On Thu, 08 Jul 2004 09:10:41 -0700, Wes wrote:

On Wed, 07 Jul 2004 19:16:36 -0400, Albert wrote:

|OK, thanks Richard.
|
|How about this.
|
|30 wavelengths per side, average conducting ground, 12 feet above
|ground.
|
|What angle do I need for best forward gain, what will the takeoff
|angle be at that angle?

Included angle from 12 to 16 degrees will give ~20 dBi at 6 deg
elevation.
|
|If I can get the entire antenna up to 20 feet above ground, how much
|better is the performance?

~ 2 dB more gain at 4 deg el.
|
|Is there any way for me to do modelling myself without burdening you
|or someone with a modeling program? I am not aware of free software
|that will model rhombics or vee's.

4nec2 is free, do a Google search

Multinec is cheap and very flexible.

|
|Thanks,
|
|Art
|
|ky1k at pivot dot net
|
|PS: Some additional info. This is for EME, which does not require much
|movement. Since it is so long, it won't be variable in the elevation
|angle anyway. But, I hope to move one wire a bit so I can steer the
|beam to the left or to the right a little (changing the angle of the
|vee at the same time).

Since I worked VK5MC to complete my 2-meter WAC and he used a slightly
steerable rhombic, I'm not going to say that this won't work, but I
must question why. If you're going to limit yourself to a few minutes
of moon time a month, why not just put up a long fixed Yagi and use it
for both transmit and receive. A forty-foot long Yagi will give the
same gain as the vee, with a *huge* improvement in the pattern and it
will have a much more tractable feedpoint impedance.

Wes

Hi Wes,

Thanks for the information, it's just what I needed.

I am not sure how dbd converts to dbi, but a 100 foot 43 element rope
yagi gives 19 dbd.

My motivation for starting this was that I had the room for a large V
beam and that it might be practical if I could get the same gain as
the big rope yagi.

The V beam is way easier to put up and much cheaper and simpler to
construct.

From looking at data from smaller rhombics, it appears they are poor
for receiving as they have some major lobes that point towards the
high noise temperature Earth, which is why I wanted the V for transmit
only. I don't think the radiation off the back of the antenna will do
any harm in transmit only, so my hope was that it does not need to be
terminated.

Your gain figures indicate this bears further investigation, even
though I would be dependent on moon set or moon rise to make my Q's.
But, people with yagis have had that limitation and live with it just
fine.

I'll look up 4nec2 and try to play with it myself. Hopefully, it will
handle large V's.

Regards,

A


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