna. The actual gain will be very much a function of height above ground, the conductivity of the ground, and the proximity and type of of surrounding opjects.
The gain "off the ends" can be very low, much worse than 2 or 3db below the maximum.

Often the proper orientation of any antenna with a small to moderate amount of directivity, such as a dipole, is a compromise. We can't always arrange for an interfering source to be 90 degrees away from the direction of the transmitter we are trying to receive signals from.
--
Art Greenberg
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Since we care about nits today, 2.15 dBi, actually, and not entirely applicable due to the parasitic coupling you mentioned. The end nulls will certainly be considerably different from a simple wire dipole because the end current nodes do not fall to zero as they must on the simple wire.

Apparently, you were not then "given a choice."
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