Do you make this screen grid or is there a type of object in 4Nec2 that does that as a choice?
I see this feature.
I have used the geometry builder in the past, but I don't really use it much anymore. More recently I tend to make this type of thing by hand, in most cases it really isn't that much slower for me anyway, and I can make certain customization that said program don't include.
Nice one The DB.. I have a 1:1 choke balun as feed point on my GPA (although initial test was using just a S0-239 not a baanced antenna so not strictly necessary as I understand) I went for a 1:1 choke/balun as it made the feed point and antenna more robust and "belt and braces" for the CMC suppression. I have yet to test the balun version but it sits here next to me in readiness.
I guess you used a mast as the radials resonance will become blurred as they hit the ground.
There are a lot of people the subscribe to the idea that balance is why CMC's happen. I am not one of them. To me, its just a matter the impedance of the different paths. AC current takes the path of least impedance.
Here is an example for you. You have a full wavelength doublet, an electrical half wavelength on either side. Their is also a matching system, say an L network, or perhaps a stub match, to match the 1000+ ohms of impedance that this antenna will have at the feed point down to 50 ohms.
In spite of it being a balanced antenna, you will almost always have a significant amount of CMC's with this antenna unless you get a very lucky feed line length. Why? Because in most cases the outside of the coax shield will present a lower impedance than the side of the antenna said shield is attached to... You can essentially see it as the same as a parallel resistor network as far as RF current is concerned...
It is important to note that with a choke you are not cutting off the current path that is the outside of the coax, you are simply raising the impedance of said coax shielding to be noticeably higher than where you want to current to flow. The antenna isn't any more or less balanced than before. All you really did was change the equation between where the currents will flow. Actually, taking this a step further, a choke right at this antenna's feed point will have little effect, the reason? There is almost no current flowing at the feed point of this antenna and RF chokes are current devices. Putting the choke about 1/4 wavelength down the coax from said feed point will be more effective.
Another spanner in the works with this antenna is that the radials are commonly bent to 43 degrees to match 50 Ohms (as is my own) and not left at 90 degrees to the radiating element. As I gather this also affects the radiation angle.
Yes said radials are commonly angled down, although not in the models I created as I used the model in sp5it's link as a base. On that page, they clearly show a 1/4 wavelength antenna with 4 horizontal radials, so that is what I used. Although in that case there was no mast, and I did use a mast...
I've done a study on this a while back, and when it comes to the idea that angling radials down from horizontal will lower an antennas radiation angle, this is a common idea that I have seen many very knowledgeable people post as a fact. I have seen some evidence against this, and none for it. I don't want to definitively say it can't happen, I'm just saying the evidence that I have seen does not support this idea. That being said, there are a lot of thing that many people think affect an antennas radiation angle than I have found at best suspect over the years, so this is no surprise to me.
I have always had a fond place in my heart for the humble GPA.. simple as it gets and it works. It may not be a high gain mega blaster of an antenna but it is an effective antenna and looks great as well.
I would be very interested to see a "ground mount" model though when radials are bent 43 degrees feed point will likely be higher up depending on what band you are using the GPA for.
Assuming by GPA, you means ground plane with angled radials, I have read in engineering texts about impedance tuning on near earth mounted antennas, including lowering an antenna's impedance by mounting the antenna some distance above the earth, angling the radials down until they reach the earth, and then runing along the earth. They also talk about raising the impedance by putting the feed point below the earth in a hole, then running the radials at an angle some distance up to the earths surface, and then running said radials along the earth. The problem here is, like any near earth mounted antenna, you really do need more than three or four radials to make the antenna efficient, and I don't mean just a few more...
The DB
No one knows it all. I only know what I know because I was selectively listening to those who I knew know much much more than I probably ever will.
