The definition of a rhombus is defined more correctly as a quadrilateral of
equal side, and two acute, and two obtuse angles.
If you want to be more pedantic, you can describe them as orthogonal angles.


The use of the word "diamond" introduces a degree of interpretation rather
than fact.


73

David W Wood 

-----Original Message-----
From: For blind ham radio operators [mailto:[log in to unmask]]
On Behalf Of Colin McDonald
Sent: Saturday, October 11, 2014 6:10 PM
To: [log in to unmask]
Subject: Re: HMMMMM a Rhombic sky fence?

I was doing a bunch of reading last night on wb8JI's website about 
Rhombic's, V-beams and curtain arrays.
Smart guy.
Interesting though how he broke down the space to gain ratio and though a 
rhombic does present significant gain over a dipole, it certainly takes up 
an awful lot of space compared to say a pair of stacked 3 element beams.
 when you start breaking it down, it doesn't present much gain on bands that

the antenna is not specifically tuned for...it will present low swr yes, but

the gain factor goes down dramatically out of band, and so it really doesn't

give you better performance than a standard dipole on those bands.  At least

from a forward gain perspective.
So, at the end of the day, a pair of 3 element beams for 40M on a tower will

take up less space, present the same forward gain, and you can use standard 
dipoles for lower bands and get similar results as the Rhombic would present

on those bands.
Of course, the trade off with a rhombic is that if you have the real estate,

it's far cheaper and easier to install a rhombic than 2 3 element 40M beams 
at 200 feet.

I checked wikipedia for definitions of a Rhombus, and it's basically a 
diamond shape with 4 equal sides.  The angle at the ends and sides does have

an effect on forward gain etc so there is an optimal shape depending on how 
many wave lengths each leg is.

There is the open ended unterminated Rhombic which presents 
bi-directionality.  Then there is the closed Rhombic terminated at the end 
opposite of the feed point with an 800Ohm resister which makes the antenna 
directional.
There is also a design which uses a low loss feedline from the resister end 
back to the feed point to maximize antenna efficiency.  A Rhombic only has 
about 47 to 50 percent efficiency over perfect earth and at a minimum of a 
half  wave length above ground.  So the extra feed line apparently feeds 
what would normally be lost at the end of the antenna back into the feed 
point and you can supposedly get much higher efficiency out of it that way.
From what I read, the optimal shape is 70Degree angles at the ends, and 
110degree angles at the sides.  Though you can widen this out a bit and get 
slightly bettter bandwidth but reduced performance on the primary band.
WB8JI does not really talk much about receiving performance of a 2 wave 
length Rhombic over say a normal Dipole.  He just refers to forward gain 
factors.  Probably because receiving performance is very very difficult to 
quantify and theorize...whereas what you get on the transmitting side can be

quantified and figured out from a scientific perspective.
I won't even get into curtain arrays because it gets mighty complicated at 
that point.

73
Colin, V A6BKX
--------------------------------------------------
From: "Mike Duke, K5XU" <[log in to unmask]>
Sent: Saturday, October 11, 2014 7:13 AM
To: <[log in to unmask]>
Subject: Re: HMMMMM a Rhombic sky fence?

> Yes, I think it's a minimum of 1 wave length per leg, but multiple wave
> lengths mean more gain.
>
> Most people would optimize their rhombic for a given band, say 20 or 15
> meters, and then use it on any of the other bands where they could get
> it to tune.
>
> On the main band, the strongest gain is in two directions. I remember
> hearing W7LUN say that his favored Indianapolis, and Melbourne,
> Australia on 15 meters.
>
> There is also a terminated rhombic, which has a resistance at the end
> opposite the feed point. That changes both the gain and the
> directivity, but I don't know exactly in what ways.
>
> The legendary Don Wallace, the original holder of W6AM, had 100 foot
> tall utility type wooden poles that held multiple rhombics, each
> favoring different directions.
>
> His family made a mint worth of money when that system was dismantled
> and the property sold to developers.
>
> -- 
> Mike Duke, K5XU