VHF Antenna Question - towertalk Forum at eHam.net

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VHF Antenna Question	Reply
by LETTERX on October 27, 2009	Mail this to a friend!
I have a VHF antenna question. I need to create an antenna for a scanner that can be attached to a glass surface (in other words, two dimensional). Obviously this is for receiving only. Which will work better? 1. A vertical dipole with the center conductor of the coax attached to a 1/4 wave element pointing up, and the shielding of the coax attached to a 1/4 wave element pointing down. 2. A 1/4 wave ground plane with only two plane elements, with the center of the coax attached to a 1/4 wave element pointing up, and the shielding of the coax attached to two horizontal 1/4 wave elements, one pointing left, and the other pointing right, so that the whole antenna looks like an inverted "T". 3. Some other configuration of 1/4 wave elements.

RE: VHF Antenna Question	Reply
by AA4PB on October 27, 2009	Mail this to a friend!
I think option 1 would give you the best omni-directional pattern. It amounts to a vertically mounted half-wave dipole.

RE: VHF Antenna Question	Reply
by WB6BYU on October 27, 2009	Mail this to a friend!
I just installed option 2 on my window. It is just as good as option 1 from a pattern standpoint (at least as modeled using EZNEC). The main choice between them is the direction that the coax cable comes off: from the side for a dipole, or from the bottom for a ground plane. Use whichever is more convenient in your situation. Another antenna I've used on a window is a rectangular loop: use the general formula for circumference but make it twice as wide as it is tall and feed it in one of the vertical (short) sides for a 50 ohm feedpoint. With any antenna there may be some detuning due to the dielectric effect of the window glass, but that hasn't been too much of a problem in practice. My latest ground plane is for 440, and it is still too long but it works well enough that I haven't bothered trimming it. This is a good time of year to stock up on small suction cups for mounting such antennas, as they are often sold to string Christmas lights around a window or hold decorations.

RE: VHF Antenna Question	Reply
by WB6BYU on October 27, 2009	Mail this to a friend!
Let me add that the two radials slope downwards on my antenna to give a better match to 50 ohms. I just soldered the coax center conductor to some flexible wire for the radiator and some stiff wire for the two radials so they stick out the sides. The antenna hangs by a string tied to the vertical wire, somewhere near the center of the window, with the coax dropping down under the bottom of the blinds.

RE: VHF Antenna Question	Reply
by WX7G on October 27, 2009	Mail this to a friend!
The ground plane configuration will have less feedline shield current but not by a lot. The dipole or the ground plane will work fine. Another design - one that has even less feedline shield current - is a one wavelength loop fed in the center of one vertical side. The downside are the nulls in the radiation pattern.

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