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-   -   Rhombic Designs - Modeled and/or Built (http://forum.tvfool.com/showthread.php?t=2865)

GroundUrMast 3-Dec-2011 8:15 AM

Rhombic Designs - Modeled and/or Built
 
There is an existing thread that started with historical rhombic information.

This thread is intended for modeled and built examples.

The following material may be of interest however. (From the earlier rhombic thread):

Quote:

Here is some additional information, including a link to "Edmund Laport's textbook Radio Antenna Engineering, published in 1952 and now out of copyright and freely available, scanned and processed into PDF by Dave Platt, AE6EO." (This is a 25 MB file)

http://www.cromwell-intl.com/radio/r...-antennas.html

Rhombic antennas are dealt with beginning on page 315.

And another collection of information re. the Rhombic: http://www.dxzone.com/catalog/Antennas/Rhombic/

GroundUrMast 3-Dec-2011 8:27 AM

A basic single Rhombic - 5 wavelength per leg. Gain > 16.6 dBi on all lower UHF CHs
 
1 Attachment(s)
The attached OpenOffice document includes screen shots of geometry, gain graph and plot, SWR and the 4NEC2 code used to produce the model.

The Rhombic has strong points and weak. It's cheap to build. It's big and difficult to turn.

The code is overly complex... I was using it to model a dual rhombic with dis-similar leg lengths. I love being able to use the SY cards to make code that can be adapted to variations of a geometry.

Credit to holl_ands for his input re. FR & RP cards.

GroundUrMast 3-Dec-2011 8:55 AM

A Quad-Stacked Rhombic - 21 dBi per 4NEC2
 
1 Attachment(s)
Here are screen shots of geometry, gain graph and plot, SWR and the 4NEC2 code used to produce the model.

There is trouble with SWR still to be resolved.

GroundUrMast 4-Dec-2011 5:16 AM

Dual Stacked 5 Wavelengths per leg. 17.9 dBi
 
1 Attachment(s)
Screen shots of geometry, gain graph and plot, SWR and the 4NEC2 code used to produce the model.

Very flat Gain across the entire UHF band and excellent SWR.

Some screen shots imply this is a 4 wavelength per leg design. This is 5 WL / leg. I scaled this up from a smaller design and failed to catch the file naming convention.

ghz24 4-Dec-2011 11:41 PM

VHF high for the 21 dBi quad stack
 
I noticed you didn't post the gains at vhf high for the quad stack above.
Code:

Freq        net gain
177        7.37
183        7.81
189        8.29
195        8.69
201        9.19
207        9.31
213        9.90

and that's not the highest the main lobe is forked with the highest gains
at +- about 20 degrees from horizontal and look like this
12.26
12.98
12.70
13.26
13.19
13.49
13.64

SWR is excessive at channel 7 but good enough every where else.

GroundUrMast 5-Dec-2011 7:27 PM

Re. the Quad Stack 9-WL ver 1.0, my motive has been one of learning how to use 4NEC2 and investigate the possibility of using a Rhombic to pursue a single weak UHF signal.

It's becoming obvious that using phasing lines (transmission lines connected as quarter-wave transformers) to combine the four sections introduces enough tuned elements to the system so that one can only hope to get 1/3 to 1/2 of the UHF band covered while keeping the SWR under 2.0.

Hybrid combiners generally offer much greater bandwidth but introduce losses that would reduce the overall gain of the system to the point that a dual stack would out perform. So now I get to go on a quest for a 'lossless wide-band combining network'.

Re. broadband versions, I have briefly looked at the VHF performance of 1 and 2 Lambda/Leg UHF rhombics. I've got real channel 13 & 14 to the SW of my location and would like a modest gain broadband antenna dedicated to those two signals. @ ghz24, Are you looking for something similar?

ghz24 10-Dec-2011 6:50 AM

I am on a quest to optimize my reception.

Commercial antennas mostly leave me cold, the manufactures have a vested interest in hiding the flaws/weaknesses of their designs and seem to excel at saying almost nothing in a way that makes their antenna sound like the Arecibo dish even though it maybe a small uhf loop with a pair of rabbit ears.

I know it's not limited to antenna makers, "lying with statistics" is very common now days and the ones doing the misleading don't seem to suffer any repercussions.
In short I think I can design and build a better (for me) antenna. Sturdier because I don't have to worry about shipping it, and better suited to the conditions where it will be used.
I started modeling to get dimensions from models others posted, and to see how complicated the program was to use.( had heard it was complicated )
After poking around for awhile and trying out the samples it didn't seem that complex.
Then became enamored with the whole, room full of engineers with slide rules
would take weeks to do what my desktop could do in 10 minutes power trip.
Now my curiosity is what drives me to model. Always what if ...
Rhombics caught my attention early on and some of my first models were rhombic designs. But ~12 foot models I considered performed lower or equal to the improved GH designs.
I guess I'll soon start a thread in reception help (not for antenna advise but to ask about amps)


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