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UHF Antenna Modification

P

Prescott Joe

I want to modify a 4bay UHF Bowtie Antenna with one monopole cut to length
for RF channel 13 and one monopole cut to length for RF channel 6. How can
I calculate/find what lengths they each need to be???
 
For a quarter wave monopole antenna, use these lengths:

Channel 6 = 0.86 meters
Channel 13 = 0.34 meters

Calculation is: (speed of light) / channel's frequency / 4
 
PTBoardOp94 said:
For a quarter wave monopole antenna, use these lengths:

Channel 6 = 0.86 meters
Channel 13 = 0.34 meters

Calculation is: (speed of light) / channel's frequency / 4
Click to expand...

In English, the formula for a dipole is: L (feet) = 468 / F (MHz)

A monopole is half that, as is each leg of a dipole. A monopole will be almost useless because it's omnidirectional, and because it's vertically polarized. A dipole without reflective elements will be one or two notches up from useless, but might work in urban areas.

A dipole cut for Channels 2-4 (63 MHz,. midpoint of Channel 3): 7'6"
For Channels 5-6 (82 MHz): 5'9"
For VHF High (195 MHz, midpoint of Channel 10): 2'4"
For the lower UHF channels 14-36 (539 MHz, midpoint of Channel 25): 10.5 inches

Each dipole will be a good match to 75 ohm coax (RG-6). They can be connected in parallel. A dipole cut for 195 MHz will also operate at its 3rd harmonic at 585 MHz (Just inside Channel 33).
 
I always put up my Antanna at the Highest Point for VHF/FM, I thought that would make a station come in better on Ch. 2-13

So I'm doing it all wrong all this time
 
MarioMania said:
I always put up my Antanna at the Highest Point for VHF/FM, I thought that would make a station come in better on Ch. 2-13

So I'm doing it all wrong all this time
Click to expand...

Height is still important (it's still line-of-sight VHF and UHF, after all), but there are other factors in the digital world that were less of a factor in the analog days. Directivity is the most important, since you want only the direct signal, with no reflections. Reflections resulted in ghosts in the analog days. They can kill a digital signal completely.

Also, if you can avoid using a preamp, do so. Preamps can overload the front end of the tuner and cause false signals due to intermodulation. And get an FM trap in the line, right after the antenna, if you live in just about any major market. Those 50-100 kW stations will also cause overload because they're picked up by the antenna just as easily as TV stations are.
 
KeithE4 said:
In English, the formula for a dipole is: L (feet) = 468 / F (MHz)

A monopole is half that, as is each leg of a dipole. A monopole will be almost useless because it's omnidirectional, and because it's vertically polarized. A dipole without reflective elements will be one or two notches up from useless, but might work in urban areas.

A dipole cut for Channels 2-4 (63 MHz,. midpoint of Channel 3): 7'6"
For Channels 5-6 (82 MHz): 5'9"
For VHF High (195 MHz, midpoint of Channel 10): 2'4"
For the lower UHF channels 14-36 (539 MHz, midpoint of Channel 25): 10.5 inches

Each dipole will be a good match to 75 ohm coax (RG-6). They can be connected in parallel. A dipole cut for 195 MHz will also operate at its 3rd harmonic at 585 MHz (Just inside Channel 33).
Click to expand...

OK, now I'm totally lost. I took PTBoardOP94's .86 and .34 and multiplied by 39, which is the approximate number of inches in a meter. I came up with just over 13 inches for RF13 and just over 33 inches for RF 6. Those numbers are for a monopole, not dipole. I always thought that the an element cut/tuned for example, RF 10, would also work fairly well for RF channels lower than RF 10 (9,8,7) but not very well for RF channels that are higher (11,12,13)or with low band VHF or UHF. Is that correct??? The channels I'm hoping to improve are full power RF 13 at 60 miles and translator RF 6 at 20 miles. They are LOS but marginal most of the time with the UHF 4 Bay antenna. Zip is 86301. I use that antenna because they are capable of receiving UHF from a wide 75 degrees. Yagi antennas are too directional for this area and application. If I position the monopoles horizontally they will not work??? Would vertical monopoles be better??? I appreciate your input.
 
Last edited:
I gave quarter-wave measurements because that is a common way to build a monopole. Keith gave half-wave measurements, because that is a common way to build a dipole.

Keith is correct that dipoles are typically better for television because the impedance match into the TV is closer to the 75 ohms standard.
 
Would there be a chart available online somewhere that would have a listing for the different ideal lengths for specific channels? I'd like to try to build an antenna that would be tuned to specific channels in the Memphis and Jackson, TN areas if that's possible. Thanks!
 
The Antennas Direct retrofit kit can be used with any model of UHF only antenna, as it comes with a combiner. It's for RF 7-13 What's on RF 6 in your area? These are usually operated as backdoor FM stations on 87.7 MHz
 
PTBoardOp94 said:
I gave quarter-wave measurements because that is a common way to build a monopole. Keith gave half-wave measurements, because that is a common way to build a dipole.

Keith is correct that dipoles are typically better for television because the impedance match into the TV is closer to the 75 ohms standard.
Click to expand...

Also, because TV stations are still using horizontal polarization AFAIK.
 
ropertex said:
The Antennas Direct retrofit kit can be used with any model of UHF only antenna, as it comes with a combiner. It's for RF 7-13 What's on RF 6 in your area? These are usually operated as backdoor FM stations on 87.7 MHz
Click to expand...

RF channel 6 is a translator for KNAZ TV, the NBC affiliate for Flagstaff and Northern AZ.
 
Prescott Joe said:
I want to modify a 4bay UHF Bowtie Antenna with one monopole cut to length
for RF channel 13 and one monopole cut to length for RF channel 6. How can
I calculate/find what lengths they each need to be???
Click to expand...

After I received some very helpful information, I cut two 13" elements for
Rf channel 13 and two more elements 33" for Rf channel 6. I tried at least
a dozen different configurations. The antenna was never able to recover
Rf 13 at 60 miles nor a translator on Rf 6 at 20 miles. MORAL: Don't waste
your time trying to modify an antenna. It just doesn't work..... unless you
are an expert.
 
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