Home-made Ground Plane Antenna

[doctordave]

Folks,

I made a ground-plane antenna, cut for 39 MHZ, out of 3/8" copper piping material....has one vertical element and 3 horizontal ground plane elements - each is 6' in length. Wondering if there's anything I should do to tweak this further.....also, how much will the resonance at 39MHZ be thrown off if I also add vertical elements for VHF-Hi and 800 range reception (figuring 3.3'ish and 18'ish inch elements).

And yet another question......I was limited to placing this in my attic.....but only 3 feet below the antenna are household 120 VAC wires(on the floor of the attic)......is it possible to somehow shield those electric wires and thereby get rid of some of the noice in my system??

Thanks much.

Dave

 

[Pro-95]

72" = 1/4wl for 39Mhz

I don't know much about your design. But I do have a question.

Why not just use one vertical and two legs @ 90* from each other pointing down. Would make for a little more compact design and you could put it against a wall in the attic.

You could use a diplexer and make a couple of flat antennas. Multi-bandwidth antennas are very complex. Which is why I elected to go several antennas and a switch.

To sheild the antenna from the electrical, how about grounded aluminum foil laid over the offending source?

 

[fourwd1]

There is (or used to be sold) a multi-band groundplane scanner antenna that had 3 verticals of different lengths (VHF Lo, VHF Hi, UHF), spaced ~4" apart on a common mount, and the standard 4 radials.
Looks something like this:


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AFAIK it worked fine.

I would not worry about the proximity of the vertical elements reacting with each other on a receiving antenna.

There is/was also a multi-band multi-element dipole scanner antenna, manufactured by a TV antenna company. Looks something like this:


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Also, the inexpensive 1/4 wave CB ground planes can be cut to other freqs, they even come with cutting charts. Years ago I cut one for 39 MHz, and I later cut it for use on 6M ham band.

 

[doctordave]

Thanks for the responses. I have essentially modeled off the old Radio Shack 20-014 monster ground plane scanner antenna w/ my design - but have cut it for 39 MHZ.....again, the vertical radial is 72" and the four ground plane radials are 72" and perfectly perpendicular to the vertical element.

Overall, it works slightly better than my scantenna on VHF-Low.

Should I keep the ground radials perpendicular to the vertical (precisely 90 degrees)....or should I flare them down at a wider angle? I ask this, because many base scanner antennas (such as the Antenna Specialists variant) seem to have about 120 degrees of angle b/w vertical & ground elements.


Thanks a lot.

Dave

 

[Pro-95]

QSL.NET 2Meter ground plane design
-or-
Alabama Railfan two legged ground plane

 

[nd5y]

It should not make any difference for scanning. The reason that most ground plane antennas have radials sloping down is because it changes the impedance. A 1/4 wave vertical over a perpendicular ground plane has an impedance of about 30 ohms. when you bend down the radials to about 45 deg. the impedance increases to about 50 ohms. It also lowers the radiation angle a little. If you continue to bend them down 180 deg parallel with the feed line (or use a sleeve instead of radials) the impedance is 70 ohms, which is the same as a dipole.

Tom ND5Y

 

[doctordave]

Thanks for the feedback. This homebrew antenna actually works nicely...made it into a triple bander, precisely cut for the freqs I mostly monitor. Subtle improvement w/ changing the ground plane radial angles from 90 to 45 degrees. Overall...actually works a bit better than my SCANTENNA.

Any thoughts on whether some other antenna would be better on VHF-Low for omni-directional reception? Have always been curious about the A/S base antenna....it is 30% smaller than this beast of a quarter-wave I've constructed....perhaps I'm optimized on VHF-Low, short of going w/ directional antennas and/or renting a 200 ft tower?