Antenna Radiation - Phase 3

RE: Antenna Radiation - Phase 3

DAB — Sat, 11 Jan 2020 20:50:03 GMT

Very good post.

DAB

RE: Antenna Radiation - Phase 3

kmikemoo — Wed, 08 Jan 2020 22:04:30 GMT

HUGE THANKS to [mention:4f57fc9d538949ad9eb336ddb9469bb8:e9ed411860ed4f2ba0265705b8793d05] and [mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05]!!! I think I can make the cardioid happen. I'm also thankful that it will be much warmer tomorrow. It started out at 5 F (-15 C) this morning.

[mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05] I took your lead and used EZNEC. I was going to see if an oscilloscope would show me my feed point phase angle difference, but why? The objective is the pattern, not the measurement method (at this point).

[mention:4f57fc9d538949ad9eb336ddb9469bb8:e9ed411860ed4f2ba0265705b8793d05] You are right. The issue is the coax and the velocity factor.

To get a good null, everything will need to be right. The distance betweeen the dipoles, the 90 degree shift by delay in the coax for the second dipole [which will be the hardest to get right], and the alignment (the dipoles need to be accurately in the same plane).

"Everything will need to be right" really kicked the wheels in motion. In RF, sometimes the "bandwidth" of the solution is very narrow.

I will try a 270 degree shift accounting for velocity factor. We shall see...

RE: Antenna Radiation - Phase 3

shabaz — Mon, 06 Jan 2020 14:20:38 GMT

Hi Mike,

If the two dipoles are being fed (they shouldn't be paralleled directly to the transmitter for the reason Jon mentioned), then it seems you're getting almost the expected result, so it is maybe a good indication that your measurement method is functioning. I also think the reasons Jon mentions are why the lobes are asymmetrical, since they would be identical if the design and feed is symmetrical.

MATLAB has antenna capability, and it's really neat (and surprisingly easy to use). It doesn't cost a huge amount for the home license (although still more than it should : (

I tried your scenario (if everything was symmetric and fed as two separate antennas) - the result is two lobes, with ~ 3 dB difference between the maximum and minimum:

[View:/resized-image/__size/882x507/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-0a8a6d18-18b9-4378-87d7-ee63eac51d84/5611.contentimage_5F00_205666.png:882:507]

[View:/resized-image/__size/881x444/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-0a8a6d18-18b9-4378-87d7-ee63eac51d84/3566.contentimage_5F00_205667.png:881:444]

I have been recently using MATLAB for an antenna design, but I have not completed it yet (it was supposed to be for the RF Project14, but I doubt it will be finished in time, since I still need to order a PCB.

RE: Antenna Radiation - Phase 3

jc2048 — Mon, 06 Jan 2020 11:20:48 GMT

Good experiments. This is fun reading these blogs.

So... where did I go wrong?

From what little I know of rf stuff, my guess is it relates to the coax feed. Your elements are physically 1/4 wavelength apart, but the wave velocity along the coax is only 2/3rds the speed of light (roughly 2/3rds, it will depend on the actual cable you've used), so you're not driving the front pair 90 degrees out of phase with the back pair (it's more like 60 degrees). See if it comes right if you simply extend the coax length by roughly half as much again [multiply by the inverse of the velocity factor for the actual cable if you have the details for it].

By the way, paralleling the dipoles like this isn't necessarily a very nice load for your transmitter. If it's a very valuable piece of kit, you may want to reconsider what you're doing.

The other thing that stands out from your pictures is how you bring the feed down near the lower dipole element. I would think you would need to move the feed further back if you want really clean radiation patterns. The lower dipole element isn't a ground - it has rf currents running in it and radiates [think of it as being driven by the top element through the electric field that forms between the two]. Having the shield of the coax nearby has as a couple of effects. It will mess up the symmetry of the electrical field between the higher and lower elements of the dipole, so skewing the radiation pattern. Also, to some extent it will passively receive and reradiate. Both will mess up your radiation patterns a bit (and in quite a complex way where you've got it coming across on the diagonal).