making one dipole element longer

[radionortheast]

Just wondering what the effect would be, if the aerial kept the same size you made the driven element longer and the ground shorter, what would the radiation pattern be?

[teckniqs]

It wouldn't change the radiation pattern I don't think, just the matching, impedance, SWR etc.

[NOYB]

I'm not so sure it won't change the radiation pattern. Thinking in extremes, medium wave normally uses a monopole with the best possible ground-plane that tries to make it equivalent to a dipole. When/if the ground plane is not perfect I'm pretty certain it causes more upward rather than outward radiation. I can't see why the same would not be true with VHF or other frequencies. My instinct says if you don't want to go with a perfect dipole, then look at a design that has been designed to be physically assymetric - like the Slim Jim etc, or alternatively if you want to feed from bottom, a shrouded dipole.

[Albert H]

The feedpoint impedance of a lengthened dipole will rise (so the SWR from a 50ohm source would worsen) and the radiation would tend to go upwards - fine if you want to talk to satellites, but not so good for coverage at ground level.

One of the most effective extended vertical aerials is the 7/8ths. This is 7/8ths of a wavelength long (actually slightly shorter than that because of the Velocity Factor), and requires a minimum of three radial ground planes, each 1/4 wavelength long. The radials need to be bent downwards at about 60 degrees from the horizontal to get a 50ohm match. From the ground, this looks like a CB aerial, so it confuses the clueless rig thieves. It also gives a reduced signal beneath the block, which fools both the thieves and the Radio "Authorities" into believing that the rig is elsewhere! The radiation pattern is very good for broadcasting.

You can also get a good match at 5/8ths of a wavelength, but the radiation pattern isn't quite as favourable.

[radionortheast]

outsideband.jpg

I'm not so sure it won't change the radiation pattern. Thinking in extremes, medium wave normally uses a monopole with the best possible ground-plane that tries to make it equivalent to a dipole. When/if the ground plane is not perfect I'm pretty certain it causes more upward rather than outward radiation. I can't see why the same would not be true with VHF or other frequencies. My instinct says if you don't want to go with a perfect dipole, then look at a design that has been designed to be physically assymetric - like the Slim Jim etc, or alternatively if you want to feed from bottom, a shrouded dipole.

thanks for that, i’m struggling at the moment trying to get swr down of a loft aerial, dipole trying to find something that will work in this situation where an aerial is effected by nearby objects.. My dipole is on top of some plastic boxes, touching the top of ridge tiles is held there by pressure, I have the coax taped to the trustes, 7 meters of urm 76. I was getting very bad swr nearly 3, 2.8 yesaday, it was showing resonance outside of the fm band. Its an old nrg dipole, i’ve managed to get it down to 1.6/1.7 by removing the end bits shortening it…the only other choice is build some end fed aerial dipole, theres a vent a tube it might be hard. i’m not sure a slim jim would work, might be too large, unless changed frequencies, i’ve been thinking about it know they have some gain.

anyway i’ve been tested it today there was about 10-20% increase , still get dips in the signal, like everyone says don’t use a loft aerial!. It dose seem to be working, even thought it maybe touching the top and bottom, theres noway the roof or the plastic will be acting as aerial its just changing the resonant frequency. I’d like to get it down to 1, the lowest reading is 1.4..

[radionortheast]

The feedpoint impedance of a lengthened dipole will rise (so the SWR from a 50ohm source would worsen) and the radiation would tend to go upwards - fine if you want to talk to satellites, but not so good for coverage at ground level.

One of the most effective extended vertical aerials is the 7/8ths. This is 7/8ths of a wavelength long (actually slightly shorter than that because of the Velocity Factor), and requires a minimum of three radial ground planes, each 1/4 wavelength long. The radials need to be bent downwards at about 60 degrees from the horizontal to get a 50ohm match. From the ground, this looks like a CB aerial, so it confuses the clueless rig thieves. It also gives a reduced signal beneath the block, which fools both the thieves and the Radio "Authorities" into believing that the rig is elsewhere! The radiation pattern is very good for broadcasting.

You can also get a good match at 5/8ths of a wavelength, but the radiation pattern isn't quite as favourable.

thought it might go upwards! so are 7/8ths are they the same as this ground wave aerial? http://www.pcs-electronics.com/66108-50 ... p-914.html

thought about making one of those, http://www.pcs-electronics.com/guide_antenna.php don’t think there would be much difference with the dipole in this situation, i’ve really struggled to built it with it not been two straight rods. would be cool if you could post pictures diagrams of the aerial if its different from the groundwave on the pcs website.

[radionortheast]

just incase anyone is wondering I found the aerial was actually touching some metal that was holding the roof trusters togeather, i’ve moved the aerial foot away it has cured the problem, I now use an aerial of the correct wavelenght. sorry feel i’m wasting peoples time with my experiments!