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80m OCFD NVIS


Paul Butzi (W7PFB)
 

Last night Rowland had an excellent question about how a 135’ OCFD antenna would work for NVIS, given that he’s made plenty of DX contacts using his OCFD.

This morning I ran some simulations of a 135 foot OCFD rigged approximately the way you’d have it if you followed the instructions from Buckmaster, with feedpoint about 33 feet AGL (Above Ground Level) and the ends about 15 feet - in other words, an inverted V.

The vertical radiation pattern you get is this:



That’s not an ideal NVIS pattern because it has more gain down near the horizontal (below, say, 30 degrees elevation) than you might like, but it’s giving you 4.5 dBi straight up so it would be a pretty good NVIS performer.

Rigged with the apex of the V up about 55 feet with the same leg drop improves NVIS performance by about 3 dB.

So what do you do if you want your OCFD to offer optimal DX performance instead of NVIS on the 80m band?

Raising the inverted V so the apex is at 45 meters gets you the following vertical pattern:


That gives you about 8.5 dBi at 26 degree takeoff angle, and about -1.5 dBi at 90 degrees.  It’s far more sensitive to both local and DX noise (think thunderstorms over Connecticut) and way less sensitive to NVIS signals.  It’s also not a circular pattern in the horizontal plane - it’s not shown in that diagram but all that gain is broadside to the axis of the antenna.  So if you decide to put your OCFD up at 145 feet, you want to think about how it’s oriented N/S/E/W.




-p W7PFB
73, Don’t forget to smile and have fun!


Rowland
 

Right now my OCFD is about 30-40' (don't know for sure). And the ends are only about 3' off the ground. 

Two questions...

1. Will adding a reflector on the ground improve my NVIS?**
2. Will raising the end wires to the 15' are improve things?

**  I will not be able to have the reflector wire exactly under the OCFD due to physical ground issues but it will be close


Thanks,

Rowland



On Fri, Apr 3, 2020 at 10:54 AM Paul Butzi (W7PFB) <w7pfb@...> wrote:
Last night Rowland had an excellent question about how a 135’ OCFD antenna would work for NVIS, given that he’s made plenty of DX contacts using his OCFD.

This morning I ran some simulations of a 135 foot OCFD rigged approximately the way you’d have it if you followed the instructions from Buckmaster, with feedpoint about 33 feet AGL (Above Ground Level) and the ends about 15 feet - in other words, an inverted V.

The vertical radiation pattern you get is this:



That’s not an ideal NVIS pattern because it has more gain down near the horizontal (below, say, 30 degrees elevation) than you might like, but it’s giving you 4.5 dBi straight up so it would be a pretty good NVIS performer.

Rigged with the apex of the V up about 55 feet with the same leg drop improves NVIS performance by about 3 dB.

So what do you do if you want your OCFD to offer optimal DX performance instead of NVIS on the 80m band?

Raising the inverted V so the apex is at 45 meters gets you the following vertical pattern:


That gives you about 8.5 dBi at 26 degree takeoff angle, and about -1.5 dBi at 90 degrees.  It’s far more sensitive to both local and DX noise (think thunderstorms over Connecticut) and way less sensitive to NVIS signals.  It’s also not a circular pattern in the horizontal plane - it’s not shown in that diagram but all that gain is broadside to the axis of the antenna.  So if you decide to put your OCFD up at 145 feet, you want to think about how it’s oriented N/S/E/W.




-p W7PFB
73, Don’t forget to smile and have fun!


Paul Butzi (W7PFB)
 

On Apr 3, 2020, at 11:25 AM, Rowland <@Rowland> wrote:

Right now my OCFD is about 30-40' (don't know for sure). And the ends are only about 3' off the ground.

Two questions...

1. Will adding a reflector on the ground improve my NVIS?**
Without modeling it it’s hard to know for sure but I’m guessing yes, it will improve NVIS performance on 80m and probably on 40m. But I’ll model it and we’ll see.

2. Will raising the end wires to the 15' are improve things?
Improve things for NVIS or DX? 80m is the only band of interest, or 80m - 6m?

Again I haven’t modeled it (I will, though) but I’m guessing raising the ends improves the radiation pattern. But it will probably also change SWR.


** I will not be able to have the reflector wire exactly under the OCFD due to physical ground issues but it will be close
I think as long as the reflector is the right length and more or less under the antenna it would not make much difference.


-p W7PFB
73, Don’t forget to smile and have fun!