FlightAware Discussions

Antenna Testing, Home Built super J pole or ¼ wave?

That would actually be a gain of 0 dBd. (no gain, no loss, with reference to unity)
1 dBd would be just a tad over unity.

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I will check the bug in simulation, but meanwhile as far as I know, the 1/2 wavelength mono-pole with ground-plane is equivalent of a full-wave dipole, not that of a half-wave dipole.

Quote from Wikipedia:
https://en.m.wikipedia.org/wiki/Monopole_antenna

Gain of dipole & monopole antennas

Note that for monopole with ground-plane, length is 1/2 of dipole.

Dipole Length in wavelengths Monopole length in wavelengths Directive gain (dBi) Notes
≪0.5 ≪0.25 1.76 Poor efficiency
0.5 0.25 2.15 Most common
1.0 0.5 4.0 Not used
1.25 0.625 5.2 Best gain (5/8)
1.5 0.75 3.5 Third harmonic
2.0 1.0 4.3 Not used

@g7pnu
@belzybob
@SweetPea11

Removed the bug in simulation.
Here are results.
Please note that these simulations are for antenna in free space. When I find time, I will simulate for antenna at 6 meters (20 feet) above real ground, and post results.

(1) Spider 1/4 wavelength Whip
Geometry: Whip = 68 mm, Radials = 8 x 68 mm, slanting 45 deg, Built on connector SO239
Gain = 1.31 dBi
Impedance = 66.1 + j 24.7
SWR 50 = 1.66

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(2) Spider 1/2 wavelength Whip
Geometry: Whip = 138 mm, Radials = 8 x 68 mm, slanting 45 deg, Built on connector SO239
Gain = 0 dBi
Impedance = 511 - j 722
SWR 50 = 30.7

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(1) Horizontal Ground-plane 1/2 wavelength Whip
Geometry: Whip = 138 mm, Radials = 8 x 68 mm, Horizontal, Built on connector SO239
Gain = 3.5 dBi
Impedance = 833 - j 658
SWR 50 = 27.1

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SIMULATION 1 OF 3: Spider, 1/4 wavelength Whip
CLICK ON IMAGE TO SEE FULL SIZE IMAGE

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SIMULATION 2 OF 3: Spider, 1/2 wavelength Whip
CLICK ON IMAGE TO SEE FULL SIZE IMAGE

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SIMULATION 3 OF 3: Horizontal Ground-plane, 1/2 wavelength Whip
CLICK ON IMAGE TO SEE FULL SIZE IMAGE

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That’s nearer the mark; at these frequencies an antenna at any reasonable height is going to be effectively ‘free space’.

Yes, a mistake in modelling caused it, possibly because I have prepared the model late in the night while I was feeling sleepy. :slight_smile:

@g7pnu

Did you try PY4ZBZ antenna?
It is very simple and easy to make.

Thanks for running the simulations and explaining why a 1/2 ground is a bad idea. I like the look of the py4zbz it looks like it could be made from ridged rod and be very stable. Thanks again @abcd567

@g7pnu
One drawback if installed outdoor: Rain or dew drops will short the two sides of hair-pin (stub), and it will perform bad, till it dries.

I used the core of some cellphone coaxial cable I had handy. About 4mm diameter. SO239 soldered directly to the matching section. The ‘chocolate block’ connectors were to allow quick changes to the antenna length.

Not pretty, but cheap and simple.

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The principle and probably sole advantage of the 1/4 wave GP (or its ‘cantenna’ variants) antenna for home builders, is that it is the easiest to replicate and thus get an effective result.

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Absolutely right.

Spider and Cantenna are naturally tuned for 50 ohm / 75 ohm system, and have lot of dimensional tolerance. These two characteristics makes them ideal DIY by a novice.

This is the reason in the thread Three Easy DIY Antennas fo Beginners, I have included only Spider, Cantenna, and Quick Spider.

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Total agree the 1/4 waves can be built again and again and you will get consistent results. They are also are easy to mount and are robust.

Thanks for posting your tests. I’m curious…looking at the photos of the J-pole I see what looks like fiberglass cloth near the coax feed point. Why is that there? Tnx.

Probably just insulation or strain relief from the coax.

Doesn’t look like it has a purpose.

Good morning. tnx for reply… Yeah, I didn’t think it added to the antennas performance :slight_smile: Maybe like you say it’s for strain relief. Have a great 2019.

No i meant it is just material that emerges when you strip the coax cable. (It could have been for strain relief in the coax but not anymore :slight_smile: )

Happy New Year.

OK…I’ve never seen material like that in the coax I use, rg-8/58/174 coax…but have seen foil used along with braid. I don’t know what the other types have under the jacket…unless it’s the shield…maybe the lighting or angle of the photo makes it look like fiberglass cloth… anyway, hope his experiments continue. I enjoy seeing what others are building.

Yeah i haven’t seen it either. But it would make sense for structural stability.

Could be a coax meant for laying directly in earth?