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Author Topic: Vertical Half Rhombic  (Read 475 times)
AB2SW
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« on: November 12, 2005, 03:37:31 AM »

Has anyone ever tried the Half Rhombic? The full Rhombic takes up so much space and wonder if it's really worth it. I know the half Rhombic is an old military antenna. Any info on dimensions and results?
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W8JI
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« Reply #1 on: November 12, 2005, 03:49:38 AM »

I was thinking of posting a question like that myself.

I had a pair of Inverted V antennas (half Rhombic on the side) for 160 and 80 on a BC tower in the 70's. Quite frankly it wasn't very good. It was about like my 1/4 wl vertical for transmitting.

I started modeling V beams and Inverted V antennas a few years ago, since I now have a 318 foot tall tower of my own.

I can't get a model of either antenna to show useful gain. I'm curious if anyone else has sucessfully modeled a V beam or Inverted V. I'd really like to install one, but can't find any combination that produces useful gain over a dipole.

73 Tom
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RobertKoernerExAE7G
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« Reply #2 on: November 12, 2005, 07:03:55 AM »

http://lists.contesting.com/archives/html/Towertalk/1998-04/msg00902.html

Not related, but an interesting read:
http://www.borg.com/~warrend/guru.html

Bob
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W5DXP
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« Reply #3 on: November 12, 2005, 03:03:50 PM »

A vertical half-rhombic? A rhombic is an end-fire antenna. Wouldn't a vertical rhombic be an ideal NVIS antenna, i.e. radiation nearly straight up?
--
73, Cecil, W5DXP
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W8JI
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« Reply #4 on: November 12, 2005, 04:42:51 PM »

They fire along the horizon Cecil.

You start out at the ground with the feedpoint, go up over  support, and back down to the ground termination.

I had trouble getting one to work, and so I ran coax to each end with matching transformers.

Instead of terminating I recombined the power back in with a T network for phase and level adjustment. It was fair doing that, but I was at a BC station in a wetland marsh and it never had the gain books predicted. (That was before modeling programs.)

I've been looking at models here for the past few years, since I have a 318 foot tower (and can go 20 feet above the light with a support pole). I can barely get models to show gain over a vertical. So I'm thinking maybe my fond memories of how it worked have been exaggerated by time. Like how the girl from HS looked, you know?

Anyway, I sure can't get a model of that antenna to work worth a hoot.

73 Tom
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WB2WIK
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« Reply #5 on: November 12, 2005, 05:37:21 PM »

>RE: Vertical Half Rhombic  Reply  
by W8JI on November 12, 2005  Mail this to a friend!  
They fire along the horizon Cecil.
You start out at the ground with the feedpoint, go up over support, and back down to the ground termination.<

::Like the B&W antenna that mostly warms tree roots?  I installed one of those once.  It had a resistive "balancing network" (yeah) and I found out 1 kW applied to that for a little while makes it very unbalanced when it melts.

>So I'm thinking maybe my fond memories of how it worked have been exaggerated by time. Like how the girl from HS looked, you know?<

::She probably was that hot at the time, but maybe didn't age well. Class reunions hold many surprises.  I found the girls that looked great when they were 17 looked great because they looked 25.  20 years later a lot of the ones who looked like little girls in HS now looked 35 and were knockouts, while the others who matured faster just kept maturing faster and looked like they should be retired.  There are exceptions, for great genes.  My memories of being a Novice are very fond, and each antenna I installed worked great.  The bands were always open and my 40m crystals were always on the same frequency as Radio Moscow, but every single memory is stellar.  I could swear my 40m folded dipole made from twin lead and fed with twin lead absolutely had 20 dB gain over my previous single-wire dipole fed with coax...

::Every time I hear of a new wire antenna idea, or almost any kind of new antenna idea, I always fall back to thinking that if it was so great, contesters with unlimited budgets and space would be using them.  

-WB2WIK/6
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KB4QAA
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« Reply #6 on: November 13, 2005, 12:16:50 PM »

We just had a lengthy discussion on this same topic back in September.  If you use the search box in upper right corner type in "rhombic" you can find all the threads on any particular topic.

http://www.eham.net/forums/Elmers/98443?page=1

Quote

If you want to put up a rhombic with gain and directionality for 28 MHz it needs to be mounted horizontally high above the ground, and at least 3 wavelengths long (+90 ft) to equal or better a dipole that is only 16 ft long. The longer the better.

-   The ARRL Antenna Book, 20th Edition, 2003, chapter 13 is entirely about long wave antennas. Figure 1 shows that you need to make a rhombic antennas in free space at least 2 wavelengths long per side to equal or better a simple dipole antenna. Therefore, you are not gaining anything by using a half-wave rhombic. The main advantage of a full wave rhombic is extremely narrow directionality of about 30 deg. Though I can't find a chart a 1/2 wave rhombic is going to be much less directional, again, essentially losing any advantage over a dipole's directionality.

…army references are tried and true antennas but often field expedient solutions using limited equipment (especially no coax) for broad frequency coverage, not necessarily the best performance.
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W8JI
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« Reply #7 on: November 14, 2005, 06:24:00 AM »

by KB4QAA
narrow directionality of about 30 deg. Though I can't find a chart a 1/2 wave rhombic is going to be much less directional, again, essentially losing any advantage over a dipole's directionality.>>

Umm, he never said 1/2 wave long Rhombic. ;-)

He said half Rhombic, or Inverted V antenna (half Rhombic turned on side so bend is at apex).


I'm also wondering if anyone has modeled a Inverted V (a TRUE Inverted V antenna, not a Inverted Vee dipole) that actually has significant gain.

73 Tom
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