Hi Guys,
The main problem I'm having with omni antennas for sat work is the mounting height. In short, this is the dilemma: - Mount it too low: horizon is obstructed badly - Mount it too high: antenna radiation pattern is distorted badly
In a lot of simulations you find online the ground systems of omni antennas (radials, screen, you name it) are modeled in the same plane as the earth surface, simulating a situation where the antenna is mounted very low. Or worse: the antenna is modeled over perfect earth, as if the ground system was solid, perfectly conducting and infinite in size (could be a good approximation at sea, but definitely not in a typical home situation). To simulate a more realistic situation I modeled the ground system (radials) at the actual mounting height above the earth surface, with the radiating elements above it.
In my case the antenna would be mounted at about 10 meters (33 feet) height to get a clear view of the horizon. What I see when I simulate that situation is that the ground system (radials) on an eggbeater works perfectly to fix the near field of the antenna, i.e. to get a nice SWR and even push the radiation pattern upwards, but it does little about the far field of the antenna. What you typically see when you mount a (partially) horizontally polarized omni at several times the wavelengths above earth, you get very deep peaks and throughs varying with elevation. This is because the signal arrives via several paths: it arrives directly, but also via earth reflections. This results in constructive or destructive interference depending on the elevation angle. This can be clearly observed in this eggbeater simulation:
Eggbeater at 10m: http://postimg.org/image/p973lofrz/
Other (partially) horizontal omni's show the same effect. For example this is a Lindenblad as I have built (EZ Lindenblad design by Tony Monteiro AA2TX):
Lindenblad at 10m: http://postimg.org/image/rrssmd1i7/
Of course these simulations don't take into account that the antenna isn't mounted in free space but that there are buildings, trees, etc. nearby, but the effect was pretty pronounced when I was testing my Lindenblad at about 10m height at a recent ARISS contact, notice the sharp signal drops every now and then. This is not caused by polarization mismatch since a Lindenblad is cross polarized for every elevation angle, unlike an eggbeater which becomes horizontal near the horizon.
Contact: https://www.youtube.com/watch?v=6H6lqluWLwc Antenna situation: https://twitter.com/tete_de_moine/status/706085371873005569
For uplink this is not a problem, since even with 5 watts the uplink power is usually orders of magnitude bigger than the downlink power and I don't really notice the peaks and throughs. But on downlink this could really be a dealbreaker on difficult QSO's, but of course as always, ymmv. The amount of peaks and throughs increases as you mount the antenna higher, so if you are able to mount the antenna very low and still see the horizon this might not be an issue.
Now I'm wondering, would you see the same effect with a yagi when it is mounted at several wavelengths height and pointed to the horizon? Especially near sea this effect should be pretty pronounced. Did anyone ever experience this? Maybe I should spend some simulation time on this..
73, Rico PA3RVG
On Wed, Mar 16, 2016 at 1:13 AM, Dick Illman [email protected] wrote:
I have successfully used UHF and VHF Egg Beaters in my attic for years. I used a TS2000 and ARR transceiver preamps (TS2000 is pretty deaf). I had 100 feet of 9913 cable.
73 Dick Illman AH6EZ/W7 _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb