help me find an FM 1/4 ground plane or 1/2 dipole diy antenna plan for broadcast?? - Page 2 - diyAudio
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Old 7th October 2012, 12:47 PM   #11
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the angle of radiation will be the diffrence
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Old 7th October 2012, 12:50 PM   #12
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Also on the usb side of things usb1 will tolerate longer cables .
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Old 7th October 2012, 12:54 PM   #13
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the dipole with only vertical elements will be approx 75 ohm . As you start to bring the ground element from 180 seg to 90 deg the impedance will change .. also this will alter the angle of radiation...
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Old 8th October 2012, 12:16 PM   #14
DF96 is offline DF96  England
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1/4 wave 'ground plane' is basically half a 1/2 wave dipole. Approximately half the feed impedance (36 ohms instead of 73 ohms). Somewhat different radiation pattern. Sloping the 'ground' elements downwards produces a hybrid with parameters partway between the two. A real 'ground plane' would be an infinite metal sheet; a few 1/4 wave counterpoise elements provide a rough approximation to this.

There is a lot on the web about antennas; some of it is helpful.
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Old 8th October 2012, 01:56 PM   #15
Pano is offline Pano  United States
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Where is Tubelab George? He knows all about these things.
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Old 8th October 2012, 09:25 PM   #16
gootee is offline gootee  United States
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Quote:
Originally Posted by peace brainerd View Post
What, in layman's terms, is the difference between a 1/4 wave ground plane


... and a 1/2 wave dipole?




The semantics throw me a bit. They are described everywhere as completely different animals.

The first is a mast element fed by the signal side, provided with a ground plane of elements connected to the ground side.

The second is a mast element fed by the signal side, provided with a ground plane of a single element connected to the ground side.

What makes them so fundamentally different beyond the fact that the main element is tuned for 1/4 or 1/2 wave?
(I think that the "mast element" is 1/4 wavelength for both cases.)

They are not extremely different from each other. The radiation patterns above zero degrees elevation should be very similar in shape. A full ground plane instead of the approximated one formed by the radials would eliminate the pattern below zero degrees elevation. So if you use the latter type, and you want your signal to be good near the antenna but at a lower altitude, then either turn the assembly upside down or bend the ground radials downward as shown, or more (which eventually would become almost like the dipole).

If this is just to get the signal to your garage, and the antenna is on an upper level, that might apply. I'm not sure if you'd get more gain with the ground plane or not. Edit: Yes, you would. With a full ground plane the pattern would have the same shape and gain, above zero degrees elevation, but with only half the input power. So the gain would be about double that of the dipole (i.e. 3 dB more), in that region, giving 5.14 dBi according to wikipedia.

I would just try the dipole first and see if you get enough signal in the garage.

Or, you could always download 4NEC2 and simulate it.

Cheers,

Tom

Last edited by gootee; 8th October 2012 at 09:41 PM.
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Old 8th October 2012, 10:04 PM   #17
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For a simple 'yardcaster' I would probably just cut a couple pieces of wire ~1/4 wavelength(~31inches,or so),and solder them right to the board. Put one on the antenna output,and one in the groundplane,and stretch them out..no need to get fancy.

It might be good to use a short piece of coax between the transmitter,and the antenna purely to get the transmitter PCB out of the RF field of the antenna..otherwise noise,hum,and other problems could result. (Putting the TX pcb in a shielded enclosure might help,if you have issues there) I would also suggest a clip-on ferrite core on the USB cable.

As far as external antennas,I have had good success with 1/4 wave ground plane type antennas,and even better success with a "J-pole" type antenna. (The J-pole kicked butt for RX/DX too!)
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Old 9th October 2012, 11:15 AM   #18
DF96 is offline DF96  England
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The J-pole uses feeder radiation, as it needs a balun but never has one. It is also fairly narrow-band, and easily disturbed by nearby objects.
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Old 25th November 2012, 12:54 AM   #19
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"What makes them so fundamentally different beyond the fact that the main element is tuned for 1/4 or 1/2 wave?"
I'm not expert, but my understanding is the different pattern they radiate, which effected by height, and other variables in each location.
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Old 27th November 2012, 11:42 PM   #20
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For everything you need to know (and much more) about antennas in (and near) the FM broadcast band:

88108 MHz

EV's Best Top Rated FM and HD Radio Antenna Guide & Reviews
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